Solar cycle dependence of High‐Intensity Long‐Duration Continuous AE Activity (HILDCAA) events, relativistic electron predictors?

Abstract

AbstractHigh‐Intensity, Long‐Duration, Continuous AE Activity (HILDCAA) events are studied using long‐term geomagnetic and solar wind/interplanetary databases. We use the strict definition of a HILDCAA event, that it occurs outside of the main phase of a magnetic storm, the peak AE is >1000 nT, and the duration is at least 2 days long. One hundred thirty‐three events have been identified from the AE indices in the 1975 to 2011 interval, a ~3½ solar cycle span. Of the 133 events, 99 had simultaneous interplanetary data available. The overwhelmed majority (94%) of these latter cases were associated with high‐speed solar wind stream (HSS) events. The remaining 6% of the cases occurred after the passage of interplanetary coronal mass ejections (ICMEs). The HSS‐related events were typically associated with large interplanetary magnetic field (IMF) Bz variances. The ICME‐related events were characterized by steady southward Bz intervals or low‐frequency fluctuations, both of which we view as possible different interplanetary phenomena. HILDCAA events have been found to have their largest occurrence frequency in the solar cycle descending phase (~6.8/year) with the second largest at solar minimum (~3.5/year). The occurrence frequencies were considerably lower in the ascending phase (~2.5/year) and at solar maximum (~2.2/year). Thus, HILDCAAs can occur during all phases of the solar cycle, with the descending phase approximately three times more likely to have an event than at solar maximum and the ascending phase. The HILDCAA events that occurred in the declining phase and at solar minimum were >20% longer in duration than those in the ascending phase and solar maximum, respectively. The events during the recent solar and geomagnetic minima, 2007–2009, were, on the average, ~17% and 14% weaker in peak AE than the events during the previous two minima of 1995–1997 and 1985–1987, respectively. The recent minimum events were ~35% and 41% shorter in durations, respectively, than the events during those previous minima. The yearly occurrence of the events exhibited statistically significant correlation (>0.70) with yearly average speed and number of HSSs. No seasonal dependence of HILDCAA was noted.