Magnetic activity describes the observable manifestations of a dynamo-generated stellar magnetic field. The most well-known example of an active star is our Sun; the 11-year solar cycle is a clear indication for its dynamo. Solar-like dynamos require (differential) rotation and convection and are, therefore, only found in stars with an interior structure analogous to that of the Sun. Nevertheless, strong signatures of magnetic activity are seen also in fully convective, very low-mass stars suggesting that some other kind of dynamo mechnism takes over.
The temperature in the atmospheres of active stars increases outward as a result of magnetic heating. The outer atmospheric layers (chromosphere, transition region and corona) exist only thanks to the stellar dynamo. They become manifest through radiation at successively shorter wavelengths, from the optical and UV to the X-ray regime. Multi-wavelength studies of magnetic activity are, therefore, a crucial tool for understanding the structure of stellar atmospheres.
After the dispersal of the accretion disk the optical and IR emission of young stars is similar to that of a main-sequence star. These so-called "weak-line" T Tauri stars are, therefore, difficult to identify. However, during the whole pre-main sequence phase -- independent of the presence or absence of circumstellar material -- the X-ray emission is up to a factor thousand stronger than on the main-sequence. This makes observations of star forming regions in the X-ray band an efficient means for finding "weak-line" T Tauri stars. X-ray surveys are, therefore, complementary to surveys at IR wavelengths which detect predominantly young stars with accretion disks (so-called "classical" T Tauri stars). Currently, we use X-ray data to establish complete samples of (1) pre-main sequence stars in star forming regions and (2) so-called "flare" stars in the solar neighbourhood.
Historically, the first indication of dynamo-generated magnetic fields in stars other than our Sun came from an empirical relation between stellar rotation rate and magnetic activity (measured through the X-ray luminosity). Nevertheless, up to now the rotation-activity relation has remained poorly constrained especially for very low-mass stars of spectral type M. M stars are fully convective, and a qualitative change of the magnetic activity with respect to solar-like stars is expected. The long rotation periods of M stars together with their low (X-ray) luminosities make the observational study of the rotation-activity relation of these objects difficult. Presently we deal with this problem through combined measurements of X-ray luminosities (as a means to assess activity) and high-precision optical light curves from the Kepler and K2 missions (from which we determine rotation periods).
Objects of spectral type M7 and later are called "ultracool dwarfs". This group comprises both stars of very low mass and brown dwarfs. The interiors of ultracool dwarfs are fully convective. Therefore, it is expected that their magnetic activity -- if any -- is of different type as that observed on solar-like stars. Moreover, the photospheres of ultracool dwarfs are characterised by a low degree of ionisation which makes the coupling between matter and magnetic field, a necessary condition for magnetic activity, questionable. We examine the activity of ultracool dwarfs with radio, optical and X-ray observations and compare the results to analogous observations of stars of spectral types GKM that have solar-like interiors.