Collisional Interaction of the Ring Current and the Plasmasphere

In the region of overlap between the ring current and the plasmasphere, two plasma populations interact with each other via multiple processes, such as wave-particle interactions and Coulomb collisions. Energy of the ring current ions is a source of free energy to excite plasma waves. This energy, in turn, will be redistributed amongst the thermal and energetic populations as the plasma waves undergo damping. Energy contained in the ring current can also be transferred to the plasmasphere through Coulomb collisions. In the following, our previous results on the coupling between the ring current ions and the plasmasphere through Coulomb collisions are presented.

In solving the kinetic equation (1), the method of fractional step is used to decompose this drift-loss model and only one process is solved at each fractional time step. The instantaneous heating rate from the ring current to the plasmaspheric species p is given by

(5)

where Dfa is the change of fa in Dt due to Coulomb collisions with species p. We calculate the volume heating rate to the thermal ions in the plasmasphere during the recovery phase of a modeled major storm similar to that which occurred in early February 1986 [Fok et al., 1995b]. Plate 2 shows the volume heating rate to the thermal ions at the equator (left panels) and the noon-midnight meridian (right panels) at (a) 2 and (b) 52 hours after the start of the recovery phase, together with the corresponding Kp values. At 2 hours, the maximum heating rate is on the order of 1 eVcm-3s-1 and is located at L shell of 2 to 2.5, with local time extending from midnight to dawn. The region of high heating rate corresponds to the location of peak density of low-energy (< 10 keV) ring current ions, which provide the main contribution to the plasmaspheric heating. This region of high heating rate drifts eastward and reaches noon in the next few hours. The isocontours of heating rate at L shells larger than 3 roughly follow the thermal density calculated from the model of Rasmussen et al. [1993]. The plasmasphere bulge located between noon and dusk during this active period can be inferred in the equatorial view of Plate 2a. The meridian view shows that the heating rate peaks at the equator near the inner edge of the ring current and is fairly uniform along field lines at high L shells. The localized heating near the Earth is a consequence of ring current ions which have an anisotropic pitch-angle distribution (peaks at 90°) caused by strong charge exchange loss at low L shells.

In late recovery (Plate 2b), isocontours of heating rate expand and fall off with L smoothly as a result of the refilling of the plasmasphere during the storm recovery. The high heating rate (on the order of 1 eVcm-3s-1), which is seen at L < 2.5 at 2 hours, is diminished due to the charge exchange losses of low-energy (< 10 keV) ions at that location. In contrast, the heating rates at high L's are higher at late recovery than at early recovery. In the meridian view, the peak heating rate at the equator is more pronounced and extends to higher L shells as a consequence of strong ring current pitch-angle anisotropy during late recovery of the storm [Fok et al., 1995a].

Plate 2. Plasmaspheric ion volume heating rate (eVcm-3s-1) from the ring current at the equator (left) and the noon-midnight meridian (right) at (a) 2 hours and (b) 52 hours after the main phase of the model storm.

Plasmaspheric Response to the Heating from the Ring Current

In order to see the plasmaspheric response to the heating from the energetic ions, the thermal ion temperatures are calculated using the heat source generated from our model for the February 1986 storm as input to the FLIP model. The volume heating rate calculated from our model is scaled by the ratio of plasmaspheric densities obtained from the model of Rasmussen et al. [1993] and FLIP. We modified the standard FLIP model slightly in order to accommodate direct ion heating from the ring current source.

The altitude profiles of ion temperature at L = 4, on the morning and evening side, at different elapsed times during the storm recovery as calculated from FLIP are plotted in Figure 3. In both cases, heating from the ring current increases the ion temperature at altitudes above the heat sink due to the neutral atmosphere, about 500 km. The ion temperature is increasing throughout the recovery until t ~ 50 hours after which it decreases. There is a two- to five-fold increase in temperature compared with no heating from ring current. For comparison, the mean DE 1/RIMS H+ temperatures during October and November of 1981 are also shown. In general, heating from the ring current can more than account for the ion temperature observed. we have clearly shown that the enhanced ion temperature in the plasmasphere cannot be explained without the heating from a magnetospheric source.

Figure 3. Altitude profiles of ion temperatures at L = 4, morning (left) and evening(right) at different times relative to the start of the recovery. Mean DE 1/RIMS data in 1981 are shown with the ®.

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