Polytropic stellar wind models with strongly localized heating

TL;DR

This paper extends polytropic stellar wind models to include more realistic non-adiabatic heating effects, relevant for understanding solar and stellar wind observations, especially with recent Parker Solar Probe data.

Contribution

It generalizes previous models by incorporating non-adiabatic heating, moving beyond the extreme adiabatic case, and demonstrates the plausibility of heating energies consistent with flare energies.

Findings

Heating energy is within plausible range compared to flare energies.

Models show non-adiabatic effects can produce observable wind stream variations.

Results are relevant for interpreting Parker Solar Probe observations.

Abstract

Polytropic models of stellar winds remain to be useful tools because they allow for a simple description of the energy balance of the expanding plasma without explicitly specifying potentially complex energy transport processes like, e.g., heat conduction or extended wave heating. Among recent applications to stellar winds and to the solar wind was a study of the consequences of strongly localized heating in the latter, possibly due to acoustic waves. Such 'nonuniform' heating can result from a time- and space-localized damping of wave modes and allows, as an extreme case, an adiabatic expansion of particular wind streams outside the heating region. The present study generalizes the modeling from the first analytical as well as numerical studies, that were limited to this extreme case, towards a more realistic non-adiabatic behaviour. The additional energy due to heating is demonstrated to be in a plausible range in view of typical flare energies and low compared to the gravitational energy of the plasma in this region. The corresponding solutions may be of interest for stellar winds, in general, and w.r.t. recent observations made with the Parker Solar Probe, which revealed strongly varying wind streams and the presence of acoustic waves near the Sun, for the solar wind, in particular. Potential observational evidence for the solar wind is discussed.