High-latitude magnetic reconnection in sub-alfv6nic flow: Interball Tail observations on May 29, 1996

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A12, PAGES 29,491-29,502, DECEMBER 1, 2001 High-latitude magnetic reconnection in sub-alfv6nic flow: Interball Tail observations on May 29, 1996 L. A. Avanov, 1 V. N. Smimov, 1 J. H. Waite Jr., 2 S. A. Fuselier, 3 and O. L. Vaisberg TM Abstract. The Interball Tail spacecraft crossed the high-latitude magnetopause near the cusp region under northward interplanetary magnetic field (IMF) conditions on May 29, 1996, with magnetic local time and magnetic latitude of- 7.3 hours and ø, respectively. Under these IMF conditions the Interball Tail spacecraft observed quasi-steady reconnection in progress and evidence for a relatively stable reconnection site at high latitudes. Sunward plasma flow observed by Interball Tail and a determination of the tangential stress balance indicated that reconnection was occurring poleward of the Earth's magneticusp, above the spacecraft's location. At these high latitudes the gasdynamic model of the solar wind/magnetosphere interaction indicates that the magnetosheath flow should be super-alfv nic, and therefore that the reconnection site should have propagated tailward. However, the spacecraft observed sub-alfv nic flow in the magnetosheath region adjacento the magnetopause current layer near the reconnection site indicating that the reconnection site may have moved in the sunward direction. These observationsuggest that the region of sub-alfv nic flow and stable, quasi-steady reconnection extend to very high latitudes under northward IMF conditions. It is shown that the thickness of the magnetopause current layer for this event (estimated as km) is consistent with that found for reconnection at the dayside magnetopause. 1. Introduction Magnetic reconnection between terrestrial magnetic field lines and the interplanetary magnetic field (IMF) is considered a principal mechanism for the transfer of mass, energy, and momentum from the shocked solar wind plasma across the magnetopause into the magnetosphere. When the IMF is northward, reconnection can occur at high latitudes poleward of the Earth's magnetic cusps [Dungey, 1963]. In this case, reconnected field lines convect over the polar caps to the dayside magnetosphere. The erosion of the magnetic flux from the lobe field lines of the magnetosphere is restored by convection of reconnected field lines from the dayside magnetosphere around the flanks. Recent global MHD simulations [Berchem et al., 1995; Richard et al., 1994; Fedder et al., 1997] quantitatively confirmed Dungey's original concept. Simulations also show that some high-latitude field lines reconnected under northward IMF conditions might be closed via reconnection in the opposite hemisphere. A consequence of such a process is the formation of the low-latitude boundary layer that was predicted by Song et al. [1994] and Le et al. [1996]. Observational evidence of high-latitude reconnection is found in the statistical study of the consequences of reconnection in ground-based observations and low-altitude cusp measurements. 1Space Research Institute, Moscow, Russia. 2Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA. 3Lockheed Martin Palo Alto Research, Laboratory, Palo Alto, Califor- nia, USA. 4Now at Laboratory for Extraterrestrial Physics, Goddard Space Flight Center, Greenbelt, Maryland, USA. Copyright 2001 by the American Geophysical Union. Paper number 2000JA /01/2000JA A polar cap convection pattern under northward IMF conditions was obtained by Maezava [1976] on the basis of a detailed statistical study. It was shown that sunward plasma convection is induced by reconnection occurring poleward of the cusp region. Other such examples of high-latitude reconnection for northward IMF conditions include sunward plasma convection observed in the cusps and a distinctive "reverse" energy dispersion of injected ions as a function of latitude [e.g., Burch et al., 1980, 1982; Woch and Lundin, 1992; Matsuoka et al., 1996]. Taken together, this evidence indicates that reconnection at high latitudes frequently occurs when the IMF is directed northward [Fuselier et al., 2000a, 2000b]. Furthermore, in situ observations of accelerated sunward ion flows near the high-latitude magnetopause [Ornel'chnko et al., 1983; Gosling et al., 1991, 1996; Kessel et al., 1996], and observations of the D-shaped ion distributions predicted by Cowley [1982] near the reconnection site [Kessel et al., 1996], directly establish the existence of reconnection at high latitudes poleward of the cusp. While reconnection at high latitudes is suggested by in situ measurements, the stability and location of the high-latitude reconnection site are less well established. When reconnection occurs at low latitude for southward IMF conditions, the reconnection site can remain stable at a fixed position for long periods of time. Such stability arises since the magnetosheath flow velocity in the subsolaregion is smaller than the Alfv6n speed, i.e., magnetosheath flow is sub-alfv6nic. The stability of the high-latitude reconnection site has been studied under steady state northward IMF by Fuselief et al. [2000a]. Observations made by Polar in the cusp were used to estimate the location of the reconnection site. It has been shown that the high-latitude reconnection site remains stable for many minutes under steady northward IMF conditions. However, according to the gasdynamic model 29,491

