Determination of the gradient magnetic field above a sunspot based on observations of the HeI and FeI infrared lines

The magnetic field strength above 36 sunspots was studied using the spectra of the HeI λ 1083.030 nm and FeI 1082.837 nm magnetosensitive lines. The value of the field was determined directly by Zeeman splitting of the lines. The advantages of our observations are as follows: (a) in the IR region, the Zeeman splitting exceeds the similar splitting in the visible region by several times, (b) both studied lines are in the same spectrum in close proximity to each other, (c) the instrumental scattering in the IR region is lower, d) the measurements are conducted close to the true continuum of intensity between the spectral lines. The relationship between magnetic field strength and residual intensity in both spectral lines is nonlinear. The gradient of the magnetic field above sunspots at heights from the photosphere to the upper chromosphere varies within 0.1–0.7 G/km for all the studied sunspots.