Alfvén waves, magnetic decreases, and discontinuities in interplanetary space (original) (raw)
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Oblique aperiodic instability driven by cross-field current in space plasmas
Advances in Space Research, 2006
Electric currents across the magnetic field can occur in the solar atmosphere because of quasi-stationary magnetic shear, plasma non-uniformity, or MHD waves. We propose a new physical mechanism that can cause the anomalous dissipation of these currents and initiate energy release in solar plasmas. In the framework of linear kinetic theory, we show that the cross-field currents are unstable with respect to low-frequency perturbations. The instability growth rate is quite high, about 2% of the ion-cyclotron frequency when the differential electron/ion velocity is 0.2 of the ion thermal speed. We identify the unstable perturbations as degenerated ion-acoustic modes coupled with backward kinetic Alfvén modes.
Stability of the kinetic Alfven wave in a current-less plasma
Advances in Space Research, 2015
The two potential theory of Hasegawa has been used to derive the dispersion relation for the kinetic Alfven wave (KAW) in a plasma composed of hydrogen, oxygen and electrons. All three components have been modeled by ring distributions (obtained by subtracting two Maxwellian distributions with different temperatures) with the hydrogen and electrons drifting, respectively, with velocities V dH and V de. For the most general case, the dispersion relation is a polynomial equation of order five; it reduces to a relation which supports only one mode when V dH = 0. For typical parameters at comet Halley, we find that both V dH and V de can drive the wave unstable; the KAW is thus driven unstable in a current-less plasma. Such an instability was found for the ion acoustic wave by Vranjes et al. (2009).
Gas acoustic and ion acoustic waves in partially ionized plasmas with magnetized electrons
Physics of Plasmas, 2007
The properties of gas acoustic and ion acoustic modes are investigated in a collisional, weakly ionized plasma in the presence of unmagnetized ions and magnetized electrons. In such a plasma, an ion acoustic mode, driven by an electron flow along the magnetic field lines, can propagate almost at any angle with respect to the ambient field lines as long as the electrons are capable of participating in the perturbations by moving only along the field lines. Several effects, including the electron-ion collisions, the perturbations of the neutral gas, and the electromagnetic perturbations, are studied in the present work. The electron-ion collisions are shown to modify the previously obtained angle-dependent instability threshold for the driving electron flow. The inclusion of the neutral dynamics implies an additional neutral sound mode, which couples to the current driven ion acoustic mode, and these two modes can interchange their identities in certain parameter regimes. The electromagnetic effects, which in the present model imply a bending of the magnetic field lines, result in a further destabilization of an already unstable ion acoustic wave. The applicability of these results to the solar and/or space and laboratory plasmas is discussed.
Nonlinear Processes in Geophysics, 2005
Alfvén waves, discontinuities, proton perpendicular acceleration and magnetic decreases (MDs) in interplanetary space are shown to be interrelated. Discontinuities are the phase-steepened edges of Alfvén waves. Magnetic decreases are caused by a diamagnetic effect from perpendicularly accelerated (to the magnetic field) protons. The ion acceleration is associated with the dissipation of phasesteepened Alfvén waves, presumably through the Ponderomotive Force. Proton perpendicular heating, through instabilities, lead to the generation of both proton cyclotron waves and mirror mode structures. Electromagnetic and electrostatic electron waves are detected as well. The Alfvén waves are thus found to be both dispersive and dissipative, conditions indicting that they may be intermediate shocks. The resultant "turbulence" created by the Alfvén wave dissipation is quite complex. There are both propagating (waves) and nonpropagating (mirror mode structures and MDs) byproducts. Arguments are presented to indicate that similar processes associated with Alfvén waves are occurring in the magnetosphere. In the magnetosphere, the "turbulence" is even further complicated by the damping of obliquely propagating proton cyclotron waves and the formation of electron holes, a form of solitary waves. Interplanetary Alfvén waves are shown to rapidly phase-steepen at a distance of 1 AU from the Sun. A steepening rate of ∼35 times per wavelength is indicated by Cluster-ACE measurements. Interplanetary (reverse) shock compression of Alfvén waves is noted to cause the rapid formation of MDs on the sunward Correspondence to: B. T. Tsurutani (bruce.tsurutani@jpl.nasa.gov) side of corotating interaction regions (CIRs). Although much has been learned about the Alfvén wave phase-steepening processfrom space plasma observations, many facets are still not understood. Several of these topics are discussed for the interested researcher. Computer simulations and theoretical developments will be particularly useful in making further progress in this exciting new area.
