Ion-scale turbulence and remote diagnostics of coronal heating

The solar corona and solar wind are turbulent plasma environments where energy transfer from large-scale magnetohydrodynamic (MHD) fluctuations to smaller kinetic scales is thought to play a critical role in coronal heating and solar wind acceleration. Despite decades of research, the properties of turbulence at ion scales, where dissipation occurs, remain poorly constrained due to limited observational data. Moreover, the precise mechanisms that heat the corona, and accelerate and heat the solar wind, remain largely open questions.

Both in-situ spacecraft measurements and remote sensing techniques provide complementary insights at distances far away from the Sun, but direct measurements close to the Sun are unavailable in situ. Solar radio bursts, particularly type III bursts, offer a unique probe of density fluctuations, providing constraints on turbulence properties from the low corona to 1 au.

The recent paper by Kontar et al 2025 finds that the magnetic fluctuations observed by Parker Solar Probe in situ and density fluctuation amplitudes obtained from radio measurements are consistent with excitation by kinetic Alfvén waves (KAWs) and/or KAW structures over a broad range of distances from the Sun. Using radio diagnostics and the KAW scenario to deduce the radial variation of magnetic fluctuation amplitudes in regions close to the Sun where in situ measurements cannot be obtained. Using this result, we have estimated the turbulence energy cascade rate near ion scales (where the wave spectrum transitions from inertial to kinetic scales), and we find that the rate is very similar to the energy transfer rate obtained in the solar wind at larger inertial scales from in-situ measurements. The radio-inferred heating rate decreases with distance quantitatively similar to in-situ measurements reported in the literature (Figure 1).

This novel approach that combines radio diagnostics of density fluctuations with in-situ measurements of magnetic turbulence to probe ion-scale turbulence amplitude from the low corona to 1 au. By linking these observations to the MHD turbulent cascade, we infer the radial evolution of magnetic fluctuation amplitudes and compute the associated energy cascade rate in regions inaccessible to spacecraft measurements. Our results reveal a consistent picture of turbulence-driven heating across three orders of magnitude in heliocentric distance, offering new insights into the fundamental processes powering the solar atmosphere and wind.

More information:
Kontar, E.P., Emslie, A.G. Clarkson, D.L. and Pitňa, A. The Astrophysical Journal Letters, 991 L57 (2025) doi:10.3847/2041-8213/ae09b0

Provided by Community of European Solar Radio Astronomers