Characterising the long-term evolution of magnetic activity on Sun-like stars is important not only for stellar physics but also for understanding the environment in which planets evolve. In the past decades, many photometric surveys of open clusters have produced extensive rotation period measurements on Sun-like stars of different ages. I use these data to infer the long-term evolution of Ca II chromospheric and X-ray coronal emission on solar mass stars combining a best fit parametric model of their rotation evolution with empirical rotation-activity relationships.
The atmospheric activity of the Sun and Sun-like stars is analyzed involving observations from HK-project at the Mount Wilson Observatory, the California and Carnegie Planet Search Program at the Keck and Lick Observatories and the Magellan Planet Search Program at the Las Campanas Observatory. We show that for stars of F, G and K spectral classes, the cyclic activity, similar to the 11-yr solar cycles, is different: it becomes more prominent in K-stars. Comparative study of Sun-like stars with different levels of the chromospheric and coronal activity confirms that the Sun belongs to stars wi…
We applied the method of continuous wavelet-transform to the time-frequency analysis to the sets of observations of relative sunspot numbers, sunspot areas and to 6 Mount Wilson HK-project stars with well-defined magnetic cycles. Wavelet analysis of these data reveals the following pattern: at the same time there are several activity cycles whose periods vary widely from the quasi-biennial up to the centennial period for the Sun and vary significant during observations time of the HK-project stars. These relatively low-frequency periodic variations of the solar and stellar activity gradually c…
Magnetic activity is a global property of the Sun; the complex processes of solar activity are connected with the solar magnetic fields. For solar-type stars and the Sun magnetic activity depends on the physical parameters of the star. In this article we study the relationships between the duration of activity cycle and effective temperature for solar-type stars and the Sun. We've tried to explain these relationships due to the existence of layer of laminar convection near the bottom of the convective zone of the star. In this layer the Rossby waves are formed. They generate the primary poloid…
The atmospheric activity of the Sun and solar-type stars is analysed involving observations from HK-project at the Mount Wilson Observatory, the California and Carnegie Planet Search Program at the Keck and Lick Observatories, and the Magellan Planet Search Program at the Las Campanas Observatory. We show that for stars of F, G and K spectral classes, the cyclic activity, similar to the 11-yr Solar cycles, is different: it becomes more prominent in K-stars. Comparative study of solar-type stars with different levels of the chromospheric and coronal activity confirms that the Sun belongs to sta…
The analysis of the effect of the hysteresis, which manifests itself in an ambiguous relationship of radiation from the photosphere, chromosphere and corona on the rise and decline phases of the solar and stellar activity cycles have done. Some indices of solar activity: the flux of the hydrogen Lyman-alpha emission, the 10.7 cm flux, the sunspot number SSN, the flux in the coronal green line 530.3 nm, the solar constant TSI and the 280 nm Mg II flux were studied. The stars with the well-determined cycles were examined to detect the effect of hysteresis between the chromosphere's S-index CaII …
The sunspot position from Greenwich Photoheliographic Results (GPR), US Air Force Solar Optical Observing Network and National Oceanic and Atmospheric Administration (USAF/NOAA), and Debrecen Photoheliographic Data (DPD) data bases in the period 1874 to 2016 were used to calculate yearly values of the solar differential-rotation parameters $A$ and $B$. The calculated differential-rotation parameters were compared with the solar-activity level. We found that the Sun rotates more differentially at the minimum than at the maximum of activity during the 1977 - 2016 epoch. An inverse correlation be…
Sunspots are a canonical marker of the Sun's internal magnetic field which flips polarity every ~22-years. The principal variation of sunspots, an ~11-year variation in number, modulates the amount of magnetic field that pierces the solar surface and drives significant variations in our Star's radiative, particulate and eruptive output over that period. This paper presents observations from the Solar and Heliospheric Observatory and Solar Dynamics Observatory indicating that the 11-year sunspot variation is intrinsically tied it to the spatio-temporal overlap of the activity bands belonging to…
We study the variability of solar activity using new photospheric proxies originally developed for the analysis of stellar magnetism with the CoRoT and Kepler photometric observations. These proxies are obtained by tracking the temporal modulations in the observations associated to the spots and magnetic features as the Sun rotates. We analyze here 21 years, spanning solar cycles 23 and 24, of the observations collected by the space-based photometric VIRGO and radial velocity GOLF instruments on board the SoHO satellite. The photospheric activity proxy Sph is then calculated for each of the th…
