Northward interplanetary magnetic field cusp aurora and high-latitude magnetopause reconnection

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. A6, PAGES 11,349-11,362, JUNE 1, 1997 Northward interplanetary magnetic field cusp aurora and high-latitude magnetopause reconnection M. Oieroset and P. E. Sandholt Department of Physics, University of Oslo, Oslo, Norway W. F. Denig Geophysics Directorate, Phillips Laboratory, Hanscorn Air Force Base, Massachusetts S. W. H. Cowley Department of Physics and Astronomy, University of Leicester, Leicester, United Kingdom Abstract. We present observations of two types of auroral forms located at different latitudes in the cusp region. Type I (south) auroras are located at - 71 ø- 75 ø MLAT and occur during intervals of southward directed interplanetary magnetic field (IMF). Higher-latitude (77 ø- 78 ø) type 2 (north) auroras are associated with northward IMF (Bz) 0). Intervals are studied which are characterized by transitions from one auroral form to the other in response to IMF switches from north (clock angle (50 ø) to south (clock angle )90 ø) and vice versa. These observations are found to be consistent with the interpretation that type south auroras are a signature of low-latitude magnetopause reconnection and that the type north auroras are associated with high-latitude reconnection, tailward of the cusp. The latter is supported by satellite (DMSP Fll) observations of particle precipitation and ionospheric convection. The simultaneous existence of type north and type south auroras is observe during intermediate IMF states (clock angle - 60ø). Observations of type north auroras may be used to study the relationship between the IMF Bx component and high-latitude reconnection. We find that type north auroras occur in the northern hemisphere for both Bx polarities. Enhanced emission intensity of the auroral green line seems to be a feature occurring predominantly during negative Bx conditions. The enhanced green line intensity is considered to be an indication of low-altitude particle acceleration in regions of strong field-aligned current intensities. Such particle accelerations (green line emission) may occur predominantly regions of upward directed IMF By-related currents in association with the type north cusp aurora. Introduction solute value of the IMF clock angle is below a certain critical value), magnetic reconnection may occur pole- The site of magnetic reconnection between the interward of the cusp. These two modes of magnetopause planetary magnetic field (IMF) and the dayside magreconnection are called low-latitude reconnection and netosphere is dependent on the orientation of the IMF high-latitude reconnection, respectively. Both processes [Dungey, 1961, 1963]. When the IMF has a southward lead to particle transfer and acceleration at the magnecomponent (Bz < 0), magnetic reconnection may octopause and associated precipitation from the boundary cur equatorward of the cusp. If the IMF has a northlayers down to the polar ionosphere is expected [e.g., ward component (Bz > 0; more specifically, the ab- Onsager et al., 1993]. Crooker [1992] proposed that there are two basic Now at International Space Science Institute, Berne, modes of quasi-steady northward IMF reconnection. Switzerland One mode is Dungey's [1963] original concept with the inclusion of dipole tilt and/or an x component of the Copyright 1997 by the American Geophysical Union. IMF, while the other mode excites the so-called lobe Paper number 97JA cell convection[reiff and Burch, 1985]. The lobe cells o14s-o?/?/? ^oo $o.oo close completely within the region of open field lines and 11,349

2 11,350 OIEROSET ET AL.: NORTHWARD IMF CUSP AURORA should be enhanced in the favored hemisphere, which is in the north if Bx < 0 and the the south for Bx > 0. roral forms located at lower latitudes (<75 ø MLAT) during intervals of low-latitude reconnection. The lat- In Dungey's [1963] original high-latitude reconnection ter consists of a narrow (_< 100 km) zone of strong red concept, merging occurs simultaneously in both hemispheres. Alternatively, merging may first occur in the favored hemisphere and subsequently occur in the unfaline emission with a sharp equatorward boundary. This luminosity was called the cusp/low-latitude boundary layer (LLBL) aurora, referring to the open magnetic vored hemisphere when the reconnected field line, which field configuration (cusp) and the plasma source (LLBL is still open at one end, is draped over the magnetopause in the subsolaregion). Part of this paper is devoted [Crooker, 1992]. The reconnected field line will then be to a more detailed study of transitions between the two closed at both ends. In this process, magnetosheath types of cusp auroral forms, occurring when the IMF plasma is effectively transferred to the magnetosphere. switches from northward to southward orientation and Both of these two modes of high-latitude reconnec- vice verse. tion may give rise to sunward convection in the favoured If the positive IMF Bz cusp aurora is a signature of hemisphere. Dungey's [1963] concept will in addition high-latitude reconnection, it may be used to investiproduce sunward convection in the unfavoured hemisphere. It is noted that also lobe cell merging may excite flow in both hemispheresimultaneously, if highlatitude reconnection happen with different IMF lines gate the relationship between the latter coupling process and the IMF B component. In this paper the particle precipitation above such an aurora is documented by combining ground and satellite observations. Data simultaneously. This will particularly occur if there is from satellite DMSP Fll show that the auroral lumia By component. In situ observations of the magnetic field and plasma signatures of high-latitude reconnection have been pubnosity, located at 77 ø MLAT, was embedded in a regime of sunward convection. A characteristic inverse energy versus latitude ion dispersion was observed, consistent lished by Gosling et al. [1991] and Kessel et al. [1996]. with observations from the Viking satellite during inter- Particle precipitation characteristics at lower altitudes vals of