Alfvén vortex filaments observed in magnetosheath downstream of a quasi-perpendicular bow shock (original) (raw)
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Journal of Geophysical Research, 2004
The Cluster satellites crossed the Earth's bow shock several times on 31 March 2001. For all these crossings the bow shock was supercritical and quasi-perpendicular. We present here the results of a detailed analysis of the magnetic field fluctuations observed downstream of the shock. We use data from the four Cluster spacecraft to determine the behavior and the geometry of these fluctuations with good accuracy. Shortly after the ramp crossing, we observed a large-amplitude nonlinear Alfvén wave, propagating along the downstream average magnetic field with a spectrum peaking at two frequencies below the proton and the alpha ion cyclotron frequencies. Farther downstream in the magnetosheath the magnetic field fluctuations took the form of three-dimensional structures which can be interpreted as cylindrical field-aligned current tubes. It is the first time that such current tubes have been observed downstream of a quasi-perpendicular shock, and they are closely associated with a quasi-monochromatic, finite amplitude Alfvén wave. We suggest that a close relation exists between the nonlinear Alfvén wave and the current tubes as a result of a filamentation instability which is expected to occur at b ! 1 and for frequencies comparable to the ion cyclotron frequencies.
Solar wind vs magnetosheath turbulence and Alfvén vortices
Nonlinear Processes in Geophysics, 2008
In this paper we give firstly a broad review of the space plasma turbulence around the ion characteristic space and temporal scales within two natural laboratories, the solar wind and the Earth magnetosheath. In both regions power law spectra of magnetic fluctuations are observed. In both regions these spectra have a break in the vicinity of the ion cyclotron frequency. A distinctive feature of the magnetosheath turbulence is the presence of Alfvén vortices at scales of the spectral break. The Alfvén vortices are multi-scale nonlinear structures. We give a review of the main theoretical features of incompressible Alfvén vortsices in the second part of the paper. Finally, we analyze the spectral properties of the Alfvén vortex solution and of the network of such vortices. We show that the observed magnetosheath spectrum in presence of the Alfvén vortices can be described, at least partially, by the vortex network model.
In situ multi-satellite detection of coherent vortices as a manifestation of Alfvénic turbulence
Nature, 2005
Turbulence in fluids 1 and plasmas 2-5 is a ubiquitous phenomenon driven by a variety of sources-currents, sheared flows, gradients in density and temperature, and so on. Turbulence involves fluctuations of physical properties on many different scales, which interact nonlinearly to produce self-organized structures in the form of vortices 2-5 . Vortex motion in fluids and magnetized plasmas is typically governed by nonlinear equations 2-5 , examples of which include the Navier-Stokes equation 1,2 , the Charney-Hasegawa-Mima equations 2-5 and their numerous generalizations 6-9 . These nonlinear equations admit solutions 2-5 in the form of different types of vortices that are frequently observed in a variety of contexts: in atmospheres, in oceans and planetary systems 2,4 , in the heliosphere 10,11 , in the Earth's ionosphere and magnetosphere , and in laboratory plasma experiments 18 . Here we report the discovery by the Cluster satellites 19 of a distinct class of vortex motion-short-scale drift-kinetic Alfvén (DKA) vortices 8,9 -in the Earth's magnetospheric cusp region. As is the case for the larger Kelvin-Helmholtz vortices observed previously 17 , these dynamic structures should provide a channel for transporting plasma particles and energy through the magnetospheric boundary layers.
Alfvén waves in the magnetosphere generated by shock wave / plasmapause interaction
Solnechno-Zemnaya Fizika, 2019
We study Alfvén waves generated in the magnetosphere during the passage of an interplanetary shock wave. After shock wave passage, the oscillations with typical Alfvén wave dispersion have been detected in spacecraft observations inside the magnetosphere. The most frequently observed oscillations are those with toroidal polarization; their spatial structure is described well by the field line resonance (FLR) theory. The oscillations with poloidal polarization are observed after shock wave passage as well. They cannot be generated by FLR and cannot result from instability of high-energy particle fluxes because no such fluxes were detected at that time. We discuss an alternative hypothesis suggesting that resonant Alfvén waves are excited by a secondary source: a highly localized pulse of fast magnetosonic waves, which is generated in the shock wave/plasmapause contact region. The spectrum of such a source contains oscillation harmonics capable of exciting both the toroidal and poloid...
Annales Geophysicae, 2008
The generation of a high-m Alfvén wave by substorm injected energetic particles in the magnetosphere is studied. The wave is supposed to be emitted by an alternating current created by the drifting particle cloud or ring current inhomogeneity. It is shown that the wave appears in some azimuthal location simultaneously with the particle cloud arrival at the same spot. The value of the azimuthal wave number is determined as m∼ω/ω d , where ω is the eigenfrequency of the standing Alfvén wave and ω d is the particle drift frequency. The wave propagates westward, in the direction of the proton drift. Under the reasonable assumption about the density of the energetic particles, the amplitude of the generated wave is close to the observed amplitudes of poloidal ULF pulsations.
