3.4 Microphysical processes
This section attempts to establish a connection between the large-scale processes shown in Figures 3 - 7 and the field-aligned currents displayed in Figure 9. Figure 10 shows the window that will be used to display the data actually shown in Figure 11. The window extends from x = -3 to -16 RE and from z =0 to 8 RE. The data is displayed in Figure 11 for t = 28.8 The display is in a color-coded format. Red corresponds to higher values and blue to the lowest, or most negative, values. The time is shortly after the onset of the expansion and dipolarization. This data has not been subject to any filtering. Figure 11a shows color-coded plots of the magnetic field magnitude. Panel a shows the total magnetic field intensity. The stronger dipole field is shown in the lower right-hand corner. Wave activity seems to occur just inside the expansion boundary and the region of large pressure gradient seen in panel f of Figure 5. Panel b of Figure 11 shows variations in By. Here there is considerable variation near the equatorial plane which seems to develop into more field-aligned structure off the equatorial plane and toward the Earth. A similar pattern is seen with the y-component of the fluid flow, Vy in panel c. The By and Vy variations seem to be about 180º out of phase. Finally, panel d shows the field-aligned current, J||. This appears to have the same type of pattern as the By and Vy, except for the higher intensity toward the Earth because of the convergence of the magnetic field lines.
The type of wave activity seen here has not been reported in MHD simulations. It is therefore likely that plasma kinetic processes are involved. For this reason it is perhaps worthwhile to examine the particle velocity distribution function in selected locations. Figure 12 shows a sample of four velocity distributions taken at the locations indicated on Figure 10. The line through the velocity distribution plots shows the local geomagnetic field direction. The velocities are in units of the lobe Alfvén velocity, of 1.2 RE/sec. Panel a of Figure 12 shows the distribution on the equatorial plane well behind the expansion front at a geocentric distance of 9 RE. Here we see two relatively cold counter-streaming plasmas. Further earthward those beams tend to merge into a single cold stationary distribution Panel b shows the distribution just behind the expansion front at 11.5 RE. The distribution shows a hot, non-gyrotropic plasma imposed on a cold core. Panel c shows the distribution also at 11.5 RE but some distance off the equatorial plane. Again, we see a double-humped, or triple-humped, distribution. This is also just behind the expansion front. Panel d shows the distribution also at 11.5 RE, but a little further displaced from the equatorial plane so it is just outside expansion front. Here we see a much more stable, albeit distorted, distribution. Other velocity distribution plots at higher latitude behind the expansion front show a colder and more isotropic distribution and a much weaker hotter component.
The pattern that emerges is that the major region of wave excitation is at low latitudes just behind the expanding dipolarization front. This suggests that the wave activity responsible for the field-aligned currents is generated in this region, and the energy is propagated along field lines away from the equator. The wave energy appears in the form of shear waves, rather than compressional waves because we do not see much in the way of variations in |B| there. The variations in |B| seen in panel a of Figure 11 do suggest some magnetosonic wave activity at higher latitudes on field lines linking to the dipolarization front.