high-latitude reconnection [Woch and Lundin, during such conditions have been documented by e.g., 1992]. We study several examples of the aurora in the Woch and Lundin [1992]. However, the detailed solar cusp region during positive Bz conditions and its relawind-magnetosphere couplin geometry and its possible tionship with the IMF B component. This is done by relationship with the IMF B component and dipole tilt are not well known at present. selecting intervals with IMF B z > 0 and with a large unidirectional B component during periods of avail- A model of magnetosheath plasma entry into the able auroral observations in the MLT sector. magnetosphere based on Dungey's [1963] concept of A comparison is made of the signatures in the auroral high-latitude reconnection occurring simultaneously in red (630.0nm) and green (557.7nm) lines for intervals both hemispheres were presented by Song and Russell with positive and negative Bx. These observationshow [1992]. Feldman et al. [1995] found that after a sharp that positive Bz aurora at 630.0nm occur in the northnorthward-dawnward turning of the IMF, precipitation ern winter hemisphere for both IMF B polarities. The of relativistic electrons occurred simultaneously in both emission intensity is, however, generally stronger during polar caps. This observation was explained terms of negative B conditions, particularly in the green line. the model of Song and Russell [1992]. Negative B is a favorable condition for high-latitude Fuselief et al. [1995] presented magnetopause ob- reconnection in the north, according to the antiparallell servations favoring an interpretation in terms of high- merging model. latitude reconnection occurring in one hemisphere only. The structure of lobe cell convection depends on the Auroral observations by Murphre et al. [1990] obtained IBy/Bzl ratio [Greenwald et al., 1995]. IMFBy-related from the Viking UV imager showed that discrete auro- zonal flows and associated field-aligned currents are ral features poleward of the cusp can exist in the north- characteristic features of the noon sector [Clauer and ern hemisphere during periods of northward IMF. They concluded that such features appear to be limited to Friis-Christensen, 1988]. The possible relationship with the auroral morphology is discussed. situations when the IMF B component is negative and the By component is positive. This is consistent with Instrumentation the idea that lobe reconnection is favored in the northern hemisphere when IMF B < 0 and in the southern The optical measurements of the dayside aurora used hemisphere when IMF Bx > 0, due to the antiparallel in this study were obtained at the station in Ny-filesund field condition (see the review by Cowley [1981]). (NYA,-geographic coordinates 78.9øN, 11.9øE; corrected Possible signatures of lobe reconnection have also geomagnetic latitude 75.7 ø). The instrumentation used been recognized in the dayside optical emission at was a meridian scanning photometer (MSP). The MSP nm as an east-west aligned auroral form at high lati- measures line-of-sight intensities of auroral emissions as tudes (77 ø- 78 ø MLAT) occurring when IMF Bz is pos- a function of zenith angle. In this paper we use observaitive /Sandholt et al., 1996a,b]. It was named the tions of the auroral red (630.0 nm) and green (557.7 nm) cusp/mantle aurora to distinguish it from different au- lines of atomic oxygen. The red emission represents

3 OIEROSET ET AL' NORTHWARD IMF CUSP AURORA 11,351 the F layer dayside auroral band caused by soft precipitation (~ ev), while the green emission is faded out near 0927 UT and was absent during the period of strongly negative IMF Bz component. The inmore sensitive to structures of higher-energy electron terval ~ 0918 to 0925 UT, showing two auroral forms loprecipitation (~0.5-1 kev) penetrating to E layer alti- cated at different latitudes, corresponds to intermediate tudes. The MSP scans along the magnetic north-south IMF conditions with small Bz component. The arrows meridian. With this instrument auroral forms within in Figure I indicate the main transitions in the IMF Bz the approximate latitude range 70 ø- 80 ø MLAT can be component and the corresponding changes in the auroobserved. Each photometer scan from north to south ral morphology. This example illustrates the fact that takes 18 s. Magnetic noon in Ny-]klesund is around the cusp aurora is extremely sensitive to changes in the 0850 UT. IMF clock angle. In the present case (Figure 1) the The ground-based optical observations are combined clock angle changed from typically ~ 45 ø in the interval with information on solar wind and interplanetary mag- of the type north aurora to ø in the interval of the netic field (IMF) data. The measurements of the IMF type south aurora (cf. Figure 5). and the solar wind velocity used in this study are 1- min resolution data from the IMP 8 and Wind (from November 1994) spacecraft while they were located in the solar wind beyond the bow shock. The solar wind velocity is needed for the calculation of the delay times between signals recorded at the satellite and associated signatures observed by the ground-based instruments in Ny-]klesund [Lockwood et al., 1989]. When available, ground observations from Ny-]klesund are also combined with complementary space data from the polar-orbiting Defense Meteorological Satellite Program (DMSP) satellites near 800km altitude. Instrumentation for the DMSP Fll satellite, which is used in this study, include an auroral particle sensor and a cold plasma detector termed the SSJ/4 and the SSIES, re- spectively. The SSJ/4 measures the differential flux of precipitating auroral electrons and ions (protons) from 30 ev to 30 kev, and the measurements from SSIES include the cross-track drifts, horizontal and vertical, of the background plasma up to 2.5 km/s. Observations In this study we concentrate on observations of type north auroras, which are possible signatures of highlatitude reconnection, occurring when the IMF Bz component is positive. The cases used were selected by choosing intervals when IMF Bz was positive, and Bx either positive or negative for a continuous period greater than 10 