Solar wind temperature measurements

¹ Christian Albrechts University at Kiel, Kiel, Germany
² Institut de Recherche en Astrophysique et Planétologie, Toulouse Cedex 4, France

^⋆ Corresponding author: berger@physik.uni-kiel.de

Received: 21 May 2025
Accepted: 6 August 2025

Abstract

Context. The solar wind has been an active research area since the beginning of the space age and the available measurements of its basic properties, i.e. solar wind density, velocity, and temperature, have fundamentally shaped our understanding of the heliosphere. Meanwhile, the data from various solar wind instruments show significant systematic differences.

Aims. We characterise systematic errors in these basic solar wind properties that are caused by instrumental limitations and put them in the context of solar wind studies.

Methods. To this end, we investigated the limitations that arise from the finite resolution of state-of-the-art solar wind instruments, namely the Proton Alpha Sensor (PAS) and the Solar Probe ANalyzer for Ions (SPAN-I). We defined two models, a virtual detector model of PAS and a more general further idealised detector with finite resolution. Virtual measurements of Maxwell-Boltzmann velocity distribution functions were compared to observations. The detailed effects of the instrumental resolution on the solar wind density, velocity, and temperature were analysed.

Results. We identify an unphysical direction dependence of the observed temperatures in data from PAS and SPAN-I. We show that both models can reproduce and explain this apparent direction dependence of the temperature observed by PAS. While the solar wind densities and the absolute values of the solar wind velocity are well determined, the directions of the solar wind velocity suffer from systematic errors and more importantly the majority of all available solar wind temperatures are systematically overestimated to varying degrees. These systematic errors are a compulsory consequence of the finite resolution of an instrument and are further enhanced by the detailed instrumental responses. In addition, limited instrumental sensitivity and field of view lead to a systematic and variable underestimation of the temperature. All observed temperatures are affected by one or more of these effects. Our results are transferable to all solar wind instruments.

Conclusions. Our results have far-reaching consequences for heliophysics. Firstly, we provide guidelines to adapt requirements for future solar wind instruments. These guidelines stem from our finding that the resolution of existing solar wind instruments is insufficient to capture all relevant underlying physics. Secondly, we discuss the impact of our results for past and future studies in various aspects of heliophysics. Even long-standing fundamental findings need to be reconsidered in the light of our results.