Radio science measurements of atmospheric refractivity with Mars Global Surveyor

Abstract

Radio occultation experiments with Mars Global Surveyor measure the refractive index of the Martian atmosphere from the surface to ~250 km in geopotential height. Refractivity is proportional to neutral density at low altitudes and electron density at high altitudes, with a transition at ~75 km. We use weighted least squares to decompose zonal refractivity variations into amplitudes and phases for observed wave numbers k=1-4 over the entire altitude range and use the results to analyze atmospheric structure and dynamics. The data set consists of 147 refractivity profiles acquired in December 2000 at summer solstice in the Martian northern hemisphere. The measurements are at an essentially fixed local time (sunrise) and at latitudes from 67deg to 70degN. Thermal tides appear to be responsible for much of the observed ionospheric structure from 80 to 220 km. Tides modulate the neutral density, which in turn, controls the height at which the ionosphere forms. The resulting longitude-dependent vertical displacement of the ionosphere generates distinctive structure in the fitted amplitudes, particularly at k=3, within plusmn50 km of the electron density peak height. Our k=3 observations are consistent with an eastward propagating semidiurnal tide with zonal wave number 1. Relative to previous results, our analysis extends the characterization of tides to altitudes well above and below the electron density peak. In the neutral atmosphere, refractivity variations from the surface to 50 km appear to arise from stationary Rossby waves. Upon examining the full vertical range, stationary waves appear to dominate altitudes below ~75 km, and thermal tides dominate altitudes above this transition region.