Low-Latitude Boundary Layer Under Different IMF Orientations

Transcription

1 WDS'05 Proceedings of Contributed Papers, Part I, , ISBN MATFYZPRESS Low-Latitude Boundary Layer Under Different IMF Orientations Š. Dušík, J. Šafránková, Z. Němeček, and L. Přech Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic Abstract. The structure of the magnetopause has been a subject of an intensive study for many years. At low latitudes, one can identify the low-latitude boundary layer (LLBL) on the magnetospheric side and rather often a depletion layer on the magnetosheath side of the magnetopause. A thickness of these layers varies from 0.2 to 1 Earth s radius but several examples of a very thick LLBL have been reported at flank parts of the magnetopause. Plasma parameters inside the LLBL are variable, the spacecraft usually observes a mixture of magnetosheath and plasma sheet plasmas. Several mechanisms including intermittent reconnection, impulsive penetration, and Kelvin-Helmholtz instability have been proposed to explain this phenomenon. The magnetopause and LLBL were crossed by the Interball-1 and Magion-4 satellite pair at various local times and with differing satellite separations. The present study employs a statistical approach to determine typical conditions for both low and high magnetic shears. Furthermore, the presented case studies reveal complicated structure of boundary layers: the depletion layer, outer and inner LLBL. The layers usually move in accord but, under some circumstances, only one boundary is unstable as it is demonstrated in this paper. We discuss the stability conditions for different boundaries within the magnetopause layer. Introduction Mass transfer across the magnetospheric boundaries remains a central issue in the magnetospheric physics because of its far-reaching implications for the magnetosphere dynamics. Although the presence of magnetopause and adjacent boundary layers has been experimentally shown about 35 years ago, the problem of their formation is still open. For better understanding, we shortly describe well known features of the magnetopause, plasma depletion layer (PDL) and low-latitude boundary layer (LLBL) and their forming mechanisms as we understand them today. The magnetopause and LLBL. This boundary is defined as the surface where the pressure of the geomagnetic field is equal to the solar wind pressure. It separates the cold and dense magnetosheath plasma from the hot and tenuous magnetospheric plasma. The LLBL is a magnetopause layer with plasma properties intermittent between the magnetosheath and the plasma sheet. Low-latitude dayside magnetopause. The subsolar magnetopause is a current sheet when the magnetic shear across the magnetopause is high (e.g., Cahill and Amazeen, 1963), while discontinuities in the plasma properties characterize the low-shear magnetopause (e.g., Paschmann et al., 1993). Reconnection flow signatures are often observed at the dayside high-shear magnetopause (e.g., Paschmann et al., 1979) but are not so often detected at the low-shear magnetopause (Paschmann et al., 1990). On the basis of in situ particle observations, Hall et al. (1991) suggested that the boundary layer lies on a combination of open and closed field lines. On the other hand, Fuselier et al. (1995) suggested that the low-latitude boundary layer lies on open field lines even when the magnetic shear across the local magnetopause is low. Song and Russell (1992) assumed that under northward interplanetary magnetic field (IMF) conditions, plasma enters the LLBL from high latitudes and Le et al. (1996) concluded that the subsolar low-shear LLBL lies on a combination of open and closed field lines. Similar results by Paschmann et al. (1993) indicated that, under conditions of low magnetic shear, the changes in plasma thermal and flow properties may be attributed to a transition from open interplanetary to closed geomagnetic field lines. Low-latitude flank magnetopause and boundary layers. The magnetopause flanks have been less explored. Evidence for reconnection there has been reported (Gosling et al., 1986), although the occurrence rate of reconnection has not been established. It has been found that the LLBL lies normally on closed field lines (Eastman and Hones, 1979). From energetic electron behavior, Mitchell et al. (1987) inferred that the LLBL flanks are closed for northward IMF but are partially opened for southward IMF. 225

