ON THE ORIGIN OF FLUX DROPOUTS NEAR GEOSYNCHRONOUS ORBIT DURING THE GROWTH-PHASE OF SUBSTORMS .1. BETATRON EFFECTS

We investigate the origin of the flux dropouts which are quasi-systema tically observed during the growth phase of substorms near the earthwa rd edge of the plasma sheet in the midnight sector. We focus on the tr ansport of energetic (50-200 keV) near-equatorially trapped ions using single-particle trajectory codes with time-varying electric and magne tic fields. We show that energetic ions which drift against the electr ic field induced by the stretching of the inner tail magnetospheric fi eld lines experience large betatron decelerations. By virtue of the Li ouville theorem, these decelerations lead to prominent decrease of flu x for energetic particles drifting around the Earth close to the synch ronous orbit. The magnitudes of the computed flux variations are compa rable to those measured near the equatorial plane at 6.6 R(E). On this basis, we argue that betatron deceleration in the presence of an indu ctive electric field is a potential mechanism for the development of e nergetic particle flux dropouts. We show that this mechanism depends l ittle upon the field variation model adopted. It reproduces well the e ssential features of flux variations during substorm growth phases for both ions and electrons.