ON THE VARIATION BETWEEN Dst AND IMF Bz DURING ‘INTENSE’ AND ‘VERY INTENSE’ GEOMAGNETIC STORMS

Intense magnetic storms are dominantly caused by the interplanetary manifestations of fast coronal mass ejections (CMEs); and are in two forms: the sheath region and the CME ejecta itself; both involving an intense and long duration southward interplanetary magnetic field component Bz. A study of the storm events which were divided into two parts of ‘intense’ –250 nT ≤ peak Dst < −100 nT) and ‘very intense’ (peak Dst < −250 nT) magnetic activities, each having four storm days spanning between 1976 and 2002 shows that an appreciable southward turning of Bz for a value >10 nT and long duration (>3 hour) would always cause a depression in the Dst magnitude, signifying an intense storm. The study reveals that ‘very intense’ storms are more likely to experience shock in the interplanetary magnetic field region faster than ‘intense’ storms with a plasma flow speed >400 km/s. This is because Dst plots shows that activity of storm sudden commencement (SSC) is not noticeable until about 7 hours to storm day under ‘intense’ storms, whereas, it is as much as 12 hours to storm day for ‘very intense’ activities. The Bz plots also shows that Bz southward orientation magnitude on the average for ‘very intense’ storms is not beyond −10 nT before the beginning of geomagnetic activity; with a northward orientation occurring less often with a magnitude less than 10 nT. However, a northward directed Bz appears more often before storm event in the case of ‘very intense’ storms with a magnitude that could be as high as 15 nT on the average and a southward orientation value not exceeding −10 nT. It was further observed from the Dst plot that ‘intense’ storms recover faster than the ‘very intense’ ones