On the estimation of solar energetic particle injection timing from onset times near earth

Abstract

A common technique for estimating the start time of solar energetic particle (SEP) injection consists of a linear fit to the observed onset time versus the inverse of particle velocity. This is based on a concept that the first arriving particles move directly along the magnetic field with no scattering. We examine the accuracy of this technique by performing numerical simulations of the transport of solar protons of different energies from the Sun to the Earth, by means of a finite difference method to numerically solve the Boltzmann equation. We then analyze the results using the inverse velocity fit. We find that in most cases, the onset times align close to a straight line as a function of inverse velocity. Despite this, the estimated injection time can be in error by several minutes. Also, the estimated path length can deviate greatly from the actual path length along the interplanetary magnetic field. The major difference between the estimated and actual path lengths implies that the first arriving particles cannot be viewed as moving directly along the interplanetary magnetic field.