Kinetic instability of ion acoustic mode in permeating plasmas
Physics of Plasmas, 2009
In plasmas with electron drift (current) relative to static ions, the ion acoustic wave is subject to the kinetic instability which takes place if the directed electron speed exceeds the ion acoustic speed. The instability threshold becomes different in the case of one quasi-neutral electron-ion plasma propagating through another static quasineutral (target) plasma. The threshold velocity of the propagating plasma may be well below the ion acoustic speed of the static plasma. Such a current-less instability may frequently be expected in space and astrophysical plasmas.
Structure and dynamics of current sheets at Alfvén resonances in a differentially rotating plasma
Physics of Plasmas, 1998
Alfvén resonances, where the local flow speed relative to the boundary is equal to the local Alfvén speed, introduce novel dynamical features in a differentially rotating plasma. The spatial structure and dynamics of current sheets in such plasmas is investigated analytically as well as numerically. The current sheets at Alfvén resonances tend to power-law singularities. The growth of current sheets is algebraic in time in the linear regime and saturates in the presence of dissipation without the intervention of nonlinear effects. These results have significant implications for forced reconnection and Alfvén wave dissipation in laboratory and space plasmas.
The effects of plasma sheet boundary flow and plasma mantle flow on the ion tearing instability
Journal of Geophysical Research, 1988
The effect of plasma bulk flow on the ion tearing mode occurring in the Earth's plasma sheet is investigated. The flow is asstuned to be confined to the region of open field lines and aligned with the magnetic field. An explidt dispersion relation is obtained for two kinds of Affv•n Mach number profiles idealizing the plasma maxxtie flow and the plasma sheet boundaxy flow. In both cases, the growth rate as well as the region of unstable wave numbers are increased as compared to the case without plasma bulk flow. The effect of plasma mantle flow on the growth rate increases as the inner boundary of the mantle approaches the plasma sheet. For the case of plasma sheet boundaxy flow, the growth rate increases rapidly as the layer where shear flows axe concentrated moves toward the center of the sheet. Possible consequences of these results for the dynamic properties of the magnetotail are discussed.
Strong space plasma magnetic barriers and Alfvénic collapse
JETP Letters, 2007
High-magnitude magnetic barriers in space and solar plasma are proposed to be attributed to the pile up of magnetic field lines and their Alfvénic collapse for MHD flows. The analysis of experimental data from both the Interball and Cluster spacecrafts shows that high-magnitude magnetic structures found in the Earth magnetosheath and near the magnetopause are supported by a nearly thermal transverse plasma flow, with the minimum barrier width being on the order of the ion gyroradius. The collapse termination at such scales can be explained by the balance between the pile up of magnetic field lines and backward finite-gyroradius diffusion. Comparison between the theoretical, modeling, and experimental data shows that the Alfvénic collapse is, in general, a promising mechanism for magnetic field generation and plasma separation.
Oblique Alfv\'en instability driven by compensated currents
Compensated-current systems created by energetic ion beams are widespread in space and astrophysical plasmas. The well-known examples are foreshock regions in the solar wind and around supernova remnants. We found a new oblique Alfv\'enic instability driven by compensated currents flowing along the background magnetic field. Because of the vastly different electron and ion gyroradii, oblique Alfv\'enic perturbations react differently on the currents carried by the hot ion beams and the return electron currents. Ultimately, this difference leads to a non-resonant aperiodic instability at perpendicular wavelengths close to the beam ion gyroradius. The instability growth rate increases with increasing beam current and temperature. In the solar wind upstream of Earth's bow shock the instability growth time can drop below 10 proton cyclotron periods. Our results suggest that this instability can contribute to the turbulence and ion acceleration in space and astrophysical fore...