Scattering of solar energetic electrons in interplanetary space

Solar energetic electrons are observed to arrive between 10 and up to 30 minutes later at 1 AU, as compared to the expectation based on their generation in a solar flare and the travel time along the Parker spiral. Both a delayed release of the electrons at the Sun and scattering of the electrons in interplanetary space are discussed as underlying mechanisms. We have investigated to what extent scattering of energetic electrons in interplanetary space does influence the arrival times of energetic electrons at a solar distance of 1 AU, as a function of electron energy and for different scattering models. A kinetic model for electrons in interplanetary space is used to study the propagation of solar-flare electrons injected into the corona. The electrons are scattered by resonant interaction with a whistler-wave spectrum that is based on observed magnetic field fluctuation spectra in the solar wind. The simulation results show a significant influence of the scattering on electron arrival times. Electrons with energies in the range of several tens of keV are delayed up to about one minute for a pure pitch-angle scattering model. It is demonstrated that this simplification is not applicable, and the full quasi-linear diffusion equation needs to be considered. This reduces the delays to values below 30 s. Thus, it follows from these numerical studies that scattering of electrons in interplanetary space cannot explain the observed delays of about 600 s, unless an unrealistic wave spectrum is assumed.