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Comparing First Ionisation Potential bias diagnostics in the solar atmosphere

Kristena D. Spruksta, David M. Long, Andy S. H. To

Abstract

Plasma composition in the solar atmosphere differs between the photosphere and corona, producing an observable difference in elemental abundance known as the FIP effect. The FIP effect is characterised 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 $\sim$1 for coronal holes, $\sim$1.5-2 for the quiet Sun, and $\sim$3 for active regions. The Extreme ultraviolet Imaging Spectrometer (EIS) onboard the \emph{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 \emph{Hinode}/EIS (Si X/S X, Ca XIV/Ar XIV, and Fe XVI/S XIII), comparing the FIP bias between Quiet Sun and an Active region. We also assume a range of signal-to-noise (SNR) cutoff values for each pixel, finding that while the SNR cutoff affects the number of useable pixels, higher (lower) SNR cutoffs 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.

Comparing First Ionisation 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 FIP effect. The FIP effect is characterised 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 1 for coronal holes, 1.5-2 for the quiet Sun, and 3 for active regions. The Extreme ultraviolet Imaging Spectrometer (EIS) onboard the \emph{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 \emph{Hinode}/EIS (Si X/S X, Ca XIV/Ar XIV, and Fe XVI/S XIII), comparing the FIP bias between Quiet Sun and an Active region. We also assume a range of signal-to-noise (SNR) cutoff values for each pixel, finding that while the SNR cutoff affects the number of useable pixels, higher (lower) SNR cutoffs 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.
Paper Structure (6 sections, 1 equation, 4 figures)

This paper contains 6 sections, 1 equation, 4 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. Results
  4. Comparing different regions of the solar atmosphere
  5. Examining the signal-to-noise ratio
  6. Discussion & Conclusions

Figures (4)

  • Figure 1: Full disk mosaics taken from 18-20 Oct 2015 by Hinode/EIS showing the fitted peak intensity (panel a), Doppler velocity (panel b), and line width (panel c) of the Fe XII 195.119 Å emission line. The red (blue) box shows the Quiet Sun (active region) region of interest further examined in Section \ref{['s:res']}.
  • Figure 2: Top row: Full disk mosaics taken by Hinode/EIS showing the derived Si X/S X (panel a, left), Fe XVI/S XIII (panel b, centre), and Ca XIV/Ar XIV (panel c, right) FIP maps. The red (blue) box shows the Quiet Sun (active region) region of interest. Bottom row shows the corresponding Kernel Density Estimator (KDE) plots of the distributions corresponding to the Quiet Sun (red) and active region (blue) from the derived Si X/S X (left), Fe XVI/S XIII (centre), and Ca XIV/Ar XIV (right) FIP maps. The legend gives the 25$^{th}$, 50$^{th}$ (i.e., median), and 75$^{th}$ percentile values for each distribution
  • Figure 3: Hinode/EIS full disk mosaics and corresponding KDE plots for signal-to-noise cutoffs of 0.1 (upper two rows) and 0.0001 (bottom two rows). In each case, the FIP bias ratios are (from left to right) Si X/S X, Fe XVI/S XIII, and Ca XIV/Ar XIV.
  • Figure 4: A comparison of fits to the Si X 258.37 Å spectral line for different ranges of the signal-to-noise (in each case given in the panel title).