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[1] Ionospheric plasma temperature variations have recently been studied based on incoherent scatter radar (ISR) observations at a lower midlatitude site, Shigaraki, in East Asia ] and Millstone Hill, a typical subauroral midlatitude site in North America [Zhang and Holt, 2004]. The French Saint Santin ISR, with a geographic latitude slightly higher but an apex latitude 14°lower than Millstone, collected bistatic and quadristatic measurements for over two solar cycles beginning in September 1965. A database of these data, containing observations between 1966 and 1987, has been used in this study in order to establish the midlatitude ionospheric climatology, in particular that of the upper atmosphere thermal status, as well as empirical models for space weather applications. This paper presents, in comparison with the Millstone Hill results, variations of ion and electron temperatures (Ti and Te) with solar activity, season, time of the day, and altitude. The F2 region Te at St. Santin is found to be lower than at Millstone between March and July, when the St. Santin electron density Ne is relatively higher. The midday Te below 300 km increases with F10.7, as at Millstone Hill. Above 300 km it tends to decrease with F10.7 at St. Santin, while it increases in summer at Millstone Hill. Ti between 250 and 350 km peaks midway between spring and summer. We have also created St. Santin ionospheric models for Ne, Te, and Ti using a bin-fit technique similar to that used for the Millstone Hill models. Comparisons with corresponding IRI predications indicate good agreement in Ti at high solar activity, and above the F2 peak, Te from the IRI tends to be higher than both the St. Santin and Millstone Hill models.

We present millimeter interferometry images of the CO J=1-0 line emission arising in the circumstellar envelope of HD 56126 (a.k.a. IRAS 07134+1005), which is one of the best studied 21-micron proto-planetary nebulae (PPNs). The CO emission extends from 1.2" to 7" in radius from the central star and appears consistent with a simple expanding envelope, as expected for a post-AGB star. We quantitatively model the molecular envelope using a radiative transfer code that we have modified for detached shells. Our best fit model reveals that two sequential winds created the circumstellar envelope of HD 56126: an AGB wind that lasted 6500 years with a mass-loss rate of 5.1x10^{-6} M_odot yr^{-1} and a more intense superwind that lasted 840 years with a mass-loss rate of 3x10^{-5} M_odot yr^{-1} and that ended the star's life on the AGB 1240 years ago. The total mass of this envelope is 0.059 M_odot which indicates a lower limit progenitor mass for the system of 0.66 M_odot, quite reasonable for this low-metallicity star which probably resides in the thick disk of the Galaxy. Comparison with images of the dust emission reveal a similar structure with the gas in the inner regions. Using 2-DUST, we model the dust emission of this source so that the model is consistent with the CO emission model and find an average gas-to-dust mass ratio of 75.Comment: 23 pages, 6 figures, 3 tables, accepted for publication in 10/2004 issue of Ap

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