2 29,492 AVANOV ET AL.: INTERBALL MAGNETOPAUSE OBSERVATIONS [Sprieter and Stahara, 1985], super-alfv6nic flow of the magnetosheath plasma should exist at high latitudes near the magnetopause; therefore the reconnection site must move tailward and the high-latitude reconnection site should not be stable. Moreover, Matsuoka et al. [ 1996] showed that the estimated flow velocity of the magnetosheath plasma adjacent to the high-latitude reconnection site was often super-alfv6nic. In this case, the sunward plasma flows of the reconnected field lines should not be observed. Unlike the rather complex explanation for apparent sunward convection of the reconnected field lines given by Matsuoka et al. [ 1996], Fuselief et al. [2000a] suggesthat a simple explanation of the stability of the reconnection site at high latitudes is the formation of a depletion layer in the magnetosheath adjacent to the magnetopause. In the depletion layer the magnetic field increases and the number density decreases. As a result, the magnetosheath flow velocity near the high-latitude reconnection site should be sub-alfv6nic. Under these conditions, Fuselief et al. [2000a] concluded that the reconnection site should be stable and does not move tailward. This paper presents measurements made by Interball Tail on its outbound trajectory while it crossed the high-latitude magnetopause on May 29, 1996, under steady northward IMF conditions. These observations indicate that the spacecraft directly observed stable high-latitude reconnection and that the reconnection site was located poleward of the spacecraft. Magnetosheath observations by Interball show that the flow observed in the vicinity of the magnetopause crossing was sub-alfv6nic because of the formation of a depletion layer in the magnetosheath region adjacent to the high-latitude magnetopause as predicted by Fuselief et al. [2000a]. 2. Instrumentation and Orbit The Interball Tail spacecraft was launched on August 3, Its orbit, with an apogee of-31 RE and inclination-62.8 ø, is suitable for investigation of Earth's high-latitude magnetosphere. We use the data from the ion spectrometer SCA-1 [Vaisberg et al., 1995], which provides three-dimensional (3-D) distributions in the energy range kev/q within-10 s, and data from the three-axis fluxgate magnetometer Multi-component Investigations of Fluctuations of Magnetic Field (MIF) [Klirnov et al., 1995], with a sampling frequency up to 4 Hz. The ion spectrometer SCA-1 consists of two identical sensor heads EU-1/1 and EU-1/2 covering two hemispheres. Each sensor head consists of a toroidal electrostatic analyzer (ESA) followed by a microchannel plate with eight anodes. An electrostatic scanner in front of each electrostatic analyzer provides measurements over a nearly 2n field of view. In the fast mode of operation, SCA-1 measures energy/charge (E/q) spectra over 15 energy steps in 64 directions: eight equally spaced (by 45 ø ) azimuthal directions by eight polar angles relative to the Sun-directed satellite spin axis: 2 ø (pointing approximately sunward), 17 ø, 40 ø, 65 ø, 115 ø, 140 ø, 163 ø, and 178 ø (pointing approximately antisunward). A narrow field of view (2 ø ) and narrow energy passband (-10%) provide differential velocity space measurements. Measurements of the complete energy angle distribution of ions are performed every 10s. 3. Observations On May 29, 1996, the Interball Tail spacecraft was traveling through the northern hemisphere of the high-latitude magneto- sphere on its outbound trajectory successively crossing lobe field lines and the high-latitude magnetopause and then entering the magnetosheath near the cusp region. The time interval of interest extends from 0300 to 0330 UT. At-0318 UT, Interball Tail encountered the high-latitude magnetopause at magnetic local time (MLT) -7.3 and magnetic latitude (MLAT) The WIND spacecraft was located -155 R E upstream of the Earth. The time lag between solar wind plasma observed by WIND and its arrival at Interball Tail was -51 min on the basis of the propagation of a solar wind discontinuity observed at the two spacecraft. The discontinuity was detected by WIND at-0232 UT and by Interball Tail at-0323 UT when it was in the magnetosheath. Observations by WIND indicate that solar wind conditions were relatively stable; the dynamic pressure was high and varied slightly between 6 and 7 npa during the time interval under study, being 3-4 times more than typically expected in the solar wind ( npa). The solar wind velocity was -350 km/s, the ion number density was -25 cm '3, and the B: component of the solar wind magnetic field was directed northward, with an average value of-15 nt. The clock angle calculated from the WIND magnetic field data (defined as arctan(by/b0) varied from -5 ø to 5 ø, indicating that these IMF conditions were favorable for reconnection at high latitudes on lobe field lines poleward of the Earth's magnetic cusps. [e.g., Russell, 1972]. For this event the high dynamic pressure causes a compression of the magneto- sphere with the subsolar magnetopause location shifted R E closer to the Earth compared to its nominal position [Petrinec and Russell, 1993]. We used the T96 [Tsyganenko, 1996] model along with observed solar wind conditions taken from the WIND spacecraft as inputs to estimate a magnetosphericonfiguration for this time interval. The resulting compressed magnetic field configuration in the noon-midnight plane is shown in Figure 1. Solid dots show Interbali's position, projected into this plane, every hour starting at 0100 UT. Figure 2 shows the dependence of magnetic latitude (MLAT) versus magnetic local time (MLT) for the spacecraft. It is seen from Figures 1 and 2 that the spacecraft probed lobe field lines starting at the prenoon sector and proceeding towards noon. Plate 1 gives an overview of plasma and magnetic field characteristics for the time period UT. From top to bottom 1996/05/29 - }))))))))))))))) XGSM, R E Figure 1. Compressed magnetospheric field configuration (T96) in the noon-midnight plane. Solid dots show the Interball Tail position projected into this plane every hour starting at 0100 UT.

