Quasi-periodic pulsations (or QPPs) are periodic intensity variations in the flare emission that occur across all wavelength bands. In this article, we review the observational and modelling achievements since the previous review on this topic by Nakariakov and Melnikov (Space Sci. Rev.149, 119, 2009). In recent years, it has become clear that QPPs are an inherent feature of solar flares because almost all flares exhibit QPPs. Moreover, it is now firmly established that QPPs often show multiple periods. We also review possible mechanisms for generating QPPs. Up to now, it has not been possible to conclusively identify the triggering mechanism or cause of QPPs. The lack of this identification currently hampers possible seismological inferences of flare plasma parameters. QPPs in stellar flares have been detected for a long time, and the high-quality data of the Kepler mission allows studying the QPP more systematically. However, it has not been conclusively shown whether the timescales of stellar QPPs are different or the same as those in solar flares.

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

The phenomenon of quasi-periodic pulsations (QPPs) in solar and stellar flares has been known for over 50 years and significant progress has been made in this research area. It has become clear that QPPs are not rare—they are found in many flares and, therefore, robust flare models should reproduce their properties in a natural way. At least fifteen mechanisms/models have been developed to explain QPPs in solar flares, which mainly assume the presence of magnetohydrodynamic (MHD) oscillations in coronal structures (magnetic loops and current sheets) or quasi-periodic regimes of magnetic reconnection. We review the most important and interesting results on flare QPPs, with an emphasis on the results of recent years, and we present the predicted and prominent observational signatures of each of the fifteen mechanisms. However, it is not yet possible to draw an unambiguous conclusion as to the correct underlying QPP mechanism because of the qualitative, rather than quantitative, nature of most of the models and also due to insufficient observational information on the physical properties of the flare region, in particular the spatial structure of the QPP source. We also review QPPs in stellar flares, where progress is largely based on solar-stellar analogies, suggesting similarities in the physical processes in flare regions on the Sun and magnetoactive stars. The presence of QPPs with similar properties in solar and stellar flares is, in itself, a strong additional argument in favor of the likelihood of solar-stellar analogies. Hence, advancing our understanding of QPPs in solar flares provides an important additional channel of information about stellar flares. However, further work in both theory/simulations and in observations is needed.

Quasi-Periodic Pulsations in Solar and Stellar Flares: A Review of Underpinning Physical Mechanisms and Their Predicted Observational Signatures

Quasi-periodic pulsations (QPPs) appear to be a common feature observed in the light curves of both solar and stellar flares. However, their quasi-periodic nature, along with the facts that they can be small in amplitude and short-lived, make QPPs difficult to unequivocally detect. In this paper, we test the strengths and limitations of state-of-the-art methods for detecting QPPs using a series of hare-and-hounds exercises. The hare simulated a set of flares, both with and without QPPs of a variety of forms, while the hounds attempted to detect QPPs in blind tests. We use the results of these exercises to create a blueprint for anyone who wishes to detect QPPs in real solar and stellar data. We present eight, clear recommendations to be kept in mind for future QPP detections, with the plethora of solar and stellar flare data from new and future satellites. These recommendations address the key pitfalls in QPP detection, including detrending, trimming data, accounting for coloured noise, detecting stationary-period QPPs, detecting QPP with non-stationary periods, and ensuring detections are robust and false detections are minimized. We find that QPPs can be detected reliably and robustly by a variety of methods, which are clearly identified and described, if the appropriate care and due diligence is taken.

Quasi-periodic pulsations in solar and stellar flares : an overview of recent results

Earth and the Universe

While a description of the quiescent chromosphere outside of flaring time intervals and away from flare locations is presented in Chap. 5, a complementary view of the chromosphere under flaring conditions is given here.

Flares: Thermal Emission

Many-site multichannel simultaneous observations of the flare star EV Lac revealed high-frequency brightness os- cillations superimposed on the mean flare light curve. Oscilla- tions occur at the earliest stage of a flare and are visible during the high state. Coherent oscillations of 26 and 13 sec were seen during three flare events. The pulsed fractions were 2.5% in the B band and 10-15% in the U band. This oscillatory phenomenon can be used to improve our understanding of the flare process.

http://aa.springer.de/papers/0364002/2300641.pdf

Detection of high-frequency optical oscillations on the flare star EV Lacertae

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Detection of high-frequency optical oscillations on the flare star EV Lacertae