En route to 3D Hanle diagnostics in stellar atmospheres: the observational perspective

Probing the unresolved, weak component of the solar magnetic field in the photosphere is a crucial yet challenging task to address many of the open questions in astrophysics. The term 'unresolved' implies that it cannot be detected through conventional diagnostic techniques like the Zeeman effect in spectro-polarimetric observations of spectral lines, which requires a non-zero magnetic flux to be present. The Zeeman effect is prone to cancellations if the magnetic field is tangled on small spatial scales. Fortunately, we are not completely blind to these fields: the Hanle effect is sensitive to this component of solar magnetism. The Hanle effect is usually detected via depolarization and rotation of spectral line polarization. To detect this change, two options are available: either a zero magnetic field reference polarization is calculated from sophisticated numerical models and compared to the observation, or many spectral lines with different Hanle sensitivities are compared, known as differential Hanle diagnostics, and therefore the need for the zero field reference is eliminated.

For many decades the Hanle effect in the photosphere was investigated without any spatial resolution, and the zero reference calculations have been sufficiently carried out with one dimensional models. However, with recently available large aperture telescopes, and high-sensitivity and high zero-level-accuracy polarimetry, we are at the dawn of a new era in solar physics, where spatially resolved Hanle observations become available, awaiting many discoveries and new exciting questions to arise. The most accurate interpretation of these observations demand the zero level calculations to transit from 1D to 3D, which implies increasing the complexity of solar models, loads of computing resources, and the development of new numerical schemes - a very ambitious task with a lot of resources needed before the first data interpretation can be carried out. In this project, I focus on the observational perspective to interpret spatially resolved Hanle observations in spectral lines using atomic differential Hanle diagnostics. The greatest advantage is that this method can generate significantly faster results with significantly less computing resources.

The project has the following objectives:

- identify suitable spectral line combinations (including observational constraints),

- demonstrate the feasibility of the atomic differential Hanle effect by reproducing results from spatially unresolved measurements of the unresolved magnetic fields,

- determine the achievable accuracy of the magnetic field diagnostic, i.e. with results from existing 3D radiative transfer codes, e.g. PORTA,

- observe the polarized spectrum of the identified spectral lines with high spatial resolution,

- generate high resolution maps of the unresolved magnetic field, compare them with realistic 3D magneto-hydrodynamic simulations including the small-scale dynamo, like CO5BOLD,

While the framework build during this project will provide the solar community a very beneficial tool to interpret solar spectro-polarimetric scattering observations in a faster way, the results will be of much broader importance: for the first time it will be possible to map the unresolved magnetic field in the photosphere, which also provides an independent result to ongoing attempts to invert the observations in the traditional way. This project creates the opportunity for Switzerland to establish a leading role in this topic in anticipation of large multi-national projects like the 4-m aperture European Solar Telescope.