Correlation between Electron Temperature and Ion Charge-state Ratios in the Solar Wind at ∼0.5 au

Abstract Whether solar wind electrons expanding into the heliosphere can preserve information about their origin in the solar corona remains an open debate. The suprathermal strahl temperature has often been postulated as an indicator of source coronal electron temperature, while the core electron temperature has also been found to correlate with the solar wind velocity in the inner heliosphere. Here we investigate how well solar wind electron populations retain imprints of coronal electron temperature. Using Solar Orbiter measurements at ∼0.5 au, we fit three components (i.e., the core, halo, and strahl) of the electron velocity distribution function and compare the resulting strahl and core temperatures with heavy-ion charge-state ratios, which serve as proxies for the coronal electron temperature. We present the first clear evidence from inner heliosphere observations that, in several individual streams, a proxy for the strahl parallel temperature, T strahl,∥ , correlates significantly and positively with the charge-state ratios O 7+ /O 6+ and C 6+ /C 5+ . However, this correlation is not universally present, implying that many electron streams are significantly affected by transport processes, such as scattering, that erase the signature. We find that, notably, the core perpendicular temperature ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">core</mml:mi> <mml:mo>,</mml:mo> <mml:mo>⊥</mml:mo> </mml:mrow> </mml:msub> </mml:math> ) also strongly correlates with the charge-state ratios. We interpret this result within the framework of the exospheric solar wind model. Our results suggest that both thermal and suprathermal electrons can at times retain coronal information, but that aggregating multiple streams can obscure the underlying relationships.