Dynamics of the Firehose Instability in the Central Plasma Sheet

There is renewed interest in the nonlinear dynamics of the firehose instability in the high beta central plasma sheet. This strong instability produces order unity magnetic fluctuations that propagate up and down the magnetic field line as Alfvenic fluctuations. A new nonlinear model of this instability is presented where the anisotropy parameter A= μ _{0(p_∥ -p_perpendicular to )/B²}, the dispersion parameter of the ion gyradius over the thickness of the central plasma sheet and the sub-grid scale damping are the three key parameters. When the anisotropy parameter A is a few percent above the critical value of unity, the magnetic turbulence strongly increases. The E x B kinetic energy remains subdominant, so this process gives a direct conversion of plasma energy to magnetic energy. We focus here on mapping out the state space for the different nonlinear states. Depending on the values of A, ρ _{i/L_z} and the sub-grid scale damping rates, we give examples of weak bursty soliton like states, weakly turbulent wave states, strong turbulence states, and nonsaturating secularly growing states The magnetic turbulence is a good candidate to explain the Pi-2 oscillations seen in association with bursty bulk flows and substorms (Sigsbee et al. 2002). A continual driving up of the parallel pressure anisotropy occurs through the inward convection of the flux tubes with their continuously shortening field line length or the discontinuous shortening with the onset of magnetic reconnection in the geotail. The Lagrangian codes of the Rice group show the firehose instability arising from the magnetic buoyancy effects which is used to explain the bursty bulk flows in Chen and Wolf (1999) and Ji and Wolf (2002). This work was supported by National Science Foundation Grant ATM-0229863. [1] Sigsbee et al., JGR, 2002. [2] Ji, S., and R. A. Wolf, JGR, 108(A5), 2003.