A new global nonlinear force-free coronal magnetic-field extrapolation code implemented on a Yin Yang grid.

The solar magnetic field dominates and structures the
coronal plasma and detailed insights are important to understand almost
all physical processes. While direct routine measurements
of the coronal magnetic field are not available, we have to extrapolate
the photospheric vector field measurements into the corona. To do so,
we developed a new code that performs state-of-the-art nonlinear force-free
magnetic field extrapolations in spherical geometry.
Our new implementation is based on an optimization principle and is
able to reconstruct the magnetic field in the entire corona, including
the polar regions.
Because of the nature of the finite-difference numerical scheme used in
the past, extrapolation close to polar regions was computationally
inefficient. In the current code, the so-called Yin Yang grid is used.
Both the speed and accuracy of the code is improved compared to previous
implementations. We tested our new code with a well known
semi-analytical model (Low and Lou solution). This new Yin and Yang
implementation is timely because
the Solar Orbiter mission is expected to provide reliable
vector magnetograms also in the polar regions within the following years.
Thus, this code can be used in the future when these synoptic magnetograms
are available to model the magnetic field of the solar corona for the entire
Sun including the polar regions.