3 AVANOV ET AL.' INTERBALL MAGNETOPAUSE OBSERVATIONS 29, May, 1996 higher speed than the positive V-parallel population. The density of the positive V-parallel population is relatively constant while,,,,,, [,,, [,,, [,,, [.,, [,,. [, 80 the density of the negative V-parallel population is inversely proportional to the speed of the population. The properties of the 60 negative V-parallel ion component do not contradict its possible magnetosheath origin. INTERBALL, 0100 UT Gosling et al. [1991] reported similar observations of two < 40 counterstreaming ion populations in the magnetospheric part of a 20 high-latitude current layer. These two ion populations were explained in terms of reconnection, the sunward directed beam (with V parallel to the magnetic field) is the magnetosheath.,. t... i i i i i,,, i, plasma entered from the reconnection site, which is located tail ward of the spacecraft. The second component, the tailward MLT moving beam (antiparallel to the magnetic field), is also magne- Figure 2. Magnetic latitude (MLAT) versus local magnetic time tosheath plasma, but it entered the magnetosphere earlier, propa- MLT. gated along the magnetic field, mirrored at low altitudes, and returned to the spacecraft. Fuselief et al. [2000a] observed twoare shown: two energy-time spectrograms for cone viewing angles 17 ø and 163 ø relative to the spacecraft-sun line, ion number density, ion temperature, bulk velocity (in red) and V, component (in black), Vy and V components, magnetic field magnitude (in beam distributions on the reconnected field lines in the highaltitude cusp with distinct low-speed and high-speed cutoffs formed, by their interpretation, due to the time-off-flight effect. Using a model proposed by Onsager et al. [ 1991 ] for precipitatred) and Bx component (in black), By and Bz. Ion flow parameters ing and mirrored ions in the plasma sheath, Fuselief et al. in GSM coordinates were calculated as the moments of the threedimensional ion distribution function at 10 s resolution, assuming that all ions are protons. The ion count rates for the top panels are color coded according to the color bar on the right. [2000a] were able to estimate a distance to the reconnection site from the spacecraft location. Unlike the events they discussed, the counterstreaming ion populations observed by Interball Tail do not display apparent cutoffs. Despite this difference, it is compelling that the antiparallel 3.1. Evidence of Reconnection Prior to the Magnetopause Crossing During the time period UT, Interball Tail observed plasma flows with mantle-like properties. This mantle-like population is always lower in density and higher in speed than the parallel population. This is consistent with the direct entry/mirroring population interpretation. The absence of cutoffs could be tentatively explained by effects such as wave-particle plasma had lower, but varied number density, from 10 to 20 cm '3, interactions and the presence of a low-energy ionosphericomand the temperature was similar to that observed in the magne- ponent propagating at low negative velocity [e.g., Fuselief et al., tosheath after -0318:30 UT and moved presumably tailward 2000a]. Also, the antiparallel population is likely a mixture of [e.g., Rosenbauer et al., 1975]. The bulk velocity was -100 kin/s, mirrored magnetosheath ions and, at low speeds, ionospheric ion outflow. and the magnetic field had typical values for this magnetospheric region with magnitude of about 90 nt and <Bx > 40, <By>- 52 In summary, we believe that the sunward ion beam moving nt and <B? close to -65 nt. Bx > 0 and B < 0 were a further in- parallel to the magnetic field is the plasma that has entered the dication that Interball Tail was on the lobe field lines at this time. However, at-0305 UT sunward flowing particles were observed magnetosphere only as a result of reconnection. Furthermore, we conclude that long before ( min) crossing the current (the Vx component became more positive) which was confirmed layer, Interball Tail sees apparent signatures of ongoing reconby increased count rates produced by particles coming from the nection at the high-latitude magnetopause. tail (the count rates in the 163 ø look direction, second panel of Plate 1). In order to see the plasma properties in more detail during this time interval, cuts through the ion distributions parallel to the magnetic field are used. In Plate 2 an example of such a cut 3.2. Magnetopause Crossing On the basis of both plasma and magnetic field data we have identified a time interval, 0315: :25 UT, as the period when Interball Tail crossed the magnetopause current layer, measured within the time interval 0312: :27 UT is shown. marked by the two vertical lines in Plate 1. The magnetic shear There are two distinct plasma populations propagating in opposite directions relative to the magnetic field. Plasma parameters obtained as a result of fitting with a drifting MaxwellJan distribution are shown at the top of Plate 2 for each ion population. The population with a positive parallel velocity has a higher density than the population with a negative parallel velocity, while the positive V-parallel population has a lower bulk speed than the negative V-parallel population. The temporal variations of the velocity, temperature, and density of the two populations are shown in Plate 3 for time interval 0305: :00 UT. These parameters were determined by fitting a drifting MaxwellJan distribution to each of the