The outermost atmosphere of the earth consists of hydrogen, the lightest gas, which is produced by the dissociation of water vapor and methane in the altitudes of above 500km, where the H atoms travel on free ballistic orbits out to distances of as much as 10 earth radii before returning. Some of the H atoms have sufficient energy to escape the earth completely, and it is in this way that the primitive earth lost the large amounts of hydrogen to make possible the present oxygen rich atmosphere. The hydrogen provides a background medium through which energetic charges particles trapped in the earth's magnetic field (the ring current and Van Allen belts) travel, and collisions with the H atoms are the main loss process for the trapped ions energized during magnetic storms. The energetic neutrals resulting from such collisions travel on straight line trajectories, and can provide, with suitable detectors, "images" of the ring current in an analogous way to photons entering a camera. Collisions with low energy ions are an important cooling process for the very numerous low energy ions that also populate the near earth space threaded by the earth's magnetic field (the Magnetosphere). Thus it is important to know the distribution of hydrogen around the earth (the Geocorona) to be able to model the interactions with the ions; interpret "images" in energetic neutrals; and quantify the escape rate. This information can be obtained by ground based observations with Fabry Perot interferometers, and a number of observers and theoreticians at three universities (Wisconsin, Michigan, and Alaska) and the Aerospace Corporation have formed a group to carry out simultaneous measurements from sites ranging from Puerto Rico to Spitzbergen, to determine the latitude variation, and the height distribution of the Hydrogen.
