Abstract
The soft X-ray emissions (hν>1.5 keV) of solar flares mainly come from the bright coronal loops at the highest temperatures normally achieved in the flare process. Their ubiquity has led to their use as a standard measure of flare occurrence and energy, although the overwhelming bulk of the total flare energy goes elsewhere. Recently Dolla et al. (Astrophys. J. Lett. 749, L16, 2012) noted quasi-periodic pulsations (QPP) in the soft X-ray signature of the X-class flare SOL2011-02-15, as observed by the standard photometric data from the GOES (Geostationary Operational Environmental Satellite) spacecraft. In this article we analyse the suitability of the GOES data for this type of analysis and find them to be generally valuable after September, 2010 (GOES-15). We then extend the result of Dolla et al. to a complete list of X-class flares from Cycle 24 and show that most of them (80 %) display QPPs in the impulsive phase. The pulsations show up cleanly in both channels of the GOES data, making use of time-series of irradiance differences (the digital time derivative on the 2-s sampling). We deploy different techniques to characterise the periodicity of GOES pulsations, considering the red-noise properties of the flare signals, finding a range of characteristic time scales of the QPPs for each event, but usually with no strong signature of a single period dominating in the power spectrum. The QPP may also appear on somewhat longer time scales during the later gradual phase, possibly with a greater tendency towards coherence, but the sampling noise in GOES difference data for high irradiance values (X-class flares) makes these more uncertain. We show that there is minimal phase difference between the differenced GOES energy channels, or between them and the hard X-ray variations on short time scales. During the impulsive phase, the footpoints of the newly forming flare loops may also contribute to the observed soft X-ray variations.
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Acknowledgements
We would like to thank the anonymous referee for the comments and suggestions that helped to improve the paper. We would like to thank Marie Dominique and Rodney Viereck for helpful discussions. HSH thanks NASA for support under contract NAS 5-98033 for RHESSI, and acknowledges warm hospitality at the University of Glasgow. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 606862 (F-CHROMA) (PJAS, LF), from STFC grant ST/I001808/1 (PJAS, LF) and ST/L000741/1 (LF).
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Solar and Stellar Flares: Observations, Simulations, and Synergies
Guest Editors: Lyndsay Fletcher and Petr Heinzel
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Simões, P.J.A., Hudson, H.S. & Fletcher, L. Soft X-Ray Pulsations in Solar Flares. Sol Phys 290, 3625–3639 (2015). https://doi.org/10.1007/s11207-015-0691-2
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DOI: https://doi.org/10.1007/s11207-015-0691-2