New Regime of Inertial Alfvén Wave Turbulence in the Auroral Ionosphere
2024
We investigate a new regime of inertial Alfvén wave turbulence observed in the very low beta plasma of the auroral ionosphere using electric and magnetic field measurements by the TRICE-2 sounding rocket. Combining the observed features of the electric and magnetic field frequency spectra with the linear properties of inertial Alfvén waves, we deduce the path of the anisotropic turbulent cascade through wave vector space. We find a critically balanced cascade through the magnetohydrodynamic scales of the inertial range down to the perpendicular scale of the plasma skin depth, followed by a parallel cascade to the ion inertial length. We infer damping of the cascade by a combination of proton cyclotron damping and electron Landau damping.
Physical Review Letters, 2008
We study the plasma turbulence, at scales larger than the ion inertial length scale, downstream of a quasiparallel bow shock using Cluster multispacecraft measurements. We show that turbulence is intermittent and well described by the extended structure function model, which takes into account the spatial inhomogeneity of the cascade rate. For the first time we use multispacecraft observations to characterize the evolution of magnetosheath turbulence, particularly its intermittency, as a function of the distance from the bow shock. The intermittency significantly changes over the distance of the order of 100 ion inertial lengths, being increasingly stronger and anisotropic away from the bow shock.
Alfvén vortices in Saturn's magnetosheath: Cassini observations
Geophysical Research Letters, 2008
1] First signatures of Alfvén vortices in the Kronian magnetosheath are presented. An Alfvén vortex is a nonlinear bi-dimensional magnetic structure associated with sheared magnetic field and velocity perturbations, propagating obliquely to the external magnetic field direction. Such structures have been recently discovered by Cluster in Earth's magnetosheath downstream of a quasiperpendicular bow-shock at scales close to 10c/w pi (where c is the speed of light and w pi the ion plasma frequency). The presence of Alfvén vortices downstream of a quasiperpendicular bow-shock of Saturn with comparable scales to the ones at Earth indicates the universality of this phenomenon. It suggests that such non-linear structures are an inherent property of plasma flow downstream of collisionless shock waves, which can have broad astrophysical applications.
Some properties of Alfvén waves: Observations in the tail lobes and the plasma sheet boundary layer
Journal of Geophysical Research, 2005
1] We report properties of substorm-related, globally excited Alfvén waves on a temporal scale of 6 to 300 s (3.3 to 167 mHz) at geocentric distances between 5 and 6 R E . The waves were observed in the tail lobes and the plasma sheet boundary layer (PSBL) by the Polar satellite. In each region we made the following observations: (1) The tail lobe Alfvén waves started at substorm onset as determined from ground magnetometer data. Hence these ULF lobe waves can possibly be used as a new substorm indicator. Although on open field lines, they often showed local standing wave signatures with a large perpendicular scale size and a near-zero net Poynting flux. We do not classify those waves as FLR but interpret them as the superposition of incident and reflected waves. The same oscillations were simultaneously recorded in ground magnetometer data. Immediately poleward of the PSBL, the lobe Alfvén waves traveled earthward (no reflection), suggesting their dissipation in the ionosphere. The lobe waves were superimposed on the signature of a field-aligned current (FAC). The onset of this FAC was simultaneous to the onset of the magnetic substorm bay. (2) The substorm-related PSBL Alfvén waves carried two to three orders of magnitude larger Poynting flux ($1 erg cm À2 s À1 ) than the lobe Alfvén waves. These PSBL waves were a mixture of standing and traveling Alfvén waves for different frequency ranges. Most Poynting flux was carried in large-scale earthward traveling waves (40-300 s). For one event, we also measured large standing wave components (>0.5 erg cm À2 s À1 ), but such events are rare. In the intermediate range (40-67 s), which overlaps with the Pi2 range, some waves showed clear standing wave signatures. At smaller periods (6-24 s), noninterfering earthward and tailward traveling waves were present with small Poynting fluxes (<0.05 erg cm À2 s À1 ). A trend for increasing E to B ratios with increasing wave frequency was observed. The PSBL waves were left-hand elliptically polarized. The wave vector was within 35°of the background magnetic field direction, suggesting that the waves were phase-mixed. The largeamplitude, substorm-related PSBL Alfvén wave events ($1 erg cm À2 s À1 ) were found in regions of upward currents. (2005), Some properties of Alfvén waves: Observations in the tail lobes and the plasma sheet boundary layer,
Alfven Vortices in Multi-Ion Plasmas
The solar wind is known to be a turbulent medium. Further on observational evidence reveals a high degree of Alfvenicity of the fluctuations at least for the fast solar wind and also the existence of nonlinear twodimensional turbulent interactions which contribute to solar wind fluctuations [fu, C.-Y. and Marsch, E., Space Science Rev. 73, No. 1-2 (1995)]. To understand fluctuations which are existing in the solar wind one has to study the behavior of nonlinear drift Alfven waves in multi-ion plasmas. For this reason we investigate the propagation of Alfven vortices in multiion plasmas.