Excitation of Flow in the Earth’s Magnetosphere-Ionosphere System: Observations by Incoherent-Scatter Radar

Abstract

The most important consequence of the coupling processes which occur at the magnetopause boundary of the Earth’s magnetosphere and in the geomagnetic tail is the large-scale convective flow that pervades the magnetosphere-ionosphere system, which dominantly determines the structure and dynamics of both the magnetospheric and the high-latitude ionospheric plasmas. The detailed properties of these flows are, however, difficult to discern from sparse single-point observations available from high-altitude spacecraft, and most of what is known has been determined from studies of the flow at ionospheric heights. Up until the early 1980s the main source of such information was obtained from data returned by low-altitude polar-orbiting spacecraft, which provide ~10 min passes over the polar regions every ~90 min for a given hemisphere. The principal result of these studies was that the flows depend strongly on the direction of the interplanetary magnetic field (IMF), thus implying that reconnection plays a central, if not exclusive, role in their generation. By combining together the data from different orbits obtained under similar IMF conditions, the overall pattern of the flow was determined. Figure 1 provides a summary of the results, adapted from the work of Reiff and Burch [1]. The flow is represented as an IMF-dependent set of steady-state patterns, driven principally by balanced dayside and tail reconnection when IMF Bz is negative (upper row), and by steady single-lobe reconnection (which does not change the amount of open flux) when IMF Bz is positive (lower row). The additional possibility of double-lobe reconnection when the IMF points close to northward, which closes open flux on the dayside and produces a time-dependent “reversed” twin-vortical flow on both open and closed flux tubes, is not represented.