Characteristics of upstream energetic (E≥50 keV) ion events during intense geomagnetic activity

Abstract

In this work we examine the statistical presence of some important features of upstream energetic (≥50 keV) ion events under some special conditions in the upstream region and the magnetosphere. The 125 ion events considered in the statistic were observed by the IMP 7 and IMP 8 spacecraft, at ∼35 RE from the Earth, during nine long time intervals of a total of 153 hours. The time intervals analyzed were selected under the following restrictions: existence of high proton flux (i.e., ≥900 p cm−2 s−1 sr−1) and of a great number of events (an occurrence frequency of ∼10 events per 12 hours in the whole statistics) in the energy range 50–220 keV. The most striking findings are the following: (1) The upstream events were observed during times with high values of the geomagnetic activity index Kp (≥3‐); (2) all of the upstream events (100%) have energy spectra extending up to energies E≥290keV; (3) 86% of these events are accompanied by relativistic (E ≥ 220 keV) electrons; and (4) the majority of the upstream ion events (82%) showed noninverse velocity dispersion during their onset phase (22% of the events showed forward velocity dispersion, and 60% showed no velocity dispersion at all when 5.5‐min averaged observations were analyzed). Further statistical analysis of this sample of upstream particle events shows that the 50‐ to 220‐keV proton flux shows a positive correlation with the following parameters: the Kp index of geomagnetic activity and the flux of the high‐energy (290–500 keV) protons and (≥220 keV) electrons. More specific findings are the following: (1) The spectral index γ for a power law distribution of ions detected by the National Oceanic and Atmospheric Administration Energetic Particle Experiment (EPE) instrument (50 ≤ E ≤ 220 keV) and The Johns Hopkins University Applied Physics Laboratory Charged Particle Measurement Experiment (CPME) instrument (290 ≤ E ≤ 500 keV) ranges between 2 and 6, with maximum probability between 4 and 5 and (2) the peak‐to‐background flux ratio of the 290‐ to 500‐keV protons and ≥220‐keV electrons increases with the time duration of upstream events. We infer that the vast majority of the upstream ion events considered in this study (under conditions of intense particle activity in the upstream region and enhanced geomagnetic activity within the magnetosphere) can be consistently explained in terms of particle leakage from the magnetosphere.