Variations of the flank LLBL thickness as response to the solar wind dynamic pressure and IMF orientation

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006ja011889, 2007 Variations of the flank LLBL thickness as response to the solar wind dynamic pressure and IMF orientation J. Šafránková, 1 Z. Němeček, 1 L. Přech, 1 J. Šimůnek, 2 D. Sibeck, 3 and J.-A. Sauvaud 4 Received 31 May 2006; revised 30 January 2007; accepted 2 April 2007; published 4 July [1] Several mechanisms have been discussed as candidates for a formation of the lowlatitude boundary layer (LLBL) magnetic reconnection between the magnetospheric and magnetosheath magnetic fields, impulsive penetration of magnetosheath plasma, and viscous/diffusive mixing of plasma populations at the magnetopause. The observed fluctuations of plasma parameters inside the LLBL are attributed either to transient nature of the phenomena forming the layer or to sweeping of deformations of the magnetopause or an inner edge of the LLBL surface along the spacecraft. We are using the Interball-1/Magion-4 satellite pair separated by several thousands of kilometers in order to distinguish between spatial and temporal changes of the LLBL plasma population. Observations of LLBL crossings invoked by sudden changes of upstream conditions show that even during a strongly northward interplanetary magnetic field (IMF), the LLBL is relatively thin and follows the compression of the magnetopause induced by changes of the solar wind dynamic pressure. The thickness of the LLBL increases significantly (from a small fraction of R E to more than 1.4 R E ) with increase in upstream density and IMF B Y component. Our results indicate that the dusk LLBL is supplied by high-latitude reconnection in the Southern (Northern) Hemisphere, when IMF B Y is negative (positive) and lies on open field lines. The change of IMF B Y direction leads to brief presence of LLBL plasma outside the magnetopause on magnetosheath field lines. Fluctuations of plasma parameters and magnetic field implicate the presence of surface waves on the inner edge of the LLBL, but no waves were observed on the magnetopause. Citation: Šafránková, J., Z. Němeček, L. Přech, J. Šimůnek, D. Sibeck, and J.-A. Sauvaud (2007), Variations of the flank LLBL thickness as response to the solar wind dynamic pressure and IMF orientation, J. Geophys. Res., 112,, doi: /2006ja Introduction [2] The low-latitude boundary layer (LLBL) was first uniquely identified in Interplanetary Monitoring Platform (IMP) 6 data more than 30 years ago [Eastman et al., 1976] after preliminary identifications of both plasma mantle and LLBL populations that were reported in 1972 by Hones et al. [1972] on the basis of VELA plasma data that lacked magnetic field data. The LLBL is defined as a region in which magnetosheath plasma population is found on the magnetospheric side of the magnetopause current layer at low latitudes. This layer has been shown to be present at almost all local times along dayside portion of the magnetopause back to the tail flanks [Eastman et al., 1976]. There is evidence that the LLBL is at times on the closed field 1 Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic. 2 Institute of Atmospheric Physics, Prague, Czech Republic. 3 NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA. 4 Centre d Etude Spatiale des Rayonnements, Centre National de la Recherche Scientifique, Toulouse, France. Copyright 2007 by the American Geophysical Union /07/2006JA lines and at times on open field lines, i.e., magnetospheric field lines which have one foot in the ionosphere and extend into the magnetosheath. Its formation on open field lines during a southward interplanetary magnetic field (IMF) is well understood in terms of dayside reconnection [Luhmann et al., 1984], while the role of reconnection in the LLBL formation during northward IMF is much less clear. Through periods of a strong northward IMF orientation, magnetosheath field lines may reconnect poleward of both cusp regions nearly simultaneously and be appended to the magnetosphere as closed LLBL magnetic field lines [Song and Russell, 1992; Reiff, 1984]. The experimental evidence for LLBL formation by reconnection in both northern and southern cusps has been reported [e.g., Le et al., 1996; Onsager et al., 2001; Fuselier et al., 2002; Němeček et al., 2003a]. Otherwise, reconnection takes place poleward of only one cusp, and open field lines filled with LLBL plasma can be appended to the magnetosphere [Fuselier et al., 1995; Fuselier et al., 1997]. [3] The thickness of the LLBL increases with increasing distance from the subsolar point (from less than 0.1 R E (Earth radius) near local noon) [Haerendel et al., 1978; Eastman and Hones, 1979] to as much as 0.5 R E at the 1of14

