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Context. The short-time (< 700 days) periodicities of both the stellar and solar activity that controls space weather are usually are discussed as manifestations of Rossby modes in tachoclines. Various interpretations of this phenomenon that have been proposed, in particular for the Sun, can be verified by considering the broad population of nonsolar-type stars. Aims. We look for surface stellar activity features, drifting in longitude, and compare their drift rates with those predicted for different low-frequency waves in stellar photospheres and tachoclines. Methods. Analogously to the Hovmöller diagrams in meteorology, we constructed a dynamic diagram of stellar activity pattern (DDSAP) to visualize the rotational variability of the stellar radiation flux as a function of rotation phase and time. We used the high-precision light curves of the fast-rotating main sequence stars, with rotation periods of 0.5 to 4 days, from the Kepler mission database. Results. We found quasi-periodic drifting lanes (DLs) of various durations and intensities in the DDSAPs for 108 stars. In the course of analysis, we carried out a correction of the stellar rotation periods by nullifying the drifts of the longest-lasing DLs that are presumably related to the long-lived starspot complexes co-rotating with the star. We discovered a clear elongated cluster of the absolute values of the DLs’ drift rates versus the stellar effective temperatures. This cluster cannot be attributed to any accidental contaminations of the light curves or manifestation of waves in the stellar photospheres, because of their extremely short timescales. An approximate equality of the absolute values of positive and negative drift rates of the considered DLs makes it impossible to interpret them in terms of Kelvin and/or magneto-Rossby waves in the stellar tachoclines. It is only global kink-type Alfvénic oscillations of the tachocline as a whole that allow us to interpret the estimated drift rates forming the above-mentioned cluster, as well as the related activity periodicities and turnover times in the convective zones. The corresponding magnetic field strength appears to be about 50 kG, which is approximately in the middle of the range of assumptions discussed in the literature. Conclusions. Alfvén waves are an important, albeit commonly ignored factor in stellar interiors. Apparently, the global tachocline’s Alfvén waves ought to play a role in triggering emergence of the magnetic flux tubes. Their manifestation in stellar activity opens up a unique way of probing the magnetic field strength in tachoclines of individual stars. Since the investigations of the tachocline waves performed thus far have been based on the shallow-fluid approximation, and also assuming a rigid fixed bottom of the tachocline layer, the global kink-type Alvénic disturbances of the whole tachocline layer have not been considered. The reported observational detection of signatures of such waves, manifested in specific longitude drifts of the stelar surface activity pattern, calls for a more detailed theoretical study.

Context. The Rieger cycles were discovered in the Sun as a specific 154-day periodicity of flare occurrence; they strongly influence terrestrial space weather. This phenomenon is far from being understood. Various proposed mechanisms for this periodicity need further verification in stars with stellar parameters different from those of the Sun. Aims. In this work, we study the Rieger-type cycle (RTC) periods PRTC of stellar activity surveyed in the photometric data of the Kepler space telescope. Methods. The processing of 1726 stellar light curves reveals statistics of PRTC values for different main-sequence stars with different effective temperatures Teff and periods of rotation P. This study uses as an index of stellar activity the squared amplitude of the first rotational harmonic A12 of the stellar light curve variability. Results. The obtained information on PRTC of the considered stars confirms the phenomenological analogy between stellar RTCs and the solar Rieger cycles. Two types of RTCs were found: (1) activity cycles with PRTC independent on the stellar rotation, which are typical for the stars with Teff ≲ 5500 K, and (2) activity cycles with PRTC proportional to the stellar rotation period P, which take place on stars with Teff ≳ 6300 K. These two types of RTCs can be driven by the Kelvin and Rossby waves, respectively. The Rossby wave-driven RTCs show a relation with the location of tachocline at shallow depths in the hot stars. This confirms the theoretical predictions of the connection of the RTC with the tachocline. At the same time, the Kelvin wave-driven RTCs do not show this connection. Apparently, both types of wave drivers of RTCs can coexist, resulting in the joint modulation of the magnetic flux tubes emergence by Kelvin and Rossby waves, and the corresponding behavior of PRTC. Conclusions. The signatures of two types of wave drivers discovered for RTCs and their different relations with the tachocline call for a revision and further elaboration of the theory of RTCs.