min. The corresponding MSP data were then studied, taking into account satellite-ground time delays. The selected cases, which are limited to the MLT sector, are given in Table 1. The times given in Table I refer to the satellite location. The latter values are approximate average values for these intervals. Table I lists seven cases of positive and nine cases of negative IMF Bx component. The total observation time is 910 min, with 430 min of positive IMF Bx, 440 min of negative B, and 40 min of near-zero B component. Two examples of the auroral observations, one corresponding to a positive and the other to a negative IMF B will now be presented. Negative IMF Bz Case Study: December 2, 1994 As an introduction to the observations we present in Figure 1 data illustrating the two main categories of auroral signature, corresponding to negative and positive IMF Bz conditions, referred to as types 1 and 2 Figure 2 shows IMF data from the Wind satellite for December 2, 1994, from 0900 to 1100 UT. The satellite location at 0900 UT was XGSM -- 23RE, YGSM RE, and ZGSM -- ORE. The signal speed [cf. Sandholt et al., 1996a]. Alternatively, these forms was ~ 500 km s-1. On this day, IMF Bz went from negmay be called type south and type north, respectively. ative to positive at 0908 UT and stayed positive until Figure 1 shows an interval of corresponding MSP and IMF Bz data from December 7, The MSP data run from 0905 to 0935 UT, and the calculated satelliteground time delay is approximately 10 min. This is 1050 UT. Bx was negative during this interval, and By was mostly positive. The signal propagation time delay between the Wind location and the dayside cusp ionosphere was estimated to be 10min around the time the sum of the signal propagation time from the satel- of the IMF transition and near 5 min for the lite location to the magnetopause and the Alfv n wave propagation delay from the subsolar magnetopause to 0950 UT period. This time delay is the sum of the solar wind propagation time from the satellite to the subsothe cusp ionosphere [cf. Lockwood et al., 1989]. Hence lar magnetopause and the Alfv n wave travel time from IMF Bz data for the interval 0855 to 0925 UT are plotted. The interval of positive Bz is characterized by a moderately intense aurora at 30 ø -40 ø north of zenith (type the magnetopause to the cusp ionosphere. A detailed discussion of such delay estimates and associated uncertainties are given by Lockwood et al. [1989]. The auroral (MSP) data ( UT) are disnorth), whereas the negative Bz interval corresponding played in Figure 3. Before 0930 UT the main auroto MSP data during the period ~0925 to 0935 UT shows ral form was located south of Ny-]klesund, showing a another, more intense auroral form located well to the marked poleward motion during the interval south of zenith (type south). The high-latitude aurora UT. We classify this as type south (BE 0) aurora.

4 11,352 OIEROSET ET AL.' NORTHWARD IMF CUSP AURORA Time(UT) 09: nm i I 09:10 09:15 09:20 09:25 09: :35 Bz (nt) N ' 60 ' 3'0 ' 0 3'0 ' ' 6'0 'S Figure 1. Corresponding 630.0nm Ny-Mesund MSP ( UT) and Wind IMF Bz ( UT) observations from December 7, The MSP plot shows auroral intensity (630.0nm) versus zenith angle and time. Type south and north auroral forms are marked. Near 0930 UT a new auroral form appeared north of sifted and moved poleward, while the type south auzenith. Both the poleward motion of the type south au- rora weakened and disappeared during the rora and the appearance of the second auroral form are UT interval. The type north aurora had a sharp poleprobably associated with the northward turning of the ward boundary and may be identified as a cusp/mantle IMF at 0908 UT. This second auroral form is classified aurora [Sandholt et al., 1996a,b]. Near 0930 UT the as a type north (Bz > 0) aurora. The two categories IMF B= component became more negative, to approxare marked in Figure 3. The type north aurora inten- imately -6 nt. It is noted that during the interval cor-

5 Table 1. Cases of Type North (Bz > 0) Aurora. OIEROSET ET AL.' NORTHWARD IMF CUSP AURORA 11,353 Date Time, UT Bz, nt By, nt B,, nt 557.7nm Dec. 5, weak Dec. 5, weak Jan. 25, weak Jan. 25, weak Jan. 26, weak Dec. 18, weak Dec. 1, strong Dec. 30, weak Dec. 2, strong Dec. 2, weak Dec. 15, weak Nov. 29, weak Dec. 2, strong Dec. 7, weak Dec. 26, strong Jan. 4, weak Jan. 24, strong The nm emission is classified as strong when the peak intensity is greater than 1 kr, and weak when it is less. responding to th e strongly negative Bx, approximately nearly perpendicular to the auroral scan at 0933:40 UT, UT, an enhanced intensity of the auroral green line (557.7nm) is observed. The onset of the strong green line is marked with an arrow in Figure 3. as indicated in Figure 4, and nearly tangent to the 77.5 ø MLAT contour poleward of Ny-]klesund. Figure 4 also includes various data features of interest, and iden- A third auroral form located slightly south of zenith tifies the locations of mapped magnetospheric regions. during the positive Bz interval, is marked in Figure 3 with the dotted line. The satellite trajectory marked in the figure is given in Subsatellite coordinates, but the plasma regions in- Figure 4 is an annotated chart of the DMSP Fll pass dicated are mapped along the field lines to an altitude across the Ny-]klesund meridian at 0933:40 UT on De- of 250 km, which is the assumed height of the 630.0nm cember 2, Note on th e chart the locations of the auroral emission. The data and the identifications in 75 ø MLAT circle, the 1200 and 1400 MLT meridian, and Figure 4 were derived from Plate 1, which is a composthe scan line of the MSP in Ny-.llesund (NYA). The ite plot of the DMSP Fll particle and field data. The hatched region along the MSP meridian is the loca- middle color panels of the figure are plots of the differtion of the type north aurora. The Fll satellite crossed ential number flux of electrons and ions, whereas the 0908 UT 10 ' ,,, I :!0930 UT!!!! I '... i... i... ' ',... ',... ':... :... Figure :.... :.,,. ': s...!... : : : ]0 Tima (h um) Sat.paa. 23, -2, 0 Wind interplanetary magnetic field (IMF) data for December 2, 1994, UT.

