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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.

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This paper studies the probable roles of interplanetary and geomagnetic parameters in the generation of ‘intense’ and ‘very intense’ magnetic storms as well as the correlation between magnetic field B and flow speed V , southward turning of Bz ( B s ) and B s duration B T . 18 storm events were observed and for analysis were divided into two sections. This include 8 ‘intense’ magnetic storm (−250 nT ≤ peak D st < −100 nT) events and 10 ‘Very intense’ storms (peak D st < −250 nT); both spanning January 1976 until May 2005. From our analysis, it was observed that the interplanetary magnetic field B z plays a prominent role alongside D st in the generation of intense storms. So also is the interplanetary electric field associated with high-speed streams and the solar wind density N p in the ring current intensification. The result further shows that over 67% of the storm events under investigation are generated from magnetic clouds which are characterized by a low beta plasma, high IMF magnitude and large scale coherent field rotations often including large and steady north-south components.As regards the geoeffectiveness of the flow speed V , the B s and B s interval ( B T ) with the magnetic field B , it was observed that generally for all the selected storm events, the flow speed is the most correlated, showing a correlation coefficient of 50.9% with B, and hence the most geoeffective. However, the statistical significance of its correlation with B is not so directly implying a higher substorm occurrence during the faster solar wind, but that it is one of the causes of substorm occurrence at a value faster than 400 km/s. Furthermore, the result shows that ‘very intense’ storms whose main feature is a plasma flow speed greater than 550 km/s has a negligible or very low correlation between the flow speed and the magnetic field B ; whereas, ‘intense’ storms have a 58.7% correlation between the two parameters. Lastly, it could be argued that all ‘very intense’ storms are likely to have a plasma flow speed greater than 550 km/s within the storm interval, but not all flow speed greater than 550 km/s are ‘very intense’ storms.

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