Outflowing ionospheric oxygen-ion motion in a reconfigurating magnetosphere

Abstract. During substorms, large-scale changes of the topology of the Earth's magnetosphere following the variation of the characteristics of the interplanetary medium are accompanied by the induction of the electric field. In this study a model of a time-dependent magnetosphere is constructed and the large-scale features of the induced electric field are described. Local-time sectors with upward or downward field-aligned component and with intense perpendicular component of the electric field are distinguished. The electric-field structure implies the existence of outflow regions particularly effective in ion energization. With the vector potential adopted in the study, the region from which the most energized ions originate is defined by the local-time sector near 2100 MLT and latitude zone near 71^° MLAT. The motion of ionospheric oxygen ions of energy 0.3–3 keV is investigated during a 5-min reconfiguration event when the tail-like magnetospheric field relaxes to the dipole-like field. As the characteristics of plasma in the regions near the equatorial plane affect the substorm evolution, the energy, pitch angle, and the magnetic moment of ions in these regions are analyzed. These quantities depend on the initial energy and pitch angle of the ion and on the magnetic and electric field it encounters on its way. With the vector potential adopted, the energy attained in the equatorial regions can reach hundreds of keV. Three regimes of magnetic-moment changes are identified: adiabatic, oscillating, and monotonous, depending on the ion initial energy and pitch angle and on the magnetic- and electric-field spatial and temporal scales. The implications for the global substorm dynamics are discussed