First ionization potential fractionation of sulfur observed with spectral imaging of the coronal environment

Hinode's extreme ultraviolet (EUV) imaging spectrometer (EIS) has advanced our understanding of elemental abundances in the solar corona, highlighting how magnetic configurations influence low first ionization potential (FIP) elements, such as iron and silicon. However, mid-FIP (MF) elements (e.g. sulfur) behave more subtly, prompting further investigation. The spectral imaging of the coronal environment (SPICE) instrument aboard Solar Orbiter offers a unique opportunity to assess sulfur abundances in the solar transition region. We developed a telemetry-efficient diagnostic to measure the relative abundance of sulfur to nitrogen using EUV spectral lines formed at 100 000 K in low-density solar regions. Adapting the linear combination ratio (LCR) technique originally designed for coronal lines, we optimize it for the SPICE data. Unlike methods using differential emission measure (DEM) inversions, which require many spectral lines across temperatures and are telemetry-intensive, our approach uses only four carefully selected lines to maintain both simplicity and accuracy. Our diagnostic relies on emissions from N iii, N iv, S iv and S v ions. We demonstrate that the LCR method is effective at obtaining relative FIP bias maps using SPICE observations. This new FIP bias diagnostic will enable routine mapping of solar composition, offering transformative insights into solar wind origins and physical processes in the solar atmosphere. This article is part of the Theo Murphy meeting issue ‘Solar atmospheric abundance in space and time’.