Dynamic cusp aurora and associated pulsed reverse convection during northward interplanetary magnetic

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. A6, PAGES 12,869-12,894, JUNE 1, 2000 Dynamic cusp aurora and associated pulsed reverse convection during northward interplanetary magnetic field P. E. Sandholt l, C. J. Farrugia 2, S. W. H. Cowley 3, M. Lester 3, W. F. Denig 4, J.-C. Cerisier, S. E. Milan, J. Moen e, E. Trondsen l, and B. Lybekk 1 Abstract. We report a study of ionospheric signatures of plasma entry and momentum transfer at the dayside magnetopause during northward oriented interplanetary magnetic field (IMF), combininground observations of the dayside aurora and ionospheric ion drift (CUTLASS HF radar) with simultaneous particle precipitation data obtained from three overflights by the Defence Meteorological Satellite Program (DMSP) F12, F13 and F14 spacecraft. The observations were taken during a 37-rain long interval of strongly northward IMF (Bz-7 nt; clock angle - 10ø-15 ø) after a rapid northward turning. The meridan scanning photometer at the ground station recorded a long stepwise poleward retraction and latitudinal widening of the band of auroral emission in the cusp region. Thus the activity includes a series of episodes which are characterized by an initial 1-2 rain poleward "step" of the auroral poleward boundary, followed by a,- 3-4 rain period of relatively steady uror l latitude. The uror l events were accompanied by bursts of"reverse" two-cell convection characterized by equatorward flow across the cusp poleward boundary. The three DMSP spacecraft, which traversed the polew rd boundary of the cusp aurora from north to south, entered into a region of auroral precipitation where electrons and ions of magnetosheath origin were present, together with equatorw rd convection. The observations are found to be consistent with a theoretical description of a sequence of bursts of lobe reconnection involving both hemispheres. This process results in the capture of magnetosheath flux tubes and thereby closed flux is added to the dayside m gnetosphere. 1. Introduction Ground observations of the optical aurora and ionospheric ion drift in the cusp region represent important tools in the study of solar wind/magnetosphere coupling processes at the magnetopause since they complement in situ measurements, and, in addition, they have the distinct advantage of continuity of coverage. Much progress has been made in recent years in identifying the auroral response in the cusp region to various Department of Physics, University of Oslo, Oslo, Norway. ' Space Science Center, University of New Hampshire, Durham. 3Department of Physics and Astronomy, University of Leicester, Leicester, United Kingdom. 4Space Vehicles Directorate, Air Force Research Laboratory, Hanscom Air Force Base, Massachusetts. 5Centre d'etude des Environnements Terrestre et Planetaires, Saint-Maur, France euniversity Cources on Svalbard, Longyearbyen, Svalbard, Norway. Copyright 2000 by the American Geophysical Union. Paper number 2000JA /00/2000JA ,869 interplanetary conditions [e.g., Murphree et al., 1990; Jankowska et al., 1990; Elphinston et al., 1993; Sandholt et al., 1993, 1996a, b; Oieroset et al., 1997; Farrugia et al., 1995; Sandholt et al., 1998b, 1999c; Sandholt and Farrugia, 1999]. It has been found from these investigations that two different cusp-type auroral forms exist depending on whether the interplanetary magnetic field (IMF) points northward southward [Sandholt et al., 1996a]. These forms are typically located at and magnetic latitude (MLAT), for IMF north and south, respectively. Both auroral forms result from plasma entry from the solar wind/magnetosheath, but the IMF-magnetosphere interconnection geometry is different. There are many studies of the cusp region auroral activities related to magnetopause plasma transfer mechanisms when the IMF is southward pointing, including the characteristic sequence of equatorward boundary intensifications/poleward moving auroral forms (PMAFs) and the IMF By-regulated prenoon-postnoon asymmetries [e.g., Sandholt et al., 1990, 1993; Sandholt and Farrugia, 1999; Moen et al., 1999]. The pattern of ionospheric ion drift in the vicinity of P MAFs has been reported in a recent study by A. Thorolfsson et al., Simultaneous optical and radar signatures of poleward mov-

2 12,870 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA ing auroral forms, submitted to Ann. Geophys., 1999 and Rich, 1993]. This sequential lobe reconnection pro- (hereinaftereferred to as Thorolfsson et al. (submitted cess is one of the ways to populate the magnetospheric manuscript, 1999)). Less information exists on the detailed structure and dynamics of the aurora occurring in the cusp region when the IMF has a strong northward boundary layer with magnetosheath plasma on closed field lines and has been modeled by Song and Russell [1992](see review by Lotko and Sonnerup [1995]). component, what has been called type 2 aurora [Sand- Pirst observations of"reverse convection" were made holt et al., 1996a]. This aurora is characterized by a by Maezawa [1976] who interpreted ground magnetomesequence of poleward boundary intensifications [Sand- ter readings in terms of lobe reconnection. More recent holt et al., 1998a]. It has been suggested that these observations of"reverse convection" were reported durforms are closely associated with magnetic reconnection ing the early part of the passage at Earth of the magat high magnetopause latitudes [Murphree et al., 1990; netic cloud on January 14-15, 1988, when the cloud Sandholt et al., 1996a; Oieroset et al., 1997; Sandholt field was strongly northward pointing [Curehock et al., et al., 1998a; Lockwood and Moen, 1999; Sandholt et al., 1999b]. The possibility of a northward-pointing IMP reconnecting with the Earth's field was first recognized by 1992; Freeman et al., 1993; Knipp et al., 1993]. Consistent with the ideas of Cowley [1981] and Crooker [1992], these authors found that strong "reverse convection" existed in the southern summer hemisphere, Dungey [1963]. He considered reconnection to occur while simultaneously, convection in the northern wintailward of the cusp, where the two fields have strong antiparallel components. In Dungey's model the IMP ter hemisphere was weak and organized only on small spatial scales. These authors also showed that the conreconnects with closed terrestrial field lines in both hemispheres simultaneously. If the process persists for some time this leads to an ionospheric convection pattern with sunward flow over the poles, so-called "reverse convection," in both hemispheres. This terminology reflects the fact that under usual conditions, with an interplanetary field that is pointed either southward or not strongly northward, the "normal convection" flow driven by low-latitude reconnection is twin celled with antisunward flow over the poles [Dungey, 1961]. Crooker [1992] refers to the "reverse convection" for northward IMP as "merging cells," a numerical model of which has recently been presented by Song dition I Bul Bz marks approximately the transition from low-latitude reconnection (I By ]> Bz) to reconnection poleward of the cusp(] By I < B ) as the IMF clock angle in the GSM Y-Z plane decreases from 60 to 30 degrees. Direct in situ satellite observations of flows during reconnection poleward of the cusp were reported by Kessel et al. [1996]. The sensitive response of reconnection of a northward-pointing IMF with the geomagnetic field to the components of the IMP, and the possibility of mode switching, was highlighted by the work of Grenwald et al. [1995], who used the Super Dual Auroral Radar Network to suggest that lobe reconnection is replaced et al. [1999]. Russell [1972] suggested the possibility by merging cell reconnection when the magnitude of the of a northward IMF reconnecting with the open lobe IMP By component goes from being larger than the Bz field in one lobe only. This merging is also accompa- component to being less than it. Small and disordered nied by reverse convection, which is decoupled between the two hemispheres. Reiff and Burch [1985] refer to this variant as "lobe merging." flows were observed when By and Bz were of comparable magnitude. This chaotic state could be a signature of transient reconnection, according to Greenwald et al. The existence of these two possibilities for reconnec- [1995]. In a large statistical study, Ruohoniemi and tion under northward IMP opens the possibility of multi Greenwald [1996] found evidence for well-ordered (almode interactions and transitions between modes. One though weak) reverse convection in both hemispheres important feature of reconnection under northward IMP when Bz >lbu I, but that the lobe cells tended to occur is the sensitivity of the process to other IMP compo- preferentially in one hemisphere only when I Byl > B. nents. As shown by Cowley [1981] and Crooker [1992], It is clear from the foregoing that more work is needed the IMP B component introduces an important variant to fully understand effects in the ionosphere of reconon the two basic topologies mentioned above. It is rec- nection under northward IMP conditions. We feel that ognized that dipole tilt and/or an IMF tilt can favor one "all-sky" auroral observations in combination with radar hemisphere over the other. Thus reconnection is more likely to occur in the summer hemisphere, since the sunmeasurements of ionospheric ion drift and magnetometer data are crucial elements in this. ward dipole tilt produces an antiparallel configuration Recent numerical simulation studies of the interacjust tailward of the cusp. Similarly, if the IMP is tilted tion between the northward oriented IMP and the mag- antisunward (IMP B < 0), the northern hemisphere netosphere-ionosphere system show the presence of highfavored over the southern as a site of reconnection, since latitude (lobe) reconnection and associated so-called the IMP is antiparallel to the geomagnetic field there. It northward B (NBZ) field-aligned currents and ionohas been suggested that reconnection in the unfavoured spheric "reverse" convection cells [Fedder and Lyon, hemisphere may eventually be activated at a later stage, 1995; Mobarty et al., 1996; Song et al., 1999]. involving the overdraped open magnetic flux resulting With the aim of testing such model predictions with from reconnection in the favored hemisphere [Crooker ionospheric observations we have selected an interval

