Statistical Study on the Decay Phase of Solar Near-Relativistic Electron Events at ACE and Ulysses

The decay phase of solar energetic particle events provides an excellent testbed to study the processes of energetic particle transport in the heliosphere. The observation of energetic particle reservoirs (defined as those time intervals when particle intensities measured by widely separated spacecraft present comparable intensities that evolve similarly in time) poses the question of how energetic particles are distributed throughout the heliosphere. We present a statistical study on the decay phase of near-relativistic (53-315 keV) electron events as observed by ACE and Ulysses throughout solar cycle 23. By fitting an exponential function (exp {-t/τ }) to the time-intensity profile in the late phase of selected solar near-relativistic electron events, we examine the dependence of τ on electron energy, electron intensity spectra, event peak intensity, event fluence, and solar wind velocity, as well as heliocentric radial distance, heliolatitude, and heliolongitude of the spacecraft with respect to the parent solar event. The decay rates are found to be either independent or slightly decrease with the electron energy. No clear dependence is found between τ and the heliolongitude of the parent solar event, with the exception of well-connected events for which low values of τ are more commonly observed than for poorly-connected events. For those events concurrently observed by ACE and Ulysses, decay rates increase at distances >3 AU. Events with similar decay rates at both spacecraft were observed mainly when Ulysses was at high heliographic latitudes. We discuss the basic physical mechanisms that control the decay phase of the electron events.