Relaxation by Collisions with Hydrogen Atoms: Polarization of Spectral Lines

Abstract

New technological improvements have made possible in recent times the development of telescopes and post-focus instrumentation for solar research capable of pushing the polarimetric accuracy of spectroscopic observations to unprecedented limits. The operation of these instruments has revealed a wealth of new phenomena, especially in the detection of puzzling signals of linear polarisation in the solar spectrum observed at small angular distances from the limb. Through its interpretation it is indeed possible to diagnose several important aspects of the physics of the higher layers of the solar atmosphere, such as the degree of anisotropy of the radiation field, and, probably the most important item, the presence and quantitative measurement of weak magnetic fields.

However, it is important to remark that weak magnetic fields act, on resonance polarization, with a general depolarization mechanism and, from this point of view, they are quite similar to depolarizing collisions. A dignostic of weak magnetic fields in the higher layers of the solar atmosphere is thus possible, from the analysis of the linear polarization solar spectrum, only if the role of depolarizing collisions is fairly understood.

A detailed treatment of collisional relaxation rates due to collisions with hydrogen atoms is presented; a particularly striking example is the sodium doublet which shows an intriguing profile in the linear polarisation solar spectrum observed very close to the the solar limb (in particular with the solar telescope THEMIS). This profile has been tentatively interpreted as due to the presence of ground level atomic polarisation in the sodium atoms of the solar atmosphere but this hypothesis has