6 11,354 OIEROSET ET AL.' NORTHWARD IMF CUSP AURORA Time(UT) lkr nm nm! lkr 09: :05 09:10 09: : : : : : :45 09:50 09:55 10:00 N '0 ' ' 6'0 'S N S Figure 3. Ny-Alesund MSP data for the interval UT on December 2, Red and green line emissions are displayed in the left and right panels, respectively. Type south and north auroral forms are marked. The dotted line in the left panel indicate a third auroral form. top two plots are the integrated energy flux and aver- The precipitating particle data in Plate I indicates age energy for each species. At the bottom of Plate 1 that the satellite was poleward of the plasma sheet are plots of the horizontal and vertical components of boundary (74 ø MLAT) from 0930:40 to 0936:50 UT, the cross-track drift. The horizontal component of the recording more soft precipitation in the high-latitude drift has been corrected for corotation. The ephemeris region. This identification is based on the high averdata at the bottom of Plate I includes the Corrected age energy ( 0.5 kev) of the precipitating electrons in Geomagnetic (CGM) coordinates of the satellite. the plasma sheet and the presence, on the afternoon

7 OIEROSET ET AL.' NORTHWARD IMF CUSP AURORA 11,355 Figure 4. The DMSP Fll trajectory in sub-satellite coordinates crossing Svalbard near 0934 UT on December 2, The optical observation site (NYA), the MSP north-south scan from Ny-Alesund, and 1200 and!400 MLT meridians are shown in the figure. The location of the type north aurora along the MSP scan is drawn as the hatched area, assuming an emission altitude of 250 km. Plasma regions mapped to a 250 km altitude are inferred from the satellite particle precipitation data and given. MA/LOBE, mantle/lobe particle precipitation; LLBL, low-latitude boundary layer particle precipitation; PS, plasma sheet particle precipitation. The ionospheric convection perpendicular to the satellite trajectory is marked with arrows. We can see that the type north aurora is located in a region of mantle/lobe precipitation and sunward onvection. side, of energetic ions. Convection within the central plasma sheet was near zero both in the prenoon and postnoon sectors although there was some modest antisunward convection detected in the postnoon boundary plasma sheet. We tentatively identify the zone on each side of noon characterized by antisunward flow, low-energy electrons (hundreds of ev), and high fluxes of < I kev ion precipitation, as the ionospheric projection of the low-latitude boundary layer (LLBL). Pole- The satellite location at 0900 UT was X RE, Y RE, Z Re, and the solar wind speed was -660 km s -. The satellite-ground time delay was approximately 10 min at the time of the IMF transition at 0915 UT. During the UT interval the time delay is estimated to be a few minutes only. Figure 6 shows MSP data for the interval UT, characterized by positive B and Bz components. We observe a steady auroral form with a sharp poleward ward of the LLBL (in the interval 0932;00 to 0935:20 boundary and a dominating nm emission, located UT) the Fll satellite intercepted a central region of approximately between 20 ø and 60 ø north of zenith in strong upward flows and modest sunward convection Ny-Jilesund. This form is similar to the one reported (0.1-I km/s). The upward flows suggesthat the Fll crossed equatorward of a region of greater electron enabove for negative IMF B conditions (cf. Figure 3) and is recognized as a typical type north (Bz > 0) aurora. ergy flux (heating) which may have been responsible However, the 557.7nm emission was relatively weak (< for the optical feature seen poleward of Ny-Jilesund. I kr) during most of the interval, except for the short We note that portions of this heating may be done by intensifications near 0826 and 0836 UT. The IMF clock Alfv n waves and Joule heating. Within the central angle was in the range 0 ø- 55 ø. region and on either side of noon, the ion energy spectrum has an increasing peak energy with latitude. Both the ion energy dispersion and the equatorward moving Discussion and Conclusions heated ionosphere are consistent with high-latitude re- An important aspect of this paper is the documentaconnection [ Woch and Lundin, 1992]. tion of the temporal/spatial evolution of the cusp auro- Positive IMF Bx Case Study: December 7, 1994 IMF data from the Wind satellite for December 7, 1994, are displayed in Figure 5. From 0745 to 0908 UT the IMF Bz component was positive, Bx was also ral morphology in response to IMF north-south transitions, such as those which occurred on December 2 and 7, 1994 (Figures I and 3). Around the time of the IMF transition on December 7, 1994 (0915 UT), the solar wind density fluctuated bepositive (approximately +5 nt), while B u was negative. tween 3 and 4cm -3, whereas the speed increased from

8 11,356 OIEROSET ET AL.: NORTHWARD IMF CUSP AURORA -5 5 Eo '---0 x Time (hours) Sat.pos. 54,-38, -2 Figure 5. Wind IMF data for the interval UT on December 7, to 700 km s -1. However, the corresponding 10 percent enhancement in solar wind dynamic pressure canwas interpreted as a signature of plasma transfer from the magnetosheath taking place at high magnetopause not explain the observed auroral transition [cf. Sandholt latitudes, tailward of the cusp, due to lobe reconnection. et al., 1994]. In the other case (December 2, 1994, 0910 Another case of south-to-north IMF transition has UT), only minor fluctuations in the solar wind speed been reported in this study. A plausible scenario for and density were observed. Thus we conclude that in the evolution of the ionospheric cusp signatures obthese cases the auroral transitions were due to the ob- served during the interval UT on Decemserved large changes in the IMF clock angle. Another objective of this study is to check if the cusp aurora