Geocoronal Imaging with Dynamics Explorer

The ultraviolet photometer of the University of Iowa spin-scan auroral imaging instrumentation on board the Dynamics Explorer-1 satellite has returned numerous images of the geocorona from altitudes of 570 km to 23,300 km. The geocoronal emissions viewed arise from the resonant scattering of solar Lyman-alpha radiation by the neutral hydrogen atoms which make up the earth's tenuous exosphere. This hydrogen exosphere extends from (TURN)500 km to greater than 80,000 km altitude, wherein the atoms travel on effectively collisionless trajectories loosely bound by the earth's gravity. In this thesis the geocoronal observations from 1981 through 1984 are compared to a spherically symmetric isothermal Chamberlain model of the exospheric density distribution. Model parameters are varied to obtain an acceptable fit. The radiative transfer equation is solved numerically for this multiple-scattering problem. Stellar intensities are monitored for inflight calibration of the DE-1 instrument. The solar Ly (alpha) flux is estimated through concurrent measurements made by the SME satellite, supplemented by published values of ground-observable solar indices. Extraterrestrial background intensities are adopted from earlier OGO-5 high-altitude measurements. The optimum fit utilizes a Chamberlain model of temperature T = 1050 K and exobase density n(,c) = 44,000 atoms cm('-3). The exobase is taken as r(,c) = 1.08 R(,E) (500 km altitude), and a satellite critical level of r(,cs) = 3.0 r(,c) is adopted. This model compares well with the DE-1 observations over the entire three years of data studied. It is concluded that the exospheric temperature does not change appreciably during this period. A definitive statement on long-term changes in the required exobase density is not made due to uncertainty in the degradation of instrument sensitivity with time. A readily observable departure from spherical symmetry is the geotail, an enhancement in the antisunward column density. Other asymmetries reported by earlier investigators such as low-altitude diurnal variations and north-south seasonal changes are not examined in detail in this analysis of the DE-1 imaging data. Further study will be necessary to evalute these effects.