The interplanetary causes of magnetic storms, HILDCAAs and viscous interaction

A review of the interplanetary causes of geomagnetic activity is presented. Intense southward interplanetary magnetic fields in the sheath region ahea d of fast interplanetary manifestations of solar CMEs (ICMEs), acid the int rinsically high B-Z fields of magnetic clouds within ICMEs, are the two mos t predominant causes of major storms with D-ST less than or equal to -100 n T. This is true during solar maximum when ICMEs dominate the interplanetary medium and also during the declining phase of the solar cycle when corotat ing streams and proto-corotating interaction regions (PCIRs) are the domina nt large scale structures. PCIRs are high magnetic field regions caused by the interaction of coronal hole high-speed streams with the upstream slow s peed streams. PCIRs cause only moderate to weak magnetic storms (rarely sto rms with D-ST less than or equal to -100 nT) because of the highly variable B-Z structure within them. It is thought that the B-Z fluctuations within the PCIR are compressed high-speed stream Alfven waves. The B-Z fluctuation s associated with nonlinear Alfven waves within the high-speed streams caus e continuous auroral activity called HILDCAAs. These HILDCAA events lead to annual AE averages that are sometimes higher during the solar cycle descen ding phase (such as in 1974) than during solar maximum (1979 or 1981). We q uantify an upper limit of the efficiency of viscous interaction energy inpu t into the magnetosphere: 1 to 3 x 10(-3) of the solar wind ram energy. Thi s is in contrast to an efficiency of 5 to 10 x 10(-2) for magnetic reconnec tion during substorms and magnetic storms. Finally, a specific mechanism of viscous interaction is explored: low latitude boundary layer (LLBL) resona nt wave-particle interactions. The waves are sufficiently intense to cross- field diffuse magnetosheath plasma onto closed field lines to create the LL BL. Pitch angle scattering will lead to auroral energy deposition of simila r to 1 erg cm(-2) s(-1), sufficient for the creation of the dayside aurora. (C) 1998 Elsevier Science Ltd. All rights reserved.