Interplanetary and magnetospheric electric fields during geomagnetic storms: what is more important, steady-state fields or fluctuating fields?

Most of the D-st variance during intense geomagnetic storms can be reproduced by changes in large-scale electric fields in the solar wind. Whether successive substorms (in other words, magnetospheric electric fields) play a direct role in the energization of storm-time ring current particles is the subject of continuing controversy. This short review proposes that, during magnetic storms, the quasi-steady component of the interplanetary electric fields is important in enhancing the ring current, while changes, or fluctuations, in the solar wind electric fields are responsible for initiating magnetospheric substorms. Thus, as in a "thought" experiment, if one were to control the solar wind, i.e., to generate purely steady southward interplanetary magnetic field (IMF), the result would be a geomagnetic storm during which no substorm expansions take place. (C) 2001 Elsevier Science Ltd. All rights reserved.