We present a brief review of observational results contributing to modern ideas on the evolution of stellar activity. Basic laws, derived for both rotation-age and activity-rotation relationships, allow us to trace how the activity of low-mass stars changes with age during their stay on the main sequence. We focus on the evaluation of the activity properties of stars that could be analogs of the young Sun. Our study includes joint consideration of different tracers of activity, rotation and magnetic fields of Sun-like stars of various ages. We identify rotation periods, when the saturated regi…
The activity of the Sun as a result of cyclic changes of the global magnetic field is studied. As a consequence of the analysis of magnetic activity of solar-type stars the following power dependencies were found: the dependence between the rotation periods and the effective temperatures, the dependence between the duration of the "11-year" cycles of activity and the effective temperatures, and the dependence between the duration of quasi-biennial cycles and the effective temperatures. It is shown that the physical nature of these dependencies associated with the observed properties of solar-t…
In previous work we identified six Sun-like stars observed by Kepler with exceptionally clear asteroseismic signatures of rotation. Here, we show that five of these stars exhibit surface variability suitable for measuring rotation. In order to further constrain differential rotation, we compare the rotation periods obtained from light-curve variability with those from asteroseismology. The two rotation measurement methods are found to agree within uncertainties, suggesting that radial differential rotation is weak, as is the case for the Sun. Furthermore, we find significant discrepancies betw…
The strong chromospheric absorption lines Ca H & K are tightly connected to stellar surface magnetic fields. Only for the Sun, spectral activity indices can be related to evolving magnetic features on the solar disk. The Solar Disk-Integrated (SDI) telescope feeds the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) of the Large Binocular Telescope (LBT) at Mt. Graham International Observatory (MGIO), Arizona, U.S.A. We present high-resolution, high-fidelity spectra that were recorded on 184 & 82 days in 2018 & 2019 and derive the Ca H & K emission ratio, i.e., the S-index. In…
The Sun and stars with low magnetic activity levels, become photometrically brighter when their activity increases. Magnetically more active stars display the opposite behaviour and get fainter when their activity increases. We reproduce the observed photometric trends in stellar variations with a model that treats stars as hypothetical Suns with coverage by magnetic features different from that of the Sun. The presented model attributes the variability of stellar spectra to the imbalance between the contributions from different components of the solar atmosphere, such as dark starspots and br…
The rotation of sunspots of 2 yr in two different solar cycles is studied with the data from the Helioseismic and Magnetic Imager on board the \it Solar Dynamics Observatory \rm and the Michelson Doppler Imager instrument on board the \it Solar and Heliospheric Observataory.\rm We choose the $α$ sunspot groups and the relatively large and stable sunspots of complex active regions in our sample. In the year of 2003, the $α$ sunspot groups and the preceding sunspots tend to rotate counterclockwise and have positive magnetic polarity in the northern hemisphere. In the southern hemisphere, the mag…
We analyze the effect of solar features on the variability of the solar irradiance in three different spectral ranges. Our study is based on two solar-cycles' worth of full-disk photometric images from the San Fernando Observatory, obtained with red, blue and Ca II K-line filters. For each image we measure the photometric sum, Sigma, which is the relative contribution of solar features to the disk-integrated intensity of the image. The photometric sums in the red and blue continuum, Sigma_r and Sigma_b, exhibit similar temporal patterns: they are negatively correlated with solar activity, with…
Long-term variations of solar differential rotation and sunspot activity are investigated through re-analyzing the data on parameters of the differential rotation law obtained by Makarov, Tlatov, and Callebaut (1997), Javaraiah, Bertello, and Ulrich (2005a, b), and Javaraiah et al. (2009). Our results indicate that the solar surface rotation rate at the Equator (indicated by the A parameter of the standard solar rotation law) shows a secular decrease since cycle 12 onwards, given by about $1\,-\,1.5\times10^{-3}$($deg\ day^{-1} year^{-1}$). The B parameter of the standard differential rotation…
