Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm53a1363m&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #SM53A-1363
Physics
[0639] Electromagnetics / Nonlinear Electromagnetics, [0654] Electromagnetics / Plasmas, [2704] Magnetospheric Physics / Auroral Phenomena, [2753] Magnetospheric Physics / Numerical Modeling
Scientific paper
Observations from the Fast Auroral SnapshoT (FAST) spacecraft indicate that a strong localized electric field often exists at the boundary separating the ionosphere from the auroral cavity in the upward current region. The localized electric field observed at the boundary usually has components that are both parallel and perpendicular to earth's magnetic field and therefore are said to be an “oblique” electric field. The boundary electric field has been modeled as an oblique BGK DL in prior efforts. In addition, FAST observations indicate that the auroral cavity has a density that is ~ 1/10 that of the ionosphere. We have used parameters derived from FAST to initialize a 1D3V (1 spatial dimension, 3 velocity dimensions) open boundary particle-in-cell simulation. We assume that a density cavity that is 1/10 - 1/20 the density of the ionosphere is initially present (as indicated by observations). However, unlike previous efforts, we do not assume that a self-consistent electric field is initially present. The initial plasma consists of anti-earthward O+ and H+ beams, hot magnetospheric H+ population, hot magnetospheric electron population and a cold dense electron population. We assume that the initial population is composed entirely of ionospheric species. At the two boundaries, plasma particles are injected based on the initial population with one key exception. The magnetospheric boundary does not include the cold electrons since observations indicate a lack of these species in the auroral cavity. We demonstrate that a DL readily forms out of the initial conditions at the interface of the dense and tenuous plasma. Furthermore the DL is quasi-stable for the entire simulation if the initial cold electrons are cold enough (~ 40 eV) and dense enough (0.3% of the total electron population). We also show that the oblique DLs are quasi-stable and also persist for the entire simulation. We find that to ensure the stability of the oblique DL, it is necessary to make the cold electrons colder and denser compared with the non-oblique DLs. The simulated DLs that we observe are quasi-stable and have no appreciable drift associated with them, as indicated by observations.In addition to the formation of a quasi-stable DL, we also demonstrate the formation and evolution of ion phase space holes and ion acoustic solitons within the newly formed auroral cavity. The ion acoustic solitons which form are highly assymetric. We show that the ion acoustic soliton may explain the mid-cavity DLs previously reported by Ergun, et al (2004).
Ergun Robert E.
Main D. S.
Newman D. L.
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