T. Qvick, L. Holappa and V. Filppa (ReSoLVE Center of Excellence, Space Climate Research Unit, University of Oulu, Finland)
The 20th century marks a period of exceptionally high solar activity, now termed the Grand Modern Maximum. Sunspot activity increased from a low level at the beginning of the century to a maximum during the solar cycle 19, then settled to a slightly lower level during cycle 20-23, and reduced to a significantly lower level during the soon-ending cycle 24. The Sun has two forms of geo-effective variability that are the main drivers of both geomagnetic activity and geomagnetic storms: coronal mass ejections (CME) and high-speed solar wind streams (and related corotating interaction regions; HSS/CIR). The occurrence frequency and heliolatitudinal distribution of these drivers vary fairly systematically over solar cycle, which is determined by the changes in the generation of new magnetic flux on solar surface and by the global structure of the solar magnetic field. CMEs closely follow sunspot activity, while HSS/CIRs arise from large coronal holes, which typically maximize in the declining phase of the solar cycle. CMEs cause most of the large storms, but HSSs produce most of geomagnetic activity in most years. New methods have been developed to extract detailed information about the relative contribution and long-term occurrence of these two drivers from the global distribution of geomagnetic activity. The dramatic variation in geomagnetic activity during the GMM has been known since the inaugural work by P. Mayaud. This variation has recently been used to obtain new information on the evolution of the solar magnetic field and coronal holes over the GMM. Less is known about the long-term evolution of geomagnetic storms. Using the extended Dst index and its local versions at the contributing stations, we can study the evolution of the properties of geomagnetic storms throughout the GMM. We determine the temporal occurrence of storms of different intensity (weak, moderate, intense and superstorms) and study the variation of their relative occurrence fractions over the GMM. We estimate the role of the two main drivers as a cause to storms of each intensity and use the local indices to study the historical asymmetry of the ring current. We also compare and combine the information obtained about the evolution of solar magnetic fields, in particular CMEs and solar coronal holes (HSS/CIRs) from these studies of long-term geomagnetic activity and geomagnetic storms.
Mode of presentation: oral (Need to be confirmed by the SOC)