Very intense geomagnetic storms and their relation to interplanetary and solar active phenomena

Abstract

We revisit previous studies in which the characteristics of the solar and terplanetary sources of intense geomagnetic storms have been discussed. In this particular analysis, using the Dst time series, we consider the very intense geomagnetic storms that occurred during Solar Cycle 23 by setting a value of Dst min 6 200 nT as threshold. After carefully examining the set of available solar and in situ observations from instruments aboard the Solar and Heliospheric Observatory (SOHO) and the Advanced Composition Explorer (ACE), complemented with data from the ground, we have identified and characterized the solar and interplanetary sources of each storm. That is to say, we determine the time, angular width, plane-of-the-sky, lateral expansion, and radial velocities of the source coronal mass ejection (CME), the type and heliographic location of the CME solar source region (including the characteristics of the sunspot groups), and the time duration of the associated flare. After this, we investigate the overall characteristics of the interplanetary (IP) main-phase storm driver, including the time arrival of the shock/disturbance at 1 AU, the type of associated IP structure/ejecta, the origin of a prolonged and enhanced southward component (Bs) of the IP field, and other characteristics related to the energy injected into the magnetosphere during the storm (i.e. the solar wind maximum convected electric field, Ey). The analyzed set consists of 20 events, some of these are complex and present two or more Dst minima that are, in general, due to consecutive solar events. The 20 storms are distributed along Solar Cycle 23 (which is a double-peak cycle) in such a way that 15% occurs during the rising phase of the cycle, 45% during both cycle maxima, and, surprisingly, 40% during the cycle descending phase. This latter set includes half of the superstorms and the only cycle extreme event. 85% of the storms are associated to full halo CMEs and 10% to partial halo events. One of the storms occurred at the time contact with SOHO was lost. The CME solar sources of all analyzed storms, but one, are active regions (ARs). The source of the remaining CME is a bipolar low-field region where a long and curved filament erupts. The ARs where the CMEs originate show, in general, high magnetic complexity; d spots are present in 74% of the ARs, 10% are formed by several bipolar sunspot groups, and only 16% present a single bipolar sunspot group. All CMEs are associated to long duration events (LDEs), exceeding 3 h in all cases, with around 75% lasting more than 5 h. The associated flares are, in general, intense events, classified as M or X in soft X-rays; only 3 of them fall in the C class, with the one happening in the bipolar low field region hardly reaching the C level. We calculate the lateral expansion velocity for most of the CMEs. The values found exceed in all cases but one the fast solar wind speed (750 km s1 ). The average lateral expansion velocity is 2400 km s1 . The spatial distribution of the solar CME sources on the solar disk shows an evident asymmetry; while there are no sources located more eastward than 12 in longitude, there are 7 events more westward than 12 . Nevertheless, the bulk of the solar sources are located near Sun center, i.e. at less than 20 in longitude or latitude. Considering the IP structures responsible for a long and enhanced Bs, we find that 35% correspond to magnetic clouds (MCs) or ICME fields, 30% to sheath fields, and 30% to combined sheath and MC or ICME fields. For only one storm the origin of Bs is related to the back compression of an ICME by a high speed stream coming from a coronal hole in the neighborhood of the corresponding CME source region. We have also found that for this particular set of storms