Firehose driven magnetic fluctuations in the magnetosphere

The nonlinear saturation of the firehose instability in the high plasma pressure central plasma sheet is shown to produce a wide spectrum of Alfvénic fluctuations in the range of Pi‐2 geomagnetic pulsations. The wave energy sources are the small p∥/p⊥ > 1 + B2/μ0p⊥ anisotropies which are created by Earthward ion convection at constant first and second adiabatic invariants. In the nonlinear state, the field‐line curvature force is weaker than the linear force. This weakening of the driving force limits the amplitude of the Alfvénic fluctuations. Away from the equatorial plane, the plasma is firehose stable, but carries large magnetic fluctuations.