The interplanetary causes of magnetic storms, HILDCAAs and viscous interaction

Abstract

A review of the interplanetary causes of geomagnetic activity is presented. Intense southward interplanetary magnetic fields in the sheath region ahead of fast interplanetary manifestations of solar CMEs (ICMEs), and the intrinsically high BZ fields of magnetic clouds within ICMEs, are the two most predominant causes of major storms with DST ≤−100 nT. This is true during solar maximum when ICMEs dominate the interplanetary medium and also during the declining phase of the solar cycle when corotating streams and proto-corotating interaction regions (PCIRs) are the dominant large scale structures. PCIRs are high magnetic field regions caused by the interaction of coronal hole high-speed streams with the upstream slow speed streams. PCIRs cause only moderate to weak magnetic storms (rarely storms with DST 〈-−100 nT) because of the highly variable Bz structure within them. It is thought that the Bz fluctuations within the PCIR are compressed high-speed stream Alfvén waves. The Bz fluctuations associated with nonlinear Alfvén waves within the high-speed streams cause continuous auroral activity called HILDCAAs. These HILDCAA events lead to annual AE averages that are sometimes higher during the solar cycle descending phase (such as in 1974) than during solar maximum (1979 or 1981). We quantify an upper limit of the efficiency of viscous interaction energy input into the magnetosphere: 1 to 3 × 10−3 of the solar wind ram energy. This is in contrast to an efficiency of 5 to 10 × 10−2 for magnetic reconnection during substorms and magnetic storms. Finally, a specific mechanism of viscous interaction is explored: low latitude boundary layer (LLBL) resonant wave-particle interactions. The waves are sufficiently intense to cross-field diffuse magnetosheath plasma onto closed field lines to create the LLBL. Pitch angle scattering will lead to auroral energy deposition of ∼ 1 erg cm−2 s−, sufficient for the creation of the dayside aurora.