Solar Wind‐Magnetosphere Coupling During High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA)

Abstract

High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA) intervals are driven by High Speed solar wind Streams (HSSs) during which the rapidly‐varying interplanetary magnetic field (IMF) produces high but intermittent dayside reconnection rates. This results in several days of large, quasi‐periodic enhancements in the auroral electrojet (AE) index. There has been debate over whether the enhancements in AE are produced by substorms or whether HILDCAAs represent a distinct class of magnetospheric dynamics. We investigate 16 HILDCAA events using the expanding/contracting polar cap model as a framework to understand the magnetospheric dynamics occurring during HSSs. Each HILDCAA onset shows variations in open magnetic flux, dayside and nightside reconnection rates, the cross‐polar cap potential, and AL that are characteristic of substorms. The enhancements in AE are produced by activity in the pre‐midnight sector, which is the typical substorm onset region. The periodicities present in the intermittent IMF determine the exact nature of the activity, producing a range of behaviors from a sequence of isolated substorms, through substorms which merge into one‐another, to almost continuous geomagnetic activity. The magnitude of magnetic fluctuations, dB/dt, in the pre‐midnight sector during HSSs is sufficient to produce a significant risk of Geomagnetically Induced Currents.

Rapid geomagnetic variations during HSS/SIR, ICME sheath and magnetic cloud-driven geomagnetic storms

“…Kim et al, 2008). Be that as it may, Milan, Mooney, et al (2023) studied HSS/SIR with HILDCAA events and also observed spikes in the pre-midnight region and stated that this type of activity is sufficient to pose a significant risk of GICs. This is in agreement with Engebretson et al (2024) observation that extreme geomagnetic disturbances in Arctic Canada at latitudes between 65°and 70°520 were associated with high-speed solar wind streams and occurred during the declining phase of the solar cycle.…”

Universal Time Effects on Substorm Growth Phases and Onsets

“…This could therefore offer an explanation of the anti‐correlation of N o and sunspot number. However, this may also be convolved with the effect of high‐speed solar wind streams that are more common in the declining and minimum phase of the cycle and have been observed to generate High‐Intensity Long‐Duration Continuous AE Activity intervals in which substorm‐like features merge into an interval of continuous substorm‐like activity (Milan et al., 2023; Tsurutani et al., 2011): this could give more onsets at these cycle phases, as defined by the FEA and N&G algorithms.…”

Time Sequence of Magnetospheric Responses to a Southward IMF Turning

“…(2021) suggested loading, then onset of tail reconnection accompanied by formation of substorm current wedge, and then tail reconnection can continue like a stationary magnetospheric convection until dayside reconnection ceases (see also Milan et al. (2023)).…”

“…Summarizing their results, some models predict loading and unloading, that is, the accumulation and reduction of the tail magnetic flux, while other models tend to predict stationary magnetospheric convection rather than the loading-unloading cycle. Milan et al (2021) suggested loading, then onset of tail reconnection accompanied by formation of substorm current wedge, and then tail reconnection can continue like a stationary magnetospheric convection until dayside reconnection ceases (see also Milan et al (2023)).…”