Soft X-Ray Pulsations in Solar Flares

Abstract

The soft X-ray emissions (hν > 1.5 keV) 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 overwhelming bulk of the total flare energy goes elsewhere. Recently Dolla et al. (2012, Astrophys. J. Lett. 749 L16) 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 paper we analyze the suitability of the GOES data for this kind of analysis and find them to be generally valuable after September, 2010 (GOES-15). We then extend the Dolla et al. result 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 characterize the periodicity of GOES pulsations, considering the red-noise properties of the flare signals, finding a range of chracteristic 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 large irradiance values (Xclass 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.

Quasi-periodic Pulsations in Solar and Stellar Flares: An Overview of Recent Results (Invited Review)

“…In some cases, the QPPs occur in both the rise phase and the gradual (decay) phase of the flare (e.g. Van Doorsselaere et al, 2011;Dolla et al, 2012;Simões, Hudson, and Fletcher, 2015). However, in some cases, the QPPs are restricted to only the rise phase (Jakimiec and Tomczak, 2012, and follow-up works), but in other cases, the QPPs are observed during the decay phase only (e.g.…”

“…in the confidence level above the power law in Fourier space (rather than the confidence above the white noise, as is commonly assumed). This was taken into account by Simões, Hudson, and Fletcher (2015), who performed a large study of 35 flares. They still found with 97.5% confidence that most flares (80% of 35 flares) had significant oscillations, above the red noise level.…”

“…Kupriyanova et al (2010); Simões, Hudson, and Fletcher (2015) found that QPPs are very common, but Mossessian and Fleishman (2012) found that they are rare. It should be investigated if there are selection effects in these articles for selecting the events, or for the period detection.…”

Oscillations Accompanying a He i 10830 Å Negative Flare in a Solar Facula

“…Based on RATAN-600 microflare observations, periods of 1.4 and 0.7 s were found in the radio emission intensity and polarization signals, respectively . Simões et al (2015) found that 28 out of 35 researched x-flares showed QPPs. Some authors relate changes in the period with the flare phases.…”