Evidence for a sunward flowing plasma layer adjacent to the tail high-latitude magnetopause during dawnward directed interplanetary magnetic field

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1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 106, NO. A12, PAGES 29,479-29,489, DECEMBER 1, 2001 Evidence for a sunward flowing plasma layer adjacent to the tail high-latitude magnetopause during dawnward directed interplanetary magnetic field D. Popescu, J.-A. Sauvaud, A. Fedorov, E. Budnik H. Stenuit, and T. Moreau Centre d'etude Spatiale des Rayonnements, Toulouse, France Abstract. We report on observations from the nterball-tail satellite of a long-lived sunward flowing plasma layer (SFPL), adjacent to the plasma mantle and affecting the high-latitude dawn northern lobe, well beyond the terminator. This observation pertains to periods when the interplanetary magnetic field was oriented mainly dawnward and northward or slightly southward. Although a plasma sheet origin of the SFPL cannot be completely excluded, we show that the most plausible source of the observed plasma is the magnetosheath. Possible entry mechanisms and entry sites are discussed. We show that the magnetosheath ions and electrons can enter the northern lobe via a rotational discontinuity (RD) affecting the high-latitude magnetopause tailward of the spacecraft location. Such a discontinuity is expected to result from the tailward convection of field lines reconnected at the dayside magnetopause. However, although the distribution functions of ions in the SFPL well fit the general entry mechanisms through RDs, the electron component is clearly heated and accelerated. Their characteristics appeal for specific acceleration and heating mechanisms related to the intrinsic structure of a RD. 1. ntroduction tary and geomagnetic magnetic fields. There are two basic models of the reconnection location. The antipar- The magnetospheric boundary layer is a transition allel model predicts reconnection at the equatorial magregion of limited thickness between the magnetosheath netopause [Dungey, 1961] for periods of southward MF and the magnetosphere that exists over the entire magwhereas for northward MF, merging should occur polenetopause surface but with different characteristics at ward of the cusps at the high-latitude magnetopause different locations. For the high-latitude portions of [Dungey, 1963; Crooker, 1979; Luhmann et al., 1984]. the magnetopause the boundary layer is also called the The component reconnection model [Sonnerup, 1974] high-latitude boundary layer or plasma mantle [Rosenpredicts that the plasma is entering near the subsolar bauer et al., 1975]. This layer is found at the interface dayside magnetopause for different MF orientations exbetween the magnetopause and the lobes. t is gencept for purely northward. erally accepted that the plasma mantle is located on Several studies [e.g., Paschmann et al., 1979; Sonopen magnetic field lines and is populated by a mixnerup et al., 1981; Gosling et al., 1991; Kessel et ture of magnetosheath plasma that entered the magal., 1996; Safrankova et al., 1998] presented observanetosphere along cusp open field lines and of ionotions of accelerated magnetosheath plasm a flowing sunspheric plasma that flowed up from the cusp and the ward and/or tailward within the high-latitude magpolar cap. For southward directed interplanetary magnetopause current layer for northward oriented MF netic field (MF), it is well established that the mag- (i.e., high magnetic shear) as an evidence for highnetosheath component of the mantle results from the latitude or midlatitude magnetic reconnection. Usnear-equatorial dayside reconnection of the interplaneing the low-inclination SEE spacecraft, Gosling et al. [1996] recently studied the lobe-magnetopause interface at seven different dates and at different locations (sun- On leave from Space Research nstitute (K), Moscow, Russia ward and tailward of the dawn-dusk terminator, in both the Northern and Southern Hemispheres and on both Copyright 2001 by the American Geophysical Union. Paper number 2001JA /01/2001JA ,479 the dawnside and duskside) and concluded that on the flank, such reconnection seemed to require essentially antiparallel fields, in contrast to reconnection closer to the subsolar point. Their observation included events