2 dawn and dusk terminators [Paschmann et al., 1993] and when the IMF is northward [Haerendel et al., 1978; Mitchell et al., 1987]. However, some observations of a very thick LLBL (several R E ) have been reported [Sauvaud et al., 1997]. Finally, Phan and Paschmann [1996] observed large variations in LLBL thickness from case to case. On the other hand, Hapgood and Lockwood [1995] presented an increase in LLBL thickness from several hundreds to several thousands of km immediately after detection of a flux transfer event (FTE). This is consistent with the theory of transient increases in the open LLBL thickness caused by a pulse of enhanced reconnection rate at the magnetopause. The reported rapid thickening was derived from measurements of two very closely separated spacecraft and lasted for about 2 min. A similar transient enhancement of LLBL thickness was reported by Faruggia et al. [2000] during a passage of magnetic cloud. [4] An additional factor that can influence LLBL thickness is the level of fluctuations in the adjacent magnetosheath. For the typical Parker spiral IMF orientation, the dawnside magnetopause lies behind the turbulent quasiparallel bow shock, whereas the duskside magnetopause does not. We therefore expect those conditions favoring the interchange instability on the dawnside magnetopause to produce a thicker dawnside than duskside LLBL. Similarly, during periods of highly variable solar wind plasma parameters and/or a near-radial IMF orientation (which would place the entire magnetopause behind the quasi-parallel shock), we would expect enhanced LLBL thicknesses at all local times. On the other hand, an increase in the magnetosheath field strength should have a stabilizing effect, which causes the LLBL to become thin, if the IMF is southward but should have little or no effect if it is northward [Book and Sibeck, 1995]. [5] Several mechanisms have been discussed in the literature as candidates for sources of magnetosheath plasma in the LLBL. These mechanisms can be divided into magnetic reconnection between the magnetospheric and the magnetosheath magnetic fields [e.g., Sonnerup et al., 1981; Song and Russell, 1992], impulsive penetration of magnetosheath plasma [Lemaire and Roth, 1978], and viscous/diffusive mixing at the magnetopause [e.g., Axford, 1964; Eviatar and Wolf, 1968; Eastman and Hones, 1979]. While impulsive penetration is only expected to be of importance at the dayside magnetopause, the other two mechanisms can act also along the magnetotail flanks. Impulsive penetration and diffusion would conserve the direction of entering plasma that is generally antisunward, whereas magnetic tension of highly curved field lines after reconnection can add a substantial velocity component to parcels of entering plasma. The observed LLBL flow velocity is usually antisunward, and this component of the flow velocity increases with distance from the subsolar point [Eastman and Hones, 1979]. However, Němeček et al. [2003b] reported a prolonged LLBL interval dominated with a sunward flow. This suggests that reconnection can sometimes be an important source of low-energy LLBL plasma, and thus the condition for plasma entry would vary with magnetic shear across the magnetopause. [6] Phan et al. [1997] studied low-latitude dusk flank LLBL topology when the local magnetic shear was low. The authors concluded that the electron parallel temperature is enhanced across the magnetopause and continues to increase through the LLBL, whereas the perpendicular temperature is constant across the magnetopause. In the LLBL, the ion and electron temperatures are well correlated with densities. The flow direction in a substantial portion of the LLBL is nearly aligned with that in the magnetosheath. The behavior of plasma distributions reported by Phan et al. [1997] suggests that the entire LLBL was on closed field lines. Their findings on topology and on LLBL plasma formation suggest that even in the absence of reconnection at the local low-shear magnetopause, the LLBL is locally coupled to the adjacent magnetosheath. Thus processes such as Kelvin- Helmholtz instability [e.g., Miura, 1987; Hasegawa et al., 2004] or diffusive entry [e.g., Sonnerup, 1980; Johnson and Cheng, 1997] have been suggested to play a dominant role in the formation and dynamics of the LLBL. On the other hand, Phan et al. [2005] discussed observations showing that reconnection can occur at the low-latitude magnetopause during northward IMF (with significant B Y component) but that low-latitude reconnection is not responsible for LLBL creation at these times and finally concluded that instead, reconnection appears to be the process of eroding a preexisting LLBL created either by diffusive entry or by nonsimultaneous double-cusp reconnection. [7] The spatial structure of the LLBL is of interest since it provides an indication as to whether or not diffusion plays a role in LLBL formation. While the LLBL exhibits a density gradient normal to the magnetopause at the flanks of the magnetopause, the dayside LLBL occasionally shows a density plateau, and the LLBL may be one of several sublayers of an overall boundary layer [e.g., Song et al., 1993]. For diffusive plasma entry, one expects relatively smooth density, temperature, and flow profiles, together with close coupling to the properties of the adjacent magnetosheath. On the other hand, sharp gradients bordering plateau profiles may be consequences of reconnection, although time-of-flight effects associated with reconnection may also give rise to gradual profiles of density and temperature [e.g., Lockwood and Hapgood, 