2 They inferred from sheath and LLBL velocities and electric fields that a viscous interaction results from diffusion across the magnetopause. Phan et al. (1997) studied the low-latitude dusk flank sheath, magnetopause, and LLBL topology for a low magnetic shear, particularly changes in plasma properties. The electron parallel temperature is enhanced across the magnetopause and further increased throughout the LLBL, whereas the perpendicular temperature is constant. In the LLBL, the ion and electron temperatures are anti-correlated with the density. The flow direction in a substantial portion of the LLBL is nearly aligned with the sheath flow. The flow tangential to the magnetopause decreases with decreasing LLBL density. The particle distributions suggest that the entire LLBL is on closed field lines. These findings suggest that even in the absence of reconnection at local low-shear magnetopause, the LLBL is locally coupled to the sheath. Plasma depletion layer. Stationary models of the magnetosheath using three-dimensional MHD simulations have demonstrated the formation of a plasma depletion layer in the subsolar region (Wu, 1992). Such layer adjacent to the magnetopause (within about 1 R E ) was also predicted from analytical treatments. From these, it was expected that IMF orientations perpendicular to the Earth-Sun line favor a PDL formation, whereas a radial interplanetary magnetic field (IMF) does not. Since the PDL occurs in the subsolar region where a significant energy transfer due to reconnection is expected, it is important to characterize the PDL and to determine the conditions under which it is formed. The plasma depletion layer is characterized by a reduction in the total plasma density, a decrease in the particle pressure, an increase in the magnetic field pressure that balances the total pressure, and an increase in the p perp /p par pressure anisotropy (Fuselier et al., 1991). Paschmann et al. (1993) found from an analysis of 22 dayside magnetopause crossings where the magnetic shear across the magnetopause was low (< 30 o ) that even there is no change in the magnetic field, the magnetopause is well defined by sudden changes in the thermal and flow properties of the plasma. Further, they found clear indications of a PDL and magnetic field pile-up region. Anderson and Fuselier (1993) reported a PDL for all orientations of the sheath magnetic field, although the density decrease and field increase were smallest for B Z < 0. By contrast, Phan et al. (1994) referred that a PDL was clearly evident only when the magnetosheath field was aligned within 60 o of the magnetospheric field, whereas they found a density enhancement in front of the high-shear magnetopause. Both groups of the authors interpreted the dependence of a PDL prevalence on the IMF orientation as an evidence for reconnection flows at the subsolar magnetopause, despite great differences in what that IMF dependence was thought to be. Anderson et al. (1997) explained this disagreement taking into account different observational conditions because all magnetopause crossings analyzed in Anderson and Fuselier (1993) occurred during intervals of high upstream dynamic pressure. The later paper suggests that the PDL is more likely for low shear, for weak solar wind field strength, and for higher solar wind pressure. Reconnection efficiency has a strong effect on plasma depletion such that plasma depletion should be more prevalent for a high magnetosheath beta. We can conclude that the PDL is created in the magnetosheath near the magnetopause when the magnetic shear across the magnetopause is low. The lack of the plasma pressure inside the layer is balanced by the increased magnetic field magnitude. Forming mechanisms Safrankova et al. (2002) showed that a layer of gradually decreased plasma density adjacent to the magnetopause can be present regardless of the IMF B Z orientation and the value of the magnetic shear. The thickness of this layer (from two-point observations) varies from 0.6 to 1 R E. The plasma depletion is not necessarily accompanied with magnetic field enhancements. High energy particles can be observed inside the layer and can play an important role in a pressure balance. Scanning of the profile of a depletion layer can be a source of fluctuations observed near the magnetopause. The mechanisms leading to the formation of the magnetopause layers for different IMF orientations are shown in Figure 1. When IMF is southward, subsolar reconnection can form the magnetopause current sheet as demonstrated by field line kinks. Inbound parts of the reconnected field lines belong to the LLBL, whereas outbound parts create sheath transition layers (Russell, 1995). Plasma properties differ in these parts due to the different history. The LLBL part originally contained a hot, tenuous magnetospheric plasma and reconnection added an accelerated sheath population. A mixture of these populations exists inside the LLBL. Sheath plasma is accelerated during reconnection but an observer outside the subsolar point can not readily identify a velocity change. However, the density is depleted inside the layer because a part of the plasma was transmitted to the LLBL. For this reason, we denote this layer as a depletion layer (DL) in the left panel of Figure

