Spectroscopy and atomic physics

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Friday, May 4, 2018

A short guide to spectroscopy is given, with emphasis upon the elementary physics needed to understand and begin modeling the radiation emerging from astrophysical plasmas using the Sun as a guide, without bias towards a particular region of the Sun's atmosphere.

The solar spectrum absorption features across the solar atmosphere image
A "classic" figure of the solar spectrum adapted from \citet{Scheffler+Elsasser1974}, highlighting emission and absorption features across the solar atmosphere. Some strong lines are deep in absorption yet have emission in the core (Mg II is the case shown here, emission being not visible at this modest spectral resolution). Notice the $\Delta n=1$ transitions of H, He I and He II, marked with asterisks, at comparable wavelengths with $\Delta n=0$ transitions of much more highly ionized species.

The Sun is the astronomical body \emph{par excellence}, upon which our tools for exploring the Universe have been most carefully developed. Observable solar plasmas span a broad range of conditions, and consequently span extremes of local thermodynamic equilibrium (LTE) deep in the atmosphere, to extreme non-LTE conditions in the corona and wind, including non-statistical equilibrium conditions. For brevity, important subjects are omitted, notably non-Maxwellian electron distributions, line broadening, polarization, particle diffusion and high-energy and collective phenomena of importance at radio and hard X-ray wavelengths.

Publication Name: The Sun: A Guide to Stellar Physics