Pages 813-822

A GEOTAIL OBSERVATION OF THE LOW-LATITUDE
BOUNDARY LAYER

M. Fujimotol, T. Mukai2, H. Kawano3, M. Nakamura4, A. Nishida2,
Y. Saito2, T. Yamamoto2, and S. Kokubun5

1Dept. Earth Planet. Sci., Tokyo Inst. Tech., Meguro, Tokyo 152, JAPAN, E-mail:   fujimoto@geo.titech.ac.jp
2ISAS, Sagamihara, Kanagawa 229, JAPAN

3IGPP, UCLA, Los Angeles, USA
4Dept. of Earth and Planet. Phys., Univ. Tokyo, Bunkyo-ku, Tokyo 113, JAPAN
5STEL, Nagoya Univ., Toyokawa, Aichi 442, JAPAN

ABSTRACT

A case study of the low-latitude boundary layer (LLBL) on the dawnside (7-9 MLT) is reported. As in previous studies, the LLBL structure is well organized if it is taken to consist of two parts, the outer-LLBL and the inner-LLBL. The inner-LLBL is where the mixing of the magnetosheath and the magnetospheric plasma is taking place on closed field lines. The outer-LLBL is where magnetosheath-like plasma is flowing tailward. Detailed analysis of particle signatures, together with the information that IMF By was the dominant component directed persistently dawnward for this interval, suggests an interpretation that the outer-LLBL is on reconnected open field lines. The positions of the reconnection sites relative to the spacecraft, and the dynamics of the flux tubes subsequent to reconnection to form the observed outer-LLBL, are also discussed.

INTRODUCTION

Because of its importance with regard to energy, momentum, and mass transport, intense observational studies of the low-latitude boundary layer (LLBL) have been done to understand its structure and dynamics since its discovery (Eastman et al., 1976). Sckopke et al. (1981) made a detailed analysis of a single crossing using 3D particle data, showing that the boundary layer consists of two parts. The two-layered structure was further investigated by Williams et al. (1985) and Traver et al. (1991). Dependence of the field line topology on the sheath Bz component was studied by Mitchell et al. (1987). It was concluded that the LLBL was on closed (open) field lines for the northward (southward) Bz.

In contrast to the above cited papers that seem to suggest the viscous interaction (e.g., Sonnerup, 1980) to be the dominant influence to LLBL dynamics at the flanks, many papers (e.g., Gosling et al., 1990a) reported events where the LLBL is certainly on open field lines that have been reconnected. Considerable knowledge and understanding that have been accumulated concerning the subsolar region (e.g., Paschmann et al., 1986; 1993; Song et al., 1990; 1993; Phan et al., 1994) shows that the physics at the dayside magnetopause differ considerably according to the presence/absence of the reconnection process. What needs to be understood now are, how the boundary layer connects from the subsolar region to the flanks, and how this connection depends on IMF Bz (and possibly on IMF By, as we will discuss in this paper).

As the GEOTAIL orbit skims along the average magnetopause, it will provide information on the MLT dependent spatial structure under stationary solar wind/IMF conditions. Furthermore, the LEP instrument (Mukai et al., 1994) provides 3D particle distributions with 12sec. time resolution, which makes a detailed analysis and uncovering previously unidentified processes of the LLBL possible. In this paper, a dataset from a skimming orbit in the dawnside LLBL (7-9 MLT) is studied. In this study, as the solar wind condition did not show significant variations, we will interpret the encounters with different large scale LLBL structure to be caused by motions of spatial boundaries over the spacecraft.

OVERVIEW

The interval of interest is from January 25 21:00 UT to January 26 03:00 UT, during which GEOTAIL was skimming the dawnside magnetopause at MLT = 7-9, slightly south of the magnetic equator. The magnetic field data of IMP-8 at XGSM = 32 RE (time lag between the two spacecraft is ~ 10 min.) denote that the By < 0 is the dominant component of IMF for the interval 21:00 UT - 00:20 UT, which is the interval of most interest (Courtesy, R. Lepping): For the interval 21:00 UT - 00:15 UT, during which GEOTAIL is in the outer-LLBL as will be discussed later, averages and standard deviations of the three components are
B = (4.0 ± 1.4, -8.4 ± 1.2, -2.2 ± 2.3), respectively, in GSM coordinates.

