Formation of the UV Spectrum of Molecular Hydrogen in the Sun

Abstract

Ultraviolet (UV) lines of molecular hydrogen have been observed in solar spectra for almost four decades, but the behavior of the molecular spectrum and its implications for solar atmospheric structure are not fully understood. Data from the High-Resolution Telescope Spectrometer (HRTS) instrument revealed that H 2 emission forms in particular regions, selectively excited by a bright UV transition region and chromospheric lines. We test the conditions under which H 2 emission can originate by studying non-LTE models, sampling a broad range of temperature stratifications and radiation conditions. Stratification plays the dominant role in determining the population densities of H 2 , which forms in greatest abundance near the continuum photosphere. However, opacity due to the photoionization of Si and other neutrals determines the depth to which UV radiation can penetrate to excite the H 2 . Thus the majority of H 2 emission forms in a narrow region, at about 650 km in standard one-dimensional (1D) models of the quiet Sun, near the τ  = 1 opacity surface for the exciting UV radiation, generally coming from above. When irradiated from above using observed intensities of bright UV emission lines, detailed non-LTE calculations show that the spectrum of H 2 seen in the quiet-Sun Solar Ultraviolet Measurement of Emitted Radiation atlas spectrum and HRTS light-bridge spectrum can be satisfactorily reproduced in 1D stratified atmospheres, without including three-dimensional or time-dependent thermal structures. A detailed comparison to observations from 1205 to 1550 Å is presented, and the success of this 1D approach to modeling solar UV H 2 emission is illustrated by the identification of previously unidentified lines and upper levels in HRTS spectra.