Global sausage oscillation of solar flare loops detected by the Interface Region Imaging Spectrograph

TL;DR

This paper reports the detection of a global sausage mode oscillation in solar flare loops using IRIS and GOES data, providing insights into flare loop dynamics, plasma density, and magnetic properties.

Contribution

It presents the first clear observation of a global sausage mode in a solar flare loop with detailed measurements of phase speed and density contrast.

Findings

Detected 25-second oscillations in flare loops in IRIS and GOES data.

Estimated phase speed of ~2420 km/s and density contrast ≥42.

Oscillation period increase possibly due to loop footpoint separation.

Abstract

An observation from the Interface Region Imaging Spectrograph reveals coherent oscillations in the loops of an M1.6 flare on 2015 March 12. Both the intensity and Doppler shift of Fe~{\sc{xxi}}~1354.08\AA{}~show clear oscillations with a period of $\sim$25 seconds. Remarkably similar oscillations were also detected in the soft X-ray flux recorded by the Geostationary Operational Environmental Satellites (GOES). With an estimated phase speed of $\sim$2420~km~s$^{-1}$ and a derived electron density of at least 5.4$\times$10$^{10}$ cm$^{-3}$, the observed short-period oscillation is most likely the global fast sausage mode of a hot flare loop. We find a phase shift of $\sim$$\pi$/2 (1/4 period) between the Doppler shift oscillation and the intensity/GOES oscillations, which is consistent with a recent forward modeling study of the sausage mode. The observed oscillation requires a density contrast between the flare loop and coronal background of a factor $\geqslant$42. The estimated phase speed of the global mode provides an lower limit of the Alfv\'en speed outside the flare loop. We also find an increase of the oscillation period, which might be caused by the separation of the loop footpoints with time.