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 ≈ 106) 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.