3 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA 12,871 of good coverage of auroral, magnetic, and ionospheric ion drift observations in the northern winter hemisphere during a 37-min period of strongly positive Bz and Bx and small By component, following a rapid northward turning. The data refer to December 16, 1998, and were acquired by (1) the optical meridian scanning photome- ters (MSPs) and an all-sky camerat Ny J lesund in the Northern Hemisphere at 760 MLAT, (2) ionospheric ion drift observations obtained by the Co-operative UK Twin Located Auroral Sounding System (CUTLASS) Finland radar, (3) ground magnetic deflections obtained by the International Monitor for Auroral and Geomagnetic Effects (IMAGE) chain of magnetometers in Svalbard and Scandinavia, and (4) the particle and ion drift detector(s) on the Defense Meteorological Satellite Program (DMSP) satellites F12, F13, and F14. The interplanetary data are from the Wind spacecraft located in the solar wind on the dawnside of the magnetosphere at (5,-28, 19) /r/6(gse coordinates). We find a good correspondence between the model of Song and Russell [1992] and data features and are also able to provide relevant timescales for the individual steps making up each reconnection episode. In this case, the ionospheric observations indicate that there was a sequence of lobe reconnection events which are marked by a characteristic expansion/contraction of the cusp aurora, the cumulative effect of which is a significant polar cap contraction and latitudinal thickening of the cusp emission. An aspect of the present case study which is different from the previously published northward IMF cases of cusp aurora is the small (negative) By component, as compared to the much larger (positive) Bx and Bz components, which is a favorable condition for exciting conjugate two-lobe reconnection, that is, the Song and Russell mechanism. The presently available observational evidence at low altitudes for the occurrence of 15ø), with Bz - 7 nt, B nt and the minor conjugate high-latitude reconnection is scarce [Feldman component By 0-2 nt. The temperature and the bulk ½t al., 1995]. The critical evidence in favor of such a sce- speed remained relatively constant at 3 x l0 s K and 540 nario in the case reported here is the poleward stepwise km s -, respectively. The period of strongly northward thickening of the band of cusp aurora and a "reverse" IMF orientation (small clock angle) lasted until two-cell convection in the northern winter hemisphere UT, after which the IMF was dominated by the B comprevailing at times when we infer reconnection at southern high latitudes to be ocurring. Previous studies of the cusp aurora and its response ponent. The change of IMF orientation at 0812 UT is also marked by a vertical line in the figure. We note that with a sizeable y component the locato northward turnings of the IMP have been largely tion of Wind is not ideal to monitor IMF effects on the based on MSP data and are thus lacking the important magnetosphere. However, comparison with the magtwo-dimensional (2-D) aspects of the dynamics [Oieroset netic field data recorded by the ACE spacecraft, loet al., 1997; Sandholt et al., 1998a, 1999a, hi. A recent study of the 2-D auroral response to a northward turning under negative B, conditions has been reported by Lockwood and Moen [1999]. In this paper we docucated at (221, 27,-14) /i œ(gse) at the time, that is, on the opposite side of the Sun-Earth line to Wind, shows a very good agreement over a separation of 55 /r/œ normal to the Sun-Earth line for the period un- ment the 2-D evolution of the individual events of cusp expansion/contraction taking place during the first 30 min after the northward turning under positive B conditions. Furthermore, these auroral observations are placed in the context of the detailed evolution of iono- spheric plasma convection from a "normal" twin cell to a "reverse" twin cell pattern in the northern winter hemi- sphere. A theoretical interpretation of the ionospheric signatures is offered for both the one-lobe and twolobe reconnection scenarios. The present study demonstrates the power of the technique of combined ground and satellite observations for studies of cusp dynamics and the associated solar wind- magnetosphere coupling processes. 2. O bservat ions The different data sets which are combined in this study, that is, (1) the IMF and solar wind plasma, (2) the aurora, (3) particle precipitation and ion drift from satellites in polar orbit, (4) radar observations of ionospheric ion drift, and (5) ground magnetic disturbances, are presented in the following subsections Solar Wind and IMF Observations Field and plasma data for the time interval from 0600 to 0900 UT from the Wind spacecraft are shown in Figure 1. During this time, the spacecraft was in the solar wind on the dawnside of the magnetosphere at (5,-28, 19) /i œ(gse coordinates). This work focuses on the sharp northward rotation of the IMF recorded by Wind at UT. The northward turning was accompanied by an enhancement of the solar wind dynamic pressure from 02 to 05 npa, the effect of which is recorded at the lower latitude IMAGE stations as a magnetic impulse event and an increase in the X component at 0729 UT. The field directional discontinuity, marked by a vertical guideline, separates two interplanetary states. The first is characterized by negative values of all IMF components, while after the transition the IMF is strongly northward pointing for 0 37 min (clock angle 010 ø- der study. Timing the arrival of IMP wave fronts between the two locations, assuming that the signal speed is given by the x component of the solar wind speed, shows that the IMF phase fronts are tilted such that they make an acute angle with positive X (sunward). Thus the IMP discontinuity front affects the Southern Hemisphere first. Our estimate of the lag from Wind

4 N 60 Ny Aolesund MSP DEC,16,, 1998,(65,O.O,nm,),--, 3O 30 - S 07'20 07:30 07'40 07:50 08'00 time (UT) 08:10 08'20 08: intensity (kr) 14 N 6O N,y A?les, und, MS, P,DEC, 16,, 19,98,(55,7.7,nm) '20 07'30 07'4-0 07'50 08'00 08' 10 08: time (UT) intensity (kr) Plate 1. Meridan scanning photometer (MSP) data for the interval UT. Line of sight intensities versus zenith angle for the (a) red and (b) green line emissions are plotted. The intensity scales are given at the bottom of each panel.

5 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA 12,873 N 6O Ny, Aele,sund MSP DEC 16, 1998 (650.0 nm)! I I I I I N S 07'40 07'50 08'00 08'10 time (UT) 08'20 O8'3O 0 ' intensity (kr) 8 Plate 2. Expanded plot of the red line MSP observations for the interval UT Same format as in Plate 1.

6 12,874 SANDHOLT ET AL.' CAPTURE OF MAGNETOSHEATH PLASMA

7 SANDHOLT ET AL.' CAPTURE OF MAGNETOSHEATH PLASMA 12,875 Wind/MFI/SWE December 16, 1998 (GSM) '1o 6o O 450 lo t t UT Figure 1. Wind plasma and magnetic field observations from the Magnetic Field Investigation and Solar Wind Experiment instruments. From top to bottom are shown the proton density, temperature, bulk speed, dynamic pressure, the total field, its GSM x, y, z components, and the IMF clock angle. to the high-latitude region in the Northern Hemisphere, 0726 UT, before a moderate rebrightening occurred durwhen field line draping is taken into account (see Dis- ing UT. Before 0730 UT ( 1030 magnetic cussion), is around zero. This is in reasonable agree- local time (MLT)) the nm aurora was located ment with the observed effect in ground magnetograms south of zenith, corresponding to the latitude at, UT of the solar wind dynamic pressure en- range ø MLAT, during the strongly southward hancement associated with the field northward turning. IMF orientation before the northward turning. Then a strong brightening at the equatorward bound Auroral Observations ary of the preexisting form occurred at 0731 UT. This Meridian scanning photometer (M SP) observations of brightening event is taken to be the first signature of the new IMF regime after the northward IMF turning auroral emissions obtained from Ny )klesund situated at 76 o MLAT for the time interval UT are dis- (see below). An even stronger brightening event occurred in both wavelengths channels during played in Plates la and b. Plates and lower lb show UT. All-sky data (not shown) reveal that the abrupt the red (630.0 nm) and the green (557.7 nm)lin emisintensification of the green line emission at 0735 UT in sion, respectively. The interval UT is characterized by a latitudinally narrow band of emission in the the MSP records corresponds to the activation of a band south. The auroral intensity is decreasing during of long auroral rays which were particularly strong in

8 12,876 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA the vicinity of the MSP meridian. We note that the latter aurora expanded poleward before contracting back almost to the preevent position at 0739 UT. The interval UT is characterized by a stepwise poleward retraction of the poleward boundary leading to a latitudinal broadening of the band of red line The green line MSP observations in Plate lb shows strong green line emission in the second brigtening event ( UT). The later events at the cusp poleward boundary (e.g., at 0757 UT) are also manifested in the green line emission. Plates 3 and 4 show nm all-sky camera obseremission. The intensity of the red line aurora decreased vations from Ny J lesund of the auroral activities obsignificantly with time during the interval served during the intervals UT and UT. Thus after the arrival of the northward oriented UT, respectively. The first interval shows a brightening IMF at 0731 UT the aurora commences a long poleward retraction which continues throughout the interval, bringing the auroral poleward boundary back to zenith ( 76øMLAT) at 0810 UT. The auroral activity up to 0810 UT consists of discrete events starting 0731, 0735, 0743, 0750, 0755, and 0804 UT. The first two events show much stronger auroral intensities than the later activity. The 37-rain interval of auroral activity observed during UT corresponds to the event expanding westward at the poleward boundary of the previous aurora during UT, which is followed by eastward contraction. Thus the brightening recorded by the MSP at UT (Plate 1) is due to the westward expansion of the westward boundary of the brightening form which thereby entered into the MSP field of view. The aurora is aligned in the magnetic east-west direction. The same general pattern of initial westward expansion followed by an eastward coninterval of strongly northward IMF orientation recorded traction is repeated during the UT event (not by Wind during UT. The auroral events af- shown). ter 0740 UT, shown expanded in Plate 2, appear in the MSP records as a sharp 1-2 rain, initial poleward leap, followed by a longer interval ( 3-4 min) of steady or slightly decreasing latitude, most evident in the poleward boundary of the red aurora. The poleward retreat of the equatorward boundary of the red aurora is smoother and slower, resulting in a latitudinal widening of the band of red emission during UT. Af- The sequence shown in Plate 4 represents the interval UT. This interval is characterized by the formation of a discrete, narrow form at the poleward boundary of the band of auroral luminosity. At 0755 UT a minimum of emission intensity is seen in the center of the image, separating two zones of stronger emission located to the west and to the east of the MSP meridian through Ny Alesund. Associated with an intensificater 0810 UT the aurora became much weaker and more tion of the aurora during the following few minutes, the eastern and western zones get connected. We note the sharp poleward boundary and the much more diffuse quiet, with its equatorward and poleward boundaries migrating poleward at equal speed. The latter development corresponds to the interval of Bx-dominated IMF orientation (Figure 1). equatorward boundary of the aurora which extends all across the camera field of view F'14 12 Figure 2. Illustration of fields of view of optical instruments Ny Alesund (NAL), beam 9 of the CUTLASS Finland radar and trajectories of the DMSP spacecraft F13 and F14 in MLAT-MLT coordinates. The field of view of the meridian scanning photometer (MSP) and all-sky camera (630.0 nm) at 0800 UT are marked by arrowed line and circle, respectively. The regions where the satellites detected magnetosheath electrons and ions are indicated by heavy bars along the trajectories. Time marks along the F13 and F14 tracks are shown for the intervals and UT, respectively. The magnetometer stations at Ny Alesund (NAL), Longyearbyen (LYR), Hopen (HOP), and Bear Island (BJN) are marked by solid circles.

