Rapid decay of storm time ring current due to pitch angle scattering in curved field line

Abstract

The storm time ring current sometimes exhibits rapid decay, as suggested from the Dst index, but the underlying mechanism is unknown. By means of a simulation with pitch angle scattering due to the field line curvature (FLC), together with the charge exchange and adiabatic loss cone loss, we investigated rapid decay of the storm time ring current for the large magnetic storm that occurred on 12 August 2000. When all three loss processes were included, the Dst (SYM-H) index showed rapid recovery with an e-folding time of ∼6 h. However, without FLC scattering, the simulated Dst (SYM-H) index showed a slower recovery with an e-folding time of ∼12 h. Overall flux of energetic neutral hydrogen with energy ≥ 39 keV was significantly reduced by the FLC scattering and is consistent with data from the high energy neutral analyzer (HENA) on board the IMAGE satellite. Power of precipitating protons showed a fairly good agreement with data from the far ultraviolet (FUV) imager on board IMAGE. These fairly good agreements with observations lead to the possible conclusion that the FLC scattering is a significant loss mechanism for the ring current ions, and the main oval of the proton aurora is likely a manifestation of the precipitating loss of the protons for this particular storm.