Figure 1 shows the magnetic field and plasma data obtained by GEOTAIL. Shown are the three components of the magnetic field (nT), the ion density (/cc) and temperature (keV), and the three components of the ion bulk flow (km/s). The magnetic field and the flow direction are expressed in GSM coordinates. 6 jumps (dashed lines) marked at 21:22, 23:00, 23:33, 00:16, 00:57, and 01:15 UT are identified, which we will interpret to be caused by passages of spatial boundaries over the spacecraft. This choice is based on the quasi-stationary IMF condition monitored by IMP-8, which would not cause significant temporal variations in the LLBL structure at large-scale. These regions are the plasma sheet, the outer-LLBL, the inner-LLBL, the outer-LLBL (again), the inner-LLBL (again), the (dayside) plasma sheet, and the inner magnetosphere, respectively.

 Fig. 1. Note: Dashed lines depict boundaries between spatial structures.

Until 21:22 UT, GEOTAIL is surveying inside the plasma sheet, the region filled with slowly earthward convecting low density (0.5/cc) hot plasma (2.5 keV). At 21:22 UT, there are sharp changes in the ion temperature, in the flow velocities, and in the Bx component of the magnetic field. The interval 21:22-23:00 UT is characterized by the well-defined tailward flow and the tailward stretched magnetic field. The presence of magnetosheath-like cold ions are responsible for this enhanced tailward flow, and the observed magnetic field is likely to result as these ions stretch the field lines tailward. Energetic electrons are observed to be depleted (not shown). One may argue that GEOTAIL is in the magnetosheath (on field lines with both the ends unconnected to the Earth) during this interval. Actually, the Bx and By components of the magnetic field are consistent with the draped dawnward directed IMF field lines over the dawnside magnetosphere. The Bz component, however, is persistently positive in contrast to IMF Bz that tends to be southward. From this feature of the Bz component, we conclude that the region is not the magnetosheath. The plasma density in this region is not uniform but is highly varying from 0.5/cc (the same as in the plasma sheet) up to 4.0/cc. The ion temperature is much smaller than in the plasma sheet, even when the density is comparably low, rejecting the possibility that low density part is the re-encountered plasma sheet. Most of these features are in accordance with previous studies if we take this region to be the "outer-LLBL", the outermost region that has magnetic connections to the ionosphere.

The next interval 23:00-23:33 UT has characteristics similar to the plasma sheet. The only difference is in the density/temperature profiles showing spiky fluctuations, which indicate the presence of plasma with higher density/lower temperature. An inspection of ion E-t diagrams (not shown) reveals that lower-energy magnetosheath-like ions constitute these cool-dense parts, and they are coexisting with, or, mixed with, the plasma sheet ions. Electron pitch angle anisotropy (not shown) depicts that the region is filled with bi-directional thermal electrons, clearly indicating the difference from the plasma sheet (-21:22 UT) (e.g., Hall et al., 1990). This region, which has been termed as the "halo" region (Sckopke et al., 1981) or the "stagnation region" (Williams et al., 1985), will be called the "inner-LLBL" in this paper.

During 23:33 - 00:16 UT, GEOTAIL encounters the outer-LLBL again. The difference from the former is that the sign of the Bx component is positive in this second outer-LLBL, so that the field lines are not directed tailward anymore. Actually, the magnetic field is weakly sunward, dawnward, and northward in this layer, and is mostly in line with the magnetospheric configuration in the dawn-dayside, southern hemisphere. As we will show in the model magnetopause coordinates, however, the magnetic field is deflected tailward in this layer compared to neighboring regions. Energetic electrons are depleted as in the first-LLBL, but thermal electrons of <500 eV are seen to show unbalanced bi-directional characteristics, with more flux in the parallel component than in the anti-parallel. This contrast in the field and the electron characteristics between the two outer-LLBL will tentatively be explained by the formation mechanism that we propose later.

