Geospace Concussion: Global reversal of ionospheric vertical plasma drift in response to a sudden commencement

Abstract

An interplanetary (IP) shock or a discontinuity in the solar wind can cause a sudden commencement (SC) in ground magnetic perturbations. It is also sometimes called sudden impulse or sudden storm commencement if followed by a geomagnetic storm (Joselyn & Tsurutani, 1990), hereinafter referred to as SC in this paper. Araki (1994) proposed a physical model to characterize the ground magnetic response during an SC. After the impinging of an IP shock, the enhanced magnetopause current produces a step-wise increase in the magnetic horizontal component dominant at low latitudes. In the magnetosphere, two pairs of field-aligned currents (FACs) with opposite sense and induced electric fields are formed after the compression of the magnetosphere by the IP shock. The FACs and associated ionospheric currents produce a two-pulse signature in ground magnetic perturbations-a preliminary impulse (PI) followed by a main impulse (MI).