The soft X-ray emissions of solar flares come mainly 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 bulk of the total flare energy goes elsewhere. Recently Dolla et al. (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. We analyze the suitability of the GOES data for this kind of analysis and find them to be generally valuable after Sept. 2010 (GOES-15). We then extend Dolla et al. results to a list of X-class flares from Cycle 24, and show that most of them display QPP in the impulsive phase. During the impulsive phase the footpoints of the newly-forming flare loops may also contribute to the observed soft X-ray variations. The QPP 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 show that there is minimal phase difference between the differenced GOES energy channels, nor between them and the hard X-ray variations on short time scales. We deploy different techniques to characterize the GOES QPP, finding no systematic patterns in spite of their general prevalence and usually no strong signature of a strict periodicity. 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 large irradiance values makes these more uncertain. We note that the QPP of the impulsive phase can result from broadband variations, for example in red-noise power law distribution, in the flare development from its basic plasma instability.
P. Simoes, H. Hudson and L. Fletcher
Wed, 10 Dec 14
Comments: 15 pages, 7 figures