Ground‐based and satellite observations of traveling magnetospheric convection twin vortices

Abstract

Close inspection of observations from the International Magnetospheric Study Scandinavian Magnetometer Array revealed the existence of a peculiar type of short‐period magnetic variations. In typical cases it can be described as a positive deflection of the geomagnetic D component from the quiet time level with the disturbance of the H component approximately representing the negative time derivative of the D component. Thus these events are similar to magnetic variations recently reported by Lanzerotti et al. (1986) and discussed in terms of possible signatures of flux transfer events as well as to observations by Friis‐Christensen et al. (1988) in connection with readjustments of the magnetopause to a sudden change in solar wind dynamic pressure. The typical duration of such a transient magnetic perturbation is about 10 min, with amplitudes of about 20 nT. Between 1975 and 1979 about 80 of these events have been detected, with a sharp occurrence maximum at magnetic forenoon. Most events were observed during moderately quiet times (KP≃2). A more detailed analysis of two such events exhibits a two‐cell ground‐equivalent current vortex structure. Simultaneous magnetic field observations from GEOS 2 for one of these events support our interpretation of these events as representing pairs of downward (in the west) and upward (in the east) field‐aligned currents in the magnetosphere, moving over the observer from the dayside to the nightside. The physical mechanism generating these traveling magnetospheric convection twin vortices and the associated field‐aligned currents is unclear. Flux transfer events, however, are not necessarily associated with the observed short‐period transient magnetic field variations.