Helium ion outflow from the terrestrial ionosphere |
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| Authors: | WJ Raitt RW Schunk PM Banks |
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| Institution: | Center for Research in Aeronomy and Department of Physics, Utah State University, Logan, UT 84322, U.S.A. |
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| Abstract: | Extensive calculations have been made of the behaviour of He+ for situations where ion outflow occurs from the topside ionosphere. For these circumstances, steady state solutions for the He+ continuity, momentum and energy equations have been obtained self-consistently, yielding density, velocity and temperature profiles of He+ from 200 to 2000 km altitude. To model the high latitude topside ionosphere, a range of background H+O+ ionospheres was considered with variations in the H+ outflow velocity, the presence of a perpendicular electric field and different peak O+ densities. In addition, the atmospheric density of neutral helium was chosen to model typical observed winter and summer densities. From our studies we have found that: (a) The outflowing He+ has density profiles of similar shape to those of H+, for basically different reasons; (b) The effect of the perpendicular electric field differs considerably for H+ and He+. This difference stems from the fact that an electric field acts to alter significantly the O+ density at high altitudes and this, in turn, changes the H+ escape flux through the O++H charge exchange reaction. A similar situation does not occur for He+ and therefore the He+ escape flux exhibits a negligibly small change with electric field; (c) The fractional heating of He+ due to the He+O+ relative flow is not as effective in heating He+ as the H+O+ relative flow is in heating H+; (d) During magnetospheric disturbances when the N2 density at the altitude of the He+ peak (600 km) can increase by a factor as large as 50, the He+ peak density decreases only by approximately a factor of 2; and (e) The He+ escape flux over the winter pole is approximately a factor of 20 greater than the He+ escape flux over the summer pole. Consequently, on high latitude closed field lines there could be an interhemispheric He+ flux from winter to summer. |
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