Magnetic activity plays a key role in the evolution of Sun-like stars and their circumstellar environment. In our research program we will compare solar and stellar magnetic activity through different observables in order to advance our understanding of the solar paradigm when studying stellar activity and vice versa. Using a wide range of stellar data we will map how the structure and strength of magnetic fields change with the variation of the stellar interior.

Even the chemical composition in solar and stellar atmospheres are related with magnetic activity, known as 'FIP' and 'inverse-FIP' effects. Studying these phenomena we will gain a deeper understanding how magnetic activity can influence the elementary composition of stellar atmospheres. For Sun-like stars the stellar wind could significantly differ from that of our Sun's depending on the stellar magnetic field and the temperature of the stellar corona.

We will survey the nearby K-M dwarf stars with known (or supposed) exoplanets to estimate their stellar wind parameters. From this data we will investigate stellar wind-exoplanet interactions in different conditions and estimate the extent of habitable zones around the stars. We will use high precision, continuous observations from space instruments (e.g. Kepler, TESS) in order to reveal such phenomena, which are hardly or not observable from the ground, e.g. how the oscillation amplitudes are damped in the presence of magnetic activity in evolved single and binary stars. This will help us to draw up the differences of the stellar evolution after the main sequence between magnetically active and inactive stars.

The role of solar-like magnetic activity in the evolution of single stars, binaries and star-planet systems OTKA T109276 (2014-2017)

Solar-like magnetic activity (i.e., phenomena that are analogous to those that we see on the active Sun) plays a key role in the evolution of late-type stars by affecting the angular momentum transfer from the inner core through the convective bulk to the surface, as well as other interactions between active stars and their environments, including companions in close binaries and in star-planet systems. When studying stellar magnetic activity in a broad time range from seconds (flare activity) through months (surface spottedness) to tens of years (activity cycles), different indirect techniques, such as Doppler imaging, and methods, such as long-term monitoring, are needed.

From Doppler imaging studies we will be able to detect and even measure surface flows (differential rotation, meridional circulation), that are responsible for solar-like dynamo action. Using Li 6708Â observations we will focus on the connection between surface lithium distribution and magnetic activity indicators. We study the possible effects of a close companion on activity. We will analyze dynamo processes using high precision data from the Kepler space telescope as well as ground-based photometric measurements. Our aim is to characterize the photospheric and chromospheric activity of late-type young stars from the Kepler-field, of ages ranging from the post-T Tauri phase (~50Myr) to the ZAMS (~100-200Myr). Continuous monitoring of different types of active stars with the Fly's Eye Camera System will give us a broader view of magnetic activity of stars of similar types but of different ages, allowing evolutionary studies.

Dramatic changes in solar and stellar magnetic activity cycles — what drives them, and can we predict them? OTKA T081421 (2010-2014)

The Sun is a variable star whose magnetic activity and total irradiance vary on a timescale of approximately 11 years. The Sun's magnetic field dominates the solar atmosphere, structures it, drives much of the atmospheric dynamics and produces all the observed energetic phenomena, like flares, jets, coronal mass ejections, etc. Magnetic field and associated solar activity reach all the way into the heliosphere, whose physical parameters, e.g. magnetic field strength and gas pressure follow the variability of solar magnetic activity.

Magnetic activity cycles are also displayed by a broad range of stars in the right half of the Hertzsprung–Russell (HR) diagram. The Sun and well­observed details of solar activity are considered the Rosetta Stone in magnetic activity studies, but the solar paradigm can be reversed: the wide variety of stellar physical conditions found in variable stars lead to great differences in magnetic field production and resulting magnetic patterns. Great differences are found in stellar magnetic cycles length and modulation. Furthermore, owing to the great number of variable stars, we can find and study stars in various phases of their modulated cycle: we can find some stars in Grand Maximum or Great Minimum (similar to the solar Maunder­minimum in the 17th century) of magnetic activity.

Solar and stellar cycle studies have a great deal of information to exchange for their mutual benefit - such a synergy is our goal.

The yearly time-scale variability of the solar and stellar dynamos OTKA T048961 (2005-2008)

From the high diversity of the physical nature we find on stars it follows, that the results derived from solar observations serve as paradigms, in a much wider context. The driving force of the solar activity is the magnetic dynamo, whose signatures can be observed on magnetically active stars as well. The working of the solar dynamo shows cyclicities on several time scales, as the long-term monitoring of stars suggest similar behaviour. Based on this, we can study the similarities between the model(s) of solar and stellar dynamos.

The differential rotation of stars is of utmost importance in understanding their magnetic activity. The rotational shear of stars with convective envelopes put constraint on the large scale structure of the magnetic field, therefore has important information on the dynamo effect taking place beneath the surface. Nevertheless, measuring differential rotation is not an easy task, the applied methods concerning most of the results, are uncertain, or do not give accurate values. In case we have exact knowledge on the changing of parameters, that describe the differential rotation on enough rich sample of active stars, we may settle, what kind of mechanisms originate those magnetic dynamos that produce much higer activity levels than that of the Sun. Similarly, we can get answer how the magnetic cycle depend on the strength of the dynamo.

In the course of the present project we plan to deal with a complex approach of the differential rotation. While data on the Sun are available both in high space and time resolutions, even the best photometric and spectroscopic measurements of stars and their cautious modellings are not able to lead us to similar results we find for the Sun. For photometric modellings we use automated telescopes, among those the recently renowed and automatized 60cm telescope of Konkoly Observatory, in Budapest. Long series of photometric data make possible to follow the changes of the spot locations, sizes and temperatures, in time. We supplement the photometric data with automated spectroscopy (STELLA1-Tenerife) in international collaboration with the Astrophysical Institute, Potsdam. In this project we plan measuring active stars with ultrashort rotational periods. The rotational periods of these objects are in the order of half a day, so it is possible to measure a full cycle during a winter night. Many active stars of our program, with longer periods, are members of binary systems; modelling these object we study the effect of binarity for the working of the dynamo. We plan to deal with explaining the long cycles of certain pulsating variables in the context of the possible magnetic activity.