Balloon Observations of MeV Electron Precipitation

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The proposed project is the preparation and flight of a series of balloon payloads to study energetic electrons precipitated from the outer zone of the Earth's radiation belts into the atmosphere, and the subsequent data analysis. The electrons under consideration are relativistic (kinetic energy greater than the electron rest mass of 511 keV). These electrons can be responsible for damage to satellites in geosynchronous orbit. They are trapped in the Earth's magnetosphere between roughly 4 and 6 Earth radii at the equator, which means they hit the Earth near the Arctic and Antarctic circles when they come down. The precipitation can be triggered by a number of causes, some associ-ated with magnetic storms and some which can occur under all conditions. The balloons study not the electrons themselves, but rather x-rays emitted by the bremsstrahlung process when they hit the atmosphere. A balloon can therefore watch all the precipitation, taking place within a hundred kilometers of its location or more, and can track the variability of the emission with time. Most satellites only observe electrons at their own position, and move too quickly through the precipitation zone to observe the evolution of the precipitation events. Balloons were used ex-tensively until the 1970s to study precipitation of non-relativistic electrons (roughly 20-200 keV). When relativistic electrons hit the atmosphere, they can produce enough ionization to create chemically important amounts of "odd nitrogen" (e.g. NO2 and other compounds). Depending on the circumstances, these compounds can either destroy stratospheric ozone or else bind up and deactivate chlorine atoms which destroy ozone even more efficiently, and thus perhaps partially protect the ozone layer. Although their role in either case is widely understood to be small compared to that of increases and decreases in chlorine introduced by human activity, that role is not well studied and may represent an important perturbation on the main chemistry. The campaign proposed for December 2004/January 2005will address the physics of the various relativistic electron precipitation processes, their patterns of occurrence in space and time, and the energy and number of electrons precipitated. The last point is relevant to the overall balance of relativistic electrons in the radiation belts and also to their impact on atmospheric chemistry.