Spectral line polarization: a new observable for planetary space environment studies
Résumé
It has recently been discovered that the emission of the terrestrial auroral oxygen red line (630nm) is linearly polarized (Lilensten et al.; GRL 2008). This discovery opens up the exciting prospect of a new observational technique in the study of the planetary atmospheres and environments. At Earth, the polarization rate depends on the energy and pitch angle of the incident precipitation. Due to the very large lifetime of the O1D state (110s) the remaining polarization rate can also give information on the collisional processes that occur between 200 and 400km in the thermosphere. So the question now confronting planetary scientists is: “Can we extend these studies to thermospheric emissions of other planets such as Jupiter, Saturn or Venus?” One example might be emission from the H3+ ion in the jovian auroral thermosphere, which is the main contributor to the aurora in the infrared (emission rates are between 10^12 and 10^13 Watts, planetwide). H3+ is created after a chemical reaction which scrambles collisional information, such that any remaining polarization can then only be due to a static anisotropy created by an electric field. Measuring this polarization can therefore provide a measure of this electric field, which is one of the main unknowns for understanding the jovian high latitude thermosphere and the very high temperatures found there. In the UV, polarization of the Lyman alpha line - which the most intense line in the auroral spectra of the giant planets – also has the potential to probe upper atmosphere phenomena. It could be a tracer of precipitating particles and, due to the fact that it is optically thick, of the diffusion processes. In the case of very high spectral resolution, the polarization of the line core versus the wing could be particularly useful. We will present our current project to carry out a planetby- planet study of which spectral lines are of potential interest for polarization measurements, what information they could provide about which processes, and how we can best carry out the measurement of those polarizations.