Studying flare-producing solar active regions with imaging spectropolarimetry in the Ca II K line

Magnetic fields in the solar atmosphere are the drivers of solar flares, but the field can only be consistently measured at the bottom of the atmosphere (the photosphere).

Determination of the magnetic field in the upper atmosphere (corona) is done through field extrapolations, which estimate the coronal field from the measured photospheric magnetic field.

The reliability of these extrapolations is limited, because assumptions made in the extrapolation do not always hold in the solar atmosphere, and the spatial resolution of the space-based observations that are normally used (typcially with SDO) is too low.Using the Swedish 1-m Solar Telescope we will perform imaging spectropolarimetry of the photosphere and chromosphere in the Fe I 630.1-630.2 nm, Ca II 8454.2, and, uniquely, in the Ca II H&K lines, with a FOV that is large enough to cover an entire active region and up to 10 times the spatial resolution of SDO.

The observations will be inverted with a non-LTE inversion code that simultaneously includes all spectral lines, yielding inverted chromospheric vector magnetic field maps of the highest quality.

With this data we will 1) study the evolution of the magnetic field in the photosphere and chromosphere before, during, and after solar flares; 2) derive the evolution of the magnetic free energy during flares, and 3) provide strong observational tests of the reliability of magnetic field extrapolations  and the fidelity of data-driven modeling of solar flares.