1997]. [8] Gradual, abrupt, and plateau-like profiles of density, temperature, and flow have been found in time series of magnetopause crossings. From single spacecraft measurements, it is difficult to determine whether the time series are representative of the spatial variations. There are indications that some of the time variations result from irregular motions of the magnetopause. Average temperature variations are smooth and gradual when plotted against density variations [Hapgood and Bryant, 1990; Hall et al., 1991]. Paschmann et al. [1990] used normal velocity v n data to deconvolve a temporal density profile into a spatial one. For one event that they analyzed, the deconvolved profile seems to be gradual. However, the method depends heavily on the accuracy of magnetopause normal determination [Paschmann, 1997]. [9] Our short survey has shown that many problems connected with LLBL structure and formation are still open and probably unsolvable unless multispacecraft measurements are used. For this reason, we employ data from two closely separated spacecraft (the Interball-1 and Magion- 4 pair) at the dusk flank of the magnetopause to analyze variations in LLBL thickness as a response to two significant increases in solar wind dynamic pressure. The IMF 2of14

3 Figure 1. Simultaneous observations of the solar wind and IMF conditions by WIND and Geotail during analyzed interval. From top to bottom: solar wind pressure from WIND; the IMF magnitude and B X component (WIND); IMF B Y and B Z components (WIND); dynamic pressure from Geotail; the IMF magnitude and B X component (Geotail); IMF B Y and B Z components (Geotail). Note that the WIND data are shifted by the estimated propagation time. orientation was favorable for formation of a thick LLBL because the IMF had a substantial B X component and B Z pointed northward. We demonstrate gradual evolution of LLBL thickness during the event as well as the motion of both boundaries, the magnetopause and the inner edge of the LLBL. We also attempt to find sources of observed sunward and tailward streaming ion populations during an interval of enhanced solar wind dynamic pressure. 2. Data Set [10] As noted, the present case study is based on measurements from two spacecraft, Interball-1 and Magion-4 on 5 March The spacecraft moved inward along nearly the same orbit with a mutual separation of 1.4 R E. Plasma properties were derived either from the ion flux measurements onboard Interball-1 [Šafránková etal., 1997] and ion energy distributions from Magion-4 [Němeček et al., 1997] or from three-dimensional electron distributions provided by the Electron spectrometer (Interball-1, Sauvaud et al. [1997]). Magnetic fields were measured by the MIF-M magnetometer (Interball-1, Klimov et al. [1997]) and by the SGR magnetometer (Magion-4, Ciobanu and Moldovanu [1995]). Corresponding upstream conditions were estimated from the Weather Information Network and Display (WIND) and Geotail magnetic field and plasma data lagged by the propagation time. Possible deceleration of the flow in the magnetosheath was omitted. 3. Upstream Conditions [11] Two distinct jumps in solar wind density were registered by the WIND spacecraft about 220 R E upstream of the Earth on 5 March 1997 between 1300 and 1700 UT. 3of14

4 At the same time, Geotail was located near the bow shock on the dusk side [at 14 UT; 1.5, 22.5, and 2.3 R E in geocentric solar magnetospheric (GSM) coordinates]. The basic parameters of the interplanetary medium, i.e., solar wind dynamic pressures and magnetic field magnitudes and components, measured by both spacecraft are plotted in Figure 1. The expected time of propagation of solar wind features from the WIND position to the Interball-1/Magion-4 location was 60 min. The solar wind velocity was nearly constant (305 km/s) through the whole interval including both density jumps (not shown). [12] The jumps of solar wind dynamic pressure that were registered at 1356 and 1610 UT occurred during clearly northward-oriented IMF, and both of them were registered by Geotail. All features were similar to those at the WIND location. A little different propagation time of the second discontinuity is probably connected with its oblique front, but the IMF orientation remained northward at the Geotail position. The IMF B X component was positive, and only B Y changed its sign several times during the interval under consideration. Note that Geotail was orbiting in the magnetosheath at the beginning of our interval, and at 14 UT, it underwent an outbound bow shock crossing corresponding to an inward motion of the bow shock in response to enhanced solar wind pressure. 