Satellite Ballistic Coefficients and the Lower Thermosphere

The observed variations in the 'estimated' ballistic coefficients (B') for low-perigee satellites in the lower thermosphere (about 200 km) during the year 2001, have been used to study their implications for neutral atmospheric densities with respect to changes in the solar activity and geomagnetic activity. The changes in B' reflect the unmodeled corrections in the atmospheric density models. It is shown that statistically, on the average, based on the 'true' ballistic coefficients (Bt), the atmospheric density models predict the 'true' density within 10 percent as a function of solar activity for quiet and mild geomagnetic periods. The observed variations in the ballistic coefficients with days exhibit a semiannual variation (SAV). The 'scaling' factor required to correctly predict the SAV in the 'true' densities from the model densities, ranges from a maximum of about 1.13 in the spring (equinox) to a minimum of about 0.93 in the summer (solstice), implying a semiannual amplitude(SAA) of (max/min) 1.2. The calculations based on the CIRA-86 models show that changes in B' during some major magnetic storms cannot be explained, as they are probably contaminated by 'high density neutral cells' observed by the S85-1 satellite at 200 km and predicted by the NCAR-TIGCM models by Crowley, et al (1995). Changes in B' due to these cells all the more require temperature/density corrections of the steady-state High Accuracy Satellite Drag Model (HASDM). It is suggested that the NCAR-TIGCM model should be used to see if it performs better in bringing the B' values closer to Bt than the steady-state models.