On the Origin of the Slow Speed Solar Wind: Helium Abundance Variations

TL;DR

This paper models how the ponderomotive force caused by Alfvén waves affects helium depletion in the slow solar wind, linking magnetic field strength and wave frequency to observed helium abundance variations.

Contribution

It introduces a model explaining helium depletion in the slow solar wind based on wave interactions and magnetic field parameters, advancing understanding of FIP effect variations.

Findings

Helium depletion increases with weaker magnetic fields.

Depletion is most significant near the loop resonance frequency.

Results align with observed variations in solar wind helium abundance.

Abstract

The First Ionization Potential (FIP) effect is the by now well known enhancement in abundance over photospheric values of Fe and other elements with first ionization potential below about 10 eV observed in the solar corona and slow speed solar wind. In our model, this fractionation is achieved by means of the ponderomotive force, arising as Alfv\'en waves propagate through or reflect from steep density gradients in the solar chromosphere. This is also the region where low FIP elements are ionized, and high FIP elements are largely neutral leading to the fractionation as ions interact with the waves but neutrals do not. Helium, the element with the highest FIP and consequently the last to remain neutral as one moves upwards can be depleted in such models. Here, we investigate this depletion for varying loop lengths and magnetic field strengths. Variations in this depletion arise as the concentration of the ponderomotive force at the top of the chromosphere varies in response to Alfv\'en wave frequency with respect to the resonant frequency of the overlying coronal loop, the magnetic field, and possibly also the loop length. We find that stronger depletions of He are obtained for weaker magnetic field, at frequencies close to or just above the loop resonance. These results may have relevance to observed variations of the slow wind solar He abundance with wind speed, with slower slow speed solar wind having a stronger depletion of He.