Thin Filaments in an Average Magnetosphere: Pure Interchange Versus Ballooning Oscillations

In the nightside region of Earth's magnetosphere, braking oscillations or buoyancy modes have been associated with the occurrence of low entropy bubbles. These bubbles form in the plasma sheet, particularly during geomagnetically disturbed times, and because of interchange, move rapidly earthward and may eventually come to rest in the inner plasma sheet or inner magnetosphere. Upon arrival, they often exhibit damped oscillations with periods of a few minutes and are associated with Pi2 pulsations. Previously we used the thin filament approximation to compare the frequencies and modes of buoyancy waves using magnetohydrodynamic (MHD) ballooning and classic interchange theory. Interchange oscillations differ from the more general MHD oscillations by assuming constant pressure along a magnetic field line. It was determined that MHD ballooning and interchange modes are similar for plasma sheet field lines but differ for field lines that map to the inner magnetosphere. This suggested that the classic interchange formulation was only valid in the plasma sheet. This paper tests the hypothesis that the agreement between MHD ballooning and classic interchange could be restored inside a bubble. We create a small region of entropy depletion in the magnetotail and compare the buoyancy mode properties. At some locations inside the bubble, the MHD ballooning buoyancy modes resemble interchange modes but with lower frequencies than those of the unperturbed background. Unstable modes are found on the earthward edge of the bubble, while at the tailward edge, MHD ballooning predicts a slow mode wave solution not seen in the pure interchange solution.

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Ballooning instability in the dipole magnetosphere: The finite transverse wavelength influence

Petrashchuk,

Mager,

Klimushkin

2024

Physics of Plasmas

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This paper is concerned with the condition for the development of ballooning instability in the dipole Earth's magnetosphere. Under investigation is the dependence of the ratio of the radial and azimuthal wave vector components squared on the wave frequency squared ω2. It is shown that this dependence is depicted by continuous curve (the dispersion curve). The part of the dispersion curve with ω2>0 corresponds to the slow magnetosonic mode modified by the coupling with the Alfvén mode, while the part with ω2<0 corresponds to the ballooning instability. The instability appears at some critical ratio of the radial and azimuthal wave vector components, the growth rate reaches the maximum value where this ratio is zero. The instability threshold is determined depending on the β parameter and the pressure gradient when the coupling of the slow and Alfvén modes is taken into account. On a given magnetic shell, at a given β value, large pressure gradient favors the instability, and vice versa, at a given pressure gradient, a large β favors instability. It is shown that in the unstable mode, the compressional magnetic field component has a phase shift with respect to the radial component. This can be used to detect unstable ballooning modes during observations in the Earth's magnetosphere.

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Thin Filaments in an Average Magnetosphere: Pure Interchange Versus Ballooning Oscillations

Cited by 3 publications

References 32 publications

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References

Buoyancy Modes in a Low Entropy Bubble

Ballooning instability in the dipole magnetosphere: The finite transverse wavelength influence

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