Comparative terrestrial planet thermospheres 3. Solar cycle variation of global structure and winds at solstices

Sw. Bougher et al., Comparative terrestrial planet thermospheres 3. Solar cycle variation of global structure and winds at solstices, J GEO R-PLA, 105(E7), 2000, pp. 17669-17692
Citations number
Categorie Soggetti
Space Sciences
Journal title
ISSN journal
2169-9097 → ACNP
Year of publication
17669 - 17692
SICI code
The comparison of planetary upper atmospheres using global databases has en tered a new era with the advent of recent aerobraking measurements of the M ars thermosphere [e.g., Keating, et al., 1998a]. The present maturity of av ailable modeling capabilities also permits us to contrast the Earth and Mar s thermosphere structures, winds, and controlling processes using global th ree-dimensional models [e.g., Bougher et al., 1999b]. This present effort f ocuses upon the comparison of the combined seasonal-solar cycle responses o f the thermospheres of Earth and Mars using the National Center for Atmosph eric Research (NCAR) Thermospheric General Circulation Model (TGCM) utility to address the coupled energetics, dynamics, and neutral-ion composition a bove similar to 100 km. Extreme thermospheric conditions are expected at so lstices, thereby revealing the changing importance of fundamental physical processes controlling the Earth and Mars thermospheric structures and winds . Seasonal-solar cycle extremes in Mars exobase temperatures are calculated to range from 200 to 380 K, giving rise to maximum horizontal winds of nea rly 215 to 400 mis. Corresponding extremes in Earth exobase temperatures ar e 700 to 1600 K, with rather small variations in global winds. The orbital eccentricities of Earth and Mars are also shown to drive substantial variat ions in their thermospheric temperatures. For Mars, dayside exobase tempera tures vary by similar to 60 K (18%) from aphelion to perihelion during sola r maximum conditions. Such large temperature variations strongly impact the rmospheric densities and global winds. The corresponding Earth dayside temp eratures also vary by 60-80 K between solstices. However, the percent tempe rature variation (5%) over the Earth's orbit and its overall impact on the thermospheric structure and winds are much smaller. Amoral activity may in fact obscure these orbital variations. Changing dust conditions throughout the Martian year modulate the aerosol heating of its lower atmosphere, yiel ding considerable variability in the height of the subsolar ionospheric pea k about its observed seasonal trend (similar to 115-130 km). Significant fu rther progress in the comparison of Earth and Mars thermospheric features a nd underlying processes must await expanded Mars global databases expected from Planet-B and Mars Express (2004-2005).