The relation between cyclotron heating and energetic particles on open coronal field lines

Space Research Laboratory, Department of Physics, 20014 Turku University, Finland

Corresponding author: R. Vainio, rami.vainio@utu.fi

Received: 5 February 2001
Accepted: 16 March 2001

Abstract

Cyclotron resonance with high-frequency Alfvén waves has been proposed as an ion heating mechanism for producing high-speed winds and large ion temperatures in coronal holes. In the simplest version of the model (cyclotron sweep model), the waves propagate without interacting until they hit a resonance with the plasma ions at distance, where the ion cyclotron frequency becomes comparable to the wave frequency. We calculate the energetic test-particle mean free path λ in solar corona heated by this mechanism. The primary result of our study is that energetic particles are efficiently coupled to the high-frequency MHD waves and may, thus, provide important feed back to wave-heating models. Detailed calculations are presented for a solar-wind stream of intermediate speed driven by the cyclotron sweep mechanism applying an extended quasi-linear theory for the mean free path. A local maximum of λ is located near , but λ is very small at distances close to the Sun () helping the acceleration of particles to high energies on open coronal field lines there. Another minimum for the mean free path is obtained near enabling ion acceleration beyond 10 MeV/n in streams of intermediate speeds by CME shock waves in the test-particle approximation. However, the presence of high-frequency Alfvén waves in amounts required for the cyclotron sweep heating has to be limited below ~10 on field lines close to the ecliptic to avoid discrepancies with the inferred arrival times of solar energetic particles. Non-linear effects are discussed as a possible mechanism for removing the wave excess from the plasma.