observed during northward IMF conditions in the northern winter hemisphere depends on the polarity ber 2, 1994 (Figure 3), will be given below. For this event, the Ny- lesund meridian was located slightly after noon, near 1230 MLT. The main auroral form in the initial state (type south aurora), say at 0915 UT, of the IMF Bx and B v components. Such information was observed near 72 ø MLAT, assuming an emission almay shed new light on the source mechanism, possi- titude of 250 km for the 630.0nm aurora. This latitude bly involving high-latitude reconnection in the north- of the cusp aurora can be associated with the observed ern hemisphere. According to one model, high-latitude IMF Bz component of approximately -2nT in the inreconnection should show a strongly interhemispheric terval before the northward transition. asymmetry, favouring Bx negative polarity in the north [e.g. Crooker, 1979 i. Studies by Sandholt et al. [1996a,b] show that the latitudinal position of the cusp aurora is strongly related to the sign of the IMF Bz component, or more specifically, the IMF clock angle, and that different auroral forms are observed during strongly positive and strongly negative Bz conditions. The data reported here add further evidence for this. It has been documented previously that the lobe reconnection process may give rise to a characteristic auroral signature in.the north- In Figure 7 we sketch the variations of the ionospheric flow, cusp ion precipitation and key boundaries for a south-to north IMF transition, based on the Cowley and Lockwood [1992] picture of flow excitation and decay in the coupled magnetosphere-ionosphere system. Specifically, we illustrate the case in which a transition occurs between an interplanetary field with negative Bz and positive By components (clock angle > 90ø), and a field which has a dominant positive Bz and weak By (clock angle 00ø), such as occurred at 0908 UT on December 2, Figure 7a shows the initial state with ern winter hemisphere [Murphree et al., 1990; Sandholt negative Bz and positive By, in which a twin-cell flow et al., 1996a]. Sandholt eta!. [1996a,b] showed that is present with a predominant westward component in a sharp transition from strqngly negative to strongly the cusp, driven by equatorial plane dayside reconnecpositive IMF Bz conditions was followed by the disap- tion [e.g., Cowley et al, 1991; Rich and Hairston, 1994]. pearance of the cusp/llbl auroral emission located The numbered dashed lines show loci of equ intervals south of 75 ø MLAT and the appearance of a new auroral form 2ø-3 ø MLAT further north, the so-called cusp/mantle aurora. The latter form was associated with sunward convection in the dayside polar cap. It of time on streamlines downstream from the dayside reconnection site (dashed part of the open-closed field line boundary), indicating the fall in the lower cut-off of the cusp ion precipitation as this time increases. We

9 ß,, OIEROSET ET AL' NORTHWARD IMF CUSP AURORA 11,357 2 DEC Ion lo s lo 4 t t..! 10 2 i : PS? LLBL? ':' MA/LOBE, ::}j;,:.. ' LLBL? PS Sunward f... : ' :40, HOR-C UT(SEC) ' HI-I:MM 9:31 9:33 NYA 9:35 9:.37 9:39 MI T meridian 77.z MLT Plate 1. The electron and ion fluxes versus energy and time for the interval UT on December 2, 1994, measured during the DMSP Fll satellite pass shown in Figure 3. The satellite crossed the Ny-Jklesund meridian from east to west at 0933:40 UT. The plasma region inferred from the particle precipitation are shown. also show a slow expansion of the open-closed field line narrower in latitudinal extent and contains only lowboundary, assuming here for definiteness that the open energy ions and electrons. After another few minutes, flux production is not fully balanced by tail reconnec- however, the northward field arrives at the reconnection tion. When the northward directed IMF arrives at the sites poleward of the cusp, and lobe reconnection starts. noon dayside equatorial magnetopause, the equatorial As shown in Figure 7c, representing the situation after dayside reconnection will cease, first in the subsolare-, 8 min, the ionospheric image of the lobe reconnection gion near noon, and then spreading with the magne- site propagates poleward from the open-closed field line tosheath flow to earlier and later local times over the next few minutes. After 3-5 min we anticipate that equatorial reconnection with the southward field will have ceased, while lobe reconnection with the northboundary into the polar cap and also expands in local time. A twin cell "reversed" flow is excited on open field lines near noon in its vicinity (the case for nearzero B u is shown), while away from noon flows driven ward field has yet to commence. In Figure 7b, therefore, by prior interval of southward field continue to decay to illustrating the situation after, 4 min (plus, 2 min, in zero. A "reversed dispersion" ion cusp is formed equathe ionosphere, to account for the Alfv n wave propaga- torward of the mapped lobe reconnection site, on field tion from the magnetopause), we show the twin-cell flow lines which may for a short interval also continue to condying away, with poleward motion of the (now wholly tain low-energy cusp ions from the prior interval. The adiaroic) dayside open-closed field line boundary as the symmetric convection patterns in Figures 7c and 7d are system moves towards an equilibrium state. The region consistent with the IMF measurements which show a to where cusp precipitation maps in the ionosphere is large Bz (, +10nT) and a small Bu for the actual time

