The rotational temperature and column density of H3 in Uranus

H₃⁺ emission from Uranus has been observed repeatedly for over a decade. However, the details of the emission mechanisms are still poorly understood. In this paper, we discuss our findings from the observations we made in September 2000 and September 2001. The spectrum of Uranus was recorded at the NASA Infrared Telescope Facility using the SpeX instrument between 3 and 5 μm, with a resolving power of 1000. The 3.4–4.1 μm range permits a determination of both the H₃⁺ column density and its rotational temperature. The H₃⁺ emission, measured at 3.986 μm in the 0.8×3.7 arcsec aperture, was 0.031 Jy in September 2000 and 0.053 Jy in September 2001. The rotational temperature was found to be 560±40 K and 640±40 K in 2000 and 2001 respectively, with corresponding column densities of 5.1 (+3.2,−1.4) 10¹¹ and 4.0 (+1.8,−1.0) 10¹¹ cm⁻². These results extend the baseline for the variability study of the H₃⁺ emission (Astrophys. J. 524 (1999) 1059). Previous observations between 1992 and 1998 seemed to indicate a correlation between the H₃⁺ intensity and the solar cycle. The current data for 2000 and 2001 appear to be consistent with this general tendency.