9 2.3. Satellite Observations: Particle Precipitation and Ion Drift SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA 12,877 Figure 2 shows the trajectories of the DMSP F13 and F14 spacecraft and the intervals during which they detected precipitation of magnetosheath ions and electrons on the dayside around 0800 UT. The latter intervals are marked by thick bars along the satellite tracks. The spacecraft passes are given in an MLAT-MLT format. The approximate field of views of the meridian scanning photometer (MSP) and all-sky cameras at Ny 'Alesund (NAL) for the red line emission (250 km altitude) at 0800 UT are shown by the arrowed line along the 1100 MLT meridian and the circle, respectively. Thus the F13 pass intersected the MSP field of view just south of the station at 0800:40 UT, at the poleward boundary of the cusp aurora. The latitudinal extent of the cusp aurora is marked by a heavy bar along the Ny ' lesund meridian (Figure 2). The or- bit of the F14 spacecraft intersected the same latitude range on the prenoon side as shown. This satellite detected a regime of similar magnetosheath precipitation as F13 and significant equatorward convection within the MLAT range 750 to 730 MLAT, around 1330 MLT, during the interval 0756:30 to 0757:20 UT. The traversal of this precipitation is also marked by a bar along the trajectory. The particle data from the F13 pass are shown in Plate 5. The number fluxes and average energy of electrons and ions (dotted) are shown in the first two panels. The third and fourth panels show color-coded electron and ion differential number fluxes as a function of en- ergy and time. The bottom panel shows the horizontal (across-track component) and vertical ion flows. We concentrate on the observations made after 0800 UT, when the satellite crossed over the auroral band within 150 south of zenith at Ny J lesund. This agrees well with the poleward boundary of the magnetosheath electron and ion fluxes, which was observed by the F13 spacecraft at øMLAT at 0800:40 UT (Figure 2). Thus the observed precipitation at this time corresponds to one of the events of auroral brightening/expansion/poleward retraction. The sharp precipitation boundary recorded by F13 at 0800:40 UT corresponds to the sharp poleward boundary of the aurora seen in Plates 1, 2, and 4. Particle precipitation data for the F14 pass is shown in Plate 6. We note the fluxes of magnetosheath origin ions and electrons during 0756: :20 UT where ion and electron fluxes are restricted to energies below 3 kev and 300 ev, respectively. Within this cusp precipitation a cross-track ion drift component of H1 km s -1 in the sunward direction was observed, together with "reverse" ion dispersion as previously noted. Plate 7 shows the particle precipitation and ionospheric ion drift observations for the interval between the first and the second major auroral brightening event discussed in the previous section, that is, the interval UT. The track of satellite F12 (see below) is very similar to that of F14 shown in Figure 2. We note the precipitation of low-energy, magnetosheath origin ions and electrons (mantle type) observed within a regime of northeastward convection (see below) within 73ø-74.6øMLAT during 0733: :20 UT. During 0734: :50 UT a patch of higher energy ions and electrons was observed to be located on antisunward conveering field lines, within MLAT. Overall, the ions are found to have the "normal" sense of dispersion during this pass, with average energies falling with increasing latitude Radar Observations: Ionospheric Ion Drift the field of view of the optical instruments Ny le- Plate 8 shows color-coded line of sight velocities obsund. From 0800:40 to 0802:20 UT, intense fluxes of served by beams 5, 7, and 9 of the CUTLASS Finland electrons and ions are encountered, both of which as the radar for the interval UT (after this time the spectrograms show are of typical magnetosheath ener- radar data become very sparse during the interval of gies (i.e., ion energy < 2 kev and electron energy < 300 interest here). Positive (blue) velocities are toward the ev, with average energies of 1-2 kev and 200 ev, re- radar, while negative (red) values are away from the spectively). After 0802:20 UT the magnetosheath com- radar. The direction of beam 9 is shown in Figure 2. ponent is strongly depleted and the higher energy elec- Beams 5 and 7 are directed 13.2 ø and 6.6 o to the west trons are enhanced (average electron energy increases of beam 9, respectively. to 1 kev). Thus after 0802:20 UT the spacecraft en- The period under study is characterized overall by recounters the boundary plasma sheet and the central organization of the convection pattern after the northplasma sheet, but these regimes are of minor concern ward turning of the IMF recorded by Wind, whose efin this study. We note the equatorward (sunward) ion fects first reached the ionosphere at UT (as inflow component observed during the interval 0800:00- dicated by the impulse recorded by the IMAGE mag- 0800:40 UT and the concurrent "reversed" dispersion of netometers to be discussed further in the next section). the cusp ions, in which the energy falls with decreas- Prior to this time strong flows away from the radar (red) ing latitude. An even stronger (1 km s - ) equatorward were recorded above 73øMLAT in the eastward part flow was observed by the F 14 spacecraft during 0756:00- of the field of view (FOV) (e.g., beam 9). After this 0757:20 UT (see below), again in the presence of"re- time, these flows migrated with the backscatter band versed" dispersion (energy decreasing with decreasing to higher latitudes, and were replaced, after 0745 UT, latitudes) in the ions. by weaker flows which were characteristically toward When the magnetosheath populations were observed, the radar in the eastward part of the FOV (green), and the poleward boundary of the cusp aurora had reached away in the westward part (yellow), indicative of a weak

10 12,878 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA westward flow. During this change the flows at lower latitudes, 71ø-73øMLAT, were punctuated by a series of bursts of equatorward flow (green and blue), which appear in concert with the auroral changes observed by the MSP. Vertical lines in Plate 8 mark the onset times of the auroral brightenings and/or poleward motions, which occur at 0731, 0735, 0743, 0750, 0755, and 0804 UT. We note that the first 2 times represent onset of transient auroral effects in which the latitude band of the aurora remains unchanged after the brightening has died away, while the last four represent the onset times of poleward "steps" which lead to the broadening of the auroral band. It is particularly evident in the data from beam 9, and to a lesser extent in beam 7, that the surges of equatorward flow occur at or slightly after the auroral event onset times. The relationship is particularly clear in the first three events, where it is also clear that the strongest equatorward flows occur at the approximate latitudinal location of the the cusp auroras, at 72ø-73øMLAT. Also evident in these plots is the stepwise poleward motion of the region of backscatter, which also often appears to occur in concert with the auroral poleward motions. Finally, we note that the poleward motion of the high-latitude limit of the HF backscatter occurs at different times on the differ Interval UT. During this interval the aurora is located within 72ø-74øMLAT. As shown in Plate 9, the ion flow is weak below 70øMLAT, variable but generally eastward between 70ø-72 ø MLAT, equatorward of the auroral band, then increasing to a strong poleward and eastward flow (1-2 km s - ) within and poleward of the auroras. This pattern is typical of IMF Bz and B u negative conditions Interval 0729: :30 UT. During this interval the first burst of equatorward flow is observed, in association with the first auroral brightenlug. The flow pattern during the interval is captured in the scan at UT, shown in Plate 9. It shows the presence of an intense equatorward flow located at 72 ø MLAT in the eastward part of the FOV. This flow is located near the equatorward border of the auroral band where the auroral enhancement took place, the latter starting (in the MSP data) H1 rain after the equatorward flow onset. Further poleward, the flows remains directed poleward and eastward as before. We interpret the auroral feature and burst of equatorward flow as being due to a first lobe reconnection event following the northward turn of the IMF, as discussed further below. The continued poleward and eastward flows at higher latitudes are then taken to be due to continued driving of the "normal" twin-cell flow as previously opened field ent beams, earlier on beam 9 and later on beam 5. This indicates a westward motion of the process from noon toward the morningside. A corresponding westward expansion of the auroral events is observed. In order to examine the flow pattern in more detail, in Plates 9-11 we show line of sight velocity scan plots over the full pattern of 14 beam directions used in the radar experiment. These were obtained over 1-min intervals lines continue to evolve into the geomagnetic tail. Overall, we typically expect such flows to die away only after an interval of rain has elapsed since open flux tube production has ceased [Cowlevi and Lockwood, 1992]. In the present case they disappear only after 0745 UT, an interval of 15 rain, as seen previously in Plate 8 and later here. every 3 min and are shown for the entire interval over Interval 0732: :30 UT. After the which useful backscatter returns were obtained. The first auroral event and before the second, the flow retime given in each panel is the start time of the corresponding scan. In these plots the dashed circles show the 70 o and 80øMLAT lines, while the radial dashed lines indicate each hour MLT, with 1200 MLT being vertically upward and the prenoon hours to the right. The line of sight velocity data are indicated using the same color code as in Plate 8, with regions of ground scatter indicated in grey. Vectors have been superposed using a beam-swinging algorithm given by t uohoniemi et al. [1989] and Villain et al. [1987]. The approximate location of the cusp auroral band has also been marked by solid lines, obtained from the optical observations. The ion drift vectors were determined by a beam-swinging technique, as described by Ruohoniemi et al. [1989]. This technique assumes that the zonal component of the plasma drift is constant across the field of view of the radar, at any particular magnetic verts to eastward and poleward essentially throughout the radar FOV, as seen in the scan at UT in Plate 9. However, the flow pattern is a bit different from those observed earlier (e.g., those starting at 0727 and 0724 UT) in that the flow below 70øMLAT is enhanced while that in the vicinity of the previous equatorward flow region is suppressed. This may represent a continuing distortion of the flow pattern caused by the auroral event, though now being insufficiently strong to cause actual flow reversal to equatorward within the affected region. Overall, the flow is still mainly associated with the previously opened flux as indicated above. The pass of the DMSP F12 spacecraft through the cusp corresponds to this interval. It shows the presence of "normally" dispersed cusp ions usually associated with IMF Bz negative, with lower energies at higher latitudes, consistent wi;h this in;erpre;afion. latitude. The validity of this assumption can be tested Interval 0735: :00 UT. The secby examining the quality of the fit between the modeled flow and the measured line of sight velocities. In the maps presented the quality of the fit was found to be adequate to allow a first-order determination of the flow pattern. In chronological sequence, these plots indicate the following features of the plasma flow. ond auroral event and burst of equatorward flow occurs in this interval, as caught in the scan at UT shown in Plate 9. The region affected is now much larger, 68ø-75 ø, spanning the auroral enhancement located between 72ø-74øMLAT(Plate 1). The flow pat- ;ern is consis;en; wi;h ;he presence of a clockwise vortex

