Magnetic storms caused by corotating solar wind streams

Geomagnetic activity at Earth due to corotating high speed solar wind streams are reviewed. High density plasma regions in the vicinity of the heliospheric current sheet in the slow solar wind impinge upon the magnetosphere and create magnetic storm intial phases. Dst increases can be higher than those associated with shocks in front of interplanetary coronal mass ejections. High speed streams following the high density interplanetary plasma interact with the upstream slow speed streams and create magnetic field compression regions called corotating interaction regions (CIRs). The southward components of the typically rapid Bz fluctuations within the CIRs, through sporadic magnetic reconnection with the Earth’s magnetic fields, lead to weak to moderate intensity magnetic storm main phases, typically Dst >−100 nT. Some CIRs (without southward component Bz fields) cause no perceptible Dst changes at all. The “recovery” phases of CIR-induced magnetic storms can last for a few days up to 27 days. The cause of these particularly long duration storm “recoveries” is near-continuous shallow plasma injections into the magnetosphere. Magnetic reconnection associated with the southward component of the Alfvén waves within the high speed streams with magnetopause fields is the cause of these injections. The auroras during these intervals are continuous and global auroral zone features. The AE/AL maxima are not substorms or convection bays. Relativistic electrons are accelerated/observed during these high speed stream intervals. The electrons first appear in the beginning of the lengthy storm “recovery” phases. Geomagnetic quiet is due to weak interplanetary magnetic fields with a lack of Alfvénic fluctuations. These interplanetary regions generally occur in the decay portion of high speed streams.