Effect of Interplanetary Disturbances and Geomagnetic Activities on Relativistic Electrons at Geosynchronous Orbit

Abstract

AbstractUsing hourly averaged data of > 2 MeV electron fluxes measured with GOES 7/9/10/11 satellites during 1988–2010, this paper investigates statistically conditions of solar wind and geomagnetic activities during relativistic electrons flux (Fe) enhancements, and describes the dependence of relativistic electrons at geosynchronous orbit (GEO) on local time, magnetic storms. The results are as follows: (1) The local time dependence of GEO relativistic electrons is influenced by solar wind and geomagnetic activities. The noon/midnight electron flux ratio grows with increasing solar wind velocity (Vsw). On conditions of the Dst index above −50 nT, relativistic electrons show regular variation with local time, while during storms with Dst< −50 nT, the maximum electrons flux may not be measured at local noon. During the declining phase of a solar cycle relativistic electron fluxes approaching maximum, Vsw, the Kp index and the cube root of solar wind density (N) show better correlations with electron fluxes after 39∼57 h, 57∼80 h and 12∼24 h, respectively. (2) Before about two years of solar minimum and near the equinoxes, occurrence frequencies of intense relativistic electron flux enhancement events, in which the daily maximum relativistic electron flux Femax ≥ 104 pfu increased, while weaker events with 104 > Femax ≥ 103 pfu exhibited no such solar cycle and seasonal dependences. For most intense events, relativistic electron fluxes begin to increase during main phases of magnetic storms, while for weaker events, relativistic electron fluxes tend to increase during recovery phases. (3) The solar wind density shows as a good indicator of subsequent relativistic electron activities. For most electron flux enhancement events, relativistic electron flux decreased to its minimum after 0∼1 days of the maximum solar wind density, Nmax, while approaching the maximum after 0∼2 days of the minimum solar wind density, Nmin. (4) Above 90% of relativistic electron events occurred on conditions of high‐speed solar wind and geomagnetic disturbances, which are as follows: Vswmax > 516 km/s, Dstmin < −31 nT, Nmin < 2.8 cm−3, Nmax > 14.1 cm−3, Bzmin < −2.9 nT, AEmax > 698 nT. (5) For all magnetic storms, occurrence probability of daily maximum electron flux above 103 pfu after Dstmin, is about 53%, the percent of intense, weak events being 36%, 64%, respectively, which is hardly affected by intensities of storms. During storms, solar wind velocity, density, and the AE index are important indicators for subsequent relativistic electron activities.