The outermost atmosphere of the earth and the other planets consists of hydrogen, the lightest gas, that is produced by the dissociation of methane and water vapor by irradiation by solar ultraviolet in the middle atmosphere above the ozone layer. The hydrogen diffuses to altitudes above 500km, where the H atoms travel on ballistic orbits out to about 10 earth radii, and form the envelope of hydrogen around the Earth known as the "Geocorona." Some of the H atoms are energized sufficiently to escape the earth altogether, mainly by collisions with the numerous low energy trapped ions confined by the earth's magnetic field, that constitute the "Magnetosphere." This process is also an important cooling process for the low energy ions. The interaction between the neutral hydrogen and another population of high energy ions is the main loss mechanism for the energy injected into the Magnetosphere in the form of a "Ring Current" of these ions during magnetic storms. Thus it is important to know the distribution of hydrogen around the earth, and both observational and theoretical efforts are under way to determine this. This award is for a modelling effort that seeks especially to take into account the effects of solar radiation pressure on perturbing the trajectories of the H atoms, and causing some to be injected into orbit, and some to escape the earth in an alternative process to collisions with trapped ions.