Context. Until now the search of peculiar exoplanetary shadows, particularly those caused by exorings, was focused on the detection of a second-order photometric difference between the ringed and ringless (circular) transiting shadows. Both scenarios involved the parameter fitting to approximate the corresponding transit light curves (TLCs). As a result, the searched difference was extremely difficult to detect in the noise of the real transit photometry signals. Aims. In this work, we look for photometric manifestations of a non-spherical obscuring matter (e.g., exorings) around different exoplanets, mainly hot Jupiters, using a principally new approach. Methods. We used the transit parameters provided in Kepler database from the NASA Exoplanet Archive, where the fitting of the TLCs gives consistent sets of parameters for the transiting objects, assuming their spherical shape. At the same time, the semimajor axes, expressed in units of the stellar radii (initially, also a subject of the fitting), finally appear to be replaced by the calculated values according the Kepler’s third law and known stellar radii and surface gravity that have been determined through other methods. In the most typical case of a spherical transiting planet, such a replacement does not break the consistency of the whole parameter set. However, in the case of a non-spherical transiter and its non-circular shadow, the real (i.e., calculated according physics) value of the orbital semimajor axis could become inconsistent with the rest of the transit parameter set defined with the standard fitting procedure. The search for such inconsistencies, manifested as the difference between the simulated and observed transit duration, constitutes one of the main goals of this work. Moreover, we elaborate on a particular technique to gain information about the shape of planetary shadow, using the derivatives of the TLC during the ingress and egress phases. Results. We checked the TLCs of 21 hot Jupiters and 2 hot Neptunes. The consistent transit parameters and quasi-circular shadows were found for 11 objects. The analysis of the TLCs of five of the objects is complicated due to the noise problems, leading to the instability of solutions and deformation of shadows due to the low resolution of the derivatives. The remaining seven objects were formally qualified as peculiar outliers and among them, the planets Kepler-45b and Kepler-840b appear to be the most intriguing targets, with the most significant inconsistency of the parameter sets and the shadows elongated along their orbital path. Conclusions. We propose a new method for probing of planetary shape that confirms the circular transiting shadows for the majority of objects on the considered list. However, several objects exhibiting peculiar shadows have been discovered. These finds could be interpreted in terms of planetary dusty envelopes or exorings. The obtained results and elaborated methodology are relevant in the context of today’s photometry space missions, such as TESS, CHEOPS, and others.

Context. Hitherto, the study of exoplanetary transit timing and duration variability has supposed transit light curves (TLCs) to be symmetric, suggesting a priori a spherical shape for the transiting object, for example, an exoplanet. As a result, the independent positions of transit borders are unknown. However, the borders of TLCs are most sensitive to the presence of exo-rings and/or dust formations of great interest. Aims. For the first time we check for a timing variability of independently treated borders of transits of different types of exoplanets. Methods. Using quadratic approximation for the start-, end-, and minimum parts of the long-cadence TLCs from the Kepler mission archive after their whitening and phase folding, we find the corresponding transit border timings: Δts, Δte, respectively, and the TLC minimum time Δtm. These parameters were found separately for each folded TLC constructed in the consequent non-overlapping time-windows with the respective medium time tw. Temporal and cross-correlation analysis of the obtained series of Δts(tw), Δte(tw), and Δtm(tw) were applied for the detection and diagnostics of variability of transit borders and TLC asymmetry. Results. Among the considered TLCs of 98 Kepler objects of interest (KOIs), 15 confirmed giant exoplanets and 5 objects with still debatable status (probably non-planets) show variations in their transit timing parameters at timescales from ≈400 to ≳1500 days. These variations are especially well manifested as an anti-correlation between Δts and Δte, indicating variability in the dimensions of transiting shadows, especially along their trajectories. There are also objects with well pronounced oscillations of transit border timing and asymmetry. Conclusions. The discovered variability of transit timing is important as an indicator of large-scale non-stationary processes in the atmospheres of KOIs, as well as dust and aerosol generation in their upper layers and in their close vicinity. These findings highlight the need for a dedicated and detailed study.

The variability of Jovian decametric emission (DAM), which is not controlled by the Io satellite (non-Io), was studied of timescales of between 1 min and 1 h with DAM records of 1991–2007 from the archive of Nancay Radio Observatory. We found that the internal structure of the non-Io radio storms has the dominating periodicity on of average 21 ± 2 min . This estimate is in close agreement with the periodicity of Io-related emission and with fundamental eigenoscillations of transversal magnetic pulsations in the Io’s plasma torus. The average duration of an arc in non-Io DAM dynamic spectra was estimated to be 7.0 ± 0.3 min, although the most probable value was 5.3± 0.7 min, which corresponds to Io-related arcs and 3rd and 4th harmonics of torus proper-oscillations. Our results could be interpreted in terms of electron acceleration in parallel electric fields of standing Alfven waves generated by Io and trapped in the Io torus. This interpretation agrees with previous arguments that the location of some sources of non-Io emission are in the magnetic shell of the Io plasma torus. However, the aurora component of non-Io DAM is not excluded, since the suggested DAM modulation by the fundamental mode of eigenoscillations inherent in aurora magnetic-lines is undetectable in our analysis because of the short duration of non-Io radio storms.

Short (S-) bursts of Jovian decametric emission show random behavior with very complicated forms in the dynamic spectrum. We first draw attention to the correlation properties of S-bursts as an indicator of their hidden regularity. Thus we calculate the linear correlation coefficient r(f − fo, Δt) between the spectral intensity in the current (f ) and the reference (fo) frequency channels with Δt time shift. The result is displayed in the form of 2D-correlation image, where the pixel brightness reflects the r estimate, and the pixel’s co-ordinates are f and Δt. The non-trivial and stabile correlation patterns are found, described, and discussed here. Apparently, there is the hidden wave-like modulation of the whole S-burst storm in the form of a correlation or anti-correlation between emissions at different radio frequencies with short time scales from 0.02 s to 0.8 s.

