Low-Energy Ion Escape from the Terrestrial Polar Regions
Abstract: 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 difﬁcult 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 outﬂow properties and distribution in space far from the Earth.In this thesis, we present a new method to measure the outﬂowing low-energy ions in those regions where they previously have been invisible. The method is based on the detection by electric ﬁeld instruments of the large wake created behind a spacecraft in a ﬂowing, low-energy plasma. Since ions with low energy will create a larger wake, the method is more sensitive to light ions, and our measured outﬂow is essentially the proton outﬂow.Applying this new method on data from the Cluster spacecraft, we have been able to make an extensive statistical study of ion outﬂows from 5 to 19 Earth radii in the magnetotail lobes. We show that cold proton outﬂows dominate in these large regions of the magnetosphere in both ﬂux and density. Our outﬂow values of low-energy protons are close to those measured at low altitudes, which conﬁrms that the ionospheric outﬂows continue far back in the tail and contribute signiﬁcantly to the magnetospheric content. We also conclude that most of the ions are escaping and not returning, which improves previous estimates of the global outﬂow. The total loss of protons due to high-latitude escape is found to be on the order of 1026 protons/s.
This dissertation MIGHT be available in PDF-format. Check this page to see if it is available for download.