Nonlinear dynamics of the firehose instability in a magnetic dipole geotail

The nonlinear dynamics of the firehose instability provides a possible explanation for the onset of the magnetic fluctuations associated with bursty bulk flows and substorms. Magnetic fluctuations called Pi2 geomagnetic pulsations are associated with bursty flows in the geotail plasma, where the plasma pressure exceeds the magnetic pressure. These strong magnetic fluctuations are often associated with substorms and start a few minutes before the arrival of the dipolarization pulse. Motivated by these observations, we have derived a nonlinear partial differential equation and developed an initial value code to investigate the firehose anisotropy-driven turbulence in the Earth's geotail. We show that Hall MHD and dispersive ion kinetic effects limit the range of unstable parallel wavenumbers and determine the frequency spectrum of the turbulence. Nonlinear saturation is caused by the nonlinear weakening at large magnetic field line distortions of the destabilizing magnetic curvature force driven by the pressure anisotropy.