The second inner-LLBL 00:16-00:57 UT is characterized by the same ion mixing feature and bidirectional electrons as the first one. Then, after a brief traversal of the dayside plasma sheet 00:57-01:15 UT, GEOTAIL goes into the ring current region.

Figure 2: The ion bulk flow and the magnetic field in the LMN coordinates. Tailward ion flow and tailward deflected magnetic field in the outer-LLBL (21:22-23:00 UT, 23:33-00:16 UT) are evident.

   Fig. 2. The ion bulk flow and the magnetic field in the LMN coordinates. Tailward ion flow and tailward deflected magnetic field in the outer-LLBL (21:22-23:00 UT, 23:33-00: 16 UT) are evident.

To demonstrate the characteristics of the magnetic and the velocity field clearly, we plot in Figure 2 the magnetic field and the plasma flow in the model magnetopause normal (LMN) coordinates (Russell and Elphic, 1978). Here, we use the magnetopause model by Roelof and Sibeck (1993) to determine the coordinates. The direction of the N-vector at every data point is determined by deforming the model magnetopause self-similarly so that GEOTAIL is on the magnetopause surface. It is reminded that the M unit vector is directed into the tailward direction parallel to the local magnetopause plane for the present dawnside case. Step-wise changes in the BM component upon entry into and exit out of the outer-LLBL (21:22, 23:00, 23:33, and 00:16 UT) clearly indicate that the field lines are more tailward deflected in the outer-LLBL compared to the neighboring regions. The tailward velocity (VM) also exhibits step-wise changes and is elevated up to ~100 km/s in the outer-LLBL. In contrast, flow in the inner-LLBL tends to be sunward.

IONS IN THE OUTER-LLBL

The dynamics of the outer-LLBL will be discussed in this section. The common characteristics of the two outer-LLBL discussed in the previous section are, (1) presence of sheath-like ions flowing tailward, (2) tailward deflected field lines, and, (3) depleted energetic electrons. On the other hand, the two encounters showed differences as, (1) the sign of the Bx component, and (2) thermal electron characteristics. A model for the formation of the outer-LLBL should be able to reproduce these common/different characters. Before proceeding to the modeling effort, here we inspect the ion velocity distributions to search for more observational clues.

Hereafter, ion velocity distributions will be presented in the BCE coordinates. The origin of the coordinates is the same as the SC frame. The coordinates consist of B, C, and E axes, and each is aligned with the magnetic field, the perpendicular component of the ion bulk flow, and the -V x B vector (presumably the MHD electric field direction), respectively. For each distribution data, these coordinates are constructed, and the data in the SC frame are re-sorted. In the procedure, ions are all assumed to be protons.

Let us concentrate on the 1 hour interval from the first outer-LLBL shown in Figure 3. Shown are the Et diagrams for the tailward and dawnward flowing ions. (These two cover most of the ions flowing tailward parallel to the magnetopause surface). The item we will highlight here is the dual-band structure in the Et diagrams, which is composed of peaks in count rates at two energies ~0.1 and ~2 keV. It is seen continuously from 22:00 UT to 22:30 UT, and then from 22:42 UT to 22:52 UT. Similar features in the Et diagrams have been occasionally noticed in the dataset from the distant tail (Hirahara et al., 1996; Seki et al., 1996), in which two energies are separated by a factor of ~16, and are understood in terms of a proton and an O+ beam streaming at almost the same velocity. Since the energy ratio of the two peaks is also close to 16 in the present case, the feature may be understood in the same way.

   Fig. 3

In Figure 4a, we show the ion distribution at the time marked by an arrow in Figure 3. Logarithms of the phase space density in m-6s3 unit are gray-scaled, and the slice of the distribution in the plane including the B-axis and the C-axis is shown. The profile on the right shows the cut of this slice along the dashed line.