10 11,358 OIEROSET ET AL' NORTHWARD IMF CUSP AURORA Time(UT) 08:10 Ilk nm nm i lkr 08:15 08: : :30 08:35 08'40 N '0 ' S N S Figure 6. Ny-hlesund MSP dat, for the interval UT on December 7, interval ( UT at the satellite). After ~10-15 min the system approaches a new equilibrium if the IMF remains steady (Figure 7d), with a fully developed "reversed" flow and "reversed" cusp dispersion. In Figure 8 we sketch the variations in latitude of the key boundaries and cusp ion characteristics anticipated on this basis along a meridian adjacent to noon (like that of the MSP on December 2, 1994) as a function of time. As for Figure 7, the numbered dashed lines show loci of equal intervals of time on streamlines down- stream from the dayside reconnection site. The Bz < 0 interval is an interval of low-latitude reconnection and the associated cusp ion dispersion signature probably show increasing energy with decreasing latitude, as indicated in the figure. In the Bz > 0 interval, low- latitude reconnection stops and the open/closed field line boundary moves poleward. High-latitude reconnection is established and the cusp ion dispersion signature now show energy increasing with latitude (cf. Plate 1). The short delay which is shown between the onset of the ionospheric changes and the switch in sense of B at the subsolar magnetopause is intended to represent the short (~2 min) Alfv n-wave propagation delay from the subsolar magnetopause to the ionosphere mentioned above. It can be seen that the basic features of the MSP data in Figure 3 are well reproduced qualitatively by this picture, including the initial poleward motion of the equatorward boundary of the type south auroras (starting between 0915 and 0920 in Figure 3), followed by the appearance at higher latitudes of the type north

11 OIEROSET ET AL.: NORTHWARD IMF CUSP AURORA 11,359 (a) Initial state /Open/closed field ; ' line boundary Merging gap loci of e.qual times since reconnection (b) After ~4 min (C) After ~8min (d) After ~12 min (~final state) Ionospheric image of lobe merging site Figure 7. Sketche showing the variations of the dayside ionospheric flow, principal boundaries, and cusp ion precipitation which occur following a switch in the direction of the IMF from one with a negative Bz and a positive By, to one with a positive Bz and a near-zero By, as occurred at UT on December 2, The solid lines show the adiaroic portions of the open-closed field line boundary, the heavy dashed lines the ionospheric projections of the magnetopause reconnection sites, the solid arrows the motion of these boundaries, the arrowed solid lines the plasma streamlines, and the numbered lighter dashed lines the loci of equal intervals of time downstream from the reconnection sites (indicating the related fall in energy of the lower cut-off of cusp ions with increasing time). (a) The initial state with asymmetric twin-cell flow, (b) he situation 04 min after the first change has occurred in the ionosphere (which is 02 min after the northward IMF first appears at the subsolar magnetopause due to the Alfv n propagation delay to the ionosphere) with the twin-cell flow dying away and the open-closed field line boundary propagating poleward, (c) the situation after 08 min with "reversed" flow cells and corresponding "reversed" cusp ion precipitation emerging near noon, and (d) the approach to a new equilibrium flow after min. aurora (near 0930 UT in Figure 3), and the poleward closed field line boundary after the field has switched to motion of their poleward border. We also indicate the north and dayside reconnection has ceased. Such laypossible existence of precipitation from a closed LLBL ers are unlikely to be present adjacent to the merging which may form due to secondary (nonreconnection) gap when dayside reconnection is in progress, because magnetopause processes just equatorward of the open- of the continual convection of closed flux tubes into the

12 11,360 OIEROSET ET AL.' NORTHWARD IMF CUSP AURORA Latitude Ionospheric image of j I B z >0 merging line I I.-- Woch and Lundin [1992] reported Viking observa-, / '.--. -_--.._- --_'Jl' energy tions during meridional passes through the polar cusp at midaltitudes for positive IMF Bz conditions, showenergy I/ increasing I-"-'-"... /,. X,.'" k.x-x... boundary ing two latitudinal zones of cusp precipitation, i.e., un- ' '0... ' '... L 'LBL closed flux accelerated magnetosheath plasma (their cusp proper) Merging gap min and a zone of accelerated and more field-aligned ions I time I and electrons further poleward (their boundary region). Bz<O ==>.! _.- Bz>O The latter component may be caused by lobe reconnec- I tion, according to Woch and Lundin. For the Bx < 0 Figure 8. Latitude variations of boundaries and cusp cases reported here the auroral form is characterized by precipitation along a meridian adjacent to noon as a function of time. The variations are due to a change enhanced green line emission (relative to the midday in IMF as described for Figure 7. The line types also gap-type aurora) and a sharp poleward boundary (Figcorrespond to the latter figure, except for the hatched ure 3). This indicates that the precipitating electrons region which shows the possible existence of a boundary have been accelerated at the magnetopause or at lower layer on closed flux tubes adjacent to the open-closed altitude. It is therefore likely that the actual auroral field line boundary when IMF Bz is positive. form (our type north aurora) corresponds to Woch and Lundin's boundary region. For cases in which Bx > 0, the green line can be almost absent, indicating that the reconnection region in these circumstances. This may particles causing these auroras are less accelerated. explain the appearance of the aurora equatorward of Examples of type north auroral signatures listed in the type north aurora marked with the dashed line in Table I are similar to those reported in Figures 3 and 6. the left panel of Figure 3. The main conclusion is that a similar auroral signature However, this