Energetics of compressive waves in the solar corona

Inelastic scattering off moving or oscillating density fluctuations leads to broadening of radio signals propagating in the solar corona and solar wind. Using an anisotropic density fluctuation model from the kinetic scattering theory for solar radio bursts, it is possible to deduce the plasma velocities (perpendicular to the line of sight) required to explain observations of spacecraft signal frequency broadening.

The kinetic energy associated with these inferred bulk velocities cascades to smaller and smaller scales, where it is ultimately dissipated through damping of ion-sound waves. The inferred energy deposition rate associated with this process compares very favorably with those required to heat the corona and drive the solar wind.

Considering a distant point source, Francesco Azzollini and others calculate the effects of inelastic scattering in the presence of motions perpendicular to the line of sight, and they derive the associated diffusion tensor for radio waves in anisotropic turbulent plasma. The research is published in The Astrophysical Journal.

The inferred velocities are consistent with motions that are dominated by the solar wind at distances ≳10 R⊙, but the levels of frequency broadening for ≲10 R⊙ require additional radial speeds of about 300 km s⁻¹ and/or transverse speeds of about (20–70) km s⁻¹.

The inferred radial velocities also appear consistent with the sound or proton thermal speeds, while the speeds perpendicular to the radial direction are consistent with non-thermal motions measured via coronal Doppler-line broadening, interpreted as Alfvénic fluctuations.

Landau damping of parallel propagating ion-sound (slow MHD) waves results in a proton heating rate that is comparable to the rates available from a turbulent cascade of Alfvénic waves at large scales, suggesting a coherent picture of energy transfer, via the cascade or/and parametric decay of Alfvén waves to the small scales where heating takes place.