3 Jun Stevo d:\wds05\pic\northsouth.ps Figure 1. A sketch of subsolar magnetopause layers for high (left panel) and low (right panel) magnetic shear (adapted from Nemecek et al., 2003). Magnetospheric ions cannot follow the kink on the field line because their gyroradius is too large but the electrons can and thus stream outward. In the LLBL, the electrons accelerated during reconnection move toward the ionosphere. Magnetic mirroring can produce counterstreaming beams. A similar feature cannot be seen in ion-energy spectra because the mirror time is comparable to the E B drift time. The LLBL almost covers the frontside below the cusp in this scenario. However, since all reconnected field lines eventually enter the mantle, it cannot explain the flank LLBL. Indeed, the flank LLBL is thinner and vanishes during southward IMF (Mitchell et al., 1987). The situation for northward IMF is illustrated in the right panel of Figure 1, as suggested by Song and Russell (1996) and Le et al. (1996). Reconnection occurs poleward of the cusp (point 1) and newly reconnected field lines cover the dayside magnetopause. They may reconnect in the opposite hemisphere (point 2), closing a mixture of sheath and magnetospheric plasma inside the magnetosphere. Convection transfers the LLBL lines tailward creating the flank. However, as the convection is slow, the plasma parameters would evolve and one can expect that parameters of the flank LLBL created by this mechanism would differ from those in the subsolar region. Separately, magnetic field pile-up creates the PDL on the dayside. The low-latitude magnetopause represents a tangential discontinuity with no direct connection between the PDL and LLBL in this case. Jun Stevo d:\wds05\pic\wds_tail.ps Figure 2. Schematic drawing of the formation of the flank LLBL (adapted from Nemecek et al., 2003). The presence of the LLBL along the flanks is probably caused by reconnection near cusp. The merging location is affected by all IMF components. The positive (negative) B Y component shifts the site to the later (earlier) local times in the northern hemisphere and newly opened field lines in the cusp region move to the flank instead of to the subsolar region. This situation is schematically depicted in Figure 2 as deducted from two-point observations by the Interball-Tail spacecraft (according to Nemecek 227

4 et al., 2003). The satellite is located tailward of the dawn terminator in the LLBL just below the neutral sheet. The field line crossing the spacecraft closes through the dawn cusps. After merging, the injected ions move along the line from the northern to southern hemisphere, and move sunward at the satellite location. A small change of the LLBL magnetic field can cause an apparent motion of the satellite from point 1 to point 2, where the ions will move tailward as expected in the LLBL. Surface waves Plasma parameters plotted across the magnetopause usually exhibit large fluctuations. In many cases, these fluctuations can be connected with the presence of surface waves. Surface waves might be generated by a train of solar wind dynamic pressure pulses (Sibeck et al., 1990) but a Kelvin-Helmholtz unstable inner edge of the LLBL is a possible cause as well. Incompressible magnetohydrodynamics predicts that the inner edge becomes Kelvin-Helmholtz unstable (Ogilvie and Fitzenreiter, 1989) when [k (V LLBL V MSPH )] 2 > ρ LLBL ρ MSPH µ 0 ρ LLBL ρ MSPH [(k B LLBL ) 2 + (k B MSPH ) 2 ] (1) where V is the velocity, B is the magnetic field and ρ is the particle density. As the wave vector k and the shear flow are perpendicular to the magnetic field at the dawn and dusk flanks, the inner edge is very susceptible to the Kelvin-Helmholtz instability there. In this paper, we are going to present a statistical study of the dependence of the magnetopause location on a local magnetic shear. From this study, we will try to deduce how surface waves can affect the uncertainties of magnetopause location predictions. We will also try to find the shape of the inner and outer edges of the LLBL and their movement by analyzing selected observations. Data set The basic data set used for our study includes a collection of magnetopause crossings observed by the Magion-4, Interball-1, and Geotail satellites. Both Interball-1 and Magion-4 (the Interball-Tail project) were launched on August 3, 1995 on an elliptical elongated orbit with the inclination of 63 o, the apogee km ( 31 R E ), the perigee 500 km and orbital period of 92 hours. Due to these orbital parameters and their temporal evolution, the satellites have scanned a broad range of local times, from the magnetospheric tail to subsolar regions. The Geotail satellite was launched on July 24, At the first phase of this project, the spacecraft orbit was very elongated with apogee km ( 200 R E ). At the second phase (from February 1995), its orbit was corrected, so the spacecraft could observe near-earth equatorial regions. In this time period, its apogee was km ( 30 R E ). Our database includes 2354 low-latitude (< 30 o ) magnetopause crossings (104 by Magion-4, 1090 by Interball-1, and 1160 by Geotail) in the range of X GSM from -20 to 12 R E. Interball-1 and Magion-4 magnetopause crossings were identified by a visual inspection of the ion and electron energy spectra (Yermolayev et al., 1997; Sauvaud et al., 1997; Nemecek et al., 1997) and the magnetic field (Nozdrachev et al., 1998). Geotail magnetopause crossings were determined automatically on the basis of the magnetic field changes (Ivchenko et al., 2000). The solar wind and interplanetary magnetic field data were taken from the Wind satellite which was launched on November 1, 1994 and monitored solar wind conditions in the interval under study. The propagation time of the solar wind features from the Wind position to the location of the particular magnetopause crossing was computed as a two-step approximation from the Wind solar wind velocity measurements (Safrankova et al., 2002). Statistical study The average shape and location of the low-latitude magnetopause can be well described by the Petrinec and Russell (1996) model (Safrankova et al., 2002). However, the distribution of the observed crossings around a model surface is rather broad. There can be many sources of this spread and we analyze a role of the IMF orientation. Since IMF is modified in the magnetosheath and the magnetospheric magnetic field has different orientations in different locations, we are using the local magnetic shear as a parameter of conditions. A high shear can indicate a local reconnection process which would erode the magnetopause. Thus, one would expect the magnetopause nearer to the Earth than in an opposite case. In Figure 3, one can see distributions of differences between predicted and observed magnetopause distances. On the X-axis, there is difference of model and observed magnetopause distances in Earth s radii. On the Y-axis, there is percentage of distributions. We compare a situation for high (> 60 o ) and low (< 40 o ) shear angles. However, the distributions in Figure 3 reveal a surprising result. The 228