2 - - 29,480 POPESCU ET AL.: SUNWARD PLASMA FLOW N THE HGH LATTUDE LOBE nterball-tail October 06 (00'00 UT), -,,, -,,, -,,, -,,, -,,, -,,, ,...,... N ,,, J.,,,.,,,. i. i i,., i, x -20 2O O Y (R ) Figure 1. nterball-tail projection (cross) at 0000 UT on October 6, 1996, in two perpendicular planes, (R - v/x 2 + y2, X) and (Y, Z ). The magnetopause ($hue et al. [1997] model) is given by the curve. as deep as X, -10 RE, as far downtail as could be MF components in GSM coordinates (Figures 2a-2c) sampled by the SEE orbit. along with the solar wind dynamic pressure (Figure 2d) n this paper we present an observation of a long measured by Wind from 1930 UT (October 5) to 0145 lasting sunward flowing plasma layer (SFPL) made on board the nterball-tail satellite at high latitudes. We detected this layer in the region adjacent to the mantle, on its lobe side, in the midtail (Xo, sm " -9 RE to -15 RE). These observations demonstrate several inter- UT (October 6). The magnetic field and plasma data displayed in Figure 2 come from the Magnetic Fields nvestigation (MF) magnetometer [Lepping et al., 1995] and 3-D Plasma and Energetic Particle Analyzer (3DP) [Linet al., 1995] on board the Wind satellite. The time esting properties of the high latitude lobes regions: (1) resolution is 46 s. The data are shifted in time acthe formation of the SFPL well tailward from the duskdawn terminator, (2) The stability of the layer, which cording to the Wind-nterball time lag. The dashed vertical lines indicate the time intervals corresponding was observed almost continuously during several hours, to the two nterball-tail encounters with sunward flowand (3) the population of the layer by ions of magne- ing plasma layer (SFPL) inside the magnetophere: The tosheath origin and by an electron population with a temperature significantly higher than that of the magnetosheath electrons. Note that such electron acceleration, although more modest, has already been reported. n section 3 we present the possible origin of the observed features. first is from, 2130 to,-2317 UT, and the second is from, 0018 to, 0105 UT (Figure 2). Note that, as a general trend, during the SFPL encounter the MF Bx component is low, while its By component is high. The equatorial projection of the field is directed tailward and dawnward. During the first SFPL encounter, Bz is positive. Then, around 2315 UT, Bz turns negative, 2. Observations and nearly simultaneously nterball-tail moves from the SFPL to the magnetosheath. For the second SFPL en- During the period from 1600 UT, October 5, 1996 to 0200 UT October 6, 1996, the nterball-tail satellite recorded several crossings of the high-latitude magnetopause far tailward from the terminator. Figure 1 illustrates the satellite position with respect to the magnetopause at 0000 UT on October 6. At that time the magnetic activity was low (Kp 10). The solar wind counter the MF Bz is close to zero and even shows two slight negative excursions. After 0105 UT, Bz undergoes a more pronounced southward turning and nearly simultaneously, nterball-tail passed into the magnetosheath (Plate 1). Note that the solar wind ram pressure was quite steady throughout the whole period with an average value of, 1.8 npa. conditions were monitored by Wind (XGsM -- 9, YO, SM Plate 1 shows an overview of electron and ion en- =-36, ZOSM -- 4 RE) and partly by Geotail (XGsM-- 13, YCSM -- 4, ZO, SM RE). Geotail moved from ergy time spectrograms, pla sma parameters and magnetic field components measured on board nterball-tail the solar wind toward the magnetosheath and crossed from 2130 UT (October 5, 1996) up to 0145 UT (Octhe bow shock during this event. For the period when both satellites were simultaneously located in the solar wind, they measured quite similar magnetic field tober 6). The electron data were obtained in the energy range kev from a top-hat quadrispherical electron spectrometer [Sauvaud et al., 1997]. The profiles, confirming thus that the MF measurements ion data were provided by the CORALL spectrometer are globally representative. Figure 2 displays the three (nearly hemispherical electrostatic analyzer) in the en-