11 ... o SANDHOLT ET AL.- CAPTURE OF MAGNETOSHEATH PLASMA 12,879 &...,.- )...,.-,,)...,

12 ß.,, F13 ' 1o12, ',. 16 Dec 1998,.,,. "';";' ' '"ø... '"'... '; Z,'t,,',. ;;',,.,,'.., ß... Ion l> o 3. lo IO "',I' '"' ','! I 91',,, "' },"]"1'" "' '" 4!,.,t III [ J ' Ii ' IJ I $ _ ' :'. -'.,r',.r..' p,j I J Jill I III -, t '.,.., ).,,,,'.... t I, I ", i I' lit?' I I" 30o0 i S w d J E!e 1o 5 lo 7,o ø z g 1o 3 _. VER HOR-C -3000,,...,,, Antisunw,'u'd :55 7:56 7:57 7:58 7:59 8:00 8:01 8:02 8: I UT(SEC) 8:04 HH:MM 68.9 MLAT 9.10 MLT Plate 5. Particle precipitation data from the DMSP F13 spacecraft for the period 0755 to 0804 UT. The panels show from top to bottom the number fluxes (ions are shown dotted), the average energy, and energy spectrograms for electrons and ions, corresponding to the color bar on the right and cross-track horizontal (violet) and vertical (green) ion flow components. The F13 trajectory is given in Figure 2. F14? to12, 16 Dec Ion ]04:,03 1o 2,..'ll,-.'. jill j jl jl ijjjjlljlj u Jlllljjl till I II r ß....:.",.;.....:,:' :.,.,%;.;..{"'.'",,,.,. ' ß ' ::':.;.,.../,.":: ' ] ' ' ";l:*:':: ' ",, '"... I I I' I I I I I I III : II l-i Ill I I I I I I III II l' I o 4 /,.111:l ti,i I "!1t'_ 'I' ', 'ø3 'I't Jl ",, I,, : ill, ii} 'i/f I, i ')11. II :, Ij,i,,Ijl I 1, "' i t, "'. II, t,i.,ll{i I..,,.,, 'I' :,, i' r, '....'"-": "j:. ::-:- 1o 5, t/, '!.,dl{. ]o 2 i.:,:'."- '.,it,. -.:'.l ::': 7. ',.. :" 1o 7.,.,,,,.,ll,,:, ll.,,ii0½,., ',,,',o4 I Ill!,,,,!,,,, I fill lo 3,o 3O00 I { I,lljt{l jj ',:,Ill 1' li 'lil: i ",, I,, i":' i]j ' J, i',i' [Jr!,!,,,l'I '.,. ' '.... Ele ]05 2 1o 3 _. VER 0 - _- -_- -- = Antisunward ilor-c i60 l (S EC ) :55 7:56 7:57 7:58 7:59 8:00 8:01 8:02 8:03 HH:MM MLAT MLT Plate 6. Particle precipitation data froin the F14 DMSP spacecraft for the period UT. The panel show from top to bottom the number fluxes (ions are shown dotted), the average energy, and energy spectrograms for electrons and ions, corresponding to the color bar on the right and horizontal (cross track) and vertical ion flow components. The F14 trajectory is given in Figure 2.

13 SANDHOLT ET AL.- CAPTURE OF MAGNETOSHEATH PLASMA 12,881 F12 16 Dec 1998 lo 12 Ioll.'> lo ß.,,,.',,,,.'.'......,,',,'.,,, ß,.,,. %, # ß, 104.J.... :...1..,,.?".,.,,,ll' _"... I ' I :',.',. t,.'-,,, ' ' "'"';1;'1'/} rs" ",, "", ';':' I ': J: '""i' :'1'" :, 1 o3" i',,:.":'i.i" '::.:,,.,...,...: ' ',-"!I,...,,' " ".".' If:, t,, r.a ", -' ',' : ',', ' --,, ;}',?, 2 -I" Iilt':'1i1",t i i 1'i'"... i)!,lill i, I'-I'I,l,'l [-i... i'i'"'"... i'i"' :'... i I' '1'1'!111' "! t!1" -l,,q,,,.,,,,,,,, q,,,., _,,,,,. 1,,,,,,,),,u ;,,,,,,t,,,,,,,,,,,,,,?,o, ].o 3 I t.i.iii I I..! _ I,,,../ I I,.I I. il 1tt '1 il 11 ii IL It,.,,..,...,.,,,.,,..,,,.,.,,..,t., t,'t, ' ' ;'-; i ' t [ ; ; J I,,,, lit t' I. ' '' ","',1 i I,'t,r ' t t.,l, ", ø [ lo 2,,t [, ',,.'-, '. ',,,,,-... '. t,,, I t l I I t li, ill,f l l,,,, tll I I I I 111,,,.,,,,,.,,,,,,,,,,,,,.,.. ß,,J t,,,,,,,,,., i. I,,,, '"' '-' ' ' 'il I '1 '.,,' ' j ' ß ' I I I Ii I I }A[.,I-,,i..., -..f,. I -ij!l,'l i lll tl rtll I'fl I 3øøø] ::---- ', 4 q 1 ::: S""'"a ' _.V R f.r.1 O "'---' ' ' Ele. FIOR UT(SEC) 7:30 7:31 7:32 7:33 7:34 7:35 7:36 7:37 7:38 7:39 HH:MM ,4 69, ,7 60,7 MLAT , ,1 11,7 11, MLT Plate 7. Particle precipitation data from the F12 DMSP spacecraft for the period UT. The panels show from top to bottom the number fluxes (ions are shown dotted), the average energy and energy spectrograms for electrons and ions, corresponding to the color bar on the right and horizontal (cross track) and vertical ion flow components. The F12 trajectory is indicated in Figure 5.

14 -- _ ,882 SANDHOLT ET AL.' CAPTURE OF MAGNETOSHEATH PLASMA u 75 -Beom I I I - ' '!l-' 8OO 6OO O o O o 75 o 70 o ø,, - ',,, - ' " % --"' ': 5:; - X--' ' a,, "" :'-- : ; ; :' ': ; :'; : : t': :...: : : : : 1... : : I '1'{ : : : t t t I t, 7 ' '- O ',,,,,,, j ' '.,.,'. l mm, m, ' - ld,,,', " :,,!, ' ' Vr ' I-"' ' "k ' ½. ß h' ' - ' '.,. ß.,,.,,e..... ' - ' &l-%,, ',...,- -,, ;,-,: 2., ',. I { l ' N!. % N..', I,,,: ' ' " 'h. I i --.,,,,,,, 2, T' {,, ', } J'},d4 ',,. ) -¾,,,,',, - = /',,,,,,, '.,,.,.-', m },",,,, ',,., ', ',,%, t,'" - ' 3,,,,,.,,,, ',' ' /,,' ' 2;,;'[ I I _ ; ; ; ; : : { : :. { ': :, :,,, {.,.,,...,I I, I,,. ;.1! - i I _ ' ' '' : {' I ' ' ' :' ' Beam 9 ' -- 1',,, -- ", ", m {., I I, _ I, I ) I', m m Im i, 1 - I I! I b ' m _ - ',., m.,-,'..:-- -.4, -. i.m I. m., ' m.., m -- 1 I,,,.', Ii _,,V' - -- { wl m ß i I I _ I 1 II'l 't -,.,.,.,/ ", ',,',,,',,,, - --, I I, I iii ' ; ',, '- I ' 5' " I."!1 '.' II I ' ii,-' tl I... -.,- ß ß! I '} '." ' ' ' ' I. 'mini,,, o UT I OO -8OO Ionospheric scot only Plate S. Line of sight ion velocities observed by (top) beams 15, 7, and (bottom) 9 of the CUTLASS Finland radar for the interval UT. Positive (blue) velocities are toward the radar and negative (red) are away from the radar. Vertical lines mark the onset of auroral events at 0731, 0735, 0743, 0750, 07515, and 0804 UT. The field of view of beam 9 is indicated Figure 2.

