The solar corona has been known to mankind for a very long time, ever since humans consciously observed the first total solar eclipse, when the corona appears as a somewhat irregularly formed "crown" around the Sun. However, the true nature of the solar corona was only recognized in the fourties of the last century, when optically observed coronal emission lines, previously erroneously attributed to a readily introduced new element "coronium," were correctly identified by Grotrian and Edlen. These emission lines turned out to be forbidden transitions of rather highly ionized iron atoms, the production of which requires temperatures in excess of 1MK. Thus, rather unexpectedly, the very outer layers of the Sun were found to be much hotter than its photosphere, and ever since the search for the dearly required heating mechanism(s) of the solar corona has become the holy grail of solar coronal physics.
Hot plasmas with temperatures in excess of 1 MK radiate most of their energy at soft X-ray wavelengths, and soon after World War II, X-ray emission from the Sun was first detected using Geiger counters onboard rockets originally developed for warfare. Yet, the overall X-ray losses of the Sun are rather weak and less than 1 part in a million of its whole energy budget is emitted at X-ray wavelengths. Thus an extrapolation of the observed solar X-ray properties to stars at large led to rather pessimistic expectations as to the detectability of stellar coronae, and indeed, none of the first couple of hundreds of extrasolar X-ray sources detected in the sixties and seventies of the last century were "normal" stars. However, the introduction of soft X-ray imaging into X-ray astronomy, first with the Einstein Observatory (operated between 1978 and 1981) and later with EXOSAT (1983-1985) and ROSAT (1990-1998), has led to the detection of X-ray emission from many thousands of stars similar to the Sun. Stellar X-ray sources were also intensely studied at extreme ultraviolet wavelengths with the EUVE satellite operated between 1992 and 2000. In particular, the first high spectral resolution observations of coronal emissions from a larger sample of stars were obtained with the spectrometers onboard EUVE. Stars were also observed with other imaging X-ray satellites such as ASCA
and BeppoSAX, but the contributions from those missions were geared mostly to other astrophysical topics. At present, the large observatories XMM-Newton and Chandra allow X-ray observations of coronal sources with unprecedented sensitivity and spectral resolution. The gratings onboard XMM-Newton and Chandra utilize the full power of spectroscopy for X-ray astronomy for the first time and have revolutionized this field of research. Since this field is so rapidly developing, we purposely leave out all detailed discussions of spectroscopy and focus on well established results mostly obtained from imaging and lightcurve analyses.
The precise physics of the mechanism(s) responsible for heating the solar corona beyond 1MK have remained rather elusive up to the very present. However, while originally nonmagnetic processes ("acoustic heating") were thought to be important, the magnetic character of solar (and stellar) coronal heating is now universally accepted, and X-ray emission from cool stars is generally considered to be a key indicator of the so-called "magnetic activity" of these objects. We note, however, that there is no general consensus or generally applied definition of solar and stellar activity. Usually one associates spots, plage, flares, spicules and related phenomena with magnetic activity on the Sun, and similar definitions apply for (cool) stars. Linsky  defines solar-like (activity) phenomena as "nonradiative in character, of fundamentally magnetic origin and almost certainly due to a magnetic dynamo operating in or at the base of a convection zone." In magnetically active regions of the Sun and the stars, one finds departures from pure radiative equilibrium caused by some kind of heating and probably momentum deposition processes.
Linsky's definition is very useful because it provides a recipe for identifying activity through searching for evidence of nonradiative heating and showing its magnetic nature. Direct measurements of coronal magnetic fields are very difficult for the Sun, and the situation is worse for stars. The observations of Zeeman broadening go along with large uncertainties because in the optical other line broadening mechanisms (most notably rotational line broadening) dominate over Zeeman splitting. However, it is straightforward to search for the heating effects associated with magnetic activity. Evidence for nonradiative heating can be obtained by observations of the thermal plasma in the UV or X-ray domain or by observations of nonthermal emission from highly energetic particles often accompanying and possibly intimately linked with the heating process(es). Further, nonradiative heating is - usually - confined in space and time, and can be diagnosed by studies of time variability and spatial structure of the emitting regions.
Therefore, X-ray observations of stars are considered a key diagnostics for magnetic activity of stars throughout the Hertzsprung-Russell (HR) diagram. At least for cool stars, i.e., stars with outer convection zones like the Sun, all magnetic activity is thought to be ultimately due to the action of an hypothesized dynamo process operating at the interface between the outer convection zones and radiative interiors of these stars. Such dynamo processes are probably fundamental for astrophysics as far as the generation of magnetic fields is concerned. Most astrophysical objects such as planets, stars, compact objects, accretion disks, jets, galaxies, and clusters of galaxies are associated with magnetic fields, and magnetic dynamos apparently produce these fields on quite different spatial scales, from planet-sized objects to large-scale galactic structures.
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