3 POPESCU ET AL- SUNWARD PLASMA FLOW N THE HGH LATTUDE LOBE 29,481 a) 1996, October 5 WND / ',,, 0... t b) --4 ' ' ' ' [ ',, ' ' ' t,,!,,, ],, [, i i i i i i...,... / i i c),,,,, i ' ' [ ',, ' ' ' [ [,,, i, ' ' i ' ' i d) --4 ',,,,, / ', J, / ' ' ' ' P q ' ' ' ' ,,,, - 3 i f ] r 3 i le :00 21 '00 22:00 25:00 00:00 01 '00 Figure 2. (a-c) MF components in GSM coordinates and ion ram pressure measured by Wind (Xc sm-- 9, YC SM ---36, ZC SM -- 4 Re) from October 5, 1996, at 1930 UT to October 6, 1996, at 0145 UT. The data are shifted in time in order to correspond to the nterball-tail location. UT ergy range kev [Yerrnolaev et al., 1997]. For both instruments, nearly full three-dimensional (3-D) distribution functions are obtained every satellite spin period (2 min). The magnetic field data, with a sampling rate of 1 vector s -, come from the fluxgate probe of the MF magnetometer [Klimov et al., 1995]. The color bars above Plate 1 indicate the different re- gions detected by the satellite, namely, red for magnetosheath, blue for mantle, and green for sunward flowing plasma. nside the magnetosphere (large Bx), the sunward flowing plasma has a density of cm -3, the ion earthward velocity is of the order of km s-, the ion temperature reaches kev, and the electron temperature is in the range ev. We identified the regions with such plasma characteristics as SFPL. The plasma observed inside the magnetosphere during the mantle crossings had a density of 0.1 cm -3, a tailward velocity of 200 km s -, an ion temperature in the range ev, and an electron tempera- ture of ev. nside the magnetosheath the plasma density reaches 3 cm -3, the ion temperature ranges between 100 and 200 ev, and the plasma flows tailward with a bulk velocity of 200 km s -. nterball-tail was located most of the time in the high-latitude northern lobe (stable magnetic field of 20 nt and mainly oriented in +X direction) with only the exception of time intervals indicated by red bars when it was in the magnetosheath (between 2317 and 2322 UT, and 2333 UT, 2347 and 0004 UT, and 0113 UT, and 0145 UT, at 0050 UT, at 0055 UT). The mantle was encountered six times: adjacent to the magnetopause (between 2320 and 2325 UT, 2335 and 2348 UT, and 0115 and 0120 UT), embedded in the SFPL (between 2235 and 2245 UT and 2255 and 2305 UT), or between the magnetosheath and the SFPL (from 0002 to 0008 UT). Plasma mantle detections are possibly related to the southward turnings of the MF (e.g., at 2320 and 2335 UT). The SFPL is observed during

4 29,482 POPESCU ET AL.: SUNWARD PLASMA FLOW N THE HGH LATTUDE LOBE long periods, i.e., between, and 2315 UT and be- sheet temperature and solar wind speed: Ti tween 0018 and 0105 UT, indicating that it corresponds Vsw. When applied to the studied case this gives to a nearly steady state earthward flowing plasma. As evidenced by the electron energy-time spectrogram of Plate 1, inside the SFPL the electrons are more energetic than in the adjacent magnetosheath (the spin modulated electron flux enhancements at energies below 30 ev correspond to photoelectrons). n Plate lc the black solid line shows the electron temperature in a plasma sheet temperature in the range kev. These values far exceed the measured ion temperature in the SFPL. However, the measured values are quite close to those deduced by Christon et al. [1989], who found the characteristic plasma sheet ion temperatures at low AE (< 100 nt) beyond 12 Re to be,,1-2 kev, i.e., close to the measured ion temperature in SFPL. the energy range ev, the dashed line displays Finally, the ion density in the SFPL is different from the average temperature in the energy range ev, which corresponds to the core of the SFPL thermal population (this point will be more explicitly discussed later in the paper), and the solid blue and red lines indicate the parallel and perpendicular temperatures in the same range. The parallel and perpendicular temperatures are displayed only for the periods when they can be reliably computed, i.e., inside the magnetosphere when the magthe plasma sheet characteristic