Guan Le and C. T. Russell
Institute of Geophysics and Planetary Physics
University of California, Los Angeles

J. T. Gosling and M. F. Thomsen
Los Alamos National Laboratory

The boundary layer is a plasma transition region located immediately earthward of the magnetopause, that is populated by plasmas from both the magnetosheath and the magnetosphere.

The boundary layer has been found to be present everywhere along the magnetopause and for both southward and northward IMF.

For southward IMF, dayside reconnection produces a boundary layer by direct entry across the open portion of the magnetopause. The boundary layer is on open field lines.

The boundary layer for northward IMF is poorly understood. Proposed entry mechanisms include:

Dungey [1963] was the first one to note that reconnection can occur at points poleward to the cusps when the IMF is northward. Figure 1(not available yet) shows Dungey's illustration for reconnection for both southward and northward IMF.

We will show in detail observations of plasma characteristics throughout the magnetopause transition region for two ISEE crossings to provide evidence for formation of LLBL by cusp reconnection for strongly northward IMF. Figure 2 shows the spacecraft trajectory for the two events.

Datasets in This Study:

ISEE 1 and 2 magnetometer data.

ISEE 2 Fast Plasma Experiment (FPE). ISEE 2 FPE provides:

Figures 3a-g show the inbound magnetopause crossing on October 27, 1977. For this crossing, the spin axis of ISEE 2 spacecraft was tilted along the y-axis and the 2D FPE measured the plasma distribution function in the XZ plane. This magnetopause crossing occurs around 19.7 deg N and 13.5 deg E in GSM coordinates. The IMF is strongly northward.

In Figure 3a the magnetic field data from ISEE2 are displayed in the upper panel in GSM coordinates. The corresponding plasma moment data from FPE are shown in the bottom panel, including ion density, 2D velocity and temperature. Figure 3b shows the energy spectra from FPE for 4 sectors of instrument look angles. In Figure 3a and 3b, the boundary layer occurs from ~ 0548 to 0559 UT (between the two solid lines). Outside the boundary layer, there is a strong density depletion layer. The dashed line separates the inner and outer boundary layer.

In the outer boundary layer, the plasma density is similar to the value of the neighboring magnetosheath. Comparing the ion distribution functions taken in the magnetosheath and outer boundary layer ( Figure 3c and Figure 3d), it is evident that the plasma in the outer BL is hotter than the sheath plasma. But the higher energy portion of outer BL plasma is not the hot magnetospheric plasma since the two distribution functions are markedly different, as shown in the slices of distribution functions in Figure 3d. It is clear that the plasma bulk flow is accelerated in the outer boundary layer associated with the plasma heating. The acceleration occurs mainly in the Vz component, or along the field-aligned direction. Thus, the heated plasma must come from elsewhere along the magnetic field lines.

Figure 3e shows that the inner BL consists of plasmas from the magnetosheath (velocity < 800 km/s) and the magnetosphere (velocity > 800 km/s). The ion distribution function in the inner BL is nearly identical to that of the trapped hot magnetospheric plasma for velocity > 800 km/s, indicating that inner BL is on closed field lines. The moment data in Figure 3a shows that the inner boundary layer is steady and uniform. The comparison of distribution functions taken in the outer edge, middle and inner edge of the inner BL ( Figure 3f and Figure 3g) confirms it.

Figures 4a-e show the outbound magnetopause crossing on July 19, 1978. The IMF is strongly northward. This crossing occurs around 23 deg N and 41 deg E in GSM coordinates. For this crossing, the FPE measured the 2-D distribution functions in XY plane.

In Figure 4a the magnetic field data from ISEE2 are displayed in the top panel in GSM coordinates. The corresponding plasma moment data from FPE are shown in the bottom panel, including ion density, 2D velocity and temperature. Figure 4b shows the energy spectra from FPE for four sectors of look angles.

From Figure 4a and Figure 4b, it is evident that the transition from magnetospheric to magnetosheath plasma, or the boundary layer, occurs over a wide range from ~ 2032 UT to 2328 UT (between the two solid lines). The magnetic field transition occurs mainly in the outer boundary layer, from ~2241 UT (the dashed line) to 2328 UT. After 2328 UT, there is a strong density depletion layer immediately sunward of the boundary layer.

Figure 4c shows contours of ion distribution functions in the magnetosphere, inner boundary layer, outer boundary layer, pulses in the outer boundary layer and the magnetosheath, respectively. In the outer BL, the bulk of the plasma is hotter than the sheath plasma. Figure 4d compares its distribution functions to that in the magnetosphere showing that the hot magnetospheric particles are not the source since their spectra are different. Accelerated bulk flows are associated with the heated outer BL plasma. Similar to the previous case, the accelerated flows are mainly in the field- aligned direction (along Y direction in this case). They must be heated sheath plasma entered the outer BL along the magnetic field lines.

In the inner BL, we see mixture of two populations. In the distribution function taken in the inner BL, both cold (velocity < 800 km/s) and hot (velocity > 800 km/s) components are present. By comparing slices of distribution functions in the magnetosphere and in the inner BL ( Figure 4e), the hot component is found to consist of trapped magnetospheric particles. The source of the cold component must be from the magnetosheath since it is absent in the magnetosphere. The energy spectra in Figure 4b show that this cold component is an extension of outer BL plasma. The inner boundary layer is clearly on closed field lines.

The observed plasma characteristics in the outer and inner boundary layers can be explained by the process of magnetic reconnection poleward of cusp regions between sheath and lobe field lines for northward IMF. Figure 5 illustrates this process. When the magnetic reconnection occurs first poleward of one cusp and later poleward of another cusp, and so on, four types of magnetic field lines are present near the magnetopause as shown in panel (d).

Outside the magnetopause, the sheath field lines (red), that are convected into the magnetosheath from the solar wind, have two ends in the solar wind. Plasmas on these field lines are cold and dense. On the other hand, the magnetospheric field lines (blue) have two ends in the ionosphere. At low latitudes, hot ring current plasmas are trapped on these field lines.

The open field lines (green) have one end in the ionosphere and one end in the solar wind. They are formed by magnetic reconnection above one cusp. The plasma on the open field lines is dominated by the magnetosheath plasma. Some sheath plasma can be heated and accelerated at the high latitude reconnection site and stream to low latitudes along the magnetic field lines. Since there is little magnetospheric plasma in the lobe near the reconnection site, no magnetospheric plasma can be found on these field lines. The open field line region is the outer boundary layer.

Earthward of the open field lines, there is a layer of closed field lines (purple) that are formed from open field lines by magnetic reconnection, first poleward of one cusp and later poleward of the other cusp. Plasmas on these field lines are mixtures of sheath plasma and hot magnetospheric plasma since the hot magnetospheric plasma can become trapped on these field lines by gradient drift. This region is the inner boundary layer.

Boundary layers are formed by reconnection poleward of cusp regions for strongly northward IMF.

The outer boundary layer contains mainly heated sheath plasma and no hot magnetospheric plasma. It is on open field lines with one end in the ionosphere and one end in the solar wind. This boundary layer is formed by reconnection between sheath and lobe field lines poleward of one cusp.

The inner boundary layer contains a mixture of sheath plasma and magnetospheric plasma. It is on closed field lines with both ends in the ionosphere, that have become closed by reconnection poleward of both cusps.

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Last modified: June 27, 1995