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Ultraviolet lines of molecular hydrogen have been observed in solar spectrafor almost four decades, but the behavior of the molecular spectrum and itsimplications for solar atmospheric structure are not fully understood. Datafrom the HRTS instrument revealed that H2 emission forms in particular regions,selectively excited by bright UV transition region and chromospheric lines. Wetest the conditions under which H2 emission can originate by studying non-LTEmodels sampling a broad range of temperature stratifications and radiationconditions. Stratification plays the dominant role in determining thepopulation densities of H2, which forms in greatest abundance near thecontinuum photosphere. However, opacity due to photoionization of silicon andother neutrals determines the depth to which UV radiation can penetrate toexcite the H2. Thus the majority of H2 emission forms in a narrow region, atabout 650 km in standard 1D models of the quiet-Sun, near the tau=1 opacitysurface for the exciting UV radiation, generally coming from above. Whenirradiated from above using observed intensities of bright UV emission lines,detailed non-LTE calculations show that the spectrum of H2 seen in thequiet-Sun SUMER atlas spectrum and HRTS light bridge spectrum can besatisfactorily reproduced in 1D stratified atmospheres, without including 3D ortime dependent thermal structures. A detailed comparison to observations from1205 to 1550 Angstroms is presented, and the success of this 1D approach tomodeling solar UV H2 emission is illustrated by the identification ofpreviously unidentified lines and upper levels in HRTS spectra.

Formation of the UV Spectrum of Molecular Hydrogen in the Sun