The effect of a solar flare on chromospheric oscillations

David C L Millar,Lyndsay Fletcher,Ryan O Milligan

doi:10.1093/mnras/stab642

Abstract

ABSTRACT Oscillations in the solar atmosphere have long been observed both in quiet conditions and during solar flares. The chromosphere is known for its 3-min signals, which are strong over sunspot umbrae, and have periods determined by the chromosphere’s acoustic cut-off frequency. A small number of observations have shown the chromospheric signals to be affected by energetic events such as solar flares, however the link between flare activity and these oscillations remains unclear. In this work, we present evidence of changes to the oscillatory structure of the chromosphere over a sunspot which occurs during the impulsive phase of an M1 flare. Using imaging data from the CRISP instrument across the H α and Ca ii 8542 Å spectral lines, we employed a method of fitting models to power spectra to produce maps of where there is evidence of oscillatory signals above a red-noise background. Comparing results taken before and after the impulsive phase of the flare, we found that the oscillatory signals taken after the start of the flare differ in two ways: the locations of oscillatory signals had changed and the typical periods of the oscillations had tended to increase (in some cases increasing from &lt;100 s to ∼200 s). Both of these results can be explained by a restructuring of the magnetic field in the chromosphere during the flare activity, which is backed up by images of coronal loops showing clear changes to magnetic connectivity. These results represent one of the many ways that active regions can be affected by solar flares.

Highlights

Much of the variation we see in our observations of the Sun is essentially random in nature, and is described as noise
Examples include coronal loop oscillations (Aschwanden et al 1999; Nakariakov & Verwichte 2005) and quasi-periodic pulsations (QPPs) seen during solar flare activity, which have been observed across the electromagnetic spectrum and at time-scales ranging from subsecond to hours (Van Doorsselaere, Kupriyanova & Yuan 2016)
The analysis presented below is based on data from the CRisp Imaging SpectroPolarimeter (CRISP: Scharmer et al 2008) instrument at the Swedish Solar Telescope (SST: Scharmer et al 2003), and the Atmospheric Imaging Assembly (AIA: Lemen et al 2012) on the Solar Dynamics Observatory (SDO: Pesnell, Thompson & Chamberlin 2012)

Summary

Introduction

Much of the variation we see in our observations of the Sun is essentially random in nature, and is described as noise. There are many sources of true periodic signals which can be identified, such as the 11-yr sunspot cycle, the p modes seen at the photosphere, and the 3-min chromospheric oscillations. These periodicities are well established and are always present in the Sun, but there are many transient phenomena which produce oscillatory signals. The 3-min chromospheric observations were first reported in the 1970s (Bhatnagar & Tanaka 1972) and their origin is thought to be linked to the photospheric p modes, driven by activity in the solar interior. The cut-off frequency in an isothermal atmosphere is given by ωc

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