We present five decades of chromospheric activity measurements in 59 Sun-like stars as time series. These include and extend the 35 yr of stellar chromospheric activity observations by the Mount Wilson Survey (1966--2001), and continued observations at Keck by the California Planet Search (1996--). The Mount Wilson Survey was studied closely in 1995, and revealed periodic activity cycles similar to the Sun's 11 yr cycle. The California Planet Search provides more than five decades of measurements, significantly improving our understanding of these stars' activity behavior. We have curated the …
A detailed investigation of the magnetic properties of young Sun-like stars can provide valuable information on our Sun's magnetic past and its impact on the early Earth. We determine the properties of the moderately rotating young Sun-like star kappa Ceti's magnetic and activity cycles using 50 years of chromospheric activity data and six epochs of spectropolarimetric observations. The chromospheric activity was determined by measuring the flux in the Ca II H and K lines. A generalised Lomb-Scargle periodogram and a wavelet decomposition were used on the chromospheric activity data to establi…
Using multi-wavelength data of Hinode, a rapid rotation of a sunspot in active region NOAA 10930 is studied in detail. We found the extra-ordinary counterclockwise rotation of the sunspot with positive polarity before an X3.4 flare. From a series of vector magnetograms, it is found that the magnetic force lines highly sheared along the neutral line accompanying the sunspot rotation. Furthermore, it is also found that the sheared loops and an inverse S-shaped magnetic loop in the corona formed gradually after the sunspot rotation. The X3.4 flare can be reasonably regarded as a result of this mo…
We use archival daily spot coverage measurements from Howard et al. (1984) to study the rotational modulation of the Sun as though it were a distant star. A quasi-periodic Gaussian process measures the solar rotation period $P_\mathrm{rot} = 26.3 \pm 0.1$ days, and activity cycle period $P_\mathrm{cyc} = 10.7 \pm 0.3$ years. We attempt to search for evidence of differential rotation in variations of the apparent rotation period throughout the activity cycle and do not detect a clear signal of differential rotation, consistent with the null results of the hare-and-hounds exercise of Aigrain et …
Relative variations in the number of sunspots and sunspot groups in activity cycles have been analyzed based on data from the Kislovodsk Mountain Astronomical Station and international indices. The following regularities have been established: (1) The relative fraction of small sunspots decreases linearly and that of large sunspots increase with increasing activity cycle amplitude. (2) The variation in the average number of sunspots in one group has a trend, and this number decreased from 12 in cycle 19 to 7.5 in cycle 24. (3) The ratio of the sunspot index (Ri) to the sunspot group number ind…
Using the Solar Optical Observing Network (SOON) sunspot-group data for the period 1985-2010, the variations in the annual mean equatorial-rotation rates of the sunspot groups are determined and compared with the known variations in the solar equatorial-rotation rates determined from the following data: i) the plasma rotation rates at 0.94Rsun, 0.95Rsun,...,1.0Rsun measured by Global Oscillation Network Group (GONG) during the period 1995-2010, ii) the data on the soft X-ray corona determined from Yohkoh/SXT full disk images for the years 1992-2001, iii) the data on small bright coronal struct…
The activity levels of the solar-twin candidates HD 101364 and HD 197027 are measured and compared with the Sun, the known solar twin 18 Sco, and the solar-like star 51 Peg. Furthermore, the absolute ages of these five objects are estimated from their positions in the HR diagram and the evolutionary (relative) age compared with their activity levels. To represent the activity level of these stars, the Mount Wilson S-indices were used. To obtain consistent ages and evolutionary advance on the main sequence, we used evolutionary tracks calculated with the Cambridge Stellar Evolution Code. From o…
We analysed the sunspot group data from Greenwich Photoheliographic Results (GPR) during 1874-1976 and Debrecen Photoheliographic Data (DPD) during 1977-2017 and studied the cycle-to-cycle variations in the values of 13-month smoothed monthly mean sunspot-group area in whole sphere (WSGA), northern hemisphere (NSGA), and southern hemisphere (SSGA) at the epochs of maxima of Sunspot Cycles 12-24, and at the epochs of maxima of WSGA, NSGA, and SSGA Cycles 12-24. The cosine fits to the values of WSGA, NSGA, and SSGA at the maxima of the sunspot number, WSGA, NSGA, and SSGA Cycles 12-24, and to th…
Frequency separations used to infer global properties of stars through asteroseismology can change depending on the strength and at what epoch of the stellar cycle the p-mode frequencies are measured. In the Sun these variations have been seen, even though the Sun is a low-activity star. In this paper, we discuss these variations and their impact on the determination of the stellar parameters (radius, mass and age) for the Sun. Using the data from maximum and minimum activity, we fitted an age for the Sun that differs on average by 0.2 Gyr: slightly older during minimum activity. The fitted ra…