15 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA 12,883 within the FOV of the radar, associated with an upward for the interval UT. These magnetometer field-aligned current. It seems reasonable to hypothe- stations belong to the IMAGE chain of stations in size that another anticlockwise vortex may be present Sval bard / S candi na via. postnoon. We note that the prenoon cusp aurora ini- A major change in the state of magnetic deflection, tially ( 0735 UT) expanded poleward and westward from (negative/positive)to(positive/negative)deflec- and later (0739 UT) collapsed back to the pre-event tions in the X/Y components, took place between 0731 form (Plate 3). and 0745 UT. The initial state corresponds to the north Interval UT. This intervalcor- eastward convection under the interval of negative IMF responds to the period after the contraction of the second auroral brightening and before the first poleward "step event." The flow has once more reverted to the B negative form which was present before UT, with mainly eastward and poleward flows, as seen in the B and By conditions observed by Wind between 0625 and 0730 UT. After 0745 UT the new, positive IMF Bz regime is reflected at all stations. A sequence of magnetic events, characterized by (positive X/negative Y) deflections, was observed in association with the auroral scans at and UT shown in Plate events during UT. The auroral events initi- 10. However, the region of stronger poleward flows has moved significantly poleward of its original location, as has the aurora. ated at 0731, 0735, 0743, 0750, 0755, and 0804 UT have been marked by vertical lines in Figure 3. There is a close correspondence between auroral brighten Interval UT. The first auroral "step" event occurs at the beginning of this interval. Equatorward flows are again initially present, with the scan at UT in Plate 10 indicating the presence of a clockwise flow vortex. A weak remnant of the poleward and eastward flows remains at the highest latitudes sampled, but this is the last time that such flows are observed. The clockwise flow vortex declines over 3-4 min, with only weak vestiges of the cell remaining in the scan at UT. The flow within the auroral band is weak and westward at the later time Interval UT. This intervalcor- responds to the second poleward "step," the aurora now extending between 72.5 ø and 75 ø. The flow remains weak and westward in the auroral region, but becoming more equatorward (green) toward the end of the interval, comparing the scans at and UT Interval UT. This intervalcor- responds to the third poleward "step" in the aurora, now including a brightening at the poleward border, though the poleward border rapidly retreats again to previous latitudes. The flow is again equatorward and westward in the auroral zone, forming the equatorward part of a clockwise vortex observed in the scan at UT shown in Plate 11. The F13 and F14 passes belong to this interval. They are consistent with a pair of small convection cells, containing sunward flow over the polar cap boundary, together with a generally westward flow on closed field lines at lower latitudes (around 70ø). The "reversed" dispersion of the ions seen on both DMSP passes is consistent with the "reversed" flow present. After the end of this interval, the CUT- LASS backscatter retreats to the westward edge of the FOV where weak poleward flows are observed and then becomes highly scattered after 0809 UT Ground Magnetic Observations Figures 3a and 3b show X and Y component magnetograms from the Svalbard stations Ny lesund(nal), Longyearbyen (LYR), Hopen (HOP), and Riornoya (BJN) ing/expansion events and specific magnetic deflections. Furthermore, the magnetic deflections correspond to activations of equatorward ionospheric ion flow events at the cusp poleward boundary, as documented in the plates described above. Equatorward ion flow (ExB drift) corresponds to poleward Hall current (in BxE direction) which gives rise to westward (negative Y component) magnetic deflection on the ground, as observed. The first in the series of magnetic events, observed at stations BJN (71.3øMLAT) and HOP (72.9øMLAT) during UT, corresponds to the first auroral brightening event after the northward turning of the IMF. Figure 4 shows X component magnetograms from IMAGE chain stations in Scandinavia, within 66.5 o- 60.8øMLAT. We note the magnetic impulse event at UT and the magnetic field perturbations during the interval UT. This interval of magnetic activity corresponds to the sequence of cusp brightenings/poleward retractions observed after the northward turning of the IMF. The magnetic impulse event and the increase of the X component at UT are most likely the magnetic signature of the rapid en- hancement of the solar wind dynamic pressure (density) recorded by Wind at 0735 UT, in association with the IMF northward turning. We note the close association between the major auroral events at 0735 and 0743 UT and the positive X component deflections at lower latitudes. 3. Summary of Observations We have presented multisite and multitechnique observations of a dayside polar cap contraction following a large, rapid northward turning of the IMF, consisting of a sequence of discrete events. After the northward turning, the IMF remained strongly northward oriented (clock angle less than 150 ) for 37 min. In this interval, a stepwise poleward retraction of the polar cusp aurora was accompanied by a sequence of bursts of equatorward ionospheric convection. The intermittent nature of the convection events are inferred from the combi-

16 12,884 SANDHOLT ET AL.' CAPTURE OF MAGNETOSHEATH PLASMA I I I I (,LU) i i i o (Do 0 0 ß o o o c o c c c (,Lu)

17 _ SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA 12,885 IMAGE X component December 16, 1998 TRO _- 7:00 7:15 7:30 7:45 8:00 8:15 8:30 8:45 9:00 UT Figure 4. X component magnetograms at 10-s resolution for the interval UT obtained from the following IMAGE chain stations in Scandinavia: TromsO, Norway (TI O; 66.5øMLAT), Kiruna, Sweden (KIl ; 64.6øMLAT), Sodankyla, Finland (SOD; 63.8øMLAT), Lycksele, Sweden (LYC; 61.3øMLAT), and Oulujarvi, Finland (OUJ; 60.8øMLAT). nation of the radar and ground magnetic observations. The particle source of the aurora is documented by overflights by the satellites DMSP F12, F13, and F14. Initially, before the IMF northward turning, a twin cell ion flow pattern existed of the form expected for negative IMF Bz and By components ("normal" circu- lation). This initial state is indicated in Figure 5a. After the IMF northward turning, affecting the ionosphere from, UT, two impulsive aurora] brightening events were observed during and UT. These events generated intense clockwise ion flow vortices in the prenoon cusp each lasting for,- 3 min. The flow configuration is indicated in Figure 5b. After each of these events the flow reverted to that appropriate to the previou southward IMF configuration (Figure which took,- 15 min to decay away completely (latest effect seen in the 0745 UT radar scan). The F12 satellite pass took place in the interval between the first two auroral pulses, and the F12 observations are consistent with flow being maintained by previously opened flux (see Figure 5c). An "old," normally dispersed (i.e., with ion energy decreasing with increasing latitudes) cusp is still present. A series of four poleward "steps" then occurred in the poleward border of the aurora, starting at, UT, when previous flow had largely decayed away. Only the first three of these events are well observed by the radar. The flows in the cusp region are consistent with a large weak clockwise convection cell in the prenoon ionosphere (as also seen by the F13 satellite) which pulses in concert with the auroral steps on few-minute timescales, and the F14 pass gives evidence of a similar anticlockwise vortex in the postnoon ionosphere (see Figure 5d). Thus the postnoon cell is assumed on the basis of logics and supporting F14 ob- servations. The particle precipitation/ion drift observed by the satellites from the cusp poleward boundary and toward lower latitudes at, and, MLT show the following characteristic features: magnetosheath-origin ions and electrons with a transition from (1) "reverse" ion dispersion/equatorward plasma flow to (2) dispersionless low-latitude boundary layer (LLBL) type fluxes

