# Quasi-periodic pulsations in a solar flare with an unusual phase shift

## TL;DR

This study investigates quasi-periodic pulsations (QPPs) during the decay phase of a solar flare, revealing their association with a post-flare loop and linking their periods to standing slow magneto-acoustic waves triggered by a Moreton wave.

## Contribution

It provides new insights into the origin of QPPs during flare decay, connecting them to energy relaxation processes and magneto-acoustic wave dynamics.

## Key findings

- QPPs with periods of 74-80 s are observed in X-ray and microwave emissions.
- QPPs in temperature and emission measure are anti-phase.
- QPPs are linked to standing slow magneto-acoustic waves.

## Abstract

Two kinds of processes could occur during the flare decay phase: processes of energy release or processes of energy relaxation. Quasi-periodic pulsations (QPPs) of the broadband emission are a good tool for the verification of mechanisms. We aim to study the processes during the decay phase of the X-class solar flare SOL2014-03-29T17:48. The observations in X-ray, microwave, and extreme ultraviolet (EUV) bands are exploited to study the fine temporal, spatial, and spectral structures of the flare. The periods, amplitudes, and phases of both the fluxes and physical parameters (emission measure, temperature) are studied using standard methods of correlation, Fourier, and wavelet analyses. It is found that the source of the QPPs is associated with the uniform post-flare loop. The X-ray source is located at the top of the arcade. QPPs with the similar characteristic time scales of $P\approx$ 74--80 s are found in the X-ray (3--25 keV) and microwave (15.7 GHz) emissions. Besides, QPPs with the same period are found in the time profiles of both the temperature ($T_e$) and emission measure ($EM$). The QPPs in temperature and the QPPs in emission measure demonstrate anti-phase behavior. The analysis reveals the quasi-periodic process of energy relaxation, without any additional source of energy during the decay phase. The periods of the QPPs are in a good agreement with second harmonic of standing slow magneto-acoustic wave in the arcade which could be triggered by a Moreton wave initiated by the flare in the direct vicinity of the arcade.