Signs of Sulphur fractionation under high magnetic field strength

Abstract

Sulphur, with a first ionisation potential (FIP) of 10.36 eV, lies at the boundary between low- and high-FIP elements, making it particularly sensitive to fractionation processes in the solar atmosphere. Sulphur exhibits variable behaviour across solar environments, with coronal remote sensing studies often observing it as a high-FIP element while in-situ measurements sometimes detect low-FIP-like enhancement. Sulphur also exhibits variable behaviour during flares and magnetic restructuring. To understand sulphur's variations, we quantify how sulphur's FIP bias depends on potential field source surface (PFSS)-derived loop properties. We analyse nine Hinode/EUV Imaging Spectrometer (EIS) raster observations using four diagnostic line pairs (Si X 258.37 A / S X 264.23 A, S XI 188.68 A / Ar XI 188.81 A, Ca XIV 193.87 A / Ar XIV 194.40 A, and Fe XVI 262.98 A / S XIII 256.69 A), with FIP biases derived using differential emission measures (DEM) calculated via regularised inversion. Our results show that abundances of low-FIP elements, including sulphur, decrease above approximately 150 G relative to the high-FIP element Ar, while showing no dependence on loop length. This provides evidence that FIP fractionation is modulated by mean magnetic field strength of coronal loops.