Energetic Particles and Corotating Interaction Regions in the Solar Wind

Abstract

This paper reviews three important effects on energetic particles of corotating interaction regions (CIRs) in the solar wind that are formed at the leading edges of high-speed solar wind streams originating in coronal holes. A brief overview of CIRs and their important features is followed by a discussion of CIR-associated modulations in the galactic cosmic ray intensity, with an emphasis on observations made by spacecraft particle telescope 'anti-coincidence' guards. Such guards combine high counting rates (hundreds of counts/s) and a lower rigidity response than neutron monitors to provide detailed information on the relationship between cosmic ray modulations and CIR structure. The modulation of Jovian electrons by CIRs is then described. Finally, the acceleration of ions to energies of ∼ 20 MeV/n in the vicinity of CIRs is reviewed. Corotating interaction regions (CIRs) are regions of compressed plasma formed at the leading edges of corotating high-speed solar wind streams originating in coronal holes as they interact with the preceding slow solar wind. They are par- ticularly prominent features of the solar wind during the declining and minimum phases of the 11-year solar cycle, but may also be present at times of higher solar activity, interspersed with slow solar wind and transient flows associated with coronal mass ejections (CMEs) (e.g., Richardson, Cane, and Cliver, 2002). This paper reviews three important effects of CIRs on energetic particles in the heliosphere. The first effect is the tendency for the galactic cosmic ray intensity to be depressed temporarily ('modulated') during the passage of a CIR and high- speed stream. This phenomenon has been studied for many years, typically using ground-based neutron monitors. However, the emphasis in this review is on obser- vations made in the inner heliosphere and near the Earth by the 'anti-coincidence' guards of certain spacecraft particle telescopes. Such guards combine a large de- tector volume and integral energy response (> several tens of MeV) to give high counting rates (hundreds of counts/s) which can provide detailed information on the relationship between cosmic ray modulations and CIR structure. The observa-