Buoyancy Modes in a Low Entropy Bubble

In the nightside region of Earth’s magnetosphere, buoyancy modes have been associated with low entropy bubbles. These bubbles form in the plasma sheet, particularly during substorm expansion, and move rapidly earthward and come to rest in the inner plasma sheet or inner magnetosphere. They often exhibit damped oscillations with periods of a few minutes and have been associated with Pi2 pulsations. In previous work, we used the thin filament approximation to compare the frequencies and modes of buoyancy waves using three approaches: magnetohydrodynamic (MHD) ballooning theory, classic interchange theory, and an idealized formula. Interchange oscillations differ from the more general MHD oscillations in that they assume a constant pressure on each magnetic field line. It was determined that the buoyancy and interchange modes are very similar for field lines that extend into the plasma sheet but differ for field lines that map to the inner magnetosphere. In this paper, we create a small region of entropy depletion in an otherwise stable entropy background profile of the magnetotail to represent the presence of a plasma bubble and determine the properties of the buoyancy modes using the same 3 approaches. In the bubble region, we find that in some regions the interchange and buoyancy modes overlap resulting in frequencies that are much lower than the background. In other regions within the bubble, we find interchange unstable modes while in other locations MHD normal mode predicts an MHD slow mode wave solution which is not found in the pure interchange solution.