plasma populations (red curves correspond to sunward flow and black ones correspond to tailward flow). It is clear from Plate 3 that the negative V-parallel population always has a lower density and a between the magnetosphere and magnetosheath adjacent to the current layer is quite large, ø. Within this time interval the Bx and B components changed their signs from a magnetospheric orientation to a magnetosheath one, approximately at a time corresponding to the center of the interval, when the magnitude of the magnetic field is depressed to -60 nt. Note that in the XZ plane the magnetic field components on both sides of the boundary are nearly antiparallel and changed sign at the center of the time interval. The ion density gradually increased within the time interval, reaching almost magnetosheath values at-0316:10 UT. In the first half of the current layer, V, had a strong sunwar direction, and the Vz component was negative. Then both components of plasma velocity changed signs almost simultaneously as the Bx and Bz components of the magnetic field changed their signs (-0317:15 UT). It should be noted that a negative Vy corn-

4 29,494 AVANOV ET AL.: INTERBALL MAGNETOPAUSE OBSERVATIONS...!,99,6/0,5/,2?... to0-3oo a) 8O m to0 ' 50 b) 20 to0 = ; 0-50 loo o -loo c) -loo m ' ' d) 4ø 20 o -50 loo loo Figure 3. Crossing of the current layer: (a) bulk velocity m d magnetic field magnitude and (b, c, and d) three components of the magnetic field and velocities in the boundary-normal reference frame. The velocity components are plotted as a histogram. ponent both inside and outside of the magnetopause current layer is consistent with a dawnside flow at this location. Observations of changes of signs of the magnetic field and velocity components, including the strong sunward plasma flow, indicate that Interball Tail crossed reconnected field lines at the high-latitude magnetopause [Gosli ng et al., 1991, 1996; Kessel et al., 1996]. If the magnetopause is open, and therefore considered as a rotational discontinuity, then there must exist a dehoffman- Teller (HT) frame in which the convection electric field is the ideally zero, plasma flows parallel to the magnetic field lines on both sides of the discontinuity, and the flow is at the local Alfv6n speed, i.e. V-VHx=VA. The test of this relationship is referred to as the Walen test, or tangentional stress balance test [e.g., Sonnerup et al., 1981; Paschmann et al., 1986]. Velocity and magnetic field vectors were used to define the HT frame by minimization of the residual electric field assuming that the HT frame is accelerating [Sonnerup et al., 1987]. In Plate 4a a scatter plot of GSE components of the convectivelectric field Ec=-V x B versus the corresponding components of the electric field Eux=-V it x B is presented. Good agreement is obtained for the time interval 0316: :50 UT with a correlation coefficient of Defined components of the HT frame velocity VHr and acceleration AHr are, in this case, [98,-251, 114] km s - and [-0.812, 0.465, ] km s '2, respectively, in GSE coordinates. In Plate 4b we present a Walen scatter plot of GSE components of V-VaT versus the corresponding components of the local measured Alfv6n velocities (V,=B/([top) 1/2, where p is the measured mass density assuming that all particles are protons and B is the measured components of the magnetic field) is shown. The regression coefficient obtained from the Walen test is 0.950, and it has a positive slope , indicating that plasma flows have a velocity of-0.61 of VA on the average within the reconnection layer and that the reconnection site is located poleward of the spacecraft position. The difference between the plasma flow velocity and Alfv6n velocity within the magnetopause current layer in the dehoffman-teller frame may result from the absence of ion composition data, by neglect of plasma anisotropy, and, most probably, due to the three-dimensional structure suggested by the observation of magnetic islands. In order to obtain a normal to the boundary we used a method of minimization of the Faraday residue (MFR) developed by Terasawa et al. [1996] in analytical form [Khrabrov and Sonnerup, 1998]. This method, as contrasted with traditional minimum variance analysis of the magnetic field, combined with maximum variance analysis of the electric field, allows us to obtain not only the normal to the discontinuity but also the velocity of the discontinuity along the normal. Using this procedure, we obtained the normal to the current layer (in GSE coordinates) as

5 , AVANOV ET AL.' INTERBALL MAGNETOPAUSE OBSERVATIONS 29,495 INTERBALL-TAIL, SCA-1, MIF 1996/05/a9 > , &,,,,,,. 1 II 7 1oo mmmmmmmmmmmmmmmmmmmmmmmmmm mmmmmmmmmmm lo 1oo 2OO too 0-1oo O o c 5O O O O 40 2O o :00 03:05 03' 10 03' 15 03:20 03:25 03:30 XGS M , ,18 YGSM- 3, ,59-3,60-3,61 ZGSM?.54?.63?.?2?,8 t?.90?.99 8,07 Plate 1. An overview of Interball Tail ions and the magnetic field data for May 29, 1996, magnetopause and crossing. From top to bottom are shown: two energy-time spectrograms of SCA-I ion spectrometer (top, sunward; bottom, antisunward), ion flow parameters number density N, ion temperature, bulk velocity (in red) and Vx component (in black), VrosM component, VmSM component, magnetic field magnitude (in red) and Bx component, BrosM component, and BzosM component. Two energy-time spectrograms are sums of the ion counting rate spectra measured along 17 ø cone and 163 ø cone relative to solar direction, respectively.

