3.1 System evolution
Figures 3 -7 show the system evolution at simulation times of t = 14.4, 21.6, 28.8, 36.0 and at 50.4, where the run was stopped. Each of the contour plots shown has 11 contour intervals. The data displayed are run through a Gaussian filter with a half width of 1.5 grid points on the curvilinear grid, because in some regions, especially near the Earth, there is too much detail to be shown in the survey plots. Since the filtering is on the grid, the plots will still show a much higher resolution near the Earth. The following subsection will follow up with more detailed views.
Panel a of Figure 3 shows the field lines crossing the equator at t = 14.4. Perhaps the most significant feature is seen in comparison with Figure 2b. The field lines have sharper radius of curvature in their equatorial crossing. Panels b shows the By component and c of Figure 3 show a pair of upward and downward field line currents in each hemisphere connecting to the inner edge of the plasma sheet with the ionosphere. Viewed from the Earth, the downward current is equatorward of the upward current. Reference to panels d and e suggests that the currents are generated by the breaking of the earthward flow in the inner edge of the plasma sheet. As the plasma runs into the stronger dipole field, ions tend to penetrate somewhat deeper than the electrons. The result is a polarization, with a positive charge earthward of the negative charge. The polarization is discharged in the ionosphere to produce the downward current equatorward of the upward current. Panels d and e show greatly increased earthward plasma flow in the near-Earth plasma sheet. The scaling is relative to the maximum and minimum values shown in the particular frame and cannot be used to infer relative values from one time to the next. The maximum velocity is 0.63 RE/sec. It will also be noticed that the flow region is considerably broader than the neutral sheet region. Panel f shows the perpendicular pressure. In this figure, the maximum value contoured was clipped, so that the contours represent the same values as in the pressure plot of Figure 2. The major change is the increased pressure in a narrow region across the neutral sheet. The parallel pressure (not shown) on the other hand is considerably broader with a profile width comparable to that of the flow speed profile of panel e. The plasma density profile across the neutral sheet does not show a significant change from the initial profile.
Figure 4 shows a similar set of frames for the simulation time of 21.6. The field line trace shows a continued thinning of the neutral sheet region. It also shows the first hint of dipolarization. Panels b and c show the field-aligned currents connecting from the ionosphere to near-Earth plasma sheet. The magnitude of the field-aligned currents has about doubled from t = 14.4. The maximum flow speed has only increased slightly from the time of the previous figure. The plasma flow arrows of panel e do show something significant, namely a sharp northward and southward diversion of the flow. Finally, panel f shows a continued increase in the perpendicular pressure in the narrow central plasma sheet. The parallel pressure profile remains broad, and the density profile does not change significantly. However, there is a continuing pressure pileup where the earthward flow stagnates.
Panel a of Figure 5 shows the onset of dipolarization, as well as a continued narrowing of the neutral sheet just tailward of the dipolarization region. The By and J-parallel plots show something different. Instead of just a simple current pair seen at each hemisphere there are now multiple current filaments. Furthermore the currents can be seen to have spread poleward. The reason for this will be explored in the more detailed analysis of the next subsection. The plasma flow speed has not changed much from the previous figure. The plasma flow arrows shown in panel e now show a clear flow reversal above and below earthward flow region. It looks as if plasma is ‘splashing’ off the dipole field. It can be seen in comparison with panel a that the apparent ‘dipolarization’ is being carried by this flow reversal. The further squeezing of the plasma sheet tailward of the dipolarization region is seen in the pressure plot of panel f.
Panel a of Figure 6 shows the continued expansion of the dipolarization region. Although not easily seen, the effects of the parallel currents near the Earth have pushed to higher latitudes. The region where the flow breaks seems to continue to coincide with the dipolarization front. Tailward of the dipolarization region the region of high perpendicular pressure is squeezed into a narrow sheet. The parallel pressure profile remains similar to the flow speed profile, and the tail density still does not change much.
Figure 7 shows the configuration at the end of the run. It is more or less a continuation of the trend seen in Figure 6