EDP Sciences
Free access
Volume 408, Number 1, September II 2003
Page(s) L1 - L4
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20031090

A&A 408, L1-L4 (2003)
DOI: 10.1051/0004-6361:20031090


Common overabundance of $\mathsf{^3}$He in high-energy solar particles

J. Torsti, J. Laivola and L. Kocharov

Department of Physics and Väisälä Institute for Space Physics and Astronomy, University of Turku, 20014 Turku, Finland
(Received 31 March 2003 / Accepted 15 July 2003)

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, ${>}15\,\mathrm {MeV\,nucleon^{-1}}$, 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 ${{\approx }}0.5$ ion per $\mathrm {(m^2\,s\;sr\,MeV\,nucleon^{-1})}$. 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  $\approx$ 0.015. This is not expected in the impulsive-gradual paradigm of solar energetic particle events. We argue that the abundance 3He/ 4He  $\approx$ 0.015 should be regarded as a "normal composition" of high-energy solar particles, instead of the previously assumed 3He/ 4He  $\approx 5\times10^{-4}$. 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.

Key words: Sun: corona -- Sun: flares -- Sun: particle emission

Offprint request: J. Laivola, jarno.laivola@srl.utu.fi

© ESO 2003