18 12,886 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA (a) Flow prior to northward turn (e.g UT) (b) Flow in first two auroral events (0730 and 0736 UT) (c) Flow between first two auroral events (0733 UT).,F12 (d) Flow during auroral "step" (after UT) Figure 5. Schematic overview illustration of auroral forms (poleward and equatorward boundaries marked by dashed curves) and associated ionospheric ion flow (arrowed curved lines) configurations representing (a) the time before the IMF northward turning (0724 UT), (b) the auroral events centered at 0731 and 0736 UT, (c) the interval between the two events in Figure 5b ( 0733 UT), and (d) the events showing poleward "steps" of the auroral poleward boundary during the interval UT. Trajectories of low-altitude satellites DMSP F12 (around 0733 UT), F13 (around 0755 UT), and F14 (around 0801 UT) are indicated in Figures 5c and 5d. flowing tailward. The latter precipitation contains a component of higher-energy magnetospheric origin particles. The presence of the transition from equatorward (sunward) to antisunward (tailward) flow both at 1100 (F13) and 1330 MLT (F14)is strongly indicative of 13 "reverse" two-cell convection. The aurora was east-west oriented with a sharp poleward boundary. Thus from the combined ground and satellite data we infer a zone of precipitation of magnetosheath-origin plasma/cusp aurora extending across the longitude range MLT, accompanied by "reverse" two-cell convection. At 0800 UT, DMSP F13 spacecraft recorded a narrow zone of electron precipitation/enhanced sunward ion flow component at 11.7 MLT/76.2øMLAT, slightly poleward of the band of magnetosheath ion precipitation. 4. Discussion The cusp convection response to rapid southward turnings of the IMF and associated enhancements of the magnetopause reconnection rate have been recently described by, for example, Greenwald et al. [1999]and Shepherd et al. [1999]. The response in the cusp aurora has been reported by, for example, Sandholt et al. [1999c] and Thorolfsson et al. (Submitted manuscript, 1999). Thorolfsson et al. (Submitted manuscript, 1999) studied the response in aurora and convection to the southward turning recorded by the Wind spacecraft at 0622 UT on December 16, 1998 (see Figure 1). Some characteristic features of the observations during the first 30 rain of the southward IMF regime are (1) equatorward motion of the sharp equatorward boundary of the aurora, (2) the auroral emission consisting of a sequence of brightenings (recurrence period 5 min) at the equatorward boundary of the preexisting luminosity followed by poleward moving auroral forms, and (3) associated bursts of northeastward (IMF By<O) convection in the vicinity of the aurora, with return flows on the sides of the auroral forms, consistent with the model predictions of Southwood [1987] and Cowley et al. [1991]. In this paper we focus on the detailed response of the aurora and convection during the interval following the IMF northward turning recorded by Wind at 0735 UT on this same day (Figure 1), when the cusp aurora retreated back to higher latitudes. Here we note that the cusp auroras observed during the intervals corresponding to southward and northward IMF orientations are different. One feature to notice is that in the former case (type 1 aurora) the equatorward boundary is generally more sharp than the poleward boundary (see Plate 1, before 0730 UT), whereas in the latter case (type 2 aurora) the poleward boundary is typically sharp while the intensity is decreasing more gradually toward the equatorward side (see the interval UT in plates 1 and 2). Based on the documented association between particle source, aurora, and convection events, we use the continuous ground-based observations in combination with the IMF data to infer essential features of the re- lated solar wind/magnetosphere coupling processes. A main feature of the auroral observations is the sequence of impulsive expansion events during the 37-min period ( UT) of strongly northward IMF after the rapid northward turning. We distinguish between two

19 . SANDHOLT ET AL.- CAPTURE OF MAGNETOSHEATH PLASMA 12, MLT 8OO 6OO 40O < 0727' UT..,.,., ' - I ' ' " '" ' I " ' I' " 1... i UT.'.." _... ;. % -8OO Ground '.. Scatter : '.',..' ' t :.,..,..., 07.3 'UT,.. :',,:';'..,"." '";' Plate 9. CUTLASS observations of line of sight velocities for the interval UT plotted in MLAT-MLT coordinates with 1200 MLT at the top of the plate. Color-coded velocity scale is given on the right side. Positive values are toward the radar (representing equatorward flow component). "Beam-swung" ion drift vectors are indicated. The position of the cusp auroral band is indicated by curved lines, as derived from the optical observations.

20 12,888 SANDHOLT ET AL- CAPTURE OF MAGNETOSHEATH PLASMA O O Ground Scatter Plate 10. Line of sight velocities for the interval UT. Same format as in Plate 9.

21 SANDHOLT ET AL.' CAPTURE OF MAGNETOSHEATH PLASMA 12, ' UT '. -,.' -800 Ground Scotter Plate 11. Line of sight velocities for the interval UT. Same format as in Plates 9 and 10.

22 12,890 SANDHOLT ET AL' CAPTURE OF MAGNETOSHEATH PLASMA sets of events. The two events before 0740 UT are char- acterized by a few-minutes interval of strong auroral intensification and poleward/longitudinal expansions and associated bursts of equatorward convection, followed by rapid auroral contraction and fading to the preevent auroral condition. The auroral "step" events observed during UT, on the other hand, were weaker in intensity and lead to a significant latitudinal widening in the band of precipitation. In the following we shall provide a theoretical discussion and interpretation of the observations. This is based upon the nature of the precipitation and flows expected when lobe reconnection occurs, though in the present case this is complicated by the additional presence of declining "normal" twin-cell flow which continues to be driven for 15 min after the northward turn of the IMF by open flux produced in the prior interval. We divide the discussion according to whether the lobe reconnection occurs in only one or in both hemispheres. In Figure 6 we show our expectations for the case in which each interplanetary field line reconnects in one hemisphere only, though in principle the process may occur simultaneously in both hemispheres (with different field lines), particularly if the interplanetary field is tilted over by an IMF By component. For simplicity, Figure 6a shows the single-lobe reconnection process occurring in the Northern Hemisphere in the case of a purely northward IMF. In this process, previously opened tail lobe field lines are disconnected from the Earth and are replaced by open lines draped over the dayside magnetopause. The amount of open flux does not change in the process, and to a first approximation the resulting flows are confined to the region of open field lines. Following reconnection, the new open tubes contract sunward over the magnetopause due to the field tension, and since they remain connected to the magnetosheath they are also carried antisunward around the magnetopause. The east-west flows will also be modified by the tension effect associated with IMF By, if such a field is present. In the incompressible ionosphere these motions give rise to "reversed" twin-vortex flow within the region of open flux, as shown in Figure 6b (which will also exhibit dawn-dusk asymmetry iflmf By is present). This figure shows the flows and cusp precipitation (hatched area) which will be produced by a single pulse of lobe reconnection. As the reconnection occurs, the region of cusp precipitation will expand poleward from the open-closed field line boundary (solid line), and will subsequently contract equatorward as the flow causes the patch to expand east-west along the boundary. The ionospheric flow due to this pulse will die away after 5-10 min, as the open field lines are carried downstream into the tail lobe, replacing the lobe field lines that were disconnected. At this stage the source of magnetosheath plasma to the ionosphere will also be switched of[, and the cusp precipitation and optical emission will correspondingly decline. Magnetosheath plasma may still enter the magnetosphere along the open tubes, but the plasma is confined mainly to the tail lobe (the mantle) due to the supersonic nature of the source. Ionospheric flow and precipitation will only be maintained if subsequent pules of reconnection occur, on timescales of less than min, as shown in Figure 6c. Each pulse of reconnection causes the precipitation region to move poleward, followed by equatorward contraction as before. The cusp ion dispersion profile will then be of "reversed" forrn, with ion energy falling with decreasing latitude, in conformity with the generally equatorward flow within the region. If a sequence of reconnection pulses then occur, the cusp precipitation will undergo recurrent poleward expansions and equatorward contractions, the effect of each lasting for 5-10 min. (a) (b) (c) (d) Latitude 10 min II / II / II I Closed field lines Time Figure 6. (a) Schematic illustration of IMFmagnetosphere field interconnection geometry for northward IMF and one-lobe reconnection in the Northern Hemisphere. Open arrows mark the direction of plasma flow along the magnetopause. (b) Cusp aurora (hatched area) and ionospheric ion flow pattern associated with an isolated event of one-lobe reconnection. (c) Same as Figure 6b during the time of a second consecutive event of lobe reconnection. (d) Cusp auroral emission (hatched area) as a function of latitude and time during a sequence of four, one-lobe reconnection events. Dashed lines mark the poleward boundary of emission associated with the individual events. The poleward boundary of the closed field line regime is marked by a horizontal line.

23 SANDHOLT ET AL.' CAPTURE OF MAGNETOSHEATH PLASMA 12,891 (a) (b) / / i i (c) (d) Latitude 10 min ', Time served flows in these cases were complicated by the presence of declining twin-cell flow as previously described, leading to the combined flow pattern sketched in Figure 5b. In Figure 7 we provide a similar description of theoretical expectations in the case where lobe reconnection occurs for a given interplanetary field line in both hemispheres. Of course, it is not necessary that northern and southern reconnections occur simultaneously, and in general they are not expected to do so. In the present case with a strong positive IMF Bx component, for example, we may expect reconnection to occur first in the Southern Hemisphere, with the reconnected tubes then draping over the northern polar magnetopause and subsequently reconnecting there. This process is qualitatively different from the singlelobe process discussed above, because the equilibrium state of the magnetosphere is changed as open tubes are disconnected from the tail lobes, and closed tubes added to the dayside magnetopause, as shown in Figure 7a. The antisunward momentum of the plasma on these tubes will be dissipated by ionospheric resistance, and because they are no longer directly connected to the essentially inexhaustible reservoir of magnetosheath momentum, they will not be rapidly carried downstream as in the case of single lobe reconnection discussed above. Nevertheless, because the process alters the state of the magnetosphere, removing open flux from the tail lobes, and adding closed flux to the dayside, "reversed" convection will again be generated as the system moves from the perturbed condition following reconnection to a new equilibrium configuration containing less open flux [Cowley and Lockwood, 1992]. The flows gener- Figure 7. (a) Schematic illustration of IMFated by a single pulse of two-lobe reconnection is shown magnetosphere field interconnection geometry for northward IMF and lobe reconnection in the two hemi- in Figure 7b, where the solid line is again the openspheres. Open arrows mark the direction of plasma closed field line boundary and the hatched area the reflow along the magnetopause. (b) Cusp aurora (hatched gion of "reversed" dispersion cusp precipitation, now on area) and ionospheric ion flow associated with an iso- newly closed field lines. The flow pattern is similar to lated event of two-lobe reconnection. (c) Same as Figure the previous case shown in Figure 6b but is now much 7b during the time of a second consecutive event of twoslower (unless the process is aided by anomalous crosslobe reconnection. (d) Cusp auroral emission (hatched field momentum transport processes at the new boundarea) as a function of latitude and time during a sequence of four, two-lobe reconnection events. Solid line ary), larger scale, and involves both open and closed marks the poleward boundary of emission associated flux tubes. If no other reconnection pulses occur, the with the individual events. patch of newly closed flux will slowly sink with the flow into the region of older closed flux at lower latitudes until the open-closed field line boundary is "circularized" (i.e., equilibrium is reached), at which point the However, as shown in Figure 6d, because the amount of open flux remains unchanged in the process, the latitudinal location of the cusp precipitation, and its overall thickness, do not change as time goes on. This scenario of poleward auroral expansion followed flow will stop. The overall timescale for this to happen is somewhat uncertain but is likely to be of order at least a few Alfven wave transit times across the system, say around 20 min. If reconnection pulses occur with greater frequency, for example, the 5-min pulses which by equatorward contraction in the presence of vigorous occur in the present interval, then the open-closed field "reversed" vortical flows, appears to provide an appro- line boundary will step successively poleward and the priate description of the first two auroral brightening band of precipitation will broaden, as shown in Figures events observed here after the northward IMF turn, be- 7c- d. Weaker, larger-scale "reverse" convection, with ginning at -,0731 and -,0735 UT. We therefore suggest less evident pulsing on few-minute timescales (because that these were produced by pulses of single-lobe reconnection in the Northern Hemisphere. However, the obeach pulse drives a slower flow which is longer lived), is a necessary corollary of this scenario. In addition,

