Comparing first ionization potential bias diagnostics in the solar atmosphere

Abstract Plasma composition in the solar atmosphere differs between the photosphere and corona, producing an observable difference in elemental abundance known as the first ionization potential (FIP) effect. The FIP effect is characterized by the ratio of low to high FIP elements, giving a number known as the FIP bias. FIP bias values vary between different regions of the solar atmosphere, with typical observed values of approximately 1 for coronal holes, approximately 1.5–2 for the quiet Sun and approximately 3 for active regions. The Extreme ultraviolet Imaging Spectrometer (EIS) onboard the Hinode spacecraft has enabled the widespread use of the Si X/S X line pair as a FIP bias diagnostic, but EIS observes other line pairs that can be used to estimate FIP bias. We consider three FIP bias diagnostics observed by Hinode/EIS (Si X/S X, Ca XIV/Ar XIVand Fe XVI/S XIII), comparing the FIP bias between the quiet Sun and an active region. We also assume a range of signal-to-noise ratio (SNR) cut-off values for each pixel, finding that while the SNR cut-off affects the number of useable pixels, higher (lower) SNR cut-offs remove (retain) a tail of high FIP bias values within the measured distribution. However, the median value of the FIP bias distribution remains largely unchanged. These results show the importance of a more nuanced view of FIP bias when using this vitally important diagnostic rather than a simplistic one-size-fits-all approach. This article is part of the Theo Murphy meeting issue ‘Solar atmospheric abundances in space and time’.