Spectropolarimetric inversions: Our key to unlocking the secrets of the solar atmosphere

Abstract

Advances in our understanding of the Sun will require the accurate and consistent measurement of the physical conditions in different regions of the solar atmosphere. The richest information, encoding the full state integrated over of the atmospheric volume in which they are formed, is contained in the detailed shapes of the numerous spectral lines present in the solar spectrum. But this integration along the line of sight is highly non-linear, which means that inferring the true physical conditions requires solving a complicated, ill-posed inverse problem. This challenge is being addressed through powerful codes that treat ever more complex aspects of the solar atmosphere -the breakdown of local thermodynamic equilibrium (or non-LTE), non-equilibrium hydrogen ionization, deviations from hydrostatic equilibrium, and multiple methods of deriving chromospheric and coronal magnetic fields. However, to reach the full potential of these techniques, the community must make a concerted, sizable effort to improve and enhance them, both in terms of their physical veracity as well as their computational tractability. These methods will be essential for fully exploiting the data from new and proposed solar facilities or missions. Reliable outputs from inversions will be a key component of more realistic extrapolations of the magnetic field of active regions and the heliosphere. These inversion methods, if accurate and robust, can become a key tool in determining the structure of stellar atmospheres, but will also become crucial for the operational prediction of the energetics of the Sun's magnetic field and the driving of space weather and changes in the solar spectral irradiance.