1.5 µm Observations and the Depth of Sunspot Penumbrae

The magnetic structure of a simple, relatively symmetric sunspot is determined using the extremely Zeeman sensitive Lande g = 3 line of Fe I at 1.5648 μm. From the measured strength and inclination of the magnetic field we estimate the fraction of the total magnetic flux of the sunspot passing through the solar surface in the penumbra. It is found that on average approximately 1/2–2/3 of the total magnetic flux of the spot emerges in the penumbra. Sunspot penumbrae are therefore deep, i.e., the τ = 1 level does not correspond to the lower magnetic boundary of the spot in its penumbra.