Physics
Scientific paper
Feb 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012jgra..11702215w&link_type=abstract
Journal of Geophysical Research, Volume 117, Issue A2, CiteID A02215
Physics
2
Magnetospheric Physics: Magnetosphere/Ionosphere Interactions (2431), Magnetospheric Physics: Mhd Waves And Instabilities (2149, 6050, 7836), Magnetospheric Physics: Numerical Modeling, Magnetospheric Physics: Plasma Sheet, Magnetospheric Physics: Substorms
Scientific paper
This paper presents a quantitative theory of “interchange oscillations,” which occur as an earthward-moving low-entropy plasma bubble slows and eventually comes to rest. Our theoretical picture is based on an idealized situation where an ideal-MHD magnetic filament moves without friction through a stationary background that represents the plasma sheet. If the relevant region of the background plasma sheet is interchange stable, then the filament usually executes a damped oscillation about an equilibrium position, where its entropy parameter matches the local background. The oscillations are typically dramatic only if the equatorial plasma beta is greater than about one. We derive an approximate analytic formula for the oscillation period, which is not simply related to slow- or intermediate-wave travel times. For an oscillation that Panov and collaborators carefully studied using THEMIS data, our simple theory, though based on an unrealistic 2D background magnetic field, predicted an oscillation period that agrees with the observations within about 40%. The simulations suggest that the ionospheric oscillation should lag behind the magnetospheric one by between 40 and 90 degrees. Ionospheric conductance affects the damping rate, which maximizes for an auroral zone conductance ∼2 S. Adding a friction force acting between the filament and the background increases the decay rate of the oscillation.
Chen Xiao-Chun
Toffoletto Frank R.
Wolf Richard A.
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