Physical significance of inhomogeneities in polar cap absorption events

Abstract

Polar cap absorption (PCA) events are caused by the bombardment of the atmosphere over the polar caps by solar flare cosmic rays. The morphology of PCA events is controlled in large part by the configuration of the magnetospheric tail. In this paper, it is argued that the tail must be longer than about 1 AU in order to account for observed spatial and time inhomogeneities for PCA events due to low-energy (∼5 Mev) protons. A magnetosphere whose polar cap field lines are directly connected to the interplanetary field should result in rather homogeneous PCA events with irregularities and inhomogeneities lasting only a few minutes. For a magnetospheric model in which there is a magnetospheric tail longer than about 1 AU and little merging between geomagnetic and interplanetary magnetic field lines, the low-energy (∼5 Mev) PCA events should take several hours to become homogeneous over the polar caps. It is concluded that available experimental data support the long magnetospheric tail in which there is negligible direct connection between geomagnetic and interplanetary fields.