The distribution shown on the left is easily decomposed into two populations, one being a beam at VB ~ 500 km/s and the other being a component sitting close to the origin of the plot. Both components have significant temperature anisotropy Tperp > Tpara.  What is peculiar about the beam is that Vc of its center is offset from the Vc of the bulk flow. (It is reminded that these should be the same for a usual situation): While the Vc of the bulk flow is calculated to be ~20 km/s, that of the beam center is ~100 km/s. A solution to this strange feature is provided by interpreting the beam to be O+ ions. As mentioned, the Figure is drawn by assuming that all the ions are protons. If the ions are O+ ions, their actual velocity must be re-evaluated by multiplying a factor of 1/4 to the value in the panel. This will reduce Vc of the beam center comparable to that of the bulk flow, as it usually should be. At the same time, this correction reduces the VB of the beam to ~130 km/s.

Now let us turn our eyes to the distribution shape of the other component. We can see that there is a significant asymmetry about the peak of this distribution (See the profile on the right). Although we have not done a quantitative multi-peak analysis, the distribution shape seems to be described by a superposition of two populations; one peak located at VB ~50 km/s (the peak in the profile itself), and the other at VB ~150 km/s. It is noted that the latter component has a VB value close to the real
VB of the O+ beam.

To summarize, the above ion distribution is interpreted to be composed of three components, O+ and proton beams having almost the same VB ~150 km/s, and a slowly flowing component with VB = 50 km/s. All the three have a common Vc of 20 km/s. Unit vectors (in the SC frame, essentially equivalent to GSE) pointing to the directions of the B and C axes are given on the lower-left of the Figure 4a. From these, the ion motions are visualized as follows: The common Vc, or the convection velocity of the flux tube is directed tailward. In addition to this motion, the ions stream tailward along the stretched field line. It is noted that this sample well represents other data obtained when the dual-band feature is evident in the Et diagram.

 Fig. 4

The second outer-LLBL is also studied in a similar way. It is found that an example shown in Figure 4b well represents most of the data obtained in the layer. In inspecting Figure 4b, what is to be contrasted with Figure 4a is the streaming direction of the sheath-like population with respect to the magnetic field. In Figure 4b, they are seen to drift at ~200 km/s anti-parallel to the field line. What we see instead in the parallel flowing hemisphere is the heat flux of magnetospheric ions, whose energy is too high to be interpreted as heated magnetosheath component. The unit vectors given on the lower-left read that the magnetic field is not stretched tailward, but is in line with the magnetospheric configuration expected in the dawn-dayside part, slightly south of the equator, and that this field line is convecting tailward. The sheath-like population travels anti-parallel to this field line in the dusk-south direction.

DISCUSSION

Let us proceed to a modeling of the outer-LLBL formation. Two major frameworks that describe the dynamics of the LLBL are the viscous model and the reconnection model. Here we discuss which model is the one that the present observations are in favor of. If we take a fluid-like description, it is not possible to make the judgement. In the viscous model, the magnetosheath ions diffuse into the magnetosphere to form the LLBL. The closed field lines are pulled most at the equator (e.g., Phan et al., 1989), resulting in a tailward deflected configuration, consistent with the observation. The evidence of depleted energetic electrons is too weak to reject the closed field of the viscous model, for some enhanced wave activities may account for them. If we proceed to the ion distribution characteristics, however, they are likely to be in favor of the reconnection model.

In the first outer-LLBL around MLT= 7-8, the field line is stretched tailward, and is emptied of energetic electrons. These features can be understood if we take the field line to have been reconnected and opened, and is draped over the dayside magnetosphere. In this picture, the magnetopause current layer is encountered by back-tracing the field line in the sunward direction. The ion distribution in Figure 4a, obtained in the first outer-LLBL, supports this idea by the following consideration of the three components identified in the previous section: The slowly streaming protons are the magnetosheath component that has been unaffected by reconnection. The proton and the O+ ion beams of the same VB ~150 km/s are those that have experienced acceleration in the magnetopause current layer. Theoretically (e.g., Cowley, 1982; Fujimoto and Nakamura, 1994), ions streaming from the current layer along the field line are expected to have field aligned velocities ~ 2 VA (VA calculated on the basis of the magnetic field component subject to reconnection) irrespective of their mass. The beam distributions fit into this picture by taking VA = 75 km/s, which is not unreasonable (Assuming 10/cc plasma density, the reconnecting component is estimated to be 20 nT).

