  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.