We analyze the X-ray emission and chromospheric activity of Sun-like stars of F, G and K spectral classes (late-type stars) studied in the framework of the HK project. More powerful coronas are possessed by stars displaying irregular variations of their chromospheric emission, while stars with regular cyclic activity are characterized by comparatively modest X-ray luminosities and ratios of the X-ray to bolometric luminosity. This indicates that the nature of processes associated with magnetic-field amplification on the convective envelope changes appreciably in the transition from small to la…
It is still unclear how common the Sun is when compared to other similar stars in regards to some of its physical properties, such as rotation. Considering that gyrochronology relations are widely used today to estimate ages of stars in the main sequence, and that the Sun is used to calibrate it, it is crucial to assess if these procedures are acceptable. We analyze the rotational velocities -- limited by the unknown rotation axis inclination angle -- of an unprecedented large sample of solar twins in order to study the rotational evolution of Sun-like stars, and assess if the Sun is a typical…
After decades of effort, the solar activity cycle is exceptionally well characterized but it remains poorly understood. Pioneering work at the Mount Wilson Observatory demonstrated that other sun-like stars also show regular activity cycles, and suggested two possible relationships between the rotation rate and the length of the cycle. Neither of these relationships correctly describe the properties of the Sun, a peculiarity that demands explanation. Recent discoveries have started to shed light on this issue, suggesting that the Sun's rotation rate and magnetic field are currently in a transi…
The present study reports the discovery of Sun-like stars, namely main-sequence stars with $T_{\rm eff}$, $\log g$ and rotation periods $P_{rot}$ similar to solar values, presenting evidence of surface differential rotation. An autocorrelation of the time series was used to select stars presenting photometric signal stability from a sample of 881 stars with light curves collected by the $Kepler$ space-borne telescope, in which we have identified 17 stars with stable signals. A simple two-spot model together with a Bayesian information criterion were applied to these stars in the search for ind…
Aims. The Sun shows strong variability in its magnetic activity, from Grand minima to Grand maxima, but the nature of the variability is not fully understood, mostly because of the insufficient length of the directly observed solar activity records and of uncertainties related to long-term reconstructions. Here we present a new adjustment-free reconstruction of solar activity over three millennia and study its different modes. Methods. We present a new adjustment-free, physical reconstruction of solar activity over the past three millennia, using the latest verified carbon cycle, 14C productio…
The application of the Wavelet analysis and Fourier analysis to the dataset of variations of radiation fluxes of solar-like stars and the Sun is examined. In case of the Sun the wavelet-analysis helped us to see a set of values of periods of cycles besides "11-year" cycle: the long-duration cycles of 22-year, 40-50 year and 100-120 year and short-duration cycles of 2-3,5 years and 1,3-year. We present a method of the chromospheric flux simulation using the 13 late-type stars, which have well-determined cyclic flux variations similar to the 11-year solar activity cycle. Our flux prediction is b…
We analyze sunspots rotation and magnetic transients in NOAA AR 11429 during two X-class (X5.4 and X1.3) flares using the data from the Helioseismic and Magnetic Imager on board the \emph{Solar Dynamics Observatory}. A large leading sunspot with positive magnetic polarity rotated counterclockwise. As expected, the rotation was significantly affected by the two flares. The magnetic transients induced by the flares were clearly evident in the sunspots with negative polarity. They were moving across the sunspots with speed of order $3-7\ \rm km \ s^{-1}$. Furthermore, the trend of magnetic flux e…
The last solar minimum was, by recent standards, unusually deep and long. We are now close to the maximum of the subsequent solar cycle, which is relatively weak. In this article we make comparisons between different global (unresolved) measures of the Sun's magnetic activity, to investigate how they are responding to this weak-activity epoch. We focus on helioseismic data, which are sensitive to conditions, including the characteristics of the magnetic field, in the solar interior. Also considered are measures of the magnetic field in the photosphere (sunspot number and sunspot area), the chr…
Asteroseismology offers the prospect of constraining differential rotation in Sun-like stars. Here we have identified six high signal-to-noise main-sequence Sun-like stars in the Kepler field, which all have visible signs of rotational splitting of their p-mode frequencies. For each star, we extract the rotational frequency splitting and inclination angle from separate mode sets (adjacent modes with l=2, 0, and 1) spanning the p-mode envelope. We use a Markov chain Monte Carlo method to obtain the best fit and errors associated with each parameter. We are able to make independent measurements …
