Energetic neutral atoms from the Sun: an alternative interpretation of a unique event^⋆

School of Physics and Astronomy, University of Birmingham, B15 2TT, UK
e-mail: gms@star.sr.bham.ac.uk

Received: 2 January 2011
Accepted: 9 May 2011

Abstract

The high temperature of the solar corona results in virtually complete ionization of the light elements and a high degree of ionization of the heavier elements. Therefore it is not expected that many neutral atoms should be emitted from the Sun, and certainly not with high kinetic energy. A particle event associated with the first major flare of the current solar cycle, on 5 December 2006, has been interpreted as containing energetic neutral hydrogen atoms (ENA) of at least a few MeV. The ENAs were identified as such on account of their arrival direction at 1 AU, which was from the solar direction; the lack of atoms heavier than hydrogen; and the timing of their arrival, which was consistent with emission at the time of the flare X-ray burst. The observations were made from the two STEREO spacecraft which were near the Earth at the time. However, the EPAM instrument on the ACE spacecraft, which is in orbit around the L₁ Lagrangian point some 1.5 × 10⁶ km away from the Earth towards the Sun, observed a pulse, or precursor, of electrons of energies of at least 50 keV but approximately one hour earlier than the pulse of ENAs at STEREO. Later ACE and STEREO detected detected a major charged particle event which is presumably associated with the 5 December flare. The relative times of the onsets of the energetic particles in both the precursor and the main solar energetic particle event at the STEREO spacecraft and ACE were consistent with corotation of the interplanetary magnetic field if the particles were the same population propagating, and probably trapped, within the field. The precursor proton intensity detected by STEREO was below the threshold of the ACE/EPAM detectors. We conclude that the interpretation of the particles seen by STEREO as energetic neutral atoms is suspect.