One item that is implicitly assumed in the above consideration is that the O+ ions originates from a cold population. With a large thermal velocity of the parent distribution, a clear beam shape as observed will not result. Another quantitative test of the reconnection model concerns with the amount of this cold O+. The beam interpreted to be composed of O+ ions has an apparent peak phase space density of ~10-12.5s3/m 6 (Figure 4a), which, multiplying by 162, is translated to a true value of ~10 -l0s3/m6.  Let us consider that the current layer acceleration transfers the peak in the parent distribution to that of the beam, with some velocity space scattering denoted by the coefficient .

By assuming that the parent distribution is a Maxwellian, its density can be estimated by


Here, n is the density in (/cc) unit, and T is the temperature of the parent distribution in keV.

Substituting fbeam,peak = 10-10 with a tentative choice of = 0.1, the above equation becomes

This equation reads that, as long as the temperature is lower than 0.1 keV, a cold parent distribution having density less than 0.25/cc is enough to explain the O+ ions in the LLBL.  Such cold O+ ions are actually detected in the dayside magnetosphere (Peterson et al., 1982).

The ion distribution in Figure 4b is taken in the second outer-LLBL around MLT=9. The magnetic field is deflected tailward but is directed weakly sunward, dawnward, and northward, in line with the magnetospheric configuration at dawn-dayside, slightly south of the equator. This magnetic field data can be reconciled with the idea of reconnection by locating the site of reconnection at a position north of the spacecraft, so that the field lines that GEOTAIL surveys has been a part of the magnetosphere, and by further assuming that the magnetosheath flow still has not yet fully stretched the field lines. The distribution shown in Figure 4b supports this idea. The sheath component is detected to drift down the tailward convecting field line, consistent with the idea that they have emanated from the reconnection site located northward of the spacecraft. The magnitude of the drift velocity (200 km/s) is again not unreasonable. (The reason for the absence of corresponding O+ ions is open). Energetic ions are detected only in the field aligned flowing hemisphere forming a heat flux. These are likely to be the magnetospheric ions that are being lost along the open field line. The fact that several keV ions, whose bounce periods are a few minutes, are not totally emptied in Figure 4b may indicate that the time of the observation is only a few minutes after the onset of reconnection. This is, however, not necessarily true, for the ions may be continuously supplied by cross-field drifts from the neighboring closed field lines.

The unbalanced bi-directional thermal electrons detected in this second outer-LLBL may be explained as follows: The magnetospheric part subject to reconnection had been the inner-LLBL filled with (balanced) bi-directional thermal electrons. Assuming a process of partially reflecting the electrons at the current layer located north of the spacecraft, electrons are envisaged to be quasi-trapped between the magnetopause current sheet and a low-altitude part in the southern hemisphere after the onset of reconnection.

To summarize, when the fluid parameter data and the particle data are coupled together, they enable us to identify the formation mechanism of the outer-LLBL. It is revealed that a reconnection site located somewhere sunward and duskward of the spacecraft is responsible for the first outer-LLBL, and that located somewhere northward is forming the second outer-LLBL. While the distances to the reconnection sites are not directly known, it would be worth noting that they may fall into the north-dawn/south-dusk quadrants of the dayside magnetosphere, where previous studies suggest that reconnection is most likely when IMF By< 0 (e.g., Crooker, 1979; Hones, 1984). See also Gosling et al. (1990b) for a result against).

ACKNOWLEDGMENTS

M. F. thanks T. D. Phan, R. A. Treumann, T. Terasawa, and K. Maezawa for their useful comments. The key parameter data of IMP-8 were provided by the NASA/GSFC data processing team.

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