6 .. 29,496 AVANOV ET AL.: INTERBALL MAGNETOPAUSE OBSERVATIONS /05/ : :27 ' ' I'" ' ' I" """ I ' ' ' I' "' ' '! ',''t, I ' : 2.23 :k0.39; Tp = 48.9 ev +_16.0; VO = km/s _+15.9.' ; Tp = 71.7 ev +_37.1' VO = 76.7 km/s :k ' Vpar, km/s Plate 2. A cut through the distribution function parallel to the magnetic field obtained in the time interval 0312: UT. Results of' a fit using a drifting Maxwellian distribution to each of' the plasma populations are shown The top line indicates the mirrored population (left peak), and the bottom line is for the transmitted magnetosheath flow 00..., m ,,..,,,, ,..,..! I '1 'W, T I m, :06 03:08 03:10 03' 1E 03:14 UT Plate 3. Plasma parameters for two populations in the magnetospheric bounda layer for (a) velocities, (b) ion temperatures, and (c) ion number densities.

7 AVANOV ET AL.: INTERBALL MAGNETOPAUSE OBSERVATIONS 29, :16:00-03:17:56 25o 10 a) b) o 100 E 50.c:_ 0 >, -5o -5 -loo -15o -10-2oo EHT in mwm, ' -25o o oo o 1 oo 200 v A in km/s Plate 4. Two scatterplots for magnetopause crossing: (a) Component-by-component comparison of the convectivelectric field Ec=-VxB versus electric field Enx=-Vnx x B. The defined Vnx components are [98, -251, 40] km/s in GSE (blue, green, and red correspond to X, Y, and Z components, respectively) and the correlation coefficient is (b) Result of tangential stress balance referred to as Walen relations. The slope is , indicating that the reconnection occurred poleward of the spacecraft. a) 40O :15:23-03:16:00 b) :17:56-03:18: E o.c_ > [ i I i ,, V^ in km/s V^ in km/s Plate 5. Walen relations for two time intervals, which correspond to (a) the magnetospheric edge of the current layer and (b) the magnetosheath edge. Format the same as for Plate 4b. See text for details.

8 _ 29,498 AVANOV ET AL.' INTERBALL MAGNETOPAUSE OBSERVATIONS a) ? O 2O i i i I i i i i i [ i i I i i i i i i i i 1996/05/29 b) m 70 c) I i i i I i i i i I i i i i i i i i I i i i I t i i i I I I ] i I I I i I_ Figure 4. (a) Number density, (b) magnetic field magnitude, and (c) Alfv6nic Mach number (M,0 for a 10-min time interval in the magnetosheath upon exit of the magnetopause current layer. For the first 4 min sub-alfv6nic flow is observed. The vertical line marks a transition from sub-alfv6nic flow to Alfv6nic/super-Alfv6nic flow. [0.686, 0.252, 0.683], and the speed of boundary U, =-8.8 km/s. The duration of the crossing (- 180 s) and magnitude of the velocity allows us to estimate a thickness of the current layer as 1600 km, suggesting reasonable agreement between this value and that for reconnection at the dayside magnetopause for high magnetic shear [Phan and Paschmann, 1996]. The structure of the magnetic field and flow velocity in the current layer is presented in Figure 3, which uses a boundarynormal coordinate system. Plasma parameters are plotted in histogram format with scales given on the right of the panels. Until 4)315:20 UT, Interball Tail was in the lobe of the magnetosphere. At 4)315:20 UT the spacecraft entered the current layer. This current layer is seen as a relatively smooth transition in the Bt. and B M components until the spacecraft enters the magnetosheath at 0318:20 UT. This corresponds to a large-scale rotation of the magnetic field across the current layer with respecto magnetospheric field lines. In the first part of the current layer, the bulk velocity is relatively constant with a value of- 200 km/s, which is close, but opposite (Plate 1), to that of the adjacent magnetosheath. The Walen test for a narrow time interval near the center of the current layer (not shown here) indicates that the HT frame moves with a value of 4) of the local Alfv6n velocity. This could be interpreted as the detection of newly injected magnetosheath plasma into the magnetosphere from the reconnection site. The velocity decreasesharply from 180 to -100 km/s at 4)316:50 UT and then smoothly increases to km/s at 4)317:50 UT. The decrease is accompanied by a depression of the magnetic field. It is seen from Plate 1 that the Vx and Vz components of the velocity change their signs almost simultaneously with corresponding components of the magnetic field inside the current layer at 4)317:25 UT. The plasma flow changes its direction from sunward/downward (Vx>0, Vz<0) to antisunward/upward (V <0, V >0), while the magnetic field components change their signs from Bx>O, B <0 to B <0, B >0 (from a magnetospheric a magnetosheath orientation). Such a velocity profile is in contrast with that observed for low-latitude magnetopause during southward IMF [Phan and Paschmann, 1996] because a shear between the plasma flow in the current layer and magnetosheath is present at high latitude. On the other hand, this is in qualitative agreement with local MHD simulations of asymmetric reconnection where the plasma and magnetic field properties are essentially different on both side of the current layer [Scholer, 1989]. Throughouthe current layer crossing, Interball Tail observes quasiperiodic structures with a period of s. These structures are better seen in B, but can be seen also in V,. These quasiperiodic structures are accompanied by decreases in magnetic field magnitude (at 4)316:15 UT, 4)316:55 UT, and 4)317:25 UT) and reveal a maximum value of db/b of 0.5 (4)317:25 UT).