4. Thickness of the Flank LLBL [13] Interball-1 was in the Southern Hemisphere near the dawn-dusk meridian (0.6, 11.3, and 5.7) R E at 1300 UT. Magion-4 was separated by (0.28, 1.37, and 0.14) R E, and both spacecraft moved in the direction opposite to the separation vector. Locations of these spacecraft with respect to estimated boundaries are shown in Figure 2. The magnetopause model of Petrinec and Russell [1996] and bow shock model of Jerab et al. [2005] are used for the plot. Three locations of both boundaries denoted by corresponding numbers use solar wind plasma and magnetic field data from WIND at 1330, 1410, and 1630 UT, respectively. Model bow shock locations are consistent with Geotail observations shown in Figure 1. Figure 2 reveals that Magion-4 and Interball-1 are in excellent positions for a study of magnetopause layers because they are moving in accord with the inward motion of the model magnetopause. [14] Both spacecraft spent a part of the interval under study in the LLBL. Figure 3 presents Interball-1 plasma and magnetic field measurements. The first part of our interval, until 1400 UT, is characterized by a quiet magnetic field with the negative B X component, low-electron density, highelectron temperature, and negative current of the Faraday cup (third panel in Figure 3). This current is positive and roughly proportional to ion flux in the magnetosheath and LLBL, but if hot electrons overcome the grid potential ( 2.4 kev), the current becomes negative. We conclude from the data that this region exhibits all features of the plasma sheet. This conclusion is in accordance with the electron energy spectrogram shown in the bottom panel of Figure 3. [15] The sharp turn of the B X component together with a high ion flux, enhanced electron density, and depressed electron temperature indicate an encounter with magnetosheath plasma (at 1402 UT). The magnetosheath interval Figure 2. Projection of the spacecraft locations onto the Y Z GSE plane. The models of Jerab et al. [2005] and Petrinec and Russell [1996] were used to determine Earth s bow shock and magnetopause locations. Three locations of boundaries are BS1 and MP1 at 1330 UT, BS2 and MP2 at 1410 UT, and BS3 and MP3 at 1630 UT. Positions of the spacecraft are shown at 1300 and 1700 UT. lasts approximately 4 min, and then INTERBALL-1 enters the region where the ion flux is low and electron temperature is fluctuating. This region is identified as the LLBL with several excursions into the plasma sheet (PS). The densities are similar in both regions (about cm 3 ), but the electron temperatures differ significantly, being several hundreds of ev in the LLBL and larger than 800 ev in the PS. A more detailed analysis of the electron energy spectrogram suggests two different states of the LLBL, (1) outer LLBL, populated with low-energy electrons; and (2) inner LLBL, characterized by a mixture of high- and low-energy populations. This LLBL feature was often mentioned in previous studies, but we do not distinguish these two regions in the present paper. We should note that the magnetopause crossing was not so simple as described. A careful analysis shows the following sequence: plasma sheet 1358 UT LLBL 1402 UT magnetosheath 1406 UT LLBL (numbers indicate approximate times of crossings). Our identification of regions is shown by color bars. This classification is based on the mean electron energy that is measured with sufficient time resolution. [16] After analysis of many electron spectrograms, we have put the boundary between the LLBL and PS to 800 ev of the mean electron energy (horizontal dashed line in the fifth panel of Figure 3). Below this limit, electron spectra exhibit clear presence of the low-temperature component that vanishes above it. This classification is further supported by a plot of the electron temperature versus electron density shown in Figure 6. This plot clearly exhibits three different slopes. The region below 100 ev can be attributed either to the magnetosheath or to outer part of the LLBL, and the temperature depends on the density 4of14

5 Figure 3. An overview of Interball-1 measurements. From top to bottom: the magnetic field magnitude and the B X component; B Y and B Z components; f is the tailward ion flux; n e is the electron density; T e is the electron temperature; and E e is the electron energy spectrogram. Vertical lines denote both pressure jumps, and an identification of different regions is shown at the bottom by color bars. only weakly in this region. The dependence becomes much stronger in the range between 100 and 800 ev, and we suppose that this part belongs to the inner LLBL. The region above 800 ev again exhibits a weak dependence of temperature on density, and we suppose that it can be considered as plasma sheet. [17] Whereas Interball-1 spent nearly the whole investigated interval in the magnetosphere (LLBL or PS), Magion-4 skimmed the magnetopause as can be seen in Figure 4. The first panel compares Interball-1 (green line) and Magion-4 (black line) magnetic field magnitudes. When both spacecraft are located in the same region, the magnetic fields agree well, whereas reduced Magion-4 magnetic field values suggest intervals when the spacecraft is located in the magnetosheath. According to the magnetopause model of Petrinec and Russell [1996] and WIND data, one would expect four magnetopause crossings (Figure 2) caused by a combination of solar wind dynamic pressure variations and spacecraft inward motion. These crossings are indeed observed, and they are denoted by vertical dashed lines in Figure 4. [18] However, a comparison of magnetic fields from Magion-4 and Interball-1 reveals other magnetopause crossings in the interval UT when Magion-4 is expected in the magnetosheath but enters the LLBL. Our identification of visited regions is given by color bars in the bottom part of Figure 4. We will show later that these additional crossings are consistent with fluctuations of solar wind dynamic pressure observed by Geotail near the bow shock (Figure 1). [19] The different regions observed by Magion-4 can be easily distinguished according to ion-energy spectrograms 5of14

