CHARACTERISTICS OF UPSTREAM ENERGETIC (E-GREATER-THAN-OR-EQUAL-TO-50 KEV) ION EVENTS DURING INTENSE GEOMAGNETIC-ACTIVITY

In this work we examine the statistical presence of some important fea tures of upstream energetic (greater than or equal to 50 keV) ion even ts under some special conditions in the upstream region and the magnet osphere. The 125 ion events considered in the statistic were observed by the IMP 7 and IMP 8 spacecraft, at similar to 35 R-E from the Earth , during nine long time intervals of a total of 153 hours. The time in tervals analyzed were selected under the following restrictions: exist ence of high proton flux (i.e., greater than or equal to 900 p cm(-2) s(-1) sr(-1)) and of a great number of events (an occurrence frequency of similar to 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 o f the geomagnetic activity index Kp (greater than or equal to 3-); (2) all of the upstream events (100%) have energy spectra extending up to energies E greater than or equal to 290 keV; (3) 86% of these events are accompanied by relativistic (E greater than or equal to 220 keV) e lectrons; and (4) the majority of the upstream ion events (82%) showed noninverse velocity dispersion during their onset phase (22% of the e vents showed forward velocity dispersion, and 60% showed no velocity d ispersion at all when 5.5-min averaged observations were analyzed). Fu rther 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 a nd the flux of the high-energy (290-500 keV) protons and (greater than or equal to 220 keV) electrons. More specific findings are the follow ing: (1) The spectral index gamma for a power law distribution of ions detected by the National Oceanic and Atmospheric Administration Energ etic Particle Experiment (EPE) instrument (50 less than or equal to E less than or equal to 220 keV) and The Johns Hopkins University Applie d Physics Laboratory Charged Particle Measurement Experiment (CPME) in strument (290 less than or equal to E less than or equal to 500 keV) r anges 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 greater than or equal to 220-keV electrons increases with the time du ration of upstream events. We infer that the vast majority of the upst ream ion events considered in this study (under conditions of intense particle activity in the upstream region and enhanced geomagnetic acti vity within the magnetosphere) can be consistently explained in terms of particle leakage from the magnetosphere.