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R.T. Clancy, B.J. Sandor (Space Science Institute), G.H. Moriarty-Schieven (James Clerk Maxwell Telescope)
Sub-millimeter line observations of CO in the Venus middle atmosphere (mesosphere) were observed with the James Clerk Maxwell Telescope (JCMT, Mauna Kea) about the May 2000 superior and July 1999, March 2001 inferior conjunctions of Venus. Combined 12CO and 13CO isotope spectral line measurements at 345 gHz and 330 gHz frequencies, respectively, provided enhanced sensitivity and vertical coverage for simultaneous retrievals of atmospheric temperatures and CO mixing ratios over the 75-105 km altitude region. Implementation of these retrievals for the dayside (superior conjunction) and nightside (inferior conjunction) periods yields a first-time definition of the vertical structure and diurnal variation of a low-to-mid latitude mesopause within the Venus atmosphere. At the times of the 1999-2001 observations, the Venus mesopause was located at a slightly lower level in the nightside (0.4 mbar, ~ 88 km) versus the dayside (0.2 mbar, ~ 91 km) atmosphere. Atmospheric temperatures above and below the Venus mesopause exhibited global-scale (> 4000 km horizontal) variations of large amplitudes (10-15K) on surprisingly short timescales (1-5 days) during the 2001 period of nightside observations. Much smaller temporal variations (5K or less) were observed during the 1999 nightside and 2000 dayside observing periods. Average diurnal variation of Venus mesospheric temperatures was of the order 5K at and just below the mesopause. Diurnal variation of Venus thermospheric temperatures began just above the mesopause at the 0.1 mbar pressure level (~ 94 km), and reached 50K by the 0.01 mbar pressure level (~ 102 km).