9 AVANOV ET AL.: INTERBALL MAGNETOPAUSE OBSERVATIONS 29,499 It is well known that the velocity of the HT frame is the velocity positive B,, value provide strong evidence that just before exiting with which the reconnected field lines slide along the rotational from the magnetopause current layer Interball Tail was on recondiscontinuity at the magnetopause during ongoing reconnection. nected field lines moving tailward from the reconnection site. Hence the defined velocity of the HT frame can be used to esti- This suggests that Interball Tail moved to the opposite side of the mate the characteristic size of such quasiperiodic structures. The X line. It also implies that the region with a high ion temperature size of these structures, defined as I V r(-260 km/s)l(20-25 s), is and lowest magnetic field magnitude, observed by Interball Tail found to be RE, which is consistent with the results ob- prior to moving to the other side of the X line, was an approach tained from MHD simulation of reconnection at the high-latitude or even crossing of the diffusion region of the reconnection site. magnetopause [Berchem at al., 1995]. As was mentioned above, we have defined the HT frame for 3.3. Sub-A!v6nic Flows in the Magnetosheath Boundary the time interval 0316: :50 UT. This time interval covers Layer the main part of the current layer crossing but not all of it. Ex- panding this time interval leads to deterioration of the quality both of the HT frame and of the Walen relation. This suggests that the reconnection process is either time-dependent or, more likely, that it has a spatial structure. We also defined the HT frame for the time interval 0315: :00 UT, corresponding to the magnetospheric edge of the current layer. The HT frame velocity in GSE coordinates in this case is V.T [-38, -363, -266]. This HT velocity differs from the one obtained for the main part of the current layer, mainly by the sign of the X component of the HT frame velocity. From this we can say that a part of the reconnected flux tube moves tailward (the X component of the HT frame is negative). The angle between the defined normal to this part of the current layer and the respective HT velocity is close to 90 ø. The Walen relations for the time interval 0315: :00 UT are shown in Plate 5a. It is also seen that the slope has the same sign through most of the current layer, i.e., plasma moves parallel to the magnetic field with-0.96 of the local Alfv6n velocity. Therefore we can conclude that at- 0316:00 UT, Interball Tail observed a reversal of convection of reconnected field lines from tailward to sunward. Sunward convection is observed throughouthe main part of the current layer. The observed reversal of convection direction of the reconnected field lines apparently reflects a possible displacement of the X line back and forth. It is interesting to note that the solar wind conditions corresponding to the crossing by Interball Tail of the current layer showed minor variations, and therefore we can conclude that the observed motion of the X line back and forth is inherent to the reconnection process. Another important feature observed by Interball Tail before exiting the current layer is the strong rotation of the magnetic field accompanied by an intense negative excursion of the Bn component (time interval -0317: :50 UT). The strongest depression of the magnetic field was observed at 0317:25 UT, when the magnetic field magnitude decreases from 70 to 35 nt. The high-resolution magnetic field data show that the depression of the magnetic field magnitude had a duration of s. Within the time interval 0317: :50 UT both the V and V components of the plasma velocity changed signs (see Plate 1 and Figure 3), and the plasma temperature started to increase, achieving its maximum at-0317:50 UT. Within the time interval 0317: :26 UT, before exiting from the current layer, plasma was observed to accelerate to a velocity exceeding that of the magnetosheath by km/s. It is important to note that the B, component of the magnetic field within this time interval is positive. The velocity of the HT frame for this time interval is estimated to move tailward with a speed of- 170 km/s, that is higher than in the main part of the current layer, and almost tangential to the boundary (the angle between the normal and HT frame velocity vector is -87ø). The Walen test for this time interval is shown in Plate 5b. The negative slope of the Walen test and At- 0318:20 UT the spacecraft entered the magnetosheath region adjacent to the magnetopause. The plasma and magnetic field parameters (the number density, magnitude of the magnetic field, and Alfv6nic Mach number for a 10-min interval just after the magnetopause crossing) are shown in Figure 4. Enhanced magnetic field magnitude and decreased ion density and temperature within the layer adjacento the magnetopause compared to the outer part of the magnetosheath indicate that this is a magnetic barrier/plasma depletion layer. The spacecraft initially observes sub-alfv6nic flow (until UT) and then a layer of flow with approximate Alfv6n velocity (until-0326 UT). The change of the regime within the depletion layer is shown by a vertical line in Figure 4. The magnetosheath flow exterior