Letter to the Editor

Common overabundance of He in high-energy solar particles

Department of Physics and Väisälä Institute for Space Physics and Astronomy, University of Turku, 20014 Turku, Finland

Corresponding author: J. Laivola, jarno.laivola@srl.utu.fi

Received: 31 March 2003
Accepted: 15 July 2003

Abstract

Solar energetic particle (SEP) observations of the ERNE instrument (the Energetic and Relativistic Nuclei and Electron experiment) onboard the Solar and Heliospheric Observatory enable measurements of the ³He intensity in the high-energy range, , with a good statistical resolution. We report an overview of the ERNE observations for the period from 8 February 1999 to 7 December 2000. Significant intensities of ³He are registered in all events with ⁴He intensity exceeding ion per . The abundance ratio ³He/⁴He is measured to vary in the range ~0.003–2, well above solar wind values. A histogram of the daily ³He/⁴He ratio reveals a sharp maximum at ³He/⁴He ≈ 0.015. This is not expected in the impulsive-gradual paradigm of solar energetic particle events. We argue that the abundance ³He/⁴He ≈ 0.015 should be regarded as a “normal composition” of high-energy solar particles, instead of the previously assumed ³He/⁴He . The ERNE registered also a number of ³He-rich events with ³He/⁴He ~ 0.1–1, which however are less frequent. We consider the common overabundance of ³He as a signature of impulsive processes, which seem unavoidable during the development of the SEP-productive solar eruptions comprising coronal mass ejections and flares.