Middle atmosphere modeling

Breaking gravity waves generate and maintain a background level of turbulence which is capable of producing substantial cooling and/or heating in the upper mesosphere and lower thermosphere. The net thermodynamic effect of breaking gravity waves is critically dependent on the eddy Prandt number (P sub t) applicable to mesospheric turbulence. When P sub t is approximately 1, the calculations of the heat budget for the mesopause region imply that the globally averaged eddy or turbulent diffusion coefficient cannot exceed .000001 sq cm/s. This upper limit on turbulant diffusion applies to both potential temperature transport and chemically inert tracer transport when radiative damping is neglible. For chemically active species larger diffusion coefficients are permitted, because the effective eddy diffusion coefficient is increased by an additive term L/2 gamma (sup 2), where L is the chemical loss rate and gamma is the vertical wavenumber. For P sub t is approximately 4 to 6, the turbulent diffusion of momentum (D sub M) is sufficiently greater than the turbulent diffusion of heat (D sub H) that the conversion of gravity wave energy to heat with high efficiency nearly balances the divergence of the downward eddy heat flux in the wave breaking zone. Therefore the heat budget of the mesopause region would no longer provide a powerful and useful constraint on D sub H. If P sub t exceeds 6 with high efficiency for energy conversion to heat, gravity waves would heat the mesosphere throughout the wave breaking region

Aeronomy of Saturn and Titan

The Saturn system presents exciting and unique objects for planetary aeronomy. The photochemistry of H2 and He leads to the formation of an ionosphere. Methane photolysis results in the formation of spectroscopically detectable amounts of C2H6 and C2H2 and in the case of Titan, C2H4. Density profiles of C2H6, C2H2, and PH3 should be indicative of the strength of atmospheric mixing processes

The Na I D resonance lines in main sequence late-type stars

We study the sodium D lines (D1: 5895.92 \AA; D2: 5889.95 \AA) in late-type dwarf stars. The stars have spectral types between F6 and M5.5 (B-V between 0.457 and 1.807) and metallicity between [Fe/H] = -0.82 and 0.6. We obtained medium resolution echelle spectra using the 2.15-m telescope at the argentinian observatory CASLEO. The observations have been performed periodically since 1999. The spectra were calibrated in wavelength and in flux. A definition of the pseudo-continuum level is found for all our observations. We also define a continuum level for calibration purposes. The equivalent width of the D lines is computed in detail for all our spectra and related to the colour index (B-V) of the stars. When possible, we perform a careful comparison with previous studies. Finally, we construct a spectral index (R_D') as the ratio between the flux in the D lines, and the bolometric flux. We find that, once corrected for the photospheric contribution, this index can be used as a chromospheric activity indicator in stars with a high level of activity. Additionally, we find that combining some of our results, we obtain a method to calibrate in flux stars of unknown colour.Comment: 12 pages, including 14 figures and 4 tables. Accepted for publication in MNRA

Magnetic properties of HO2 thin films

We report on the magnetic and transport studies of hafnium oxide thin films grown by pulsed-laser deposition on sapphire substrates under different oxygen pressures, ranging from 10-7 to 10-1 mbar. Some physical properties of these thin films appear to depend on the oxygen pressure during growth: the film grown at low oxygen pressure (P ~= 10-7 mbar) has a metallic aspect and is conducting, with a positive Hall signal, while those grown under higher oxygen pressures (7 x 10-5 <= P <= 0.4 mbar) are insulating. However, no intrinsic ferromagnetic signal could be attributed to the HfO2 films, irrespective of the oxygen pressure during the deposition.Comment: 1