densities: higher than that of the hot plasma sheet and lower than that of the recently discovered cold, dense plasma sheet, n > 1 cm -3 [Fujimoro et al., 1998]. Furthermore, cold, dense plasma sheet is generally associated with tailward flowing ions. As a preliminary conclusion, we can state that there is no clear indication that the plasma forming the SFPL has a plasma sheet origin. netic field was stable during one spin period. n the magnetosheath, where the magnetic field rapidly fluctuates, a reliable diagonalization of the pressure tensor is not possible. Plate lc clearly displays electron tem Magnetosheath Origin? For a magnetosheath source, earthward flowing plasma perature anisotropy at low energies ( ev), with implies an entry site tailward of the observation point (X < -15 Re) through an open magnetopause, i.e., a rotational discontinuity (RD). A mechanism that can 3. Discussion create RDs is magnetic reconnection. deal MHD is violated at the diffusion region where magnetic fields Although, for clarity and brevity, we presented in this paper only the period from 2130 to 0145 UT, the SFPL reconnect. Once reconnection is achieved, open field lines are convected with the magnetosheath flow, thus was first detected on board nterball before 2130 UT. The observation of SFPL for hours poses an important constraint on the physical process at its origin, which should be nearly stationary. Possible sources are the plasma sheet and the magnetosheath Plasma Sheet Origin? For slightly northward MF Bz but substantial B u the sunward ionospheric flow of the two-cell convection can occur at quite high magnetic latitudes [Weimer, 2001]. n order to check if the ionospheric projection of nterball could correspond to the plasma sheet, i.e., to the sunward flowing part of the two convection cells, the ionospheric projection of the satellite was computed according to the magnetic field model of Tsy 7anenko [1989]. nterball maps at an invariant latitude of 82.5 ø and at a magnetic local time (MLT) of 3 hours. For the MF conditions given by the Wind satellite this does not correspond to a sunward convection site but to the northern part of the morning convection cell where convection is directed antisunward; that is, this corre- sponds to lobe convection (Figure 2 of Weimer [2001]). Furthermore, the plasma temperature measured inside SFPL is largely different from that of the hot plasma sheet (i.e., T,. 5 kev). However, it must be stressed that the plasma sheet temperature can largely vary from event to event in the near-earth region. n the X range from to-22.5 Re, Borovsky et al. [1998] statistically found the following relation between plasma leading to the formation of an open magnetopause por- tion. For a high-latitude nightside lobe reconnection process to directly account for the SFPL observation, it has to occur tailward of the satellite location, i.e., tailward of X Re. The question arises then whether steady state reconnection may occur on the high-latitude midtail magnetopause. n classic two-dimensional (2-D) models, for steady state reconnection to exist at least two conditions must be fulfilled [Cowley and Owen, 1989]: (1) the magnetic field components on either side of the magnetopause must be antiparallel and (2) in order to lead to energy transfer from the field to the plasma (J.E > 0), the X line must be oriented such that the reconnected field lines move away from the re- connection site in opposite directions (otherwise energy is converted from the flow to the field in the current sheet; J.E < 0). Reconnection occurring tailward of the satellite would produce two field lines bent at the reconnection site; one line with an end connected to the Earth and the other one completely detached from the Earth. Since the kink (bend) in the magnetic field lines generated by reconnection moves at the local Alfv(]n velocity in the frame of the magnetosheath flow, the motion of the reconnected field lines is directed tailward for a super- Alfv nic magnetosheath flow. n such a case a stationary reconnection is not possible (the second condition is not fulfilled). This is confirmed by two-dimensional

