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L. M. Trafton (U. Texas), S. Miller (UCL)
At electron densities typical of the Uranian ionosphere, ~104 cm-3, the chemical lifetime of H3+ is between 103 and 104 seconds (Trafton et al. ApJ 524, 1059; 1999), much less than the rotational period of the planet. This suggests that H3+ concentrations on the night side should be low; and the day side should exhibit a significant time-of-day effect, with H3+ emission ramping up from the sunrise limb to reach a broad maximum in the late afternoon, towards the evening limb. Such a diurnal lag in the peak emission would be useful for characterizing the structure of the Uranian ionosphere, especially the electron concentration produced by the daily EUV pulse. We report preliminary results of a search for evidence of a dawn-dusk asymmetry about Uranus' central meridian in the H3+ emission intensity.
We obtained imaging observations of Uranus' H3+ emission in selected lines of the fundamental-band that were obtained in July 1999 at the IRTF with NSFCam. Neither Uranus’ continuum nor the ring continuum were detected at these wavelengths. The seeing for the observations reported was 0."5 to 0."7 vs pixel size 0."15 square. On the night of July 12, images obtained in the relatively bright Q(1) and Q(3) lines at 3.953 and 3.986 \mu, respectively, were obtained using the continuously variable filter to isolate them in wavelength. Although the emission appears symmetric along the central meridian, it exhibits the predicted behavior from the AM to PM limb. Preliminary measurement indicates a phase lag of 33 deg, uncorrected for seeing. We will discuss the extraction of the day side ionospheric electron density and the departure from thermal equilibrium of H3+.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.