An intense geomagnetic storm associated with slow solar wind

Results of earlier studies on intense geomagnetic storms appear to suggest the interplanetary manifestation of fast (v = 500km/s) coronal mass ejections (CMEs) as the dominant interplanetary phenomenon causing intense magnetic storms. However, the intense storm of April 1-2, 1973 appears to be due to a slow solar wind. Presently a study has been made of the April 1-2, 1973 geomagnetic storm in order to identify the probable roles played by the various solar and interplanetary factors in the generation of major geomagnetic storms. The storm is summarized using the low-latitude magnetic index, Dst and is interpreted using available interplanetary data: proton number density, solar wind flow speed, plasma temperature, interplanetary magnetic field southward component Bz, plasma beta and dawn-dusk electric field. Our results show the magnetic storm is a double step event: a moderate-to- weak storm which occurred during the period 20:00 UT March 31- 13:00 UT April 1 and is due to the weakly compressed magnetic fields in the sheath, and the April 1-2 intense storm which is due a magnetic cloud. Furthermore, the present results show that the peak solar wind velocity occurred on April 2, a day Dst after attained its peak value of -211 nT and confirms the results of Sawyer and Haurwitz (1976) that geomagnetic activity is highest on the day preceding peak velocity in the high-speed stream. Keywords: solar wind, Interplanetary Magnetic Field (IMF), coronal mass ejections (CME), low-latitude magnetic index (Dst), geomagnetic storm, Plasma beta Nigerian Journal of Physics Vol. 17, 2005: 27-3