Bounce-averaged stability of compressional modes in geotail flux tubes

The stability of low‐frequency drift magnetosonic waves in the nightside magnetosphere between the outer ring current and the distant neutral line is investigated. As low‐frequency Pi2 oscillations are seen before, during, and after substorm onset, these compressional waves are important for understanding the connection between the two substorm onset mechanisms of (1) the near geosynchronous orbit (NGO) mechanism and (2) the near‐Earth neutral line (NENL) mechanism. It is found that there are two different regions of parameter space where drift‐compressional modes can be unstable: (1) when pressure gradients become sufficiently steep to reverse the magnetic‐guiding center drift and (2) when the temperature gradient is in the opposite direction to the density gradient. Nonlocal bounce‐averaged eigenmode equations are solved with a simple analytic model of the magnetospheric magnetic field and with the Tsyganenko and Stern [1996] magnetic field model. Resulting growth rates, frequencies, and eigenmode structures are reported.