Are high-intensity long-duration continuous AE activity (HILDCAA) events substorm expansion events?

High-intensity, long-duration, continuous AE activity (HILDCAA) are magnetospheric/ionospheric events that occur during high-speed solar wind streams. The AE increases are caused by intermittent magnetic reconnection between southward components of interplanetary Alfvén wave fluctuations and magnetopause magnetic fields. During solar minimum, corotating interaction regions (CIRs) created by corotating stream interactions with slow-speed streams cause relatively short duration moderate to minor magnetic storms. These storms are followed by lengthy (days to weeks) of HILDCAA intervals characterized by low levels of enhanced ring current activity ( D ST). Shorter HILDCAA intervals are also noted following interplanetary coronal mass ejections (ICME)-related storm events. Two intervals were chosen to study in detail using POLAR UVI imaging data to identify substorms. The first was an interval not associated with a storm. The second was a short interval following an ICME-related magnetic storm. Although substorms were detected during the HILDCAA intervals, there was little or no relationship between substorm occurrences and AE (−AL) increases. One possible explanation is that prompt penetration of interplanetary electric fields associated with magnetic reconnection lead to enhanced ionospheric westward electrojet current densities (−AL increases). These same dawn-to-dusk electric fields could lead to the convection of nightside outer magnetospheric/plasmasheet plasma, causing particle injection and D ST decreases. Substorms must be caused by other processes, such as self-organized criticality.