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

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 r = 2 R⊙, but λ is very small at distances close to the Sun (r < 1.3 R⊙) helping the acceleration of particles to high energies on open coronal field lines there. Another minimum for the mean free path is obtained near r = 10 R⊙ 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 R⊙ 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.