to the depletion layer is super-alfv6nic. 4. Discussion In this paper we have presented measurements made by the Interball Tail spacecraft during one pass through the high-latitude magnetopause on its outbound trajectory under steady northward IMF B. The plasma and magnetic field data indicate that the satellite first crossed reconnected field lines upstream of the X line. After crossing this magnetic structure the satellite approached the diffusion region and then partly crossed the field tubes on the dawn side of the X line. Figure 5 shows the effective motion of the satellite relative to the (moving) structure of the current layer. The HT frame and Walen relations are evidence of ongoing reconnection at the high-latitude magnetopause. The excellent fit defining the HT frame (Plate 4), the slope in velocity (plasma moves with velocity of-0.6 VA in the HT frame), and the -1 km s '2 acceleration are consistent with observations of recon- nection at lower latitudes [e.g., Sonnerup et al., 1987]. The positive slope in the Walen relation (Plate 4b) also shows that reconnection occurred poleward of the spacecraft. The magnetopause current layer shows a complex structure with possible multiple magnetic islands passing the spacecraft during the current layer crossing and flow direction reversals at the edges of the current layer. The structure of the current layer at high latitude during reconnection is quite different from that at low latitudes, in agreement with simulations for asymmetri conditions [Scholer, 1989]. The difference between plasma flow velocity and Alfven velocity within the magnetopause current layer in the dehoffman-teller frame may result from the unmeasured ion composition differences, by neglect of plasma anisotropy, and, most probably, as a result of the three-dimensional structure (suggested by the observation of magnetic islands). There are some details of the magnetopause crossing that are not sufficiently understood. They include the unusual magnetopause normal direction and the positive normal velocity in the part of the magnetopause current layer that lies on the magnetospheric side. The latter fact can be partly explained by outward

10 29,500 AVANOV ET AL.: INTERBALL MAGNETOPAUSE OBSERVATIONS Magnetosheath flow 0317:15 UT 0317:25 UT 0318:20 UT / Magnetospheric Lobe 0315:20 UT I Figure 5. A sketch showing the effective motion of the spacecraft relative to the structure of the current layer. motion of a (preexisting) ion component moving opposite to the magnetic field line direction. However, these details of the magnetopause crossings are most likely associated with the dynamics of the reconnected field tube, which is moving opposite to the magnetosheath flow direction. In contrasto previous observations of high-latitude reconnection made by ISEE 2 [Gosling et al., 1991] and Hawkeye [Kessel et al., 1996], Interball Tail observed a single magnetopause crossing with a duration of- 3 min. We believe this is the result of the stability of solar wind conditions during this magnetopause crossing and the geometry of the encounter with the magnetopause. Interball also observed the structure of the high-latitude current layer, in which them are quasiperiodical variations of the B, component of the magnetic field. Finally, after the magnetopause crossing, a depletion layer with lower density and a higher magnetic field, resulting in sub-alfv nic flow near the magnetopause boundary, are encountered prior to entering the magnetosheath proper. We can identify this region as the magnetospheric high-latitude boundary layer. Within the time interval -0305: :00 UT (10-12 min before the magnetopause crossing in Figure 1), Interball Tail observed ion distribution functions with two magnetosheath-like plasma populations propagating in opposite directions (Plate 3) relative to the magnetic field. Such distributions are not observed before this time period. This suggests that both mirrored and direct magnetosheath plasma distributions are observed for several minutes prior to the magnetopause crossing with an origin that is most likely associated with high-latitude reconnection poleward of the spacecraft. The relatively long duration of these magnetosheath distributions is taken as evidence for stability of the reconnection site at high latitudes. Previous measurements made at high latitudes near the magnetopause showed no evidence of such long-term signatures of reconnection [Gosling et al., 1991; Kessel et al., 1996]. Interball Tail also observed distinct changes in plasma convection suggestive of high-latitude convection during IMF B north. Before observation of the boundary layer with its associated sunward moving ion component, Interball Tail observed quasi-regular plasma flows that propagate tailward. The plasma beta for this time interval is low, [ < 0.1; hence the plasma motion perpendicular to the magnetic field is controlled by the convection of magnetic field lines. At 0306 UT the direction of plasma convection became sunward and duskward. The change of the convection direction is probably related to the boundary between closed- and open-lobe magnetic field lines, and therefore this part of the high-latitude boundary layer is on open-lobe field lines. Changing