›The Problem of the Magnetic Field Gradients in Sunspots

In sunspots, the measured gradients respectively vertical (dBz/dz) and horizontal (dBx/dx+dBy/dy) largely differ in the observations, whatever the methods, instruments or spectral lines are. All the measurements agree on a vertical gradient of about 3 G/km on the one hand, and an horizontal gradient of about 0.3 G/km on the other hand, which surprisingly questions the vanity of the apparent divB computed from these values. Thus S. Solanki wrote in a review devoted to sunspots in 2003, p. 184 (S. Solanki, Astronomy and Astrophysics Review, 2003, 11, 153): "No satisfactory solution has been found as yet for the unexpectedly small vertical gradients obtained by applying the divB=0 condition”.

The workshop is devoted to investigate the following question: do these measurements enable us to conclude about the local divergence of the magnetic field, and, if not, what effect explains the departure from zero of the observed value corresponding to a larger scale?

The workshop will be organized as follows: after presentation (V. Bommier), reports on the problem itself as observation result (H. Balthasar) and Stokes inversion and radiative transfer (E. Landi Degl’Innocenti), the discussion will remain free and open around a table, for together investigation of possible solutions or artifacts. Any other contribution prepared in advance is welcome.

Depending on the conclusions, the workshop would eventually be followed by a second one more aimed towards theory, in the following year.