5 Figure 3. Distributions of differences between predicted and observed magnetopause locations for high (top) and low (bottom) local magnetic shears. Right hand panels show this distribution for all nightside crossings, whereas subsets for X < 5 R E are plotted in the left hand side. distribution of differences between the model and observed low-latitude magnetopause locations does not Jun Stevo d:\wds05\pic\bigpic1.ps depend on the magnetic shear (compare upper and lower panels) but it exhibits two peaks symmetrically located around zero. These peaks are more distinct for crossings located more tailward (X < 5 R E, left panels). Figure 4. Model distributions of differences between predicted and observed magnetopause locations for increasing random component of the magnetopause motion. We suggest that such distribution can be a result of a combination of a periodic magnetopause motion (surface waves) with random fluctuations. In order to determine a typical surface wave amplitude Jun Stevo d:\wds05\pic\bigpic2.ps and random component, we have carried out a small numerical experiment. The results are shown in Figure 4. The first left panel shows the distribution of differences if only a periodic component with the amplitude of 2 R E is present. Consecutive panels show distributions with the Gaussian noise with 0.25, 0.5, and 0.75 R E added to the magnetopause location. One can see that these combined distributions rather well describe distributions of observed crossings. We can thus conclude that a quasiperiodic motion is probably a frequent feature of the low-latitude magnetopause. Nevertheless, this way of the magnetopause motion can have two sources - an intrinsic instability or fluctuations of the solar wind, or magnetosheath pressures. 229

6 Case studies We have chosen two events where both Interball-Tail spacecraft scanned structures near the magnetopause. We define the boundaries of these structures and analyze their shapes and motions in time. April 12, 1997 event Jun Stevo d:\wds05\pic\wds_spectra2a.ps Figure 5. Observations of the Interball-1 spacecraft on April 12, From top to bottom: ion flux f 0 ; total magnetic field B; magnetic field components B X, B Y, B Z, and magnetic shear angle. Conditions in the solar wind (not shown in this plate) were relatively stable. The first event was observed on April 12, 1997 (Figure 5). The Interball-1 satellite was moving from the magnetosheath to plasmasheet, crossed the magnetopause and scanned the depletion layer and LLBL. A more detailed view on the structure of the whole region brings the plot of an electron temperature vs density (T-n plot) in Figure 6. We have broken entire interval into several parts and distinguished the data from these parts with levels of the gray scale. The magnetosheath interval ( UT, not shown in Fig. 5) is black, the interval with several crossings from 1330 to 1401 UT is dark gray, the LLBL interval from 1401 to 1430 is light gray, and the following hour (not shown in Fig. 5.) is marked with gray diamonds. The resulting profile is very clear and we can identify several distinct regions or plasma properties. Clouds of points with a low temperature and high density can be attributed to the magnetosheath proper. The points lying at the part with a rapidly decreasing density and slowly increasing temperature were measured in the depletion layer that is still on magnetosheath field lines. Note that this depletion is clearly pronounced in a T-n plot, whereas the temporal profile suggests no presence of the depletion layer. The slope of the T-n curve sharply increases at the magnetopause when the satellite enters the LLBL but even there we can identify two parts - the outer LLBL with a strongly increasing temperature, and the inner LLBL where the temperature still increases as the density decreases but the slope is not so steep. Such profile is not unique, it can be found everywhere along the low-latitude magnetopause. 230

