Highly Geo-effective Solar Transients and Their Associated Geomagnetic Activities

Abstract

The solar Cycle 23 has shown some peculiar features, i.e. slow and prolonged decline phase. When it is combined with the ascending phase of Cycle 24, it provides a long phase during which the overall magnetic activity was very low. During this interval the average sunspot number appeared on the solar disk was very low and signified the weak polar magnetic fields, and solar wind streams mainly originating from coronal holes. The study investigated the relationship between magnetic structure of coronal holes and/or coronal mass ejection (CME) source region and their influence on Earth's geomagnetic field, i.e. storms and sub storms. The study is performed mainly considering very intense geomagnetic storms that occurred during Solar Cycle 23. The disturbance storm time index Dst is taken as an indicator of geomagnetic activity by setting a value of Dstmin ≤ −200 nT as threshold. By examining halo CMEs that erupted between 2000 and 2008, we selected 07 events associated with M-class and X-class solar flares. Furthermore, as the geomagnetic field (BGeomag) puts a lower cutoff rigidity (Rc) to the entry of cosmic particles in to the earth's atmosphere. It depends upon the nature of geomagnetic activity and also upon the reconnection of field with the plasma regions formed due to the solar ejections. Sometimes when this entry of charged particles exhibits very sudden sharp and short lived increases in cosmic ray intensities, registered by the neutron monitor, it is termed as Ground-level enhancement (GLEs). These enhancements are known to take place during the result of powerful solar eruption. In present investigation GLE event associated with solar flare and coronal mass ejection (CME) occurred during the study period are also studied. The spacecraft data acquired by STEREO mission and those provided by SOHO, ACE and geomagnetic data from the geomagnetic stations like WDC-Kyoto are utilized in the study. It is found that the GLE's are well associated to X-class solar flares. The average speed of GLE associated CME was much faster than the average speed of non GLE associated CMEs.