Polhemsalen, Ångström Laboratory, UppsalaCold magnetospheric plasma flows: Properties and interaction with spacecraft
The ionosphere constantly loses matter to the surrounding magnetosphere through different outflow processes, and it is probably the main source of plasma supply to the magnetosphere. The ionospheric plasma has low energy and when flowing out from the Earth along diverging magnetic field lines, the density decreases. This will make the plasma ions difficult to detect with spacecraft, since the low density ensures a high spacecraft potential, which the low-energy ions will not be able to overcome. Therefore, only few observations of tenuous, cold plasma have been made, in spite of its abundance in the magnetosphere.
We present a new method for detecting and studying cold plasma with the double-probe electric field instrument EFW on the Cluster spacecraft. In cold flowing plasmas EFW observes a negatively charged spacecraft wake, which can be used to derive the flow speed of the cold plasma. The method has been verified for a case in the magnetotail at 18 Earth radii away from Earth, where a very special and unusual setup of the four Cluster spacecraft allowed simultaneous measurements of the ions with particle detectors. We have then applied the method for an initial statistical study of three months of Cluster data in the magnetotail lobes. The resulting flow parameters show agreement with previous measurements of ion outflow at lower altitudes, and the method opens up for observing cold flowing ions in regions where they previously have been inaccessible to spacecraft. To better understand the observed wake fields in EFW data, we have studied properties of enhanced wakes by numerical simulations, theoretical reasoning and data analysis.Created 2006-03-10 14:34:44 by Rick McGregorLast changed 2006-03-10 14:34:44 by Rick McGregor