Comparison of Inversion Codes
One of the most powerful and successful tools to infer the thermodynamic, kinematic and magnetic properties of astrophysical plasmas is the analysis of the polarization properties of the light it emits in different absorption/emission atomic/molecular lines. Obtaining reliable information from the polarization signals (aka: Stokes profiles) is not an easy and direct task, because the observed Stokes profiles depend in a deep and intricate manner on the physical properties we aim at investigating. A crucial breakthrough was achieved in the last 30 years with the development of non-linear inversion codes that, with the aid of relatively simple models, are able to reliably extract information about the thermodynamical, kinetic and magnetic properties of the plasma. This technique has been systematically applied to the study of the solar Photosphere and Chromosphere. Many inversion codes are nowadays available to the Solar Physics community and their application has provided us with a better understanding of the outer layers of the solar atmosphere. Every inversion code is based on a specific model (with different degrees of complexity) and therefore the inferred physical properties have to be interpreted with some care and are conditioned to the reliability of the proposed model. Since a variety of models are employed to interpret observations, it is crucial to investigate, through a controlled and systematic experiment, which model is favored to produce inferences closer to the real physical conditions. To this end, we propose the following steps:
(1) use the thermodynamic and magnetic parameters, obtained from recent 3D non-grey MHD simulations (of the quiet Sun, active regions and sunspots), to compute synthetic Stokes profiles that will be fed to several existing inversion codes
(2) each inversion code will analyze this data and attempt to retrieve the physical parameters, which will be then compared to the real values provided by the numerical simulations
(3) analyze the dependence of the results on the model assumed. Our team will comprise several experts and developers of the most widely used inversion codes, which guarantees the successful completion of this project.
Scientific rationale and goals
A large fraction of our knowledge of the thermodynamical and magnetic properties of solar plasmas comes from the interpretation of spectro-polarimetric observations carried out during the last decades. Very sensitive spectro-polarimeters mounted on ground-based telescopes have revolutionized the field by providing polarimetric sensitivities at the order of and sometimes better than 10-4 (this means detecting rates of one polarized photon per 10000). Ground-based telescopes and their success has been recently accompanied by a large interest on observations carried out with space-borne observatories, like the spectropolarimeter SP aboard Hinode or the IMaX magnetometer aboard the Sunrise balloon. These telescopes offer data without the influence of the atmosphere, with continuous observing periods and with unprecedented spatial resolutions. Altogether, ground-based and space-borne facilities are offering and will deliver data of very good quality that we should exploit to understand better the physical process taking place in the solar atmosphere.
The development of complex inversion codes to extract information from observations has evolved almost in parallel to the development of the instrumentation. This has served to deepen our understanding of the role the magnetic field plays in all sorts of solar structures: granulation, inter-network, sunspots, prominences, spicules, etc. Initial efforts resulted in inversion codes that were able to infer physical parameters proposing really simple models based on simple quantities that could be estimated observationally. As soon as the full Stokes vector was made observationally available, these codes evolved towards more complex models based, the majority of them, on a Milne-Eddington model atmosphere. Later, atmospheres in local (or non-local) thermodynamical equilibrium were used, producing the present state-of-the-art computer codes. In spite of their success, no systematic analysis of the capabilities of such codes has never been carried in controlled conditions. It is now the moment to analyze and compare different inversion codes, specially considering how wide-spread they are, and due to the vast amounts of data that already is, and will be available in the near future. In our study, the team proposes the following goals:
- - Inter-comparison of codes and comparison of their inferences with numerical simulations
- - Find a compromise between model complexity and reliability
- - Applications to current and future instrumentation
You can download the proposal here
News :
We had a great first meeting. Now it is time to work. A picture of the group is available here.