of the convection direction is consistent with a global convection pattern for northward IMF [Crooker, 1979]. The reconnection site at high latitudes during northward IMF has been assumed to be unstable by the gasdynamic model, but observations made by Interball Tail do not supporthis paradigm. In fact, according to the gasdynamic model [Sprieter and Stahara, 1985], magnetosheath flow at high latitudes must be essentially super-alfv nic [see also Rodger et al., 2000]. In the super- Alfv nic flow the reconnection site must move tailward so that the plasma flow in the HT frame of reference is Alfv nic. The tailward speed of the reconnection site would increase as the bulk flow velocity of the magnetosheath plasma along the magnetopause increases. In contrast, the observed reconnection site is comparatively stable for the magnetopause crossing studied here. Furthermore, the convection flow was sunward, not tailward. Using Polar measurements in the cusp, Fuselief et al. [2000a] showed a similar example where the reconnection site location remained fixed for many minutes under steady state northward IMF and did not move tailward. It has been suggested that the reconnection site could be stable at locations where the gasdynamic model predicts that it cannot be stable. The stability is attributed to formation of a plasma depletion layer that leads to increasing Alfv n velocity along the magnetopause, thus forming the sub- Alfv nic flow in the magnetosheath region close to the magneto- pause. Onsager et al. [1995] have used the Sprieter and Stahara [ 1985] gasdynamic model to parameterize plasma parameters and magnetic field as a function of distance (XGSM) along the magnetopause with nominal subsolar magnetopause standoff taken as 10 RE. Since measurements by Interball (Plate 1) were made during a time when the solar wind dynamic pressure was high, a change in the X distance scale is needed in order to compare the observations with the gasdynamic model. Plasma and magnetic field measurements on Interball Tail were averaged within two time intervals in order to compare the observations with the gas dynamic model [Sprieter and Stahara, 1985]. The first is from 0318:30 UT till 0322:30 UT when the spacecraft was in the depletion layer, and the second from 0323:00 UT to 0330:00 UT measured in the magnetosheath proper. In Figure 6 normalized plasma parameters observed in the magnetosheath compared to gasdynamic predictions are presented. The plot shows the ratios of number density (N/Nsw), temperature (T/Tsw), velocity (V/Vsw) and magnetic field (B/Bsw), respectively, along the magnetopause (noon-midnight meridian) for the gasdynamic model (solid line) and the observed parameters for Interball Tail (open squares) for May 29, Them is an agreement in number density, though values measured by Interball Tail are slightly lower than predicted by the model, but the temperature and magnitude of the magnetic field are higher and the flow velocity is lower than derived from the model. The result is sub-alfv nic flow near the magnetopause. The sub-alfv nic flow appears to be the result of a combination of lower ion density and higher magnetic field, which raise the Alfv n speed and lower the gasdy-

11 AVANOV ET AL.: INTERBALL MAGNETOPAUSE OBSERVATIONS 29, O lo o o.o 6 Interball May _ UT '. 0 ß,, Gasdynamic Model 0330 UT,,,. X GSM (R E) Figure 6. Parameters observed in the magnetosheath compared to gasdynamic predictions (see text for details). namic magnetosheath flow velocity. This lower flow velocity may also be an artifact of the depletion layer, which affects the plasma density in the magnetosheath [Denton and Lyon, 2000]. 5. Summary Analysis of the high-latitude magnetopause/boundary layer crossing by Interball Tail under a stable northward IMF provided strong evidence of the stability of the reconnection site location. These measurements also show that under these IMF conditions the flow within the depletion layer/magnetic barrier at this location is sub-alfvenic and allows the reconnection site at the lobe field lines to remain stationary. This effect is not predicted in the gasdynamic model in which the flow is supersonic at high solar zenith angles. The relative stability of the reconnection site leads to the development of sunward convecting field lines, as observed by Interball Tail. Reversal of the convection direction within the high-latitude boundary layer is explained by a transition from closed to open field lines. Acknowledgments. The authors wish to thank K. W. Ogilvie and the SWE team for providing of WIND data. This research was supported by NASA grant NAG Janet G. Luhmann thanks Ramona L. Kessel and Ayako Matsuoka for their assistance in evaluating this paper. simulation, in Physics of the Magnetopause, Geophys. Monogr. Ser., vol. 90, edited by P. Song, B. U. O Sonnerup, and M. F. Thomsen, p. 205, AGU, Washington, D.C., Burch, J. L., P. H. Reiff, R. A. Hellis, R. W. Spiro, and S. A. Fields, Cusp region particle precipitation and ion convection for northward interplanetary magnetic field, Geophys. Res. Lett., 7, 393, Burch, J. L., P. H. Reiff, R. A. Heelis, J. D. Winningham, W. B. Hanson, C. Gurgiolo, J. D. 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