Synthetic Emissions of the Fe xxi 1354 Å Line from Flare Loops Experiencing Fundamental Fast Sausage Oscillations

Abstract

Inspired by recent IRIS observations, we forward model the response of the Fe XXI 1354Å line to fundamental, standing, linear fast sausage modes (FSMs) in flare loops. Starting with the fluid parameters for an FSM in a straight tube with equilibrium parameters largely compatible with the IRIS measurements, we synthesize the line profiles by incorporating the non-Equilibrium Ionization (NEI) effect in the computation of the contribution function. We find that both the intensity and Doppler shift oscillate at the wave period (P ). The phase difference between the two differs from the expected value (90 • ) only slightly because NEI plays only a marginal role in determining the ionic fraction of Fe XXI in the examined dense loop. The Doppler width modulations, however, posses an asymmetry in the first and second halves of a wave period, leading to a secondary periodicity at P/2 in addition to the primary one at P . This behavior results from the competition between the broadening due to bulk flow and that due to temperature variations, with the latter being stronger but not overwhelmingly so. These expected signatures, with the exception of the Doppler width, are largely consistent with the IRIS measurements, thereby corroborating the reported detection of a fundamental FSM. The forward modeled signatures are useful for identifying fundamental FSMs in flare loops from measurements of the Fe XXI 1354Å line with instruments similar to IRIS, even though a much higher cadence is required for the expected behavior in the Doppler widths to be detected.

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…Inspired by the work of Tian et al (2016b), Shi et al (2019) forward-modeled the response of the IRIS Fe XXI line to fast sausage modes in flare loops under NEI conditions. They found that the synthetic spectral signatures, in particular the phase difference between the intensity and Doppler-shift oscillations, are mostly consistent with the IRIS observations.…”

Magnetohydrodynamic Fast Sausage Waves in the Solar Corona

“…This requirement is more stringent than that the wave period P = 2π/ω be much longer than the ionization and recombination timescales (1/ν C and 1/ν R ). Non-EI effects are therefore seen even in the study on the Fe XXI 1354 Å line emitted from flare loops, despite the high densities therein (Shi et al 2019a). That said, the non-EI computations differ from their EI counterparts only in intensity variations, whereas the Doppler shifts and Doppler widths hardly experience any influence.…”

“…The issue with temporal cadence arises from the short period of FSMs, which led AVD13 to propose that the ionic fractions (fq) may not respond instantaneously to the temperature variations to maintain ionization equilibrium. Indeed, it was shown by Shi et al (2019a) that non-equilibrium ionization (non-EI) effects are negligible only when the angular frequencies ω of FSMs are far below the ionization and recombination frequencies (ν C and ν R ). This requirement is more stringent than that the wave period P = 2π/ω be much longer than the ionization and recombination timescales (1/ν C and 1/ν R ).…”

“…Indeed, AVD13 showed that the wave modulation of spectroscopic quantities is substantial, particularly for the Doppler width. Summing up AVD13, Shi et al (2019c) and Shi et al (2019a), one expects the following (E)UV signatures of fundamental trapped FSMs. Firstly, the intensity variations consistently possess the same period as the FSM.…”

Fast Sausage Oscillations in Coronal Loops with Fine Structures

“…Inspired by the fundamental FSMs measured in flare loops in T16, we forward-modelled the numerical results by using the FoMo code ) to obtain their observational signals. We perform the synthetic process by using the Fe XXI 1354 Å spectral line, which has been considered by Shi et al (2019) in their forward modeling effort associated with the above-mentioned IRIS measurements. In our case, however, the coronal loop with substructures therein is a more realistic consideration.…”