Excitation of Polar Substorms by Northward Interplanetary Magnetic Field
It has been generally recognized that the activity of the polar substorm is strongly influenced by the N-S component B z of the interplanetary magnetic field. The relation between B z and the substorm activity is usually expressed as’ substorms occur when B z is southward’ or’ substorms follow north-to-south changes in B z ’. However, we can find cases of substorms during intervals of the northward B z , and these cases appear to form exceptions if the relation is expressed as the above. In order to resolve this difficulty some suggestions have been made. The first is that substorms may appear to result from the northward B z because of the time delay between B z and the corresponding substorm activity, but the time delay has been found to be 1 to 2 hr and is not likely to explain the occurrence of substorms during intervals of prolonged (5 hr or more) northward B z.The other suggestion has been that interplanetary parameters other than B z may also be important, but we have not yet seen convincing evidence, either theoretical or observational, in favor of that view.
KeywordsInterplanetary Magnetic Field Plasma Sheet Magnetic Field Data Reconnection Model Substorm Activity
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