5 ... POPESCU ET AL.: SUNWARD PLASMA FLOW N THE HGH LATTUDE LOBE 29, oooo b) > 1000, Log de Log JE 2OO '""' > 1000, t,, "' w ' ;,,,,.,,,,,',,,',,.,,,,,,,,,, JE ,,, 100 -,, ".ooo -i loo - E O... z '-" - O.OlO ' : :. i.;. i, ; loooo :.:, : :,...:. :., E x , 400,.,-400 : : : : : :' '- 20--: lo ;x- O.., Bz,..,...',,,..,,....'. '.,'... UT 21:50 22:00 22:50 25:00 25:50 00:00 00:50 01:00 01:50 Xgsm Ygsm Zgsm Plate 1. Particle and magnetic field measurements made on board nterball-tail during the time interval 2130 UT (October 5, 1996) to 0145 UT (October 6, 1996). (a, b) Electron energy-time spectrograms measured by the sunward and tailward looking detectors, (c) electron temperatures in the range ev (black solid line) and ev (dashed line), parallel (red line) and perpendicular (blue line) temperatures in the range ev, (d, e) ion energy-time spectrograms for sunward and tailward looking directions, (f) ion density, (g) ion temperature, (h) XGSM component of the ion bulk velocity, and (i) the three GSM components of the magnetic field (B, red; B, green; B:., blue). Particle energy fluxes are expressed in kev (cm ' s sr kev) -1. The colored horizontal bars above Plate 1 indicate the regions crossed by the spacecraft: red, magnetosheath; blue, mantle; green, SFPL (see text).

6 29,484 POPESCU ET AL.: SUNWARD PLASMA FLOW N THE HGH LATTUDE LOBE,1-24 b) 1-25., ".. VdHT Bm (D Vsh'[,- Bsh/ VA -6OO V L, km s !r- - ',,,!,,,,,,,,,,,, L.,16 "24 c),400 o VdHT Bm 1 E - 0 Vm / > e n. 1 ( OO V L, km s -1. 1E27 Plate 2. Projections of the proton distribution functions in the LM plane (boundary normal coordinates with L parallel to Bin). V L and V M are the velocity components along and perpendicular to the magnetic field. The measured magnetic fieldirections are shown by the red axis. (a) Magnetosheath distribution; the black point indicates the de Hoffman-Teller velocity, and the red circle designates the magnetosheath bulk velocity. Red thin curved lines are schematic contours of constant space density. (b) Sketch showing the transformation of the distribution function across the rotational discontinuity (magnetopause). (c) SFPL distribution; the blue circle shows the SFPL bulk flow. The missing circular part in the center of the distribution function corresponds to the lower threshold of the count rate of the instrument.

7 POPESCU ET AL.: SUNWARD PLASMA FLOW N THE HGH LATTUDE LOBE 29,485 MHD simulations in more realistic conditions, i.e., in the presence of a shear flow and symmetric or asymmetmeasurements, we deduced the Alfv{]n velocity (VA) and the plasma bulk flow in the magnetosheath (Vsh). ric density across the current sheet [La Belle-Hammer These quantities have been reported on Plate 2 and et al., 1995]. The only case where reconnection could allow to fix the position of V HT (along the magneoccur for super-alfv{}nic plasma flow is when the X line itself moves tailward with speeds comparable to the tosheath B field), i.e., VdHT = Vsh - bshv (bsn is the unit vector along the magnetosheath magnetic field, magnetosheath flow. Nevertheless, this situation can V HT is the de Hoffmann-Teller speed and V is the hardly be quasi-steady since the reconnection site, as it drifts tailward, encounters different plasma densities, flow velocities, and magnetic field strengths. n our example the ion flow velocity, directly measured on board Alfv n velocity in the magnetosheath). Plate 2b illustrates geometrically the transformation of the distribution function in the de Hoffman-Teller frame, i.e., across the RD. Here particles do not gain any energy, and af- nterball-tail, inside the magnetosheath (not shown) ter crossing the discontinuity, they keep the same bulk was largely super-alfv{]nic (2 < MA < 6). Nightside velocity (Vtm, along the magnetospheric B field) still lobe reconnection can thus hardly account for our ob- equal to the Alfv{]n one (Vtm- Vtsh - V, where servations. Another possibility is that the SFPL can result from dayside reconnection (where the magnetosheath plasma flow is sub-alfv( nic) if (1) this process produces a RD in the region located tailward of the satellite and (2) the topology of the RD allows the particles to reach the the primed quantities are measured in the de Hoffman- Teller