Firehose driven magnetic fluctuations in the magnetosphere

Abstract

[1] 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 k /p ? > 1 + B 2 /m 0 p ? 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.

Firehose instability near substorm expansion‐phase onset?

“…A similar firehose instability was also noted in the bubble‐filament simulation of Ji and Wolf [2003]. The present simulation was terminated ∼30 s after onset of the instability for two reasons: (1) the simulation had by this time lost its fidelity to the original differential equations, because the wavelength of the dominant simulated firehose ripples is ∼4 mass points and we know our numerical method underestimates growth rates for such ripples [ Ji and Wolf , 2003]; (2) more detailed theoretical work by Horton et al [2003] suggests that the dominant firehose ripples should have k −1 ∼ion gyroradius or inertial length, which are comparable for β∼1, and these finite‐Larmor‐radius effects are not in our MHD equations. Incidentally, the ion gyroradius is ∼0.05 R E for our case, ∼1/3 of the distance between our adjacent mass points.…”

“…(2) more detailed theoretical work by Horton et al [2003] suggests that the dominant firehose ripples should have k À1 $ion gyroradius or inertial length, which are comparable for b$1, and these finite-Larmor-radius effects are not in our MHD equations. Incidentally, the ion gyroradius is $0.05 R E for our case, $1/3 of the distance between our adjacent mass points.…”

Review of Pi2 Models

“…First, the wave activity does not have the typical short duration of only a few wave cycles, and second, the frequency changes more randomly throughout the interval. Perhaps related to these observations are the theoretical results of Horton et al (2004), showing that ion pressure anisotropies p > p ⊥ (firehose instability), possibly associated with the flow braking of magnetotail plasma, produce a wide spectrum of Alfvénic fluctuations in the Pi2 band.…”

A Case Study of Connection Between Ground Magnetic Field Perturbations and Tail Current Sheet Bursty Flows at X = −60 RE

“…With such anisotropy, a system becomes firehose unstable when the condition of C f = 1–0.5 · ( β || – β ⊥ ) < 0 is satisfied, where β = 2 μ 0 P / B 2 . It has been suggested from simulations that firehose instability can be a mechanism for ULF waves, including Pi2 pulsations (e.g., Horton et al, ). Other mechanisms proposed for Pi2 pulsations include SWC, periodic flow bursts, periodic reconnection bursts, and other instabilities such as ballooning instability (Keiling & Takahashi, ).…”