aurora may also be a type south au- is observed both for positive and negative IMF B conrora in view of the dominant IMF By component in this ditions (Bz _ 0). However, there may be a relationship time interval ( UT). The IMF clock angle between the IMF B polarity and the emission intenfor this interval is - 60 ø, which implies an intermediate sity, and spectral properties of the auroral signature. magnetic shear condition at the subsolar magnetopause. For strongly negative IMF B conditions there seems in Under such circumstances, type south and north auro- general to be a stronger 557.7nm (green) emission than ras may be simultaneously present, constituting a dual for positive B values (see example in Figure 3). This mode of cusp auroral emission (hybrid cusp), possibly is an indication of more energetic electrons penetrating due to simultaneous low- and high-latitude reconnecto lower altitudes, below approximately 150km, durtions, [see Weiss et al., 1995]. ing the former condition. For a positive IMF Bx the The different states of the cusp aurora observed dur- auroral signature appears to be somewhat weaker and ing the UT interval on December 2, 1994, more dominated by the 630.0nm (red) emission in gen- may correspond to the following three regions of IMF clock angle: 0 > 90 ø UT 0-0 ø UT 0-60 ø UT The DMSP data in Plate 1 support the interpretation given in Figures 7 and 8. The particle signature (1 kev ions and 200eV electrons) in the vicinity of the type north auroral form (north of Ny-. lesund) and the associated sunward convection are consistent with highlatitude reconnection as the source. In addition, the energy versus latitude ion dispersion recorded from 0932 to 0936 UT is the opposite of that ascribed to lowlatitude reconnection, and is consistent with the the- ory of high-latitude reconnection. In that process, ions may be accelerated by magnetic tension forces somewhere poleward of the cusp and move sunward along reconnected field lines. The highest-energy ions reach the ionosphere first, that is at highest latitudes, making the characteristic inverse dispersion signature seen in Plate 1. These observations may be described by two lobe cells, as shown in Figure 7d. eral. The green line can be almost absent (< 0.5 kr) in some cases. From Table I we can see that "strong" green line emission is observed (five cases) during the following IMF conditions: B < 0; By > 0; B, << 0; By = 0; B << 0; By < 0; clock angle. < 55 ø (three cases) (1 case in prenoon sector) (1 case in postnoon sector) The IMF conditions during intervals of "weak" green line emission (12 cases) are B > 0; By < 0; clock angle < 55ø(six cases) B < 0; By > 0; (four cases) Bx > 0; By > 0; (one case; prenoon) B -- 0; By < 0; (one case; prenoon) According to these limited data, the preferred IMF condition (three out of five cases) for "strong" green line emission in the type north aurora is B 0 and By O, whereas a weak green emission often occurs (6 out of 12 cases) when B is positive an(]. By is negative.

13 OIEROSET ET AL.' NORTHWARD IMF CUSP AURORA 11,361 Murphree et al. [1990] concluded from their satellite by R. Lepping and B. Ignacio, Goddard Space Flight Center, observations (Viking) that auroral features poleward of Greenbelt, Maryland. IMP 8 and Wind solar wind plasma. data were kindly provided by A. J. Lazarus, MIT, Camthe normal dayside aurora can exist in the northern bridge, Massachusetts. hemisphere during periods of northward IMF when Bx The editor thanks A. S. Rodger and Frederick J. Rich for is negative and By is positive. This statement is rea- their assistance in evaluating this paper. sonable in view of their limited sensitivity. They were able to observe only the strongest events, i.e., those with References green line intensities above a few kilo-rayleighs. Clauer, C. R., and E. Friis-Christensen, High-latitude day- The possible relationship with By may occur via the side electric fields and currents during strong northward geometry of field-aligned currents in the cusp region interplanetary magnetic field: Observations and model [e.g., Clauer and Friis-Christensen, 1988; Watanab et simulations, J. Geophys. Res., 93, 2749, al., 1996]. Positive (negative) By is generally associ- Cowley, S. W. H., Magnetospheric and ionospheric flow and the interplane tary magnetic field, in The Physical Basis of ated with westward (eastward) lobe cell convection and the Ionosphere in the Solar-Terrestrial System, no. 295, upward (downward) currents in the north and down- pp , NATO, Neuilly sur Seine, France, ward (upward) return-currents at lower latitudes. This Cowley, S. W. H., and M. Lockwood, Excitation and decay current geometry may explain why Bx O, By 0 of solar wind-driven flows in the magnetosphere-ionosphere is a preferred condition for electron acceleration events system, Ann. Geophys., 10, 103, Cowley, S. W. H., J.P. Morelli, and M. Lockwood, Depen- (stron green line emission) in the type north cusp audence of convective flows and particle precipitation in the rora. high-latitude dayside ionosphere on the X and Y com- The relationship between occurrence of discrete auro- ponents of the interplanetary magnetic field, J. Geophys. ral forms and supercritical upward directed field-aligned Res., 96, 5557, current (FAC) densities is well known. The association Crooker, N. U., Dayside merging and cusp geometry, J. Geophys. Res., 8, 951, of narrow discrete auroral forms in the cusp region and Crooker, N. U., Reverse convection, J. Geophys. Res., 97, strong field-aligned currents during northward IMF con- 19,363, ditipns has been documented by Sandholt [1991](see Dungey, J. W., Interplanetary magnetic field and the auroral also Torb err and Carlson [1980]). Thus the different zones, Phys. Rev. Left., 6, 47, green line emission