24 12,892 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA because the newly reconnected field lines are closed, trapping previous magnetosheath plasma in a boundary layer adjacent to the magnetopause, cusp precipitation and optical emission is expected to be longer lived on respectively. The following model predictions are relevant to the present observations: (1) the entry of magnetosheath plasma into the LLBL of the magnetosphere takes place near the subsolar magnetopause via capture these tubes (as shown in Figure 7d). In the single-lobe of magnetosheath flux tubes by high-latitude reconnecreconnection case, injected ions are able to undertake one bounce only before flowing out along the open field lines away from the Earth, and the plasma source is then switched off after the tubes are swept into the tail. In the two-lobe case, the lifetime of the cusp precipitation may only be effectively limited by wave processes which scatter the particles into the loss cone, and by transport of the plasma in local time around the boundary. tion, (2) the expansion along the magnetopause of the high pressure newly closed flux tube [see also Song et al., 1994], (3) plasma convection in the polar cap ionosphere ("reverse" twin cell) is driven by the entry of solar wind plasma and the associated NBZ field-aligned currents that map to the ionosphere in the cusp region (NBZ currents are generated by the distortion of the magnetic field in the region of plasma entry, as described Comparing these expectations with our observations, by Song et al. [1994]),(4)convection in the outer magit seems reasonable to conclude that the "stepped" poleward progression of the auroras observed after 0740 UT is due to a sequence of two-lobe reconnection events which "capture" closed flux tubes at the dayside magnetopause. This is indicated primarily by the sequential broadening of the latitude band occupied by the aurora and the slowly decaying nature of the optical intensity at a given latitude. The weaker, broader-scale flows obnetosphere, at subcusp latitudes, which is driven by closure of the NBZ currents by Pedersen currents in the ionosphere. It is suggested that features 1 and 2 above are reflected in the ionosphere as a corresponding longitudinal expansion of the cusp aurora. Such auroral expansions (westward in the prenoon cusp region) are observed in the present case. Prediction 3 above is evident served by the radar and the generally weaker nature of in the "reverse" twin-cell convection identified from the the flow pulses associated with the auroral "steps" both support this interpretation. One point of apparent disground and satellite ion drift observations in our case study. It is suggested that prediction 4 is reflected in crepancy is that the observed auroral band (as seen, e.g., in Plate 2) also drifts slowly poleward at its equatorward boundary, rather than moving slowly equatorward as expected in Figure 7d. However, it seems likely that this is due to concurrent open flux destruction by simultaneous substorm activity in the particular case of the present study. The reported auroral step" events are thus consistent with the specific scenario of solar wind/magnetosphere the magnetic field oscillations observed at subcusp latitudes during the interval UT (see Figure 4) whose frequency matches that of the cusp events. The predicted association between plasma entry and plasma convection in the cusp region ionosphere is reflected in the observed close association between auroral activ- ity and impulsive convection events documented in this study. A global MHD simulation of the IMF-magnetosphere coupling described by Song and Russell [1992](see In- interconnection geometry for an IMF orientation similar troduction), if one includes the overdraped lobe hypoth- to that observed in the present case (but for a different esis of Crooker [1992], the latter being necessitated by season) is shown in Figure 5 of Russell ½t al. [1998]. the season (winter) and sunward tilt of the IMF (Bx-6- The presence of magnetic X lines (null points) both in 7 nt). A brief report on this interpretation of the "step" the southern and northern lobes is indicated. Related events has been given by Sandholt et al. [1999a]. The to this we note that the narrow zone of electron precipiinterconnection geometry of the present case has also tation and enhanced sunward flow component poleward been described and illustrated in Figure 3a of lrleiff and of the zone of magnetosheath ions recorded by DMSP Burch [1985] and more specifically in Figure 9 of Le F13 at 0800 UT (11.7 MLT/76.2øMLAT) may repreet al. [1996]. The duration of the reconnection bursts sent the ionospheric image of the reconnection site in may be estimated from the duration of the auroral pole- the northern lobe in our case (see Plate 5). ward "step" phase (1-2 min). A transition toward a more quiet cusp aurora (ab- Aspects of our observations are also consistent with sence of poleward leap events) took place at 0810 UT, the in situ low-latitude boundary layer observations in association with the IMF rotation toward more radial of Leet al. [1994, 1996] and the recent global 3-D (Bx dominated) orientation. This IMF condition gives magnetohydrodynamic (MHD) modeling of solar wind/ rise to a weak solar wind forcing of the magnetosphere magnetosphere interactions for a northward directed with a correspondingly weak and steady cusp aurora in IMF [Song et al., 1999]. In their Plate 3 they consider the Northern Hemisphere. three major topological regions in the magnetosphere: In summary, we have documented the association bethe inner core or the plasmasphere, the outer magne- tween the dynamic cusp aurora and pulsed "reverse" tosphere, and the LLBL/tail region. These regions are two-cell convection in the northern winter hemisphere separated by separatrix surfaces, corresponding to to during strongly northward IMF. The observations are the plasmapause, the boundary between the outer mag- explained in terms of a scenario of sequential capture of netosphere and the LLBL/tail, and the magnetopause, magnetosheath plasma by the magnetosphere via high-

