Abstract
Abstract In this Letter we report the detection of chromospheric 3-minute oscillations in disk-integrated EUV irradiance observations during a solar flare. A wavelet analysis of detrended Lyα (from GOES/EUVS) and Lyman continuum (from Solar Dynamics Observatory (SDO)/EVE) emission from the 2011 February 15 X-class flare (SOL2011-02-15T01:56) revealed a ∼3 minute period present during the flare’s main phase. The formation temperature of this emission locates this radiation at the flare’s chromospheric footpoints, and similar behavior is found in the SDO/Atmospheric Imaging Assembly 1600 and 1700 Å channels, which are dominated by chromospheric continuum. The implication is that the chromosphere responds dynamically at its acoustic cutoff frequency to an impulsive injection of energy. Since the 3-minute period was not found at hard X-ray (HXR) energies (50–100 keV) in Reuven Ramaty High Energy Solar Spectroscopic Imager data we can state that this 3-minute oscillation does not depend on the rate of energization of non-thermal electrons. However, a second period of 120 s found in both HXR and chromospheric lightcurves is consistent with episodic electron energization on 2-minute timescales. Our finding on the 3-minute oscillation suggests that chromospheric mechanical energy should be included in the flare energy budget, and the fluctuations in the Lyα line may influence the composition and dynamics of planetary atmospheres during periods of high activity.
Highlights
Quasi-periodic pulsations (QPPs) are widely reported in emission from solar flares. These are regular fluctuations in the flare radiation intensity, which are very clear in hard X-rays (HXRs) and radio waves generated by non-thermal electrons (Nakariakov & Melnikov 2009; Inglis et al 2016 and Van Doorsselaere et al 2016), but are detected over a wide range of wavelengths
QPPs with periods of 1 minute have been found in UV/EUV/SXR flare emission (e.g., Dolla et al 2012; Simões et al 2015), perhaps signaling MHD oscillations in post-flare coronal loops, or coronal loop-filling by heated plasma expanding from a periodically heated chromosphere
The 180 s period is not apparent in the quiescent, full-disk signal from GOES/EUV Sensor (EUVS)-E or Solar Dynamics Observatory (SDO)/EUV Variability Experiment (EVE), presumably due to the incoherence of the signal in disk-integrated emission, longer-period (300 s) oscillations are apparent in non-flaring regions of Atmospheric Imaging Assembly (AIA) 1600 and 1700 Å images
Summary
Quasi-periodic pulsations (QPPs) are widely reported in emission from solar flares. These are regular fluctuations in the flare radiation intensity, which are very clear in hard X-rays (HXRs) and radio waves generated by non-thermal electrons (Nakariakov & Melnikov 2009; Inglis et al 2016 and Van Doorsselaere et al 2016), but are detected over a wide range of wavelengths. QPPs in non-thermal signatures are widely interpreted as either revealing the magnetohydrodynamic (MHD) oscillation modes of the flare’s magnetic environment or reflecting an oscillatory driver for electron acceleration. Convincing observational evidence for impulsive excitation of oscillations at the acoustic cutoff frequency was recently presented by Kwak et al (2016), who detected periods of 2.7–3.3 minutes in response to a strong downflow event detected in Hα+0.5 Å. This was seen in spatially resolved chromospheric (Mg II, Ca II) and transition-region (C II, Si IV) lines measured by IRIS.
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