Energetic neutral atom observations during recurrent magnetic storms

Abstract

Recurrent storms are triggered by Earth's encounters with corotating interaction regions (CIR) in the solar wind, they occur most frequently in the declining phase of the solar cycle. CIRs are regions of strong magnetic field accompanied by high pressure. They form in the interplanetary medium by the interaction of low- and high-speed solar wind streams co-rotating with the Sun. Typically, CIRs are not fully developed at 1 AU. A forward shock is commonly lacking, and a reverse shock is absent in the majority of events. Compared to interplanetary coronal mass ejection (ICME) driven events this results in weak storms, with minimum Dst excursions that can fall short of even -50 nT. On the other hand, the recovery of Dst to background levels during high-speed stream (HSS) intervals following a CIR can take many days, much longer than for storms caused by ICMEs. Recovery is accompanied by prolonged intervals of Auroral Electroject index (AE) activity, indicating sustained levels of substorm and plasma sheet activity during Dst recovery. In this paper we present IMAGE energetic neutral atom (ENA) observations of recent CIR/high-speed stream intervals, contrasting those observations with recent findings of ENA imaging during ICME-induced storms. We discuss the presence and composition (H + and O + ) of the ring current during recurrent storms, characterize the magnetospheric activity during the prolonged recovery of Dst, and describe the overall magnetospheric convection associated with the solar wind Alfven wave activity associated with those events.