Astronomy seminar via zoom: Magnetic sensitivity of the H I Lyman-α scattering polarization wings in the solar disk radiation

The linear polarization signals arising from scattering processes in the H I Lyman-α line in the solar disk radiation represent a valuable observable for probing the chromosphere-corona transition region (TR) and the underlying chromospheric plasma. Whereas the line-center signal encodes information on the magnetic activity and geometry of the TR, the broad large-amplitude scattering polarization signals in the wings, produced through the joint action of partial frequency redistribution and J-state interference, had generally thought to be insensitive to the influence of magnetic fields. However, our investigations show that such Q/I and U/I wing signals are in fact sensitive to the presence of chromospheric magnetic fields with a significant longitudinal component. This sensitivity arises from the magneto-optical (MO) effects that introduce a coupling between the transfer equations for Stokes Q and U and it becomes appreciable for field strengths similar to those for which the Hanle effect operates in the line core (approximately between 5 and 100 gauss), being well below those required for sizable signals to be produced through the familiar Zeeman effect.

In this talk, we begin by giving a primer on the main physical mechanisms that give rise to polarization phenomena in this line. We then show the results of a series of numerical radiative transfer calculations, highlighting the sensitivity to MO effects both in its wing and frequency-integrated linear polarization signals. The theoretical results presented here further enhance the diagnostic interest of the unprecedented spectropolarimetric and broadband observations obtained by the Chromospheric Lyman-Alpha Spectropolarimeter (CLASP) sounding rocket mission.