Firehose instability near substorm expansion‐phase onset?

Ji, Shuai; Wolf, R. A.

doi:10.1029/2003gl017981

Cited by 3 publications

(6 citation statements)

References 17 publications

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“…[20] In a related work Ji and Wolf [2003b] argue that the appearance of the anisotropy is a product of the near-Earth neutral line formation. Consequently, the observations of Pi-2 turbulence described here along the auroral field lines may be a signature of the flow braking from fast Earthward flows produced by a near-Earth neutral line.…”

Section: Discussionmentioning

confidence: 99%

Firehose driven magnetic fluctuations in the magnetosphere

Horton

Wong

2004

Geophysical Research Letters

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[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.

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Section: Discussionmentioning

confidence: 99%

Firehose driven magnetic fluctuations in the magnetosphere

Horton

Wong

2004

Geophysical Research Letters

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“…The firehose instability driven by p ∥ > p ⊥ pressure anisotropy is typically created by the Fermi acceleration mechanism due to rapid shortening of the flux tube length by convection or by bursty bulk flows. Bursty bulk flows increase the parallel pressure faster than the perpendicular pressure in the high‐β central plasma sheet driving the firehose instability [ Ji and Wolf , 2003a, 2003b]. A sufficiently large dawn‐to‐dusk electric field would also be expected to trigger the firehose instability and produce turbulent magnetic fluctuations by the conversion of anisotropy particle energy stored in the plasma pressures in the high‐β regions of the plasma sheet.…”

Section: Discussionmentioning

confidence: 99%

“… Ji and Wolf [2003a, 2003b] established that the firehose mechanism is set off during the rapid Earthward convection associated with bursty bulk flows. Other studies have suggested how the Pi2 pulsations and flow braking are related [ Shiokawa et al , 1998; Kepko and Kivelson , 1999, 2001].…”

Section: Discussionmentioning

confidence: 99%

“…There are apparently three varieties of Pi2 fluctuations according to Kepko and Kivelson [1999, 2001], with one type of Pi2 turbulence well correlated with the high‐speed Earthward flow burst v x = E y / B x ≈ 800 km/s to 1000 km/s. Such fast Earthward plasma flows are expected to drive up the parallel pressure of the ions faster than the perpendicular pressure, producing the firehose instability [ Ji and Wolf , 2003a, 2003b]. Thus we seek to correlate these particular Pi2 fluctuations with the nonlinear firehose electromagnetic turbulence derived here.…”

Section: Comparison Of Driven Firehose Turbulence With Pi2 Fluctuationsmentioning

confidence: 99%

“…The present paper investigates the nonlinear dynamics of the firehose instability in a magnetic dipole geometry. The firehose instability driven by p ∥ > p ⊥ pressure anisotropy is typically created and maintained by the Fermi acceleration mechanism due to the rapid shortening of the flux tube length by convection or by bursty bulk flows [ Ji and Wolf , 2003a, 2003b]. Geotail flux tubes with pressure deficits accelerate toward the Earth as described by Chen and Wolf [1999] In section 2 we briefly describe the kinetic dispersion relation for low‐frequency electromagnetic modes propagating in a uniform plasma parallel to the equilibrium magnetic field.…”

Section: Introductionmentioning

confidence: 99%

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Nonlinear dynamics of the firehose instability in a magnetic dipole geotail

Horton

Wong

et al. 2004

J. Geophys. Res.

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[1] 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.

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Cluster observations of multiple dipolarization fronts

et al. 2011

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[1] We present Cluster observations of a series of dipolarization fronts (DF 1 to 6) at the central current sheet in Earth's magnetotail. The velocities of fast earthward flow following behind each DF 1-3 are comparable to the Alfvén velocity, indicating that the flow bursts might have been generated by bursty reconnection that occurred tailward of the spacecraft. Based on multispacecraft timing analysis, DF normals are found to propagate mainly earthward at 160-335 km/s with a thickness of 900-1500 km, which corresponds to the ion inertial length or gyroradius scale. Each DF is followed by significant fluctuations in the x and y components of the magnetic field whose peaks are found 1-2 min after the DF passage. These (B x , B y ) fluctuations propagate dawnward (mainly) and earthward. Strongly enhanced field-aligned beams are observed coincidently with (B x , B y ) fluctuations, while an enhancement of cross-tail currents is associated with the DFs. From the observed pressure imbalance and flux tube entropy changes between the two regions separated by the DF, we speculate that interchange instability destabilizes the DFs and causes the deformation of the midtail magnetic topology. This process generates significant field-aligned currents and might power the auroral brightening in the ionosphere. However, this event is associated with neither the main substorm auroral breakup nor the poleward expansion, which might indicate that the observed multiple DFs have been dissipated before they reach the inner plasma sheet boundary.

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Firehose instability near substorm expansion‐phase onset?

Cited by 3 publications

References 17 publications

Firehose driven magnetic fluctuations in the magnetosphere

Firehose driven magnetic fluctuations in the magnetosphere

Nonlinear dynamics of the firehose instability in a magnetic dipole geotail

Cluster observations of multiple dipolarization fronts

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