6 Figure 4. An overview of Magion-4 observations. From top to bottom: the magnetic field magnitude from Magion-4 (black) and from Interball-1 (green); Magion-4 magnetic field components in the GSM system; Ei0, ion spectrograms measured by the analyzer oriented along the satellite spin axis toward the Sun; Ei90, the spectra measured by the analyzer oriented perpendicularly to the spin axis; and the Ei180 analyzer measures spectra of sunward-streaming ions. Vertical lines denote magnetopause crossings predicted by the model of Petrinec and Russell [1996] and WIND solar wind observations, whereas our identification of different regions is shown at the bottom by color bars. in the bottom panels. The Ei0 panel shows spectrograms as measured by a narrow electrostatic analyzer oriented approximately along the satellite spin axis toward the Sun. The Ei180 analyzer measures spectra of sunward-streaming ions. The Ei90 spectra were measured by the analyzer oriented perpendicularly to the spin axis. This analyzer scans a broad range of pitch angles and will be used for a determination of the ion-flow direction later in this paper. [20] Plasma regimes can be classified as follows: (1) the region occupied by high-energy nearly isotropic ions seen at the top of the spectrograms is the PS, (2) dense low-energy ions in the Ei0 spectrogram and with a clear regular spin modulation in the Ei90 spectrogram can be attributed to the magnetosheath, and (3) intervals belonging to the LLBL. [21] A careful analysis of Interball-1 and Magion-4 measurements enables us to sketch the LLBL profile plotted in Figure 5. Straight lines show the radial R YZGSM coordinates of both spacecraft as a function of time. Colors distinguish different regions: the magnetosheath (red), LLBL (black), and plasma sheet (green). The actual model magnetopause location computed from WIND is shown by a light blue line, and that using Geotail data is shown by a dark blue line. Note that Geotail was moving in the magnetosheath until 1405 UT. Source data for this drawing were crossings of boundaries by the Magion-4/Interball-1 pair, and the lines connecting consecutive crossings were roughly approximated by an attempt to follow how variations of upstream conditions affect the model magnetopause location. We could see that observed magnetopause crossings exactly coincide with model predictions if the near-earth s solar wind monitor (Geotail) is used, and there is no indication of separate surface waves. On the other hand, the inner edge of the LLBL is less stable with respect to surface waves [e.g., Ogilvie et al., 1984; Němeček et al., 2003b], and this boundary was crossed many times during the investigated interval. For this reason, the location of this boundary is known from the experiment with sufficient accuracy. The sketch reveals the LLBL thickness to be less or comparable to the spacecraft separation (1.2 R E ) during nearly the entire time interval under study. However, between 1515 and 1530 UT, then between 1635 and 1645 UT, and again at the end of our interval, both 6of14

7 Figure 5. A sketch of locations of magnetospheric regions as a function of time (see text for detailed explanation). spacecraft were inside the LLBL. These intervals of a thick LLBL are too long to be attributed to a passage of FTE or similar transient effects. We suppose that the LLBL thickness evolves as shown schematically in Figure 5 because of the increase in the upstream pressure (Figure 1). [22] The LLBL thickness before 1400 UT is under question because there is no LLBL observation at this time; both spacecraft were in the plasma sheet. After an upstream pressure jump, Interball-1 observes the LLBL plasma for 4 min prior to the magnetopause crossing. The velocity of the magnetopause motion determined from twopoint observations was 37 km/s, and that leads to an LLBL thickness estimate of 1 R E. However, there is no indication of LLBL plasma in the Magion-4 data during its path from the PS to the magnetosheath, and that fact suggests a very thin LLBL. We think that this difference is caused by a nonuniform motion of the magnetopause and/or the inner edge of the LLBL. An analysis of the ion convection velocity (not shown) based on measurements of VDP Faraday cups oriented perpendicularly to the Sun- Figure 6. The scatterplot of electron density versus temperature from Interball-1. 7of14