intensities observed during different Dungey, J. W., The structure of the exosphere, or adventures in velocity space, in Geophysics' The Earth's Envi- B and By conditions may be a signature of different ronment, edited by C. DeWitt, J. Hieblot, and A. Lebeau, FAC densities and particle accelerations corresponding pp. 503,550, Gordon and Breach, New York, to slightly different solar wind-magnetosphere coupling Feldman, W. C., et al., Possible conjqgate reconnection at modes as indicated above. the high-latitude magnetopause, J. Geophys. Res., 100, In view of the limited data available to this study the 14,913, Fuselief, S. A., B. J. Anderson, and T. G. Onsager, Partipossible relationship between the imf B polarity and cle signatures of magnetic topology at the magnetopause: the presence of electron acceleration events at the cusp AMPTE/CCE observations, J. Geophys. Res., 100, 11,805, poleward boundary should be examined further, based on a larger statistical material. Gosling, J. T., M. F. Thomsen, S. J. Bame, R. C. Elphic, and Finally, we point out that an alternative particle en- C. T. Russel, Observations of reconnection of interplanetary and lobe magnetic field lines at the high-latitude try related to the type north cusp auroral emission magnetopause, J. Geophys. Res., 96, 14,097, may occur along open overdraped flux, equatorward of Greenwald, R. A., W. A. Bristow, G. J. Sofko, C. Senior, the high-latitude reconnection site [cf. Crooker, 1992]. J.-C. Cerisier, and A. Szabo, Super Dual Auroral Radar Song and Russell [1992] used this mechanism to explain Network radar imaging of dayside high-latitude convechow the LLBL is populated during northward IMF con- tion under northward interplanetary magnetic field' Toward resolving the distorted two-cell versus multicell conditions. However, in such a case an additional mechatroversy, J. Geophys. Res., 100, 19,661, nism for particle scatter into the loss cone and subse- Kessel, R. L., S.-H. Chen, J. L. Green, S. F. Fung, S. A. quent penetration to ionospheri altitudes is needed [cf. Boardsen, L. C. Tan, T. E. Eastman, J. D. Craven, and Feldman et al., 1995]. According to Crooker [1992] re- L. A. Frank, Evidence of high-latitude reconnection durconnection signatures in the winter/b -unfavored hemi- ing northward IMF: Hawkeye observations, Geophys. Res. Left., 23, 583, sphere is driven not by tail lobe reconnection, but by Lockwood, M., P. E. Sandholt, S. W. H. Cowley, and T. internal reconnection involving the inner layer of the Oguti, Interplanetary magnetic field control of dayside auoverdraped lobe flux (cf. her Figures 4 and 6). The roral activity and the transfer of momentum across the overdraped flux may lead to subsequent reconnection dayside magnetopause, Planet. Space Sci., 37, 1347, in the north during positive Bx conditions. Murphree, J. S., R. D. Elphinstone, D. Hearn, and L. L. Cogger, Large-scale high-latitude dayside auroral emis - Acknowledgments. This work has been supported by sions, J. Geophys. Res., 95, 2345, the Research Council of Norway. The optical campaign Onsager, T. G., C. A. Kletzing, J. B. Austin, and H. MacK- in Ny-] lesund benefited from the financial and technical support from the Norwegian Polar Research Institute. We greatly appreciate the IMP 8 and Wind IMF data provided iernan, Model of magnetosheath plasma in the magnetosphere: Cusp and mantle particles at low-altitudes, Geophys. Res. Left., 20, 479, 1993.

14 11,362 OIEROSET ET AL.: NORTHWARD IMF CUSP AURORA Reiff, P. H., and J. L. Burch, IMF By-dependent plasma Weiss, L. A., P. H. Reiff, E. J. Weber, H. C. Carlson, M. flow and Birkeland currents in the dayside magnetosphere. Lockwood, and W. K. Peterson, Flow-aligned jets in the 2. A global model for northward and southward IMF, J. magnetospheric cusp: Results from the Geospace Envi- Geophys. Res., 90, 1595, ronment Modeling Pilot program, J. Geophys. Res., 100, Rich, F. J., and M. Hairston, Large-scale convection pat- 7649, terns observed by DMSP, J. Geophys. Res., 99, 3827, Woch, J., and R. Lundin, Magnetosheath plasma precipita tion in the polar cusp and its control by the interplanetary Sandholt, P. E., Auroral electrodynamics at the cusp/cleft magnetic field, J. Geophys. Res., 97, 1421, poleward boundary during northward IMF, Geophys. Res. Left., 18, 805, Sandholt, P. E., et al., Cusp/cleft auroral activity in relation S. W. H. Cowley, Department of Physics and Astronomy, to solar wind dynamic pressure, interplanetary magnetic Leicester University, Leicester LE1 7RH, United Kingdom field Bz and By, J. Geophys. Res., 99, 17,323, ( swhcl@ion.le.ac.uk) Sandholt, P. E., C. J, Farrugia, M Oieroset, P. Stauning, and W. F. Denig, PL/CAG, Geophysics, Phillips Laboratory, S. W. H. Cowley, Auroral signatures of lobe reconnection, 29 Randolp Rd., Hanscorn Air Force Base, MA 01731~3010. Geophys. Res. Left., 23, 1725, 1996a. ( denig@plh.af. mil) Sandholt, P. E., C. J. Farrugia, P. Stauning, S. W. H. Cow- P. E. Sapdholt, Department of Physics, University of Oslo, ley, and T. Hansen, Cusp/cleft auroral forms and activ- P. O. Box 1058 Blindern, N-0316 Oslo, Norway. ( ities in relation to ionospheric convection: Responses to p. e. uio. no ) specific changes in solar wind and IMF conditions, J. Geo- M. {3ieroset, International Space Science Institute (ISSI), phys. Res., 101, 5003, 1996b. Hallerstrasse 6, CH-3012 Bern, Switzerland. ( Song, P., and C.:T. Russell, Model of the formation of the oieroset@issi.unibe.ch) low-latitude boundary layer for strongly northward interplanetary magnetic field, J. Geophys. Res., 97, 1411, Totbert, R. B., and C. W. Carlson, Evidence for parallel electric field particle acceleration in the dayside auroral (Received May 20, 1996; revised February 6, 1997; oval, J. Geophys. Res., , accepted February 14, 1997.)