Investigation of isolated substorms: Generation conditions and characteristics of different phases

Abstract

Characteristics of isolated substorms selected by variations in the 1-min values of the AL index are analyzed. The substorms were divided into several types with respect to the behavior of the Bz component of the interplanetary magnetic field (IMF) during the expansion phase. The probability of observations of substorms associated with the northward turn of the Bz component of IMF was ~19%, while the substorms taking place at Bz < 0 were observed in 53% of cases. A substantial number of events in which no substorm magnetic activity was observed in the auroral zone after a long (>30 min) period of the southward IMF and a following sharp turn of the Bz component of IMF before the north was detected. The data suggest that a northward IMF turn is neither a necessary nor sufficient condition for generating substorms. It has been shown for substorms of the both types that the average duration of the southward IMF to moment T ₀ and the average intensity of the magnetic perturbation in the maximum are approximately the same and amount to ~80 min and–650 nT, respectively. However, for substorms at Bz < 0, their mean duration, including the expansive and recovery phases, is on average 30 min longer than that at a northward turn of IMF. Correlations between the loading–unloading processes in the magnetosphere in the periods of magnetospheric substorms were investigated with different functions that determine the efficiency of the energy transfer from the solar wind to the magnetosphere. It has been shown that the highest correlation coefficient (r = 0.84) is observed when the function suggested by Newell et al. (2007) is used. It has been detected that a simple function VB _S yields a high correlation coefficient (r = 0.75).