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Session: Heliophysics and the Solar System

Name: Prof. Alexander Nindos (University of Ioannina)
Coauthors: No coauthors were included.
Type: Oral
Title: Recent advances in our understanding of the solar chromosphere
Abstract:

The solar chromosphere is traditionally defined as a ~2000-km-thick layer lying above the photosphere. Its emission can be detected in strong optical and UV spectral lines as well as in infrared, millimeter-wavelength (mm-λ) and submillimeter-λ continua. The pertinent spectral observations indicate that the chromosphere is highly inhomogeneous and dynamic. Although several of the observational building blocks of the chromosphere have been established a long time ago, the physics dictating their properties and dynamics is not. This is because the chromosphere is intrinsically complex. It is the layer of the solar atmosphere where the transition from a plasma-dominated regime to a magnetic-field-dominated regime takes place. It is also a region where interactions between ions and neutrals can be relevant. Furthermore, the formation of the chromospheric spectral lines are associated with nonequilibrium effects. Observations of the radio continuum at mm-λ provide a unique chromospheric diagnostic. The non-flaring Sun continuum mm-λ emission is of thermal origin. The emission mechanism is free-free and electrons are almost always in LTE; hence, there is no need to consider non-LTE effects, abundances, ionization and excitation equilibria, that are common sources of uncertainty in other parts of the spectrum. However, old mm-λ observations of the Sun were underexploited due to the lack of appropriate instruments. With the advent of the Atacama Large Millimeter and submillimeter Array (ALMA), chromospheric data at mm-λ with unprecedented angular resolution (a few seconds of arc or less), temporal resolution (1-2 s) and sensitivity have been accumulating. In this talk I review recent advances in our understanding of the chromosphere using ALMA observations. Special emphasis will be given on the new results on the temperature structure of the chromosphere, the chromospheric network and spicules, small-scale dynamic phenomena and oscillations, prominences and filaments, as well as sunspots.