Solar cosmic rays

spaceweb@oulu.fi - last update: 6 March 2000 (RR)

Introduction

The solar cosmic rays (SCR) aka solar energetic particles (SEP) originate mostly from solar flares. Coronal mass ejections and shocks in the interplanetary medium can also produce energetic particles. SCR particles have energy of up to several handred MeV/nucleon (sometimes up to few GeV/nucleon). Composition is similar to galactic cosmic rays: mostly protons, about 10% of He and <1% heavier elements. During strong solar flares (if optimally located on the Sun), the flux of CR at the Earth orbit can increase for some hundred percent during hours/days leading to a SEP event.

Solar cosmic rays were firstly discovered experimentally on 28 February 1942, as a sudden increase of Geiger counters counting rate was associated with a large solar flare. Since that time detectors, set up to monitor cosmic rays, have occasionally seen sudden increases in the intensity of the radiation (sometimes as large as several hundred per cent) associated with outbursts on the Sun, mostly with visible flares. The cosmic ray intensity returns to normal within tens of minutes to hours, as the acceleration process ends and as accelerated ions disperse throughout interplanetary space. The short increases of cosmic ray detectors count rate associated with solar particles arrival are called GLE - Ground Level Enchancement / Ground Level Events. So far, there are 58 registered GLEs (see the table).

The solar cosmic rays cause also the polar cap absorption (PCA) events measured by riometers.

References

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See also

• Space Physics Text Book of Oulu