Abstract

We find evidence that magnetic storms are not only unnecessary for geosynchronous relativistic electron enhancements but also not directly relevant to the electron enhancements even if the enhancements are accompanied by magnetic storms. What is crucial for electron enhancements at geosynchronous orbit are sustained south-oriented or north-south fluctuating interplanetary magnetic field (IMF) B-z that drives sufficiently large substorm activity and small solar wind density N-sw that likely leads to low loss rate of relativistic electrons to the ionosphere and/or to the magnetopause for an extended time period. Specifically, almost all the abrupt, large electron increases in our data set took place under the condition of average AE>235nT and average N(sw)5cm(-3). Examination of detailed time profiles clearly shows that electron flux starts to increase quite immediately with arrival of the right IMF and solar wind conditions, regardless of a magnetic storm, leaving the accompanied magnetic storms merely coincident.