Other
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsm52b..04s&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SM52B-04
Other
2431 Ionosphere/Magnetosphere Interactions (2736), 2704 Auroral Phenomena (2407), 2736 Magnetosphere/Ionosphere Interactions, 2776 Polar Cap Phenomena
Scientific paper
Ionospheric outflows are the result of a multi-stage process. The ions must be accelerated to a sufficiently high energy so that their parallel velocity exceeds the escape velocity, usually through wave heating. But other processes must act so as to increase the effective scale height of the ionosphere. Two mechanisms are often cited as causes of ion upwelling. One is Joule dissipation associated with ionospheric Pedersen currents, which results in heating of the ions in the ionosphere. The second process is soft electron precipitation, which increases the ionospheric electron temperature, in turn enhancing ion upwelling through the resultant ambipolar electric field. A statistical analysis of ion outflows in the dayside cusp region using data from the FAST spacecraft shows that the outflows are most strongly correlated with the density of precipitating electrons, supporting the hypothesis that soft electron precipitation is the dominant controlling factor. Nevertheless, it is not possible to completely discount ionospheric Joule dissipation as measured by downward Poynting flux because of the strong correlation between different parameters. Furthermore, Poynting flux can be carried by both the quasi-static field-aligned current system and Alfvén waves. Large amplitude Alfvén waves have been observed at FAST altitudes in the dayside cusp region and in the nightside boundary plasma sheet. Alfvén waves are more likely to increase ion outflows by increasing the precipitating flux of soft electrons, rather than ion heating, since most of the Alfvén-wave Poynting flux is reflected at the ionosphere because of the impedance mismatch. Through comparison between dayside and nightside ion outflows we can begin to address the relative importance of Alfvén waves in driving ionospheric outflows. Our preliminary analysis indicates that soft electron precipitation is the dominant controlling factor for both dayside and nightside, and Poynting flux, be it Alfvénic or quasi-static, is probably of secondary importance, augmenting the ion outflow rate through additional particle heating.
Carlson Carl W.
Ergun Robert E.
Strangeway Robert J.
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