Physics – Plasma Physics
Scientific paper
Nov 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996jgr...10124565d&link_type=abstract
Journal of Geophysical Research, Volume 101, Issue A11, p. 24565-24572
Physics
Plasma Physics
21
Ionosphere: Ionosphere/Magnetosphere Interactions, Ionosphere: Polar Cap Ionosphere, Ionosphere: Topside Ionosphere, Space Plasma Physics: Charged Particle Motion And Acceleration
Scientific paper
The ionospheric convection electric fields that occur at high latitudes cause plasma to drift across the cusp region and the polar cap. Since the magnetic field at high latitudes is close to vertical, pointing downward (upward) in the northern (southern) hemisphere, the convecting plasma experiences a centrifugal acceleration as it crosses the polar region because of the diverging magnetic field geometry. The centrifugal force is directly proportional to the mass of the plasma particles, and it is reasonable to ask whether this force has an effect on polar plasma outflow, particularly for the more massive ion O+. To date, a number of studies have addressed this question, but the theoretical models used in these studies were either overly simplified (i.e., neglected processes known to be important in the polar ionosphere) or else did not use appropriate boundary conditions or take account of the time variability of the problem. The results of these prior investigations were often contradictory. In order to overcome the limitations of these earlier studies, we have used a macroscopic particle-in-cell (PIC) code, which is sophisticated in the sense that a broad range of physical processes are incorporated in its description, in conjunction with time-varying boundary conditions obtained from a time-dependent, three-dimensional, hydrodynamic model of the polar ionosphere. This enables us to properly account for the variation of boundary conditions along a flux tube trajectory. Initially, our macroscopic PIC model was solved for steady state conditions. This allowed us to compare results from our code with those of a prior study of centrifugal acceleration that uses a PIC formulation. Also, by obtaining steady state solutions for both low and high electron temperatures, we have been able to directly compare the effects of electron temperature and centrifugal force on the polar plasma outflow, a comparison that a time-dependent simulation might obscure. Then time-dependent PIC solutions were obtained for the plasma in a convecting flux tube, using solutions to a time-dependent, three-dimensional, hydrodynamic model to provide realistic boundary values for the electron and ion temperatures and the H+ and O+ densities and drift velocities along a flux tube trajectory. Both steady state and time-dependent solutions indicate that centrifugal acceleration does not significantly contribute to the loss of plasma from the polar ionosphere.
Barakat Abdallah R.
Demars Howard G.
Schunk Robert W.
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