Thermal bifurcation in the upper photosphere inferred from heterodyne spectroscopy of OH rotational lines

Abstract

We have obtained low noise (S/N > 10³), high spectral resolution (ν/Δν ≈ 10 ⁶) observations of two pure rotation transitions of OH from the solar photosphere. The observations were obtained using the technique of optically null-balanced infrared heterodyne spectroscopy, and consist of center-to-limb line profiles of aυ = 1 and a υ = 0 transition near 12 μm. These lines should be formed in local thermodynamic equilibrium (LTE), and are diagnostics of the thermal structure of the upper photosphere. We find that theυ = 0R22(24.5)e line strengthens at the solar limb, in contradiction to the predictions of current one-dimensional photospheric models. Our data for this line support a two-dimensional model in which horizontal thermal fluctuations of order ±800 K occur in the region τ₅₀₀₀ ≈ 10⁻³–10⁻². This thermal bifurcation may be maintained by the presence of magnetic flux tubes, and may be related to the solar limb extensions observed in the 30–200 μm region.

Observations of theυ = 1R11(29.5)f line, at 885.643 cm⁻¹, show that it is anomalously weak in the photospheric spectrum. We argue that the source function in the core of this line has been substantially increased by interaction with the 9j-7i transition of Mgi at 885.524 cm⁻¹, which is itself too weak to appear in the disk center spectrum.