frame). n the rest frame, Vm = V tht +bm VA (bin is the unit vector along the magnetospheric magnetic field). The red curves in Plate 2b show the part of the magnetosheath distribution, which enters inside the magnetosphere. Plate 2c illustrates the distribution satellite location. function measured inside the magnetosphere; it fits rea- Concerning the first point, recent MHD simulations sonably well the shape of the distribution function ex- [White et al., 1998] and full kinetic 3-D simulations pected from the transformation across a RD. Only par- [Nishikawa, 1998] clearly revealed that for a large By component of the MF a deep, narrow B field minimum runs along the high-latitude dayside flanks. For negative By, like for the event presented here, the minimum, which is topologically a magnetic merging line, runs from the dayside northern cusp along the dayside ticles with positive (earthward directed) velocities in the rest frame are able to reach the satellite from a tailward site. However, the distribution observed at the satellite location contains particles that entered the magnetosphere at different locations of the magnetopause, earthward and tailward of the satellite position. A fracdawnward flank. Conditions are thus fulfilled to cre- tion of the particles with negative velocities (see Plate ate a rotational discontinuity at the high-latitude flank magnetopause. Concerning the second point, we can directly check if the observed ion distribution function fits the gen- 2b) issued from the RD part located earthward of the satellite can still reach it owing to convection inside the magnetosphere [e.g., Fedorov et al., 2000]. This results into the part of the distribution function with negative eral model proposed by Cowley and Owen [1989], which velocities displayed in Plate 2c. Note, however, that the predicts a D-shaped distribution function of the trans- expected velocity cutoff, which depends on the satellite mitted ions. This model is deduced from the one- distance to the magnetopause, cannot be precisely dedimensional tangential momentum balance across a RD fined here owing to the coarse angular resolution of the (Bn 0 across the current layer). Plates 2a and 2c show detector. Although the shape of the ion distribution in slices of ion distribution functions taken on both sides the SFPL well fits that expected from the penetration of the magnetopause. They are displayed in an L M plane. This plane is perpendicular to the local normal (N) of the magnetopause with L being parallel to the lobe magnetic field. The times when the distributions in Plates 2a and 2c were measured are indicated by black triangles in Plate le. Plate 2a shows the ion distribution measured in the magnetosheath at, UT. Plate 2c gives the ion distribution function measured in the magnetosphere at 0030 UT. Following Cowley's method, we will check if the disof magnetosheath plasma trough a rotational discontinuity, the ion phase space density observed in SFPL is 1-2 orders of magnitude lower than that expected at those locations in velocity space based on the observed magnetosheath distribution and the simple reconnection construction presented above. A similar result has been found by Onsager et al. [1995], who inferred from data and model comparisons a factor 10 between the incident and transmitted ion fluxes. At present, the reason for this large reflection coefficient is not clear. tribution shown in Plate 2c can result from the trans- This could be because of complex nonadiabatic motion formation of the magnetosheath distribution of Plate 2a through a rotational discontinuity. n the de Hoffman- Teller frame, moving along the magnetopause, the elecof particles inside the local magnetopause as suggested by Delcourt et al. [2000]. t must be stressed that in the model described above, tric field vanishes on both sides of the discontinuity, the electrons should not be strongly affected as their and the plasma bulk flow is field aligned and Alfv{]nic thermal speed is much higher than the de Hoffman- [de Hoffman and Teller, 1950]. Using the nterball-tail Teller speed. However, inside magnetospheric boundary

