Astrophysical Plasmas

In this chapter, we discuss the application of spectro-polarimetry diagnostics to the investigation of astrophysical plasmas. We first present an overview of why polarization is expected in the spectral-line radiation that we receive from a large variety of cosmic objects, and then treat in some detail specific atomic models (e.g., the 0–1 and 1–0 two-level atoms), which illustrate how physical and electro-dynamical properties of the emitting plasma can be inferred by studying the polarized radiation in the corresponding spectral lines. The practical applications described in this chapter are taken exclusively from the realm of solar physics, mainly for two reasons: (a) from a historical point of view, the Sun was the first cosmic object to which polarization analysis of radiation was successfully applied, proving the existence of solar magnetic fields, and demonstrating the diagnostic potential of radiation phenomena involving resonance polarization, and (b) because spectro-polarimetric signals are generally very weak, their detection with a sufficient signal-to-noise ratio is possible only for strong radiation sources. In particular, a plethora of atomicpolarization effects (magnetic and collisional depolarization, alignment-toorientation conversion, level crossing and anti-crossing interferences) could be detected in the polarized light from the Sun only because of the high sensitivity that can be attained in solar observations. As the light-collecting capabilities of night-time astronomical instrumentation keep growing, it is expected that the diagnostic techniques illustrated in this chapter will become increasingly available for the investigation of plasma properties all over the universe.