Solar near-relativistic electron observations as a proof of a back-scatter region beyond 1 AU during the 2000 February 18 event

¹ Department of Physics, University of Helsinki, 00014 Helsinki, Finland
² Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA e-mail: nagueda@ssl.berkeley.edu
³ Applied Physics Laboratory, The Johns Hopkins University, Laurel, MD 20723, USA
⁴ Departament d'Astronomia i Meteorologia & Institut de Ciències del Cosmos, Universitat de Barcelona, 08028 Barcelona, Spain

Received: 23 December 2009
Accepted: 3 May 2010

Abstract

Aims. We study the near-relativistic (NR; >30 keV) electron event observed on 2000 February 18 by near-Earth spacecraft. Previous works have explained this event by assuming that the propagation of NR electrons is essentially “scatter-free” at heliocentric radial distances r < 1 AU, and that beyond 1 AU particles are “back-scattered” by magnetic field irregularities.

Methods. Our aim is to re-visit this interplanetary propagation scenario and infer the injection profile at the Sun by fitting the electron directional intensities observed by the Advanced Composition Explorer.

Results. We use a Monte Carlo transport model to explore this approach. We assume that the interplanetary magnetic field is an Archimedean spiral and that the interplanetary transport of NR electrons is characterized by a large radial mean free path ( > 0.5 AU) and anisotropic pitch-angle scattering for r <1 AU, and a small radial mean free path ( < 0.5 AU) and isotropic scattering in the back-scatter region.

Conclusions. The event cannot be explained without assuming a back-scatter region beyond 1 AU. The best fit is obtained by assuming = 3.2 AU in the inner heliosphere and a back-scatter region characterized by a small mean free path = 0.2 AU located beyond 1.2 AU.