8 Earth line did not show any motion in the +YGSM direction in the LLBL between 1359 and 1401 UT; thus we can expect that the magnetopause was nearly stationary during this interval and then begins a rapid compression. This effect leads to the apparently thicker LLBL recorded by Interball-1. [23] An alternative explanation of Interball-1 and Magion-4 observations can be a short time thickening of the LLBL induced by a reconnection burst during the magnetopause displacement, similar to that reported by Hapgood and Lockwood [1995]. Nevertheless, both interpretations lead to a very thin LLBL prior to the first jump of the upstream dynamic pressure. [24] We have mapped Interball-1 locations along the Tsyganenko and Stern model magnetic field lines [Tsyganenko and Stern, 1996] onto the Earth s surface and found that the same region was crossed by Defense Meteorological Satellite Program (DMSP) F12 and F13 satellites several times during the investigated interval (Figures 8, 9, and 10). These DMSP observations will be discussed later, but we would like to point out here that at 1350 UT, DMSP-13 did not observe LLBL-type precipitation. This is the only DMSP pass through the investigated region with this feature during the interval under study. We think that this fact again supports our conclusion about a very thin LLBL prior to the first pressure jump. 5. A source of LLBL Plasma [25] Upstream conditions varied in a broad range during our time interval, and that allows us to estimate the relationship between LLBL and upstream plasma parameters. A direct comparison is complicated by strong fluctuations in the LLBL, and thus we have divided the whole interval into four parts for further analysis: (1) plasma-sheet interval from 1300 to 1355 UT, (2) magnetosheath ( UT), (3) LLBL1 from 1410 to 1600 UT, and (4) LLBL2 from 1610 to 1700 UT. Note that the spacecraft spent a significant (30 min) part of the LLBL1 interval in the PS (Figure 3). The regions can be distinguished by electron temperatures for magnetosheath below 50 ev, for LLBL between 50 and 800 ev, and for PS above 800 ev. All intervals are plotted by different colors in Figure 6. The scatterplot of electron density versus temperature reveals that plasma-sheet electrons in the first and third intervals exhibit the same ranges of temperatures and densities, whereas LLBL electrons are significantly more dense in the fourth (LLBL2) interval. Since solar wind density was larger by a factor of 1.5 for the LLBL2 interval, we can consider the immediate increase in the LLBL density by a similar factor as evidence of direct coupling between the magnetosheath and LLBL. On the other hand, the same densities in the plasma-sheet and LLBL1 intervals indicate that the plasma sheet is, in some sense, partly decoupled from the upstream region. Our observations are consistent with a study of Borovsky et al. [1998], who found that a correlation between solar wind and plasma-sheet densities peaks with a lag of several hours. [26] Figure 3 shows that plasma-sheet and LLBL magnetic fields are parallel because there is no change of magnetic field components at the boundary, and thus the particle transport is very limited under these conditions. Figure 7. Projections of actual magnetic field vectors measured by Interball-1 in the time interval from 1500 to 1540 UT (magnetic vectors are mapped according the model of Tsyganenko and Stern [1996]). However, one can expect that the degree of coupling can be enhanced by surface waves on the inner edge of the LLBL. Such waves can be inferred from fluctuations of electron energy in Figure 3 that indicate quasiperiodic crossings of this boundary. Even a better demonstration of the presence of surface waves can be found in Figure 7 where projections of actual magnetic field vectors measured by Interball-1 are shown. The magnetic field vectors exhibit a periodic grouping that shows periodic changes of the magnetic field direction caused by waves passing along the spacecraft. Since the wave amplitude is low, their contribution to plasma transport would be small, and that explains why plasma-sheet density does not directly respond to changes of upstream density, although there is an interval of about 2 hours between the two consecutive plasma-sheet observations (Figure 3). [27] The direct coupling between the magnetosheath and LLBL suggested above can be mediated by reconnection, and thus we scrutinize the observations to check consistency with this mechanism. If we compare the Ei0 (tailward streaming ions) and Ei180 (sunward ions) panels in Figure 3, we can note a variable proportion between these two ion populations in the LLBL. For example, counts in the Ei0 channel dominate during UT interval, whereas sunward streaming ions prevail between 1552 and 1557 UT. IMF pointed northward during the whole time interval, and thus a possible source of the LLBL plasma is reconnection tailward of the cusp. However, the IMF B Y component would shift a reconnection site in the Northern Hemisphere to the dusk side when B Y is positive and to the dawn side when B Y is negative. The sense of the shift would be opposite in the Southern Hemisphere. On the other hand, Němeček et al. [2003b] have shown that even reconnection 8of14

9 Figure 8. F12 DMSP LLBL observations of precipitating ions and electrons during dawnward-pointing IMF ( UT). Note intensive precipitation in the morning sector and less intensive precipitation in the afternoon sector that originates in the opposite hemisphere. 9of14

