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

The emission from the gas behind the front of a stationary shock wave is calculated for the conditions in the atmospheres of red dwarf stars for velocities u0 from 30 to 100 km/s. Energy exchange between the electron and atom-ion components is taken into account through elastic collisions, free-free, bound-bound, and bound-free collisional and radiative transitions of hydrogen in the radiation field of a star’s photosphere. Cooling by the following chemical elements is included: C, N, O, Mg, Si, S, Ca, and Fe. The following results are obtained: the post-shock gas remains transparent in the optical range of the continuum throughout the emission time; hence, it cannot be a source of the black-body radiation that appears at times during flares. The recombination and bremsstrahlung radiation of the transparent gas, as well as the flux in the Balmer series lines represent a few percent of the energy flux of matter through the viscous jump. The ratio of the fluxes in the spectrum lines and the continuum depends on u0 and on the parameters of the atmosphere. These results are consistent with the idea of multicomponent emission in flares, specifically, line emission is determined by the shock wave in layers above the photosphere, while black-body radiation comes from the photosphere heated by a flux of suprathermal particles.

Properties of Radiative Shock Waves in the Atmospheres of Red Dwarf Stars

Results of cooperative observations of the flare star EV Lac in September 1993 are presented. One of the about 30 optical flares detected was powerful enough to permit a quantitative analysis of its intrinsic radiation with the colour-colour technique. Sinusoidal brightness variations due to spottedness of the stellar surface was found to have an amplitude Δ V = 0.m0.24. Behaviour of the K band stellar brightness during strong and weak U band flares are considered. The upper limits of very fast optical brightness variations were estimated during both a moderate flare and quiet state of the star. No decametric bursts were observed during the campaign that could be certainly attributed to flare activity.

Coordinated observations of the red dwarf flare star EV Lac in 1998

Regular photometric observations of the two active flare stars EV Lac and AD Leo were carried out in 1996–1999. We have compared the location of the intrinsic radiation of the strongest flares in UBVRI two-color diagrams with those predicted by various models for the radiation sources. Our statistical analysis of flares in EV Lac confirms the 7.5-year activity cycle reported earlier.

Rapid UBVRI photometry of the active flare stars EV Lac and AD Leo

In 1967-1973 and in 1986-1995, two sets of international cooperative programmes were initiated by the Crimean Astrophysical Observatory for studying UV Cet-type flare stars. The first was devoted to photometric observations with the purpose of studying the temporary distribution of flares and their power; during the second series of campaigns the photometric monitoring of one of the most active stars of this type EV Lac was accompanied by various observations (spectral, polarimetric, ultraviolet, infrared and radio) of this object. A brief list of data received, some results on their astrophysical analysis and its later development are given.

Some results of international campaigns initiated by the crimean astrophysical observatory for investigations of flare stars

A program of Many-sites Multi Channel simultaneous observations of flare stars by a network of telescops was initiated in 1998. The purpose of this program is to investigate the fine structure of the flares light-curve in UBVRI, depressing atmospheric and instrumental noises. In this paper we present the results of the observations on the Flare star EV Lac in the B color, which were carried out at the Stephanion observatory during the campaign of 1999. Discrete Fourier Transform analysis of the B-light curve of the largest flares reveal the presence of high frequency small amplitude oscillations with periods ranging between 6.9 sec and 20 sec in around the flare maximum phase. The oscillation amplitude ranges from 1.4 to 2.6 % of the star brightness in accordance with the results of the campaign of 1998 (Zhilyaev et al. 2000). In addition there is a tendency the higher frequencies to persist in the late phases of the flare evolution. The observations are consistent with the phenomenology of the evolution of a fast mode magneto-acoustic wave generated at the impulsive phase of the flare and traveling through the magnetic loop. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Detection of high‐frequency optical oscillation during the flare phase of EV Lac in 1999

The flares fine structure lies often below the detec- tion limit for photometry data. UBVRI high-speed observa- tions combined with synchronous monitoring at two telescopes situated far from each other allowed us to detect new small- amplitude features in the flare activity of EV Lac. Digital fil- tering of the light curves with a Gaussian window was used. We found that some flare events are composed of a flare and an intensity drop having a crater-shaped structure centered at the flare peak. Fading amplitudes extend from 0.1 to 0.01 mag.

http://aa.springer.de/papers/8334003/2300931.pdf

New features in the EV Lacertae flares discovered by fast high precision UBVRI photometry

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Coordinated observations of the red dwarf flare star EV Lac in 1998

Rapid UBVRI photometry of the active flare stars EV Lac and AD Leo

Detection of high‐frequency optical oscillation during the flare phase of EV Lac in 1999

New features in the EV Lacertae flares discovered by fast high precision UBVRI photometry