New model for ULF Pc5 pulsations: Alfvén cones

Bellan, Paul M.

doi:10.1029/96gl01735

Cited by 18 publications

(21 citation statements)

References 21 publications

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“…We find that the exhaust structure in Figure 2 is controlled by kinetic effects and has the attributes of both whistler cone as well the kinetic Alfvén wave (KAW) cone. The latter cone, like the cones for the slow MHD and whistler modes, is associated with the maximum group‐velocity angle of shear Alfvén waves (SAWs) [ Morales et al , 1994; Gekelman et al , 1997; Bellan , 1996; Singh , 1999]. The kinetic and inertial Alfvén waves are special cases of SAWs depending on plasma β .…”

Section: Introductionmentioning

confidence: 99%

Group velocity cones in diverging magnetic reconnection structures

Singh

2007

J. Geophys. Res.

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[1] In a typical geometry of magnetic reconnection, the so-called exhaust regions diverge out of the localized diffusion region. The diverging reconnection structure is conical in shape with its apex near the x-line. Such conical regions have been seen in both MHD and kinetic simulations of reconnection as well as in satellite observations. We demonstrate here that depending on the reconnection regime, the cone angle of the exhaust regions found in numerical simulations and as well as in satellite observations compare well with the maximum angles of group-velocity cones associated with the slow MHD mode, or the whistler mode, or the kinetic Alfvén wave (KAW). For each of these three reconnection regimes, we give a quantitative description of the group velocity cones. In the MHD case the cone angle is small and increases almost linearly with the plasma b < 2. In the whistler regime the cone angle is a constant of 19.5°. In the KAW regime the cone angle depends on plasma b as well as on the timescale associated with the diffusion process in the current sheet. The close agreement between the observed and simulated exhaust cone angles and the group-velocity cone angles in all the three reconnection regimes is highly suggestive that ''at least'' the geometrical feature of the exhaust region is determined by the group velocity of the disturbances, which propagate out of the diffusion region.

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Section: Introductionmentioning

confidence: 99%

Group velocity cones in diverging magnetic reconnection structures

Singh

2007

J. Geophys. Res.

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“…Recently Semeter and Blixt [2006] reported fine structures in auroral imagery as evidence of wave dispersion typical of AWRC. AWRCs radiated by antennas have been predicted theoretically [ Morales et al , 1994; Bellan , 1996; Singh , 1999], and their existence is verified in laboratory experiments [ Gekelman et al , 1994].…”

Section: Introductionmentioning

confidence: 99%

Scattering of long wavelength shear Alfvén waves by a localized density cavity

Singh

Khazanov

2007

Geophysical Research Letters

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We demonstrate here, for the first time, that the scattering of a shear Alfvén wave (SAW) having a long perpendicular wavelength (λ⊥), with a localized density cavity, generates perturbations in the field structures like that expected from the radiation pattern of an Alfvén wave resonance cone (AWRC) emitted from localized sources. Such a SAW‐cavity interaction provides a novel mechanism for generating narrow structures of inertial Alfvén waves like those observed from satellites and variously called solitary, kinetic and dispersive Alfvén waves.

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“…The parallel acceleration of finite-mass electrons explicitly determines the parallel current (this physics is neglected in ideal MHD). Superposition of the k spectrum generated by a localized source excites resonance cones [9,10], and it has recently been shown that small-scale auroral shear Alfvén signals are consistent with resonance cone excitation by a distant, pulsed, localized source [11].…”

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Fine-Scale Cavitation of Ionospheric Plasma Caused by Inertial Alfvén Wave Ponderomotive Force

Bellan

Stasiewicz

1998

Phys. Rev. Lett.

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Deep, very narrow magnetic-field-aligned density depletions were observed by the Freja spacecraft during auroral oval crossings. These cavities have perpendicular width of the order of the electron skin depth c͞v pe and are associated with low-frequency electromagnetic perturbations and with discrete auroral structures. We demonstrate here that these cavities are likely produced by the ponderomotive force of inertial Alfvén waves. [S0031-9007(98) [7].The purpose of this Letter is to show that these density cavities result from the ponderomotive force created by the large field-aligned oscillating current [7] of an inertial Alfvén shear mode. Unlike previous models which tended to ignore dynamics parallel to the equilibrium magnetic field, we find that parallel electric fields and the resulting parallel electron motion (i.e., electron inertia effects) are of fundamental importance. We demonstrate in this Letter that very weak oscillating parallel electric fields associated with the inertial Alfvén shear mode can lead to substantial density depletions in magnetized plasma; this behavior is analogous in some ways to the density depletion associated with large amplitude lower hybrid resonance cones [8] and has important implications for the creation of striated cavities and other nonlinear phenomena in low b space plasmas.The inertial Alfvén shear mode is a cold-plasma wave which exists when the plasma b is smaller than the mass ratio m e ͞m i (or, equivalently, the Alfvén velocity y A is larger than the electron thermal velocity y Te ). This mode involves finite parallel electric field and finite electron mass (in contrast to ideal MHD where both these quantities are assumed to be vanishingly small). The parallel acceleration of finite-mass electrons explicitly determines the parallel current (this physics is neglected in ideal MHD). Superposition of the k spectrum generated by a localized source excites resonance cones [9,10], and it has recently been shown that small-scale auroral shear Alfvén signals are consistent with resonance cone excitation by a distant, pulsed, localized source [11].We begin by presenting in Fig. 1(a) typical Freja measurements of the transverse magnetic field B Ќ ; these perturbations have been interpreted as being due to inertial Alfvén resonance cones [7]. Figure 1(a) shows that the magnetic structure is irregular and occasionally has sharp gradients. Each time increment of 1 sec in Fig. 1 corresponds to Freja traveling a distance of 6.7 km perpendicular to the equilibrium magnetic field. Figure 1(b) shows measurements of the corresponding electron density. The sharp density cavities evident in Fig. 1(b) are correlated with the magnetic field gradients in Fig. 1(a). These measurements were made in auroral regions at an altitude of 1500 km where T e 0.5 1 eV, B 0 2.6 3 10 4 nT, and the oxygen content of the plasma was typically 50%-75% with the balance being mainly hydrogen. Thus, b ø m e ͞m i so that the shear Alfvén modes are

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New model for ULF Pc5 pulsations: Alfvén cones

Abstract: It is proposed that nightside auroral Pc 5 signals are inertial Alfvén resonance cones excited by localized field‐aligned current pulses in the distant magnetotail.

Cited by 18 publications

References 21 publications

Group velocity cones in diverging magnetic reconnection structures

Group velocity cones in diverging magnetic reconnection structures

Scattering of long wavelength shear Alfvén waves by a localized density cavity

Fine-Scale Cavitation of Ionospheric Plasma Caused by Inertial Alfvén Wave Ponderomotive Force

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