8 29,486 POPESCU ET AL.- SUNWARD PLASMA FLOW N THE HGH LATTUDE LOBE 1996, October t Sheath SFPL Cold SFPL x x x104 v [km/$] x104 4x104 6x104 8x104 V [km/s] Plate 3. Electron distribution functions measured in the magnetosheath at 2351:38 UT (blue curve), in the mantle at 2241:40 UT (green curve), and in the SFPL for two crossings corresponding to different temperatures, low, at 2306:02 UT (dashed red curve), and higher, at 0041:37 UT (solid red curve). The X axis gives the electron velocity parallel to the magnetic field. The insert displays more details on the electron distribution functions at low velocities. October 6, 1996 NTERBALL-Auroral 87 ev electrons 90-11, :24 00:26 00:28 00:30 00:32 00:34 00:36 00:38 00:40 00:42 00:44 UT Plate 4. Successive 47r distributions of the core electrons (- 87 ev) of the SFPL between 0024 and 0045 UT on October 6, The vertical axis gives the polar angle, 0, ranging from -90 ø to +90 ø, which is covered by the eight anodes of the electron experiment. The horizontal axis gives the azimuthal or spin angle b and the corresponding universal time. The white diamonds indicate the direction of the measured magnetic field, which appears in a given (0, b) cell every spin period (2 rain). The electrons are preferentially detected along the +B and -B directions.

9 . =, ß POPESCU ET AL.: SUNWARD PLASMA FLOW N THE HGH LATTUDE LOBE 29, nitial distribution o i f i i i i i i... i... i... i Vi (x 104 krn/s) 1.5 Field-aligned deceleration 1.5 Field-aligned acceleration E v o llllllljiljl!... i... i Vii (x104 kin/s) 1.5 d) !... i... i...! Adiabatic deformation... i... i... i Vii (X10 4 kin/s) /:",q......,!, i 'i''' ':":': '"':" ' ,..., V,, (x104 krn/s) Figure 3. (a-d) Sketch representing the changes in electron phase space density contours at the magnetopause using an electrostatic potential well first decelerating by,-.100 V an initial Maxwellian distribution (n = 3 cm -3, kt = 100 ev) (Figure 3b), then accelerating it by the same potential (Figure 3c). The electrons with negative velocities do not enter the magnetopause. nside the magnetosphere the distribution is adiabatically deformed (B1/Bp.- 1.5) along the field line (Figure 3d). The dashed contours in Figurel 3d represent reflected electrons (at their mirror point). As a global result, the potential well repeals the lowest-energy electrons and produces a distribution with highest fluxes at low energy and small pitch angles. layers the electrons are clearly heated when compared to the magnetosheath ones [e.g., Sauvaud et al., 1997]. For the case studied here, Plate 3 displays electron distribution function cuts for the SFPL, the mantle, and the magnetosheath. The X axis gives the electron parallel velocity. The dashed red curve displays a distribution measured at 2306:02 UT, where the electron component of the SFPL shows a lower temperature (hence the name of "cold SFPL" in Plate 3), for example, compared to preceding SFPL crossing (from, to, UT, Plate 1). The ion component, however, does not show significant temperature variation. Note first that elec-

10 29,488 POPESCU ET AL.: SUNWARD PLASMA FLOW N THE HGH LATTUDE LOBE tron distributions have the same general shape. However, the temperature of the core electrons of each distribution increases from the magnetosheath to the SFPL. This is also apparent in Plates la and lb. All distriitive or slightly negative MF Bz and dawnward oriented MF, presents the following characteristics: (1) t has a long duration (several hours), suggesting that the mechanism at its origin is steady or slowly time varybutions show a suprathermal component (V) ing, (2) the electrons inside this layer have temperatures 104 km s - ), which in the case of the magnetosheath above the adjacent magnetosheath, and (3) the shape can be attributed to the well-known halo distribution of the ion distribution functions is in agreement with of solar electrons. Undoubtedly, the lowest-energy part of the magnetosheath electrons is missing in both the mantle and SFPL; this can be better appreciated in the insert of Plate 3. Thus any processes for electron transfer from the magnetosheath to the magnetosphere have to explain this disappearance (or extremely strong flux reduction) of the lowest-energy