There are theoretical and experimental arguments for a modulation of Jovian decametric S-emission by standing Alfven waves in the regions of radio sources. One effect of such modulation is the concentration of S-burst emission around certain frequencies (Sbands) in dynamic spectra of radio emission. It is argued that S-bands are generated at the local electron cyclotron frequencies, around nodes of a standing Alfven wave. To test this hypothesis, we derived several verifiable predictions: the anti-correlation of radio-flux in adjacent S-bands at short time scales ( 0. 1s ); and the transformation of the broadband S-storm into S-bands. We searched for and found these effects experimentally. Because the predictions are confirmed, the standing wave modulation of Jovian S-emission appears valid.

The variability of Jovian decametric emission (DAM) is studied at time scales from 1 min to 1 h with DAM records of 1991–2007 from the archive of Nancay Radio Observatory. We found that the internal structure of the Io-related radio storms has the dominating periodicity of 23±2 min on average. This estimate practically coincides with the fundamental eigenoscillations of transversal magnetic pulsations in the Io plasma torus. Our autocorrelation analysis confirms the excess of DAM variations with the time scale of the fundamental and first harmonic periods of the Io torus. Moreover, the time scale of arc pattern in DAM dynamic spectra is estimated to be 5.4 min, which corresponds to Io’s Alfven wing diameter or to the 3rd and 4th harmonics of torus proper oscillations. These results could be interpreted in terms of electron acceleration in field-aligned electric fields of standing Alfven waves trapped in the Io torus. There is an analogous modulation of auroral kilometric radiation of the Earth by magnetic pulsations at field line resonances in the terrestrial magnetosphere (Hanasz et al. 2006, J. Geophys. Res., 111, A03209).

The flux variability of Jovian decametric radio emission (DAM) is analyzed to search for its possible modulations by magnetohydrodynamic waves of ultra-low frequencies (ULF). The ULF modulating waves are found in DAM dynamic spectra as a moving pattern (moire) on the background of Jovian millisecond radio bursts (S-bursts). Their frequency drift (~55 MHz/s) corresponds to the wave motion to Jupiter with Alfven velocity (~ 4 $\times$ 10 4 km s -1 ). There are whistler-like drifting details in the dynamic power spectra of ULF variations of S-burst emission flux at fixed radio frequency. Their frequency drift rates are consistent with whistlers that are dispersed mainly in the Io torus. Formally our analysis of DAM reveals the specific 2s-modulation of S-burst emission corresponding to the known magnetic pulsations near gyrofrequencies of heaviest ions (SO$_{2}^{+}$ and SO + ) in the Io torus.

Most modulation lanes in dynamic spectra of Jovian decametric emission (DAM) are formed by radiation scattering on field-aligned inhomogeneities in the Io plasma torus. The positions and frequency drift of hundreds of lanes have been measured on the DAM spectra from UFRO archives. A special 3D algorithm is used for localization of field-aligned magnetospheric inhomogeneities by the frequency drift of modulation lanes. It is found that some lanes are formed near the magnetic shell of the satellite Amalthea mainly at longitudes of $123^{\circ}\leq\lambda_{\rm III}\leq140^{\circ}$ (north) and $284^{\circ}\leq\lambda_{\rm III}\leq305^{\circ}$ (south). These disturbances coincide with regions of plasma compression by the rotating magnetic field of Jupiter. Such modulations are found at other longitudes too (189° to 236°) with higher sensitivity. Amalthea's plasma torus could be another argument for the ice nature of the satellite, which has a density less than that of water.

System of periodic peaks is found in the power spectra of ULF flux oscillations of Jovian decametric emission, which is observed at fixed frequencies. This result could be interpreted in terms of an ionospheric Alfven resonator near Jupiter. Standing Alfven wave could explain the system of emission bands in the dynamic spectrum of decametric emission. The space period of such bands and the frequency of standing waves have been used to estimate electron number density in the low magnetosphere of Jupiter. The result is consistent with radio occultation data. Hence, the ULF modulation hypothesis appears validated.

Aims. We interpret long-periodic (minutes) modulations detected in solar microwave emission during flaring events as signatures of large-scale transverse oscillations of coronal loops. Methods. Our data analysis method is based methodologically on a sliding-window Fourier transform combined with the VignerWille technique. We analyze three different events where TRACE detected post-flare oscillating loops (on Mar. 23, 2000; Sep. 15, 2001; Sep. 07, 2001) Results. For the transverse large-scale oscillatory motion of a loop, a properly located observer, in addition to the modulation caused by the emission diagram pattern motion at the main frequency of the loop oscillation, may detect a modulation at twice the frequency, produced by the varying magnetic field during each inclination of the loop. Our main result consists in identification of these “modulation pairs” in the dynamic spectra of solar microwave emission and their association with the observed oscillating coronal loops.