Relativistic electron acceleration during high-intensity, long-duration, continuousAEactivity (HILDCAA) events: Solar cycle phase dependences

Abstract

High-intensity, long-duration, continuous AE activity (HILDCAA) intervals during solar cycle 23 (1995–2008) have been studied by a superposed epoch analysis. It was found that HILDCAA intervals order the solar wind velocity, temperature and density (characteristic of high-speed solar wind intervals), the polar cap potential, and various other geomagnetic indices well. The interplanetary magnetic field Bz is generally negative, and the Newell solar wind coupling function is high during HILDCAA events. The HILDCAA intervals are well correlated with an enhancement of magnetospheric relativistic (E > 2 MeV) electron fluxes observed at geosynchronous orbit with a delay of ~1.5 days from the onset of the HILDCAAs. The response of the energetic electrons to HILDCAAs is found to vary with solar cycle phase. The initial electron fluxes are lower for events occurring during the ascending and solar maximum (AMAX) phases than for events occurring during the descending and solar minimum (DMIN) phases. The flux increases for the DMIN phase events are >50% larger than for the AMAX phase events. Although the solar wind speeds during the DMIN phases were slightly higher and lasted longer than during the AMAX phases, no other significant solar wind differences were noted. It is concluded that electrons are accelerated to relativistic energies most often and most efficiently during the DMIN phases of the solar cycle. We propose two possible solar UV mechanisms to explain this solar cycle effect.