The problem of the height dependence of magnetic fields in sunspots

TL;DR

This paper investigates the conflicting measurements of magnetic field gradients in sunspots, highlighting discrepancies between different methods and discussing potential explanations for these differences.

Contribution

It compares various methods of determining magnetic field stratification in sunspots and discusses possible reasons for their inconsistent results.

Findings

Spectral line analysis yields steep magnetic gradients of -2 to -3 G/km.

Horizontal derivatives and divB=0 suggest shallower gradients around -0.5 G/km.

Including chromospheric lines and radio measurements consistently indicates shallow gradients.

Abstract

To understand the physics of sunspots, it is important to know the properties of their magnetic field, and especially its height stratification plays a substantial role. There are mainly two methods to assess this stratification, but they yield different magnetic gradients in the photospheric layers. Determinations based on the different origin of several spectral lines and the slope of their profiles result in gradients of -2 to -3G/km, or even steeper. This is similar for the total magnetic field strength and for the vertical component of the magnetic field. The other option is to determine the horizontal partial derivatives of the magnetic field, and with the condition divB=0, also the vertical derivative is known. With this method, gradients of -0.5 G/km and shallower are obtained. Obviously, these results do not agree. If chromospheric spectral lines are included, only shallow gradients around -0.5G/km are encountered. Shallow gradients are also found from gyro-resonance measurements in the radio wave range 300 - 2000GHz. Some indirect methods are also considered, but they cannot clarify the total picture. An analysis of a numerical simulation of a sunspot indicates a shallow gradient over a wide height range, but with slightly steeper gradients in deep layers. Several ideas to explain the discrepancy are also discussed. With no doubts cast on Maxwell's equations, the first one is to look at the uncertainties of the formation heights of spectral lines, but a wider range of these heights would require an extension of the solar photosphere that is incompatible with observations and the theory of stellar atmospheres. The problem of the height gradient of the magnetic field in sunspots is still not solved.