On the origin of flux dropouts near geosynchronous orbit during the growth phase of substorms: 1. Betatron effects

Abstract

We investigate the origin of the flux dropouts which are quasi‐systematically observed during the growth phase of substorms near the earthward edge of the plasma sheet in the midnight sector. We focus on the transport of energetic (50–200 keV) near‐equatorially trapped ions using single‐particle trajectory codes with time‐vary ing electric and magnetic fields. We show that energetic ions which drift against the electric field induced by the stretching of the inner tail magnetospheric field lines experience large betatron decelerations. By virtue of the Liouville theorem, these decelerations lead to prominent decrease of flux for energetic particles drifting around the Earth close to the synchronous orbit. The magnitudes of the computed flux variations are comparable to those measured near the equatorial plane at 6.6 RE. On this basis, we argue that betatron deceleration in the presence of an inductive electric field is a potential mechanism for the development of energetic particle flux dropouts. We show that this mechanism depends little upon the field variation model adopted. It reproduces well the essential features of flux variations during substorm growth phases for both ions and electrons.

The causal sequence investigation of the ring current ion-flux increasing and the magnetotail ion injection during a major storm

“…Sauvaud and Winckler (1980) further showed that this equatorward and earthward motion of the trapped outer boundary is the result of the stretching of the magnetic field lines towards a more tail-like configuration. During each substorm period when magnetic field stretching (blue arrows in Figure 8) and energetic ion-flux decline at geosynchronous altitudes (green arrows in Figure 9) occurred, this phase constituted a "growth phase dropout" (Sauvaud et al, 1996). Magnetic field dipolarization (red arrows in Figure 8) was accompanied by plasma convective flows turned Earthward (red arrows in Figure 9), and the energetic ion-flux also began to increase rapidly (blue arrows in Figure 9).…”

Effect of multiple substorms on the buildup of the ring current

“…The second major difference is that a flux dropout around geosynchronous orbit occurs for case 2 during the last interval of southward IMF as shown in Figure 8c. As the tail stretches (substorm growth phase), conservation of the third adiabatic invariant causes drift shells to expand to higher L and at the same time particles lose energy causing a flux dropout [Baker and McPherron, 1990;Delcourt and Sauvaud, 1994;Sauvaud et al, 1996;Fok et al, 1999]. Future studies will consider under what conditions flux dropouts occur in the combined model.…”

Modeling of inner plasma sheet and ring current during substorms

“…Not surprisingly, there is no significant substorm-associated flux depletion at 2200 and 0000 MLT. The plasma sheet flux dropouts and enhancements during substorms have been previously modeled using our singleparticle code [Sauvaud et al, 1996;Delcourt and Sauvaud, 1994]. representing low-energy (1-5 keV) flux, green representing medium-energy (5-40 keV) flux, and blue representing highenergy (40-300 keV) flux.…”