Magnetic clouds and the quiet-storm effect at Earth

Abstract

In this review, we discuss first magnetic clouds in the context of other interplanetary causes of geomagnetic storms. We then describe work on the global field line topology of magnetic clouds, focussing on information gained by the use of energetic particles. We then give a summary of theoretical and simulation work on the dynamics of magnetic clouds. In one approach, based on self-similar evolution of radially expanding magnetic flux ropes, the role of electrons is central. A section on the boundaries of magnetic clouds is followed by one on magnetic field line draping around these ejecta, including the formation of a magnetic barrier. In the aspect of the study dealing with the geomagnetic response to magnetic clouds, we discuss effects on the dayside magnetosheath; ionosphere; and nightside magnetosphere at geostationary orbit and beyond, utilizing primarily observations made during Earth's encounter with a magnetic cloud on January, 13 - 14, 1988. A case study is mentioned where solar energetic particles, injected into a magnetic cloud and then guided along its helical field lines, entered the magnetosphere through interconnection of the cloud's field lines with those of Earth. Simulation work on the geomagnetic response to magnetic clouds is briefly reviewed. We finally consider studies specifically correlating magnetic clouds, in isolation or as part of compound streams, with geomagnetic storm activity. Throughout, we indicate areas where further work is needed.