ACE SWICS observations of solar cycle variations of the solar wind

TL;DR

This study uses ACE/SWICS data to analyze how solar wind properties vary with the solar cycle, revealing that wave-driven heating and acceleration effects diminish over the cycle, especially in slow wind.

Contribution

It provides a detailed analysis of solar wind property variations with the solar cycle, highlighting the differing impacts on slow and fast winds and the persistent role of wave-based processes.

Findings

All wind properties, except differential speeds, vary with the solar cycle.

Wave-driven heating and acceleration decrease in effectiveness over the cycle.

Slow wind is more affected by solar cycle variations than fast wind.

Abstract

In the present work we utilize ACE/SWICS in-situ measurements of the properties of the solar wind outside ICMEs in order to determine whether, and to what extent are the solar wind properties affected by the solar cycle. We focus on proton temperatures and densities, ion temperatures and differential speeds, charge state distributions and both relative and absolute elemental abundances. We carry out this work dividing the wind in velocity bins to investigate how winds at different speeds react to the solar cycle. We also repeat this study, when possible, to the subset of SWICS measurements less affected by Coulomb collisions. We find that with the only exception of differential speeds (for which we do not have enough measurements) all wind properties change as a function of the solar cycle. Our results point towards a scenario where both the slow and fast solar wind are accelerated by waves, but originate from different sources (open/closed magnetic structures for the fast/slow wind, respectively) whose relative contribution changes along the solar cycle. We also find that the signatures of heating and acceleration on one side, and of the FIP effect on the other, indicate that wave-based plasma heating, acceleration and fractionation remain active throughout the solar cycle, but decrease their effectiveness in all winds, although the slow wind is much affected than the fast one.