Statistical analysis of solar wind parameters and geomagnetic indices during HILDCAA/HILDCAA∗ occurrences between 1998 and 2007

Abstract

In this paper, we investigated the interplanetary conditions during 124 less strict high-intensity, long-duration, continuous AE activity (HILDCAA∗) events between the years of 1998 and 2007. The HILDCAA∗ events were chosen by following three “traditional” criteria of high-intensity, long-duration, continuous AE activity (HILDCAA) events which are characterized with peak of AE intensities equal or greater than 1000nT; and a minimum of 2days length where AE values occur outside the main phase of geomagnetic storms. However, we include a small modification in the following criterion: “the AE values should not drop below 200nT for more than 2h at a time”. This criterion is modified by changing “2” to “4h at a time” in which the AE values should not drop below 200nT. Our results shows that the temporal distribution of HILDCAA∗ events during the solar cycle presents a pattern of double peak, where the first peak is seen around the rising phase and the maximum of the sunspot cycle 23, with the second peak in its descending phase. This kind of temporal behavior is also observed in HILDCAAs in earlier studies. After the definition of HILDCAA∗ events, a comparison of solar wind parameters and geomagnetic indices among HILDCAAs, HILDCAAs∗, and the background condition is performed using a statistical approach. It is shown that interplanetary causes of HILDCAAs and HILDCAA∗s are the same. The advantage of the usage of HILDCAA∗s is that the number of events available for study will be ∼3 times higher.