Magnetosphere–ionosphere coupling: selected topics

Abstract

A broad outline is presented of what has been learned over the past decade concerning magnetosphere–ionosphere (M-I) coupling, dynamic interchanges of particles and electromagnetic energy between magnetically conjugate regions of near-Earth space. Although the concept of M-I coupling is useful for relating characteristics of distant source regions to ionospheric signatures, it is fundamentally incomplete, and must include connections to the interplanetary medium. With single satellite missions “we’ve gone about as far as we can go”. Ground-based magnetometer, optical and/or radar measurements are now routinely integrated with complementary data acquired by satellites to interpret electrodynamic signatures. Simultaneous measurements in the ionosphere and magnetosphere show that Alfvén waves at Pi 2 frequencies are important sources of M-I coupling near times of substorm onsets. The same measurements suggest that the braking of bursty bulk flow (BBF) structures is not important for triggering substorms. M-I coupling signatures of BBFs in the nightside ionosphere are compared with those of flux transfer and impulsive penetration events on the dayside. We then explore some implications of the hypothesis that BBF plasma is initially accelerated near the equatorial plane. Subsequent braking would result from electrodynamic coupling which redistributes energy and momentum to plasma in the high latitude parts of flux tubes and in the ionosphere.