Thin filament simulations for Earth's plasma sheet: Interchange oscillations

Abstract

[1] 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.

Anharmonic oscillatory flow braking in the Earth's magnetotail

“…Asymmetry in the radial background plasma sheet profiles of magnetic field, pressure, and flux tube volume leads to an asymmetric potential around the equilibrium position that causes anharmonicity [Wolf et al, 2012]. In the case of a strong asymmetry, harmonic generation is expected to occur.…”

“…Estimates of bursty bulk flow oscillation periods can be derived analytically in the MHD approximation using typical background plasma sheet parameters [Wolf et al, 2012]. Farther out in the tail where field lines are longer, the oscillation period increases [Wolf et al, 2012;Panov et al, 2014a].…”

Amplitude and size scaling for interchange motions of plasma filaments

“…Recent measurements of magnetospherical plasmas at Saturn [30] and analytic theory [31] suggest that plasma bubbles propagating through plasma sheets in these regions are also driven by buoyancy.…”

Period and damping factor of Pi2 pulsations during oscillatory flow braking in the magnetotail

“…As BBFs decelerate, they may oscillate [Semenov and Lebedeva, 1991;Chen and Wolf, 1999;Panov et al, 2010;Birn et al, 2011]. Their oscillation periods can be derived by analytical MHD approximation using oscillating flux tube and background plasma sheet parameters [Wolf et al, 2012a[Wolf et al, , 2012b. More recently, it was shown that oscillating BBFs may cause a modulation of the current wedge's magnetic disturbance on the ground through alternating parallel currents, which indeed may lead to magnetic pulsations [Panov et al, 2013c].…”

“…All Rights Reserved. Wolf et al [2012a], damping factor , THEMIS footprints' geographic latitude and longitude ranges predicted by the AM-03 model by Kubyshkina et al [2011], observed period of Pi2 pulsations at THEMIS footprints' ranges Pi2 , damping factor of the Pi2 pulsations Pi2 , THEMIS probe used to calculate the Alfvén travel time A (the time required for an Alfvén wave to travel from the equatorial plasma sheet to the corresponding ionospheric footprint), and 4 A . Wolf et al [2012a], [see Panov et al [2013b] for details], damping factor and THEMIS footprints' geographic latitude and longitude ranges predicted by the AM-03 model.…”