Recent data on superflares on sun-like stars and radiocarbon data on solar activity in the past are both indicative of transient epochs of unusually high magnetic activity. We propose an explanation for the grand activity maxima in the framework of a solar dynamo model with fluctuating parameters. Solar-type dynamos are oscillatory because of the combination of the solar-type differential rotation with positive (in the Northern hemisphere) alpha-effect. An artificial reversal of the sign in the alpha-effect changes the dynamo to a steady regime with hundreds of times larger magnetic energy com…
The Kepler mission has been providing high-quality photometric data leading to many breakthroughs in the exoplanet search and in stellar physics. Stellar magnetic activity results from the interaction between rotation, convection, and magnetic field. Constraining these processes is important if we want to better understand stellar magnetic activity. Using the Sun, we want to test a magnetic activity index based on the analysis of the photo- metric response and then apply it to a sample of M dwarfs observed by Kepler. We estimate a global stellar magnetic activity index by measuring the standar…
Multi-wavelength studies of solar analogs (G0-5 V stars) with ages from ~50 Myr to 9 Gyr have been carried out as part of the "Sun in Time" program for nearly 20 yrs. From these studies it is inferred that the young (ZAMS) Sun was rotating more than 10x faster than today. As a consequence, young solar-type stars and the early Sun have vigorous magnetohydrodynamic (MHD) dynamos and correspondingly strong coronal X-ray and transition region / chromospheric FUV-UV emissions. To ensure continuity and homogeneity for this program, we use a restricted sample of G0-5 V stars with masses, radii, T(eff…
The variability of the spectral solar irradiance (SSI) over the course of the 11-year solar cycle is one of the manifestations of solar magnetic activity. There is a strong evidence that the SSI variability has an effect on the Earth's atmosphere. The faster rotation of the Sun in the past lead to a more vigorous action of solar dynamo and thus potentially to larger amplitude of the SSI variability on the timescale of the solar activity cycle. This could led to a stronger response of the Earth's atmosphere as well as other solar system planets' atmospheres to the solar activity cycle. We calcu…
This paper focuses on the observations of sunspots made by Ruđer Bošković in 1777. We derived the expressions needed to calculate the elements of the Sun's rotation and period from observations. We used modern ephemeris data in the processing of the observation results. Obtained results are very similar to Bošković's original calculations. In addition to historical significance, they also provide scientifically valuable data on the Sun's differential rotation, which plays a significant role in generating and maintaining solar magnetic activity.
The Sun provides the energy necessary to sustain our existence. While the Sun provides for us, it is also capable of taking away. The weather and climatic scales of solar evolution and the Sun-Earth connection are not well understood. There has been tremendous progress in the century since the discovery of solar magnetism - magnetism that ultimately drives the electromagnetic, particulate and eruptive forcing of our planetary system. There is contemporary evidence of a decrease in solar magnetism, perhaps even indicators of a significant downward trend, over recent decades. Are we entering a m…
During the solar magnetic activity cycle the emergence latitudes of sunspots change, leading to the well-known butterfly diagram. This phenomenon is poorly understood for other stars as starspot latitudes are generally unknown. The related changes in starspot rotation rates caused by latitudinal differential rotation can however be measured. Using the set of 3093 Kepler stars with activity cycles identified by Reinhold et al. (2017), we aim to study the temporal change in starspot rotation rates over magnetic activity cycles, and how this relates to the activity level, mean rotation rate, and …
It is well known that the combination of barotropic rotation and radiative equilibrium are mutually incompatible in stars. The Sun's internal rotation is far from barotropic, however, which allows at least the theoretical possibility that the Sun's thermal balance is one of radiative equilibrium in the region of the tachocline near the outer boundary of the radiative zone. We show here that (i) the constraint of radiative equilibrium leads to a straightforward ordinary differential equation for the Sun's rotation profile, and (ii) solutions of this equation can be found that, to within current…
