Low-Energy Ion Escape from the Terrestrial Polar Regions

Sammanfattning: The contemporary terrestrial atmosphere loses matter at a rate of around 100,000 tons per year. A major fraction of the net mass loss is constituted by ions, mainly H+ and O+, which escape from the Earth’s ionosphere in the polar regions. Previously, the outflow has only been measured at low altitudes, but to understand what fraction actually escapes and does not return, the measurements should be conducted far from the Earth. However, at large geocentric distances the outflowing ions are dif?cult to detect with conventional ion instruments on spacecraft, since the spacecraft electrostatic potential normally exceeds the equivalent energy of the ions. This also means that little is known about the ion out?ow properties and distribution in space far from the Earth.In this thesis, we present a new method to measure the out?owing low-energy ions in those regions where they previously have been invisible. The method is based on the detection by electric ?eld instruments of the large wake created behind a spacecraft in a ?owing, low-energy plasma. Since ions with low energy will create a larger wake, the method is more sensitive to light ions, and our measured out?ow is essentially the proton out?ow.Applying this new method on data from the Cluster spacecraft, we have been able to make an extensive statistical study of ion out?ows from 5 to 19 Earth radii in the magnetotail lobes. We show that cold proton out?ows dominate in these large regions of the magnetosphere in both ?ux and density. Our out?ow values of low-energy protons are close to those measured at low altitudes, which con?rms that the ionospheric out?ows continue far back in the tail and contribute signi?cantly to the magnetospheric content. We also conclude that most of the ions are escaping and not returning, which improves previous estimates of the global out?ow. The total loss of protons due to high-latitude escape is found to be on the order of 1026 protons/s.