Temporal Evolution of Chromospheric Oscillations in Flaring Regions - A Pilot Study

TL;DR

This study investigates how chromospheric oscillations in active regions change during solar flares, revealing power enhancements and depletions that could serve as early flare indicators and reflect energy conversion processes.

Contribution

It provides new insights into the temporal evolution of chromospheric oscillations during flares using high-cadence H-alpha observations, highlighting potential early warning signals.

Findings

Low-frequency power increases before and after flares.

Power depletion at flare maximum varies with frequency and time.

Potential link between wave power changes and energy conversion during flares.

Abstract

We have analyzed H-alpha intensity images obtained at a 1 minute cadence with the Global Oscillation Network Group (GONG) system to investigate the properties of oscillations in the 0-8 mHz frequency band at the location and time of strong M- and X-class flares. For each of three sub-regions within two flaring active regions, we extracted time series from multiple distinct positions, including the flare core and quieter surrounding areas. The time series were analyzed with a moving power map analysis to examine power as a function of frequency and time. We find that, in the flare core of all three sub-regions, the low-frequency power (~1-2 mHz) is substantially enhanced immediately prior to and after the flare, and that power at all frequencies up to 8 mHz is depleted at flare maximum. This depletion is both frequency and time dependent, which probably reflects the changing depths visible during the flare in the bandpass of the filter. These variations are not observed outside the flare cores. The depletion may indicate that acoustic energy is being converted into thermal energy at flare maximum, while the low-frequency enhancement may arise from an instability in the chromosphere and provide an early warning of the flare onset. Dark lanes of reduced wave power are also visible in the power maps, which may arise from the interaction of the acoustic waves and the magnetic field.