The solar chromosphere, a region between the visible photosphere and the tenuous corona, is still not well understood. No comprehensive models exist for the chromosphere and it remains extremely challenging to derive physical parameters, such as magnetic fields and velocities, from observations. A deeper understanding of the chromosphere is crucial to solve the coronal heating problem and to understand highly dynamic events rooted in this region, such as flares or coronal mass ejections (CMEs). One challenge is that each of the few chromospheric polarimetric instruments has their own reduction procedure and ways to overcome observational limitations, such as seeing, image alignment, and polarization calibration, making it hard to combine data from various instruments. But exactly this would be desirable to obtain a more complete picture of the chromosphere and would require all people involved to discuss their data together. Another problem is the lack of suitable interpretation methods. Inversion codes that “convert” the observed polarization signals into physical variables, such as magnetic field, velocity, and atmospheric parameters are rare because of the non local thermal equilibrium (NLTE) conditions. Recent instrument developments and the advance in understanding the physics involved in radiative transfer in the chromosphere allow us to get to know this solar region better.

In this international team, we would like to combine the knowledge of the experts who interpret chromospheric spectropolarimetric data and of the experts developing and using inversion codes, especially for the chromosphere. With our combined experience we will make progress in explaining the unexpected signatures that can be seen in the polarimetric data and we will advance the tools necessary to study this important solar layer. This is especially important, considering that future missions and telescopes (SOLAR-C, ATST, EST) will include chromospheric spectropolarimeters, whose data currently cannot be used without a large effort for the analysis.