25 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA 12,893 latitude reconnection in both hemispheres and associated electromagneti coupling to the ionosphere at cusp and subcusp latitudes. Steady state (time averaged) aspects of this IMF - magnetosphere- ionosphere coupling process are in agreement with the MHD simulation model of Song ½t al. [1999]. A central feature of our interpretation of the multiinstrument observations is that a phase of one-lobe reconnection was followed by a phase of two-lobe events which terminated when the IMF became radial. Acknowledgments. We thank Keith Ogilvie and Ronald Lepping, the principal investigators on the SWE and MFI instruments, respectively, on Wind for use of the solar wind and IMF data. This work is supported in part by NASA grant NAG The IMAGE magnetometer data used in this study were collected as a German-Finnish- Polish-Norwegian project conducted by the Technical University of Braunschweig and the Finnish Meteorological Institute. The auroral observation program on Svalbard is supported by the Norwegian Research Council and the Norwegian Polar Research Institute. The CUTLASS radar is funded by the Particle Physics and Astronomy Research Council on grant PPA/R/R/1997/00256 and S. E. Milan on grant PPA/G/O/1997/ Michel Blanc thanks Hermann Opgenoorth and Christopher T. Russell for their assistance in evaluating this paper. References Cowley, S. W. H., Magnetospheric and ionospheric flow and the interplanetary magnetic field, in The Physical Basis of the Ionosphere in the Solar - Terrestrial System, Rep. 295 AGARD-CP, pp. 4 (1)-4 (14), NATO, Neuilly-sur-Seine, Cowley, S. W. H., and M. Lockwood, Excitation and decay of solar wind-driven flows in the magnetosphereionosphere system, Ann. Geophys., 10, 103, Cowley, S. W. H., M. Freeman, M. Lockwood, and M. F. Smith, The ionospheric signature of flux transfer events, in Cluster Dayside Polar Cusp, edited by C. I. Barron, Eur. Space Agency Spec. Publ., ESA SP-$$0, pp , 199!. Crooker, N. U., Reverse convection, J. Geophys. Res., 97, 19,363, Crooker, N. U., and F. J. Rich, Lobe cell convection as a summer phenomenon, J. Geophys. Res., 98, 13,403, Cumnock, J. A., R. A. Heelis, and M. R. Hairston, Response of the ionospheric convection pattern to a rotation of the interplanetary magnetic field on January 14, 1988, J. Geophys. Res., 97, 19,449, Dungey, J. W., Interplanetary magnetic field and the auroral zones, Phys. Rev. Lett., 6, 47, Dungey, J. W., The structure of the exosphere, or adventures in velocity space, in Geophysics: The Earth's Environment, edited by C. DeWitt, J. Hiebolt, and A. Lebeau, pp , Gordon and Breach, Newark, N.J., Elphinstone, R. D., D. J. Hearn, J. S. Murphree, L. L. Cogger, M. L. Johnson, and H. B. Vo, Some UV dayside auro- ral morphologies, in Auroral Plasma Dynamics, edited by R. L. Lysak, Geophys. Monogr. Set., vol. 80, pp , AGU, Washington, D.C., Farrugia, C. J., P. E. Sandholt, S. W. H. Cowley, D. J. Southwood, A. Egeland, P. Stauning, R. P. Lepping, A. J. Lazarus, and T. Hansen, Reconnection - associated auroral activity stimulated by two types of upstream dynamic pressure variations: Interplanetary magnetic field Bz O, By<<0 case, J. Geophys. Res., 100, 21,753, Fedder, J. A., and J. G. Lyon, The Earth's magnetosphere is 165 RE long: Self-consistent currents, convection, mag- netospheric structure, and processes for northward interplanetary magnetic field, J. Geophys. Res., 100, 3623, Feldman, W. C., et al., Possible conjugate reconnection at the high-latitude magnetopause, J. Geophys. Res., 100, 14,913, Freeman, M. P., C. J. Farrugia, M. R. Burlaga, L. F. Hairston, M. E. Greenspan, J. M. Ruohoniemi, and R. P. Lepping, The interaction of a magnetic cloud with the Earth: Ionospheric convection in the northern and southern hemispheres for a wide range of quasi-steady interplanetary magnetic field conditions, J. Geophys. Res., 98, 7633, Greenwald, R. A., W. A. Bristow, G. J. Sofko, C. Senior, J.- C. Cerisier, and A. Szabo, Super dual auroral radar network radar imaging of dayside hogh-latitude convection under northward interplanetary magnetic field: Toward resolving a distorted two-cell versus multicell controversy, J. Geophys. Res., 100, 19,661, Greenwald, R. A., J. M. Ruohoniemi, K. Baker, W. A. Bristow, G. J. Sofko, J.P. Villain, M. Lester, and J. Slavin, Convective response to a transient increase in dayside reconnection, J. Geophys. Res., l O d, 10,007, Jankowska, K. R., R. D. Elphinstone, J. $. Murphree, L. Cogger, D. Hearn, and G. Marklund, The configuration of auroral distribution for interplanetary magnetic field Bz northward, 2, Ionospheric convection consistent with Viking observations, J. Geophys. Res., 95, 5805, Kessel, R. L., $.-H. Chen, J. L. Green, S. F. Fung, S. A. Boardsen, L. C. Tan, T. E. Eastman, J. D. Craven, and L. A. Frank, Evidence of high-latitude reconnection during northward IMF: Hawkeye observations, Geophys. Res. Lett., 23, 583, Knipp, D. J., et al., Ionospheric convection response to slow, strong variations in a northward interplanetary magnetic cloud: A case study for January 14, 1988, J. Geophys. Res., 98, 19,273, Le, G., C. T. Russell, and J. T. Gosling, Structure of the magnetopause for low Mach number and strongly northward interplanetary magnetic field, J. Geophys. Res., 99, 23,723, Le, G., C. T. Russell, J. T. Gosling, and M. F. Thomsen, ISEE observations of low-latitude boundary layer for northward interplanetary magnetic field: Implications for cusp reconnection, J. Geophys. Res., 101,27,239, Lockwood, M., and J. Moen, Reconfiguration and closure of lobe flux by reconnection during northward IMF: Possible evidence for signatures in the cusp/cleft auroral emissions, Ann. Geophys., 17, 996, Lotko, W., and B. U. O. Sonnerup, The low-latitude boundary layer on closed field lines, in Physics of the Magnetopause, edited by P. Song, B. U. O. Sonnerup, and M. F. Thomsen, Geophys. Monogr. Ser., vol. 90, pp , AGU, Washington, D.C., Maezawa, K., Magnetospheric convection induced by positive and negative z component of the interplanetary magnetic field: Quantitative analysis using polar cap magnetic records, J. Geophys. Res., 81,2289, Mobarry, C. M., J. A. Fedder, and J. G. Lyon, Equatorial plasma convection from global simulations of the Earth's magnetosphere, J. Geophys. Res., 101, 7859, Moen, J., H. C. Carlson, and P. E. Sandholt, Continuous observation of cusp auroral dynamics in response to an IMF By polarity change, Geophys. Res. Lett., 26, 1243, 1999.

26 12,894 SANDHOLT ET AL.: CAPTURE OF MAGNETOSHEATH PLASMA Murphree, J. S., R. D. Elphinstone, D. Hearn, and L. L. Cogger, Large-scale high-latitude dayside auroral emissions, J. Geophys. Res., 95, 2345, {3ieroset, M., P. E. Sandholt, W. F. Denig, and S. W. H. Cowley, Northward interplanetary magnetic field cusp aurora and high-latitude magnetopause reconnection, J. Geophys. Res., 102, 11,349, Reiff, P. H., and J. L. Burch, IMF By-dependent plasma flow and Birkeland currents in the dayside magnetosphere, 2, A global model for northward and southward IMF, J. Geophys. Res., 90, 1595, Ruohoniemi, J., and R. Greenwald, Statistical patterns of high-latitude convection obtained from Goose Bay HF radar observations, J. Geophys. Res., 101, 21,743, Ruohoniemi, J. M., R. A. Greenwald, K. B. Baker, J.P. Villain, C. Hanuise, and J. Kelly, Mapping high-latitude plasma convection with coherent HF radars, J. Geophys. Res., 94{, 13,463, Russell, C. T., The configuration of the magnetosphere, in Critical Problems of Magnetospheric Physics, edited by E. R. Dryer, pp. 1-16, Nat. Acad. Sci., Washington, D. C., Russell, C. T., et al., Entry of the Polar spacecraft into the polar cusp under northward IMF conditions, Geophys. Res. Lett., 25, 3015, Sandholt, P. E., and C. J. Farrugia, On the dynamic cusp aurora and IMF By, J. Geophys. Res., 104, 12,461, Sandholt, P. E., M. Lockwood, T. Oguti, S. W. H. Cowley, K. S.C. Freeman, B. Lybekk, A. Egeland, and D. M. Willis, Midday auroral breakup events and related energy and momentum transfer from the magnetosheath, J. Geophys. Res., 95, 1039, Sandholt, P. E., J. Moen, A. Rudland, D. Opsvik, W. F. Denig, and T. Hansen, Auroral event sequences at the dayside polar cap boundary for positive and negative interplanetary magnetic field By, J. Geophys. Res., 98, 7737, Sandholt, P. E., C. J. Farrugia, M. {3ieroset, P. Stauning, and S. W. H. Cowley, Auroral signature of lobe reconnection, Geophys. Res. Lett., 23, 1725, 1996a. Sandholt, P. E., C. J. Farrugia, P. Stauning, S. W. H. Cowley, and T. Hansen, Cusp/cleft auroral forms and activ- ities in relation to ionospheric convection: Responses to specific changes in solar wind and interplanetary magnetic field conditions, J. Geophys. Res., 101,5003, 1996b. Sandholt, P. E., C. J. Farrugia, and S. W. H. Cowley, Pulsating cusp aurora for northward IMF, J. Geophys. Res., 103, 26,507, 1998a. Sandholt, P. E., C. J. Farrugia, J. Moen, {3. Noraberg, B. Lybekk, T. Sten, and T. Hansen, A classification of dayside auroral forms and activities as a function of IMF orientation, J. Geophys. Res., 103, 23,325, 1998b. Sandholt, P. E., C. J. Farrugia, S. W. H. Cowley, W. F. Denig, M. Lester, and B. Lybekk, Capture of magnetosheath plasma by the magnetosphere during northward IMF, Geophys. Res. Lett, 26, 2833, 1999a. Sandholt, P. E., C. J. Farrugia, S. W. H. Cowley, M. Lester, J. Moen, B. Lybekk, and E. Trondsen, Excitation and decay of magnetospheric lobe cell convection and its associated aurora, Geophys. Res. Lett., 26, 3597, 1999b. Sandholt, P. E., C. J. Farrugia, and B. Lybekk, The dynamic cusp aurora on Nov. 30, 1997: Response to southward turning of the IMF, Ann. Geophys., 17, 1155, 1999c. Shepherd, S. G., R. A. Greenwald, and J. M. Ruohoniemi, A possible explanation for rapid, large-scale ionospheric responses to southward turnings of the IMF, Geophys. Res. Lett., 26, 3197, Song, P., and C. T. Russell, Model of formation of the low - latitude boundary layer for strongly northward interplanetary magnetic field, J. Geophys. Res., 97, 1411, Song, P., T. E. Holzer, C. T. Russell, and Z. Wang, Modelling the low-latitude boundary layer with reconnection entry, Geophys. Res. Lett., 21,625, Song, P., D. L. DeZeeuw, T. I. Gombosi, C. P. T. Groth, and K. G. Powell, A numerical study of solar wind - magneto- sphere interaction for northward interplanetary magnetic field, J. Geophys. Res., 104, 28,361, Southwood, D. J., The ionospheric signature of flux transfer events, J. Geophys. Res., 92, 3207, Villain, J.P., R. A. Greenwald, K. B. Baker, and J. M. Ruohoniemi, HF radar observations of E region plasma irregularities produced by oblique plasma streaming, J. Geophys. Res., 92, 12,327, J.-C. Cerisier, Centre d'etude des Environnements Terrestre et Planetaires, 4 Avenue de Neptune, Saint- Maur Cedex, France. S. W. H. Cowley, and M. Lester, Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, United Kingdom. W. F. Denig, Space Vehicles Directorate, Air Force Research Laboratory, Randolph Road, Hanscom AFB, MA C. J. Farrugia, Space Science Center, University of New Hampshire, Durham, NH B. Lybekk, P. E. Sandholt, and E. Trondsen, Department of Physics, University of Oslo, P. O. Box 1048, Blindern, N-0316, Norway. (p.e.sandholt@fys.uio.no) J. Moen, University Courses on Svalbard, P.O.Box 156, N-9070 Longyearbyen, Norway. (Received November 15, 1999; revised January 20, 2000; accepted January 28, 2000.)