7 Jul Stevko c:\work\wds05\pic2\tn.ps Figure 6. Electron temperature as a function of the electron density. February 23, 1996 event Based on the analysis of the previous event, we can conclude that the structure of the LLBL is very robust. The event analyzed in this section documents that the quasiperiodic features often observed there are due to to scanning of this structure. The scanning is caused by a nearly permanent motion of the whole region along the spacecraft. The motion can be induced either by external forces (magnetosheath fluctuations) or by self-excited waves. The following example presents that both effects should be considered. The event observed by Interball-1 and Magion-4 on February 23, 1996 (Figure 7) clearly shows that the outer edge of the LLBL (magnetopause) and its inner edge can move independently. The left panel in Figure 7 shows that upstream conditions were relatively stable. There are two different parts in the plot. Until 1400 UT, both spacecraft observe the same fluctuations. This suggests a simultaneous motion of magnetopause layers. After this time, Interball-1 remained in the magnetosheath, whereas Magion-4 is located at the inner LLBL edge and observes a strong motion of this boundary (the right panel in Figure 7). The smooth profile observed by Interball-1 suggests that the magnetopause is standing. We present a simple sketch of the whole situation in Figure 8. Note that both spacecraft would cross the magnetopause only once in the interval under study according to the Petrinec and Russell (1996) model, whereas multiple crossings are observed. We believe that this disagreement can be attributed to magnetosheath fluctuations. Conclusion The shape of the low-latitude nightside magnetopause is well described by the Petrinec and Russell (1996) model. Using this model, we have shown in Figures 3 and 4 that the uncertainty of the magnetopause location is not connected with the IMF orientation but it is probably caused by a quasiperiodic motion of magnetopause layers complemented with small random displacements. The mean amplitude of a quasiperiodic magnetopause motion is of the order of 1 R E (determined from Figure 3). A further study would define a proportion between two sources - magnetosheath fluctuations and surface waves. The random component does not correlate with fluctuations of upstream parameters as can be seen from Figures 7 and 8 where the magnetopause location computed from upstream parameters does not match the observed crossings. We suggest that this uncertainty would be probably attributed to magnetosheath fluctuations. An analysis of several events suggests that magnetosheath waves influence the whole region, whereas the surface waves are typical for the inner edge of the LLBL. However, the surface waves on the inner edge of the LLBL can probably result in the magnetopause motion in the cases of a very thin LLBL. Acknowledgments. The present work was supported by the Czech Grant Agency under Contract 205/05/

8 Jun Stevo d:\wds05\pic\spectracb.ps Figure 7. The event observed on February 23, Both satellites were again moving from the magnetosheath to plasmasheet. Left panels from top to bottom: Interball-1 ion flux; total magnetic field B; components of the magnetic field B X, B Y, B Z ; ion flux observed by Magion-4; predicted magnetopause distance (according to the Petrinec and Russell (1996) model). Right panels from top to bottom: Interball-1 ion flux; components of the magnetic field; the magnetopause distance; electron spectra; ion spectra. Jun Stevo d:\wds05\pic\model.ps Figure 8. A simple sketch of scanning of the magnetopause and LLBL. Points represent crossings of both boundaries with respect to Magion-4 and Interball-1 Y-coordinates. Marked lines show different regions (dark gray is the magnetosheath and light gray is the magnetosphere), and full lines show actual distances of the model magnetopause (according to Petrinec and Russell, 1996) computed from upstream parameters for both Interball spacecraft. Hatched area denotes the estimated LLBL location consisted with observations of particular crossings. References Anderson, B. J., and S. A. Fuselier, Magnetic pulsation from 0.1 to 4.0 Hz and associated plasma properties in the Earth s subsolar magnetosheath and plasma depletion layer, J. Geophys. Res., 98, 1461, Anderson, B. J., T.-D. Phan, and S. A. Fuselier, Relationship be- tween plasma depletion and subsolar recon- 232

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