Ring current ion motion in the disturbed magnetosphere with non-equipotential magnetic field lines

Abstract

Transport of ring current ions during the main phase of the geomagnetic storm is modeled. Particle trajectories are simulated by the Lorentz equation for dipole and Tsyganenko magnetic field models. The convection electric field is described by variations on the K p dependent Volland–Stern model structure in the equatorial plane. Out of that plane the electric field is assumed to be the same as in the equatorial plane at least at the low latitudes. This consideration implies the possibility of non-equipotentiality of geomagnetic field lines at least for L⩾6 during strong magnetic storms. In our modeling energetic protons, typically of several tens of keV, start on the night side at L=4 or at L=7, and move initially under gradient magnetospheric drift largely confined to the equatorial plane. However, soon after crossing the noon–night meridian, the protons rather abruptly depart from the equatorial plane and deviate towards high latitude regions. This latter motion is essentially confined to a plane perpendicular to the equator, and it is characterized by finite periodic motion. The calculations indicate a slow violation of the first adiabatic invariant at the point of ion departure from the equatorial region, with slower non-adiabatic variation later along the orbit. The greater the convection electric field, the higher is the energy of the protons participating in this off equatorial divergent flow. The more energetic ions, of hundreds of keV and higher, however, rather continue their magnetic drift around the Earth uninterruptedly and these ions form the symmetric ring current ion population. The numerical calculations described herein explicitly indicate that the perpendicular divergent ion flow can contribute to the morning–evening component of the magnetic field perturbation during magnetic storm conditions, and can result in populating the high latitude and tail regions by the energetic protons.