Relativistic Electron Enhancements at GEO: Magnetic Storms vs. Substorms

Abstract:

We find evidence that magnetic storms are not only unnecessary for relativistic electron enhancements at geosynchronous orbit (GEO) but also not directly relevant to the electron enhancements at GEO even if the enhancements are accompanied by magnetic storms. What is crucial for electron enhancements at GEO are sustained south-oriented or north-south fluctuating IMF Bz that drives sufficiently large substorm activity and small solar wind density Nsw 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 > 235 nT and average Nsw ≤ 5 cm^-3. Examination of detailed time profiles clearly shows that electron flux at GEO 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 co-incident, while the prolonged, intense substorms are critical.