The uppermost fully ionised and magnetically dominated part of the atmosphere of the Sun,the solar corona, has attracted researchers’ attention for several decades. The main motivation of solar coronal studies is connected with the decisive role the corona plays in solar– terrestrial relations. Also, being a natural plasma laboratory, the corona provides us with a wide range and combination of plasma parameters, allowing us to gain knowledge important for controlled fusion efforts. Moreover, our interest in the corona is motivated by the fundamental questions of space physics and plasma astrophysics: why is the temperature of the coronal plasma two orders of magnitude larger than that of the solar surface? What are the mechanisms for the acceleration of solar and stellar winds? How is the vast amount of energy released in solar flares and coronal mass ejections, making them the most powerful physical phenomena in the solar system? The difficulties in answering these, and many other questions posed by the corona, are partly associated with the lack of precise and often even minimal knowledge of the physical conditions and parameters in this part of the solar atmosphere. Indeed, remote diagnostics of the hot, dynamic, sometimes non-equilibrium and often transparent plasma is a non-trivial task.
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