To shed more light on the nature of the observed Lyα absorption during transits of HD 209458b and to quantify the major mechanisms responsible for the production of fast hydrogen atoms (the so-called energetic neutral atoms, ENAs) around the planet, 2D hydrodynamic multifluid modeling of the expanding planetary upper atmosphere, which is driven by stellar XUV, and its interaction with the stellar wind has been performed. The model selfconsistently describes the escaping planetary wind, taking into account the generation of ENAs due to particle acceleration by the radiation pressure and by the charge exchange between the stellar wind protons and planetary atoms. The calculations in a wide range of stellar wind parameters and XUV flux values showed that under typical Sun-like star conditions, the amount of generated ENAs is too small, and the observed absorption at the level of 6%-8% can be attributed only to the non-resonant natural line broadening. For lower XUV fluxes, e.g., during the activity minima, the number of planetary atoms that survive photoionization and give rise to ENAs increases, resulting in up to 10%-15% absorption at the blue wing of the Lyα line, caused by resonant thermal line broadening. A similar asymmetric absorption can be seen under the conditions realized during coronal mass ejections, when sufficiently high stellar wind pressure confines the escaping planetary material within a kind of bowshock around the planet. It was found that the radiation pressure in all considered cases has a negligible contribution to the production of ENAs and the corresponding absorption.

We study the short-periodic component of stellar activity with a cycle periods P cyc < 10 3 days using the Kepler mission photometry of fast-rotating (1 < Ρ < 4 days) stars with spectra of M4V to F3V. Applying the originally developed two non-spectral methods, we measured the effective period of stellar cycles in 462 objects. The obtained results are in accordance with previous measurements by , do not seem to result from a beating effect. The performed measurements of P cyc cluster in a specific branch which covers the previously unstudied region in the Saar-Brandenburg (1999) diagram, and connects the branch of inactive stars with the area populated by super-active objects. It is shown that the formation of the discovered branch is due to the cu-quenching effect, which saturates the magnetic dynamo and decreases the cycle periods with the increase of inverted Rossby number. This finding is important in the context of the discussion on catastrophic quenching and other heuristic approximations of the non-linear α-effect.Subject headings: stars: activity -stars: magnetic field -(stars:) starspots -stars: statistics IntroductionStellar activity is a key factor of space weather and a valuable information channel on processes in stellar interiors (e.g., differential rotation, circulation and convection). At the same time, the connection of these processes with surface activity is poorly known. Various theories of magnetic dynamos need further observational tests and verification. That is why the empirical approach plays an important role in studies of stellar activity phenomenology. The matter of this approach consists in the search for the relation between physical parameters of stars and their activity parameters (e.g., the cycle periods P cyc and the average activity levels A) and comparison of that with theoretical predictions.An interesting view on stellar activity is given in Fig. 1 (reproduced from Saar & Brandenburg, 1999, Fig. 5; Herein after SB diagram). It shows that the stars in relation to their ω cyc /Ω and the inversed Rossby number Ro -1 =2τ MLT Ω are clustered in 3 branches: inactive (I), active (A) and super-active (S) objects. Here ω cyc = 2π/P cyc is the angular velocity of the magnetic cycle, Ω = 2π/Ρ is the angular velocity of the stellar rotation, Ρ is the rotation period of star and TMLT is the turnover time according to the mixing length theory (MLT; Bohm-Vitense, 1958).Unfortunately, the limited sample of the considered stars in the SB diagram does not cover the region of log(ω cyc /Ω) > -2 at Ro -1 >1, which still has to be explored. The uncovered region corresponds to the fast rotating stars and the shortest stellar activity cycles. In the present paper, we investigate this unstudied area using new data from the Kepler mission.Our study is focused on the stars with 1 < Ρ < 4 days and shortest scale of activity cycles P cyc < 10 3 days. A few similar objects with short activity cycles were initially investigated using Earth-based data (Vida & Olah, 2006; Olah et al., 2009;Vida et al., 2013)...

The current knowledge on plasma mixing in stellar interiors is based on theoretical modeling, which calibrates the empirical relations between the mixing and observable stellar parameters. Hence, a model-independent method of studying the mixing process is a desirable, but yet unfounded approach. Here, using a new spectral-correlation method, verified for the Sun, we show that deep mixing is manifested in stellar photometric light curves. We measure the timescales of the stochastic change in the spectral power of rotational variability of hundreds of main-sequence stars from the Kepler mission archive. The discovered proportionality of these timescales to the turnover time of the standard mixing length theory, their consistency with Kolmogorov's theory of turbulence, and the relation with stellar activity provide a new approach to the probing of stellar interiors. This opens a broad perspective for experimental studies of deep mixing and magnetic dynamos in stars.