Observation and theoretical modeling of anomalous circular polarization signals in strong chromospheric lines

At Istituto Ricerche Solari Locarno (IRSOL), high resolution spectropolarimetric observations can be carried out using the Zürich Imaging Polarimeter (ZIMPOL). Since almost two decades, this instrument is playing a leading role in the field of solar spectropolarimetry, allowing us to detect polarization signals with an accuracy of 10-5. Within the framework of a fruitful collaboration with the Kiepenheuer Institut for Solar Physics (KIS) in Freiburg, a ZIMPOL-III system is being permanently installed at GREGOR, the 1.5m German solar telescope in Tenerife.The aim of this postdoctoral project is to investigate anomalous circular polarization signals that have been recently observed in strong solar chromospheric lines using ZIMPOL, both at IRSOL and at GREGOR. The investigation of these signals is particularly important for two reasons. On one side, it will allow us to improve our understanding of the complex physics of the generation and transfer of polarized radiation. On the other, it can unveil a new powerful diagnostic tool for investigating the magnetism and dynamics of the solar chromosphere.Our research activity will follow a double path: on one hand, we will analyze such signals in more details by carrying out new observational campaigns. On the other hand, taking into account the indications provided by the observational work, we will investigate their physical origin, trying to identify the mechanisms responsible for their appearance.The observed circular polarization Stokes V/I signals are considered anomalous since they do not show the typical antisymmetric profiles due to the longitudinal Zeeman effect, but they show two peaks of the same sign, and are characterized by a non-zero frequency-integrated signal (net circular polarization). Up to now, they have been detected in close-to-the-limb observations of the NaI D1 and D2 lines, as well as in the BaII line at 4554 Å. From the observational point of view, we aim at clarifying which spectral lines can produce this kind of signals, and whether they can be associated to particular dynamic or magnetic structures in the solar atmosphere. Our preliminary observational results provide strong hints suggesting that a key physical ingredient for interpreting these signals is the possible presence of atomic polarization in the lower level of the line transitions.