Sunspot light bridges are known to exhibit a variety of dynamic and persistent phenomena such as surges, small-scale jets etc. in the chromosphere and transition region. While it has generally been proposed that magnetic reconnection is responsible for this small-scale dynamism, persistent flaring activity lasting several hours from the same spatial location on a sunspot light bridge, has rarely been reported. We combine observations from the Atmospheric Imaging Assembly and the Helioseismic Magnetic Imager on board the Solar Dynamics Observatory to investigate homologous flaring activity over…
The rotational profile of the Sun is considered to be one of the key inputs in a solar dynamo model. Hence, precise and long-term measurements of this quantity is important for our understanding of solar magnetism and its variability. In this study, we use the newly digitized, white light sunspot data (1923 -- 2011) from Kodaikanal Solar Observatory (KoSO) to derive the solar rotation profile. An automated correlation based sunspot tracking algorithm is implemented to measure the rotation parameters, $A$, the equatorial rotation rate and $B$, the latitudinal gradient. Our measurements of $A=14…
Long-term records of sunspot number and concentrations of cosmogenic radionuclides (10Be and 14C) on the Earth reveal the variation of the Sun's magnetic activity over hundreds and thousands of years. We identify several clear periods in sunspot, 10Be, and 14C data as 1000, 500, 350, 200 and 100 years. We found that the periods of the first five spherical harmonics of the slow magnetic Rossby mode in the presence of a steady toroidal magnetic field of 1200-1300 G in the lower tachocline are in perfect agreement with the time scales of observed variations. The steady toroidal magnetic field can…
Context. Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship. Aims. We aim to identify possible reasons for the Sun to be currently outside of th…
Recent work on improving and revising estimates of solar activity [Clette et al., 2014] has resulted in renewed interest in what has been called the longest running 'Science Experiment'. We compare four reconstructions of solar activity as reflected in the number of sunspot groups ('active regions') constructed by different authors using very different methods. We concentrate on the period since AD 1900 where the underlying solar and geomagnetic data are plentiful and of sufficient quality and find that all four methods yield essentially the same Sunspot Group Number series. We take that as in…
The oscillation frequencies observed in Sun-like stars are susceptible to being shifted by magnetic activity effects. The measured shifts depend on a complex relationship involving the mode type, the field strength and spatial distribution of activity, as well as the inclination angle of the star. Evidence of these shifts is also present in frequency separation ratios which are often used when inferring global properties of stars in order to avoid surface effects. However, one assumption when using frequency ratios for this purpose is that there are no near-surface perturbations that are non-s…
Rotational shear in Sun-like stars is thought to be an important ingredient in models of stellar dynamos. Thanks to helioseismology, rotation in the Sun is characterized well, but the interior rotation profiles of other Sun-like stars are not so well constrained. Until recently, measurements of rotation in Sun-like stars have focused on the mean rotation, but little progress has been made on measuring or even placing limits on differential rotation. Using asteroseismic measurements of rotation we aim to constrain the radial shear in five Sun-like stars observed by the NASA Kepler mission: KIC0…
Existing methods for conversion between synodic and sidereal rotation velocities of the Sun are tested for validity using state of the art ephemeris data. It is found that some of them are in good agreement with ephemeris calculations, while the other ones show a discrepancy of almost 0.01 deg per day. The discrepancy is attributed to a missing factor and a new corrected relation is given.
We quantify the effect of observational spectroscopic and asteroseismic uncertainties on regularised least squares (RLS) inversions for the radial differential rotation of Sun-like and subgiant stars. We first solved the forward problem to model rotational splittings plus the observed uncertainties for models of a Sun-like star, HD 52265, and a subgiant star, KIC 7341231. We randomly perturbed the parameters of the stellar models within the uncertainties of the spectroscopic and asteroseismic constraints and used these perturbed stellar models to compute rotational splittings. We experimented …
The 11-year activity cycle of the Sun is a consequence of a dynamo process occurring beneath its surface. We analyzed photometric data obtained by the CoRoT space mission, showing solar-like oscillations in the star HD49933, for signatures of stellar magnetic activity. Asteroseismic measurements of global changes in the oscillation frequencies and mode amplitudes reveal a modulation of at least 120 days, with the minimum frequency shift corresponding to maximum amplitude as in the Sun. These observations are evidence of a stellar magnetic activity cycle taking place beneath the surface of HD49…