10 Figure 9. Footprints of the orbits of analyzed spacecraft (for the northern hemisphere). Depicted time intervals are (1) 1346: :30 UT, (2) 1528: UT, (3) 1345: UT, and (4) 1527 UT 1544 UT. near one cusp can supply precipitation in both hemispheres. Since presented observations were made close to equinox, one can expect similar plasma conditions (namely Mach number) above both cusps and thus similar reconnection rates. The upstream density was high above an average value, thus LLBL precipitation would be clearly distinguishable in the DMSP data. We are showing one example of DMSP observations (Figure 8) in order to demonstrate that the LLBL could be clearly identified in the DMSP data. An identification of regions shown below the ion spectrogram in Figure 8 is a product of the automated classification; thus the interval around 1540 UT is classified as unclear. However, a careful inspection of ion and electron spectra shows that this interval belongs to the LLBL, too. Since the solar wind speed was low during the event, the energy of LLBL ions is exceptionally low, but similar energy is observed by MAGION-4 near the magnetopause. We would like to note that the same precipitation patterns were observed in the dusk sector of the auroral oval several minutes earlier on the same DMSP pass. [28] The mapping of DMSP passes that occurred within the investigated interval onto the Earth s surface is shown in Figures 9 and 10. The passes are numbered, and the corresponding times are given in figure captions. The LLBL intervals are distinguished by orange-color lines, and one can note that the LLBL was observed in both hemispheres at morning as well as at afternoon local times regardless of the IMF B Y direction. This feature can be explained if reconnection in one hemisphere supplies precipitation in both hemispheres. We would like to draw the attention to a projection of the Interball-1 orbit in Figures 9 and 10. Its strange behavior is caused by changes of magnetospheric configuration due to variations of upstream conditions, and that can explain why the LLBL was observed for such a long time interval. [29] Magion-4 is located in the Southern Hemisphere not far from the cusp (Z GSM = 5 R E ). However, as we have pointed out above, the observed ions can come from either hemisphere. The actual source can be identified by an analysis of ion flow directions. Three types of pitch angle distributions can be found throughout the analyzed interval, (1) ions parallel to the magnetic field, (2) ions streaming against the magnetic field, and (3) counterstreaming ions. These three types are shown in Figure 11 as energy-pitch angle diagrams. Each of them can be attributed to a particular IMF orientation. Negative IMF B Y is associated with field-aligned low-energy ions (Figure 11a). Ion velocities antiparallel to the magnetic field (Figure 11b) are observed during intervals of positive IMF B Y, and counterstreaming populations (Figure 11c) are formed after an IMF turn or during intervals of fluctuating IMF B Y. We should note that IMF direction can be used for orientation only 10 of 14

11 Figure 10. The same as Figure 9 but for the southern hemisphere. Depicted time intervals F12 and F13 DMSP spacecraft LLBL measurements are (1) UT, (2) 1439: UT, (3) 1620: UT, (4) 1255: UT, (5) 1435: UT, and (6) :30 UT. because the field direction just above the magnetopause is important for reconnection but is usually unknown because of magnetosheath fluctuations. For this reason, we are showing the pitch angle distributions measured during intervals of the strongest upstream B Y component in Figures 11a and 11b. 6. Discussion [30] We have analyzed one Interball-1/Magion-4 pass through the dusk LLBL from two points of view. Our analysis of possible influence of the upstream pressure on the LLBL thickness is schematically summarized in Figure 5. This drawing suggests that the LLBL is very thin for this pass at the Interball-1/Magion-4 location near the dawndusk meridian if the upstream pressure (density) is low, although the IMF orientation is favorable for LLBL formation (IMF B Z > 0). A stepwise increase in the upstream pressure to 5 npa (without any significant change of the IMF direction) led to a gradual increase in the LLBL thickness that finally exceeded the spacecraft separation (1.4 R E ). Intervals when both spacecraft moved within the LLBL were too long (10 15 min) to be caused by transient effects like FTEs [Hapgood and Lockwood, 1995]. Nevertheless, since one spacecraft was near the magnetopause and the second near the inner LLBL edge, LLBL thickness was probably close to their separation distance. The following increase in upstream pressure to 8 npa is followed by an IMF turn and, although the LLBL seems to be even thicker in Figure 5, it is difficult to speculate about a source of this thickening. We are discussing observed effects in terms of upstream pressure but cannot exclude the possibility of a large density in combination with velocities well below the mean. These conditions can enhance the amount of solar wind plasma entering the magnetosphere (as we will discuss later) and thus can lead to a thick LLBL. The quantification of our conclusions depends on the definition of LLBL boundaries. Whereas the outer boundary, the