Transforming in-situ observations of CME-driven shock accelerated protons into the shock's reference frame.

I M Robinson,G M Simnett

doi:10.5194/angeo-23-1931-2005

Abstract

Abstract. We examine the solar energetic particle event following solar activity from 14, 15 April 2001 which includes a "bump-on-the-tail" in the proton energy spectra at 0.99 AU from the Sun. We find this population was generated by a CME-driven shock which arrived at 0.99 AU around midnight 18 April. As such this population represents an excellent opportunity to study in isolation, the effects of proton acceleration by the shock. The peak energy of the bump-on-the-tail evolves to progressively lower energies as the shock approaches the observing spacecraft at the inner Lagrange point. Focusing on the evolution of this peak energy we demonstrate a technique which transforms these in-situ spectral observations into a frame of reference co-moving with the shock whilst making allowance for the effects of pitch angle scattering and focusing. The results of this transform suggest the bump-on-the-tail population was not driven by the 15 April activity but was generated or at least modulated by a CME-driven shock which left the Sun on 14 April. The existence of a bump-on-the-tail population is predicted by models in Rice et al. (2003) and Li et al. (2003) which we compare with observations and the results of our analysis in the context of both the 14 April and 15 April CMEs. We find an origin of the bump-on-the-tail at the 14 April CME-driven shock provides better agreement with these modelled predictions although some discrepancy exists as to the shock's ability to accelerate 100 MeV protons. Keywords. Solar physics, astrophysics and astronomy (Energetic particles; Flares and mass ejections) – Space plasma physics (Transport processes)

Highlights

Interplanetary energetic particles represent a valuable tool for the remote sensing of solar activity
We focus on the 15–18 April 2001 solar energetic particle event which was the largest ground level event (GLE) of the current solar cycle, reaching GeV proton energies during its early stages
One might conceive of a scenario where proton velocities v1 and v2 require modification in order to reflect the bulk properties of the observed proton pitch angle distribution around the time the shock arrives at L1, which for example may have been peaked around a 45◦ pitch angle

Summary

Introduction

Interplanetary energetic particles represent a valuable tool for the remote sensing of solar activity. The Sun drives the majority of energetic particle events seen at L1, either through solar flares or the release of coronal mass ejections (CMEs). Both phenomena are known to accelerate charged particles the details of the acceleration processes remain a matter of current research. Additional diagnostics are available if one investigates particle ionisation states and abundance ratios in the interplanetary medium These have been shown to distinguish between flare and CME particle populations (see Reames, 1999, for a review) doubts have been raised (Klein and Trottet, 2001) as to the validity of such approaches at energies above ∼10 MeV/nucleon. The total output from the flare is available to the CME, which may provide further acceleration and re-distribution (Torsti et al, 2002; Tylka et al, 2002)

Results

Discussion

Conclusion