solar electrons. Several models of RD in a collision-free plasma have an electric potential barrier/well as an intrinsic part of the transformation of the magnetosheath distributions through a rotational discontinuity. We examined the possibility that such a layer be formed by plasma sheet particles and found no clear evidence supporting this origin. More likely, the SFPL is the result of magnetopause reconnection. The existence of the SFPL for an extended time period (several hours) during quite stable solar wind conditions implies a large-scale quasi-stationary reconnectheir structure [e.g., $u and $onnerup, 1968; Lee and tion driven by the global interaction between solar wind Kan, 1982]. n our case the electron distributions call for a potential well at the boundary layer that would and magnetosphere rather than patchy reconnections determined by local conditions at the magnetopause. reflect the lowest-energy electrons (the core of the magnetosheath distribution function), leading to the observed decrease in the density of the transmitted electrons. Only electrons with sufficient parallel energy will pass the decreasing negative potential part and will be re-accelerated in the increasing potential part of the well, which favors the alignment of the electrons with the magnetic field. Figure 3 schematically illustrates the effect of such a potential well. For typical magnetopause conditions the electric fields normal to the layer predicted by Lee and Kan [1982] are of the order of 1 mv m -. n the SFPL the electrons have characteristic energies Ee ev. n order to repel ev electrons with a 0.5 mv m - electric field, a km thick layer at least would be necessary, which is about the size of the ion gyroradius. Nevertheless, a supplementary mechanism is needed to account for the electron heating. This mechanism could be, for instance, based on particle interaction with waves inside the boundary layer (e.g., lower-hybrid like instabilities). Finally, Plate 4 illustrates the measured distributions of the core electrons (,- 87 ev) of the SFPL. They show a clear flux increase along the +B and -B directions, in agreement with the simple sketch presented in Figure 3. This kind of anisotropy (although stronger) is also systematically observed inside the low-latitude bound- ary layer [e.g., Paschmann et al., 1993; $auvaud et al., 1997]. t must be stressed that a potential barrier of the order of V is not expected to have a drastic effect on the ion population, which has a flow energy 10 times larger, of,. 1 kev. 4. Summary and Conclusion We presented experimental evidence for the existence of sunward flowing plasma layer near the highlatitude magnetopause, adjacent to the mantle and with a source tailward of X RE. This event, which occurred during quiet geomagnetic conditions for pos- This favors the hypothesis of dayside magnetopause reconnection as a plausible mechanism that creates RD stripes in the tail magnetopause, allowing the plasma to penetrate in the distant tail without the requirement of local reconnection. However, if the RD model can grossly explain the behavior of transmitted ions, it does not simply account for the electron behavior. The reflection of the core magnetosheath electrons, the acceleration of transmitted electrons requires modest parallel electric fields, and electron heating can result from wave-particle interactions associated with smallscale processes linked to the structure of the rotational discontinuities. Although the SFPL is often observed by nterball-tail at the high-latitude tail flanks, a further understanding of the mechanisms at its origin clearly requires further theoretical modeling of the plasma penetration processes across the magnetopause. Acknowledgments. The authors thank S.. Klimov and S. A. Romanov for providing nterball-tail magnetic field data. This research program has been financed by CNES (contract CNES-08). Thanks are due to E. Penou (CESR), to J. Durand and the data center division from CNES at Toulouse, and to E. Gavrilova and the data center from K for reduction and visualisation software of the ON data. The work of E. B. and A. 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