A. E. Hedin

Models of Venus neutral upper atmosphere - Structure and composition

Abstract Models of the Venus neutral upper atmosphere, based on both in-situ and remote sensing measurements, are provided for the height interval from 100 to 3,500 km. The general approach in model formulation was to divide the atmosphere into three regions: 100 to 150 km, 150 to 250 km, and 250 to 3,500 km. Boundary conditions at 150 km are consistent with both drag and mass spectrometer measurements. A paramount consideration was to keep the models simple enough to be used conveniently. Available observations are reviewed. Tables are provided for density, temperature, composition (CO 2 , O, CO, He, N, N 2 , and H), derived quantities, and day-to-day variability as a function of solar zenith angle on the day- and nightsides. Estimates are made of other species, including O 2 and D. Other tables provide corrections for solar activity effects on temperature, composition, and density. For the exosphere, information is provided on the vertical distribution of normal thermal components (H, O, C, and He) as well as the hot components (H, N, C, O) on the day- and nightsides.

The atmospheric model in the region 90 to 2000 km

A comprehensive revision of the MSIS-83 earth atmosphere model was undertaken to extend predictions to the mesopause, on the basis of temperature, density, and composition data obtained by sounding rockets, satellites, and incoherent scatter radars. Attention is presently given to a semiempirical model similar to MSIS-83, which incorporates additional terms in order to represent seasonal differences in the morphology of composition variations under both quiet and disturbed polar region conditions. The data base is expanded to include both composition and temperature data from the Dynamics Explorer-2 satellite.

On the structure and dynamics of the thermosphere

Abstract Thermospheric temperature, composition and wind measurements from the Dynamics Explorer satellite (DE-2) are interpreted using a three dimensional, multiconstituent spectral model. The analysis accounts for tides driven by the absorbed solar radiation as well as energy and momentum coupling involving the magnetosphere and lower atmosphere. We discuss phenomena associated with the annual tide, polar circulation, magnetic storms and substorms.

Neutral winds derived from IRI parameters and from the HWM87 wind model for the sundial campaign of September, 1986

Meridional neutral winds derived from the height of the maximum ionization of the F2 layer are compared with values from results of the HWM87 empirical neutral wind model. The time period considered is the SUNDIAL-2 campaign, 21 Sept. through 5 Oct. 1986. Winds were derived from measurements by a global network of ionosondes, as well as from similar quantities generated by the International Reference Ionosphere. Global wind patterns from the three sources are similar. Differences tend to be the result of local or transient phenomena that are either too rapid to be described by the order of harmonics of the empirical models, or are the result of temporal changes not reproduced by models based on average conditions.

Intercomparison of neutral composition measurements from the satellites Esro 4, Aeros A, Aeros B, and Atmosphere Explorer C

During the past few years the number of satellites carrying neutral gas mass spectrometers for in situ geophysical measurements in the thermosphere has increased significantly. In order to make optimum use of the data thus obtained it is essential to make an intercomparison of the absolute number densities from the various measurements. We therefore performed a direct comparison at ‘crossover points’ of the orbits of the satellites Esro 4/Aeros A, Esro 4/AE-C, and AE-C/Aeros B in the altitude range 150–300 km, and with few exceptions the agreement is within the experimental errors. This result lends confidence to the use of the broad data base obtained by these satellites since December 1972.

Composition changes and empirical models

Abstract Empirical model descriptions of the response of the neutral thermosphere to magnetic activity have become increasingly sophisticated over the last two decades. The latest CIRA model (MSIS-86) includes a dependence of the thermospheric response to magnetic activity on altitude, latitude, local time, season, longitude, UT, and time history of magnetic activity. A primary assumption is that density changes are directly related to the measured magnetic activity (and its history) and the goal is for the model to be representative of the typical or average response for any given level of magnetic activity under specified ancillary conditions. Unfortunately, available measurements for the larger storms are too sparse to give good averages and storm response may not be uniquely represented by current magnetic indices. Point by point comparisons with the MSIS-86 model have standard deviations of 20% for O and 30% for N 2 and He at high latitudes during storm conditions.