magnetopause, is defined rather well, our definition of the inner boundary is based on electron temperature and n-t plot. However, further investigations are needed in order to find if this boundary is sharp and distinct or diffusive. [31] The second topic of our analysis explains several observational facts, (1) a very thin LLBL prior to 1400 UT (first pressure jump), (2) immediate responses of LLBL plasma parameters to upstream changes, and (3) changes of ion flow direction inside the LLBL in reaction to changes in IMF orientation. [32] From the suggested sources of LLBL plasma, point 3 excludes impulsive penetration because the plasma entering the magnetosphere should conserve its antisunward flow direction, but detailed comparison of count rates in sunward and antisunward directions revealed indication of the pres- 11 of 14

12 Figure 11. Pitch angle distributions of ions measured by Magion-4 in the dusk LLBL for (a) positive, (b) negative, and (c) variable IMF B Y. ence of sunward velocity component of magnetosheath-like ions in the LLBL. These intervals can be identified by enhanced counts in the Ei180 energy spectrogram complemented with diminishing counts in the Ei0 spectrogram in Figure 4. [33] Standard diffusion is not fast enough to explain point 2 and thus cannot result in the above mentioned flow reversal. On the other hand, diffusion can be enhanced by surface waves, and sunward-moving plasma can be observed during crossings of well-developed Kelvin- Helmholtz vortices as suggested by Otto and Fairfield [2000]. However, such vortices are likely to be observed much farther down the tail but not at the dawn-dusk meridian. Moreover, all observed magnetopause crossings can be explained by fluctuations of upstream parameters, and thus surface waves cannot contribute significantly to the observed LLBL plasma. [34] On the other hand, all observed features could be explained by magnetopause reconnection at nearly antiparallel sites. This is indeed supported by the presented observations. They suggest a very thin or missing LLBL at the Interball-1 location until 1400 UT, which can be caused by either low upstream pressure (density) or the specific IMF orientation (IMF B Y was nearly zero prior 1400 UT). It is impossible to discern these two factors, as the increase in solar wind dynamic pressure and IMF B Y magnitude occurred simultaneously. Nevertheless, we should note that a zero or very small IMF B Y above the cusp means that magnetic reconnection takes place near the noon-midnight meridian, and the newly reconnected line lies on the dayside magnetopause near the local noon. These lines can eventually lead to an emergence of dayside LLBL on closed field lines [Song and Russell, 1992]. Because such lines are drawn toward the nightside by magnetospheric convection, they can hardly be observed at the dawn-dusk meridian. [35] The presence of the IMF B Y component shifts reconnection toward dawn or dusk, as schematically shown in Figure 12. If IMF B Y is negative, reconnection proceeds duskward of the cusp in the Southern Hemisphere, and Magion-4 observes the field-aligned ions as shown in Figure 11a. Reversal of this component shifts the source region to the Northern Hemisphere, and ions with large pitch angles are observed (Figure 11b). This scenario leads to the LLBL on open field lines and thus does not explain the counterstreaming ions shown in Figure 11c. Ions entering in one hemisphere are admittedly reflected in the opposite hemisphere, but the time needed for their transport along field lines is rather long (15 20 min), and thus the entering and reflected populations cannot be observed at the same place simultaneously in this case. On the other hand, the transport time is very important when IMF B Y abruptly changes sign. The flux tube reconnected, for example, in the Northern Hemisphere is filled with magnetosheath plasma proceeding against the magnetic field. These magnetic field lines are in a proper location for reconnection in the Southern Hemisphere after the IMF B Y reversal. Reconnection would supply this flux tube with field-aligned ions, and Figure 12. A schematic illustrating the flow directions observed by Magion of 14

13 thus a counterstreaming flow would be observed at the Magion-4 location (Figure 12) for a short time. We would like to point out that this second reconnection opens the field lines completely, and thus these field lines lie in the magnetosheath in spite of the fact that they contain LLBL plasma. 7. Conclusion [36] Our case study of detailed two-point observations of a dusk flank LLBL pass during northward oriented IMF has shown that: [37] The mean LLBL thickness can exceed 1.4 R E during intervals of enhanced solar wind dynamic pressure (4 8 npa) presumably if the solar wind speed is low and density is high. [38] The thickness of the LLBL could be a rising function of the solar wind dynamic pressure under the above circumstances. [39] LLBL density follows upstream density with no delay or small delay if other parameters are constant. [40] The source of the low-energy component of LLBL plasma is probably high-latitude reconnection. 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