scholarly journals Interacting vorticity waves as an instability mechanism for magnetohydrodynamic shear instabilities

2015 ◽  
Vol 767 ◽  
pp. 199-225 ◽  
Author(s):  
E. Heifetz ◽  
J. Mak ◽  
J. Nycander ◽  
O. M. Umurhan
Keyword(s):  
Magnetic Field ◽  
Phase Locking ◽  
Unstable Flow ◽  
Background Magnetic Field ◽  
Non Local ◽  
Action At A Distance ◽  
Shear Instabilities ◽  
Basic States ◽  
The Waves ◽  
Stable Flow

AbstractThe interacting vorticity wave formalism for shear flow instabilities is extended here to the magnetohydrodynamic (MHD) setting, to provide a mechanistic description for stabilising and destabilising shear instabilities by the presence of a background magnetic field. The interpretation relies on local vorticity anomalies inducing a non-local velocity field, resulting in action at a distance. It is shown here that the waves supported by the system are able to propagate vorticity via the Lorentz force, and waves may interact. The existence of instability then rests upon whether the choice of basic state allows for phase locking and constructive interference of the vorticity waves via mutual interaction. To substantiate this claim, we solve the instability problem of two representative basic states, one where a background magnetic field stabilises an unstable flow and the other where the field destabilises a stable flow, and perform relevant analyses to show how this mechanism operates in MHD.

Parametric excitation of magnetoacoustic waves by a pump magnetic field in a high-β plasma

1977 ◽  
Vol 17 (1) ◽  
pp. 93-103 ◽  
Author(s):  
N. F. Cramer
Keyword(s):  
Magnetic Field ◽  
Parametric Excitation ◽  
Acoustic Waves ◽  
Magnetic Pressure ◽  
Gas Pressure ◽  
Alfven Wave ◽  
Fast Wave ◽  
Magnetoacoustic Waves ◽  
Background Magnetic Field ◽  
The Waves

The parametric excitation of slow, intermediate (Alfvén) and fast magneto-acoustic waves by a modulated spatially non-uniform magnetic field in a plasma with a finite ratio of gas pressure to magnetic pressure is considered. The waves are excited in pairs, either pairs of the same mode, or a pair of different modes. The growth rates of the instabilities are calculated and compared with the known result for the Alfvén wave in a zero gas pressure plasma. The only waves that are found not to be excited are the slow plus fast wave pair, and the intermediate plus slow or fast wave pair (unless the waves have a component of propagation direction perpendicular to both the background magnetic field and the direction of non-uniformity of the field).

scholarly journals Multi-spacecraft determination of wave characteristics near the proton gyrofrequency in high-altitude cusp

2005 ◽  
Vol 23 (3) ◽  
pp. 983-995 ◽  
Author(s):  
D. Sundkvist ◽  
A. Vaivads ◽  
M. André ◽  
J.-E. Wahlund ◽  
Y. Hobara ◽  
...  
Keyword(s):  
Magnetic Field ◽  
High Altitude ◽  
Nonlinear Effects ◽  
Linear Wave ◽  
Wave Growth ◽  
Ion Cyclotron Waves ◽  
Background Magnetic Field ◽  
Ion Cyclotron ◽  
The Waves ◽  
Proton Gyrofrequency

Abstract. We present a detailed study of waves with frequencies near the proton gyrofrequency in the high-altitude cusp for northward IMF as observed by the Cluster spacecraft. Waves in this regime can be important for energization of ions and electrons and for energy transfer between different plasma populations. These waves are present in the entire cusp with the highest amplitudes being associated with localized regions of downward precipitating ions, most probably originating from the reconnection site at the magnetopause. The Poynting flux carried by these waves is downward/upward at frequencies below/above the proton gyrofrequency, which is consistent with the waves being generated near the local proton gyrofrequency in an extended region along the flux tube. We suggest that the waves can be generated by the precipitating ions that show shell-like distributions. There is no clear polarization of the perpendicular wave components with respect to the background magnetic field, while the waves are polarized in a parallel-perpendicular plane. The coherence length is of the order of one ion-gyroradius in the direction perpendicular to the ambient magnetic field and a few times larger or more in the parallel direction. The perpendicular phase velocity was found to be of the order of 100km/s, an order of magnitude lower than the local Alfvén speed. The perpendicular wavelength is of the order of a few proton gyroradius or less. Based on our multi-spacecraft observations we conclude that the waves cannot be ion-whistlers, while we suggest that the waves can belong to the kinetic Alfvén branch below the proton gyrofrequency fcp and be described as non-potential ion-cyclotron waves (electromagnetic ion-Bernstein waves) above. Linear wave growth calculations using kinetic code show considerable wave growth of non-potential ion cyclotron waves at wavelengths agreeing with observations. Inhomogeneities in the plasma on the order of the ion-gyroradius suggests that inhomogeneous (drift) or nonlinear effects or both of these should be taken into account.

scholarly journals Steepening of magnetosonic waves in the inner coma of comet 67P/Churyumov–Gerasimenko

2021 ◽  
Vol 39 (4) ◽  
pp. 721-742
Author(s):  
Katharina Ostaszewski ◽  
Karl-Heinz Glassmeier ◽  
Charlotte Goetz ◽  
Philip Heinisch ◽  
Pierre Henri ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Interaction Region ◽  
Occurrence Rate ◽  
Solar Wind Interaction ◽  
Background Magnetic Field ◽  
The Waves ◽  
Comet 67P ◽  
The Magnetic Field ◽  
Field Enhancements

Abstract. We present a statistical survey of large-amplitude, asymmetric plasma and magnetic field enhancements detected outside the diamagnetic cavity at comet 67P/Churyumov–Gerasimenko from December 2014 to June 2016. Based on the concurrent observations of plasma and magnetic field enhancements, we interpret them to be magnetosonic waves. The aim is to provide a general overview of these waves' properties over the mission duration. As the first mission of its kind, the ESA Rosetta mission was able to study the plasma properties of the inner coma for a prolonged time and during different stages of activity. This enables us to study the temporal evolution of these waves and their characteristics. In total, we identified ∼ 70 000 steepened waves in the magnetic field data by means of machine learning. We observe that the occurrence of these steepened waves is linked to the activity of the comet, where steepened waves are primarily observed at high outgassing rates. No clear indications of a relationship between the occurrence rate and solar wind conditions were found. The waves are found to propagate predominantly perpendicular to the background magnetic field, which indicates their compressional nature. Characteristics like amplitude, skewness, and width of the waves were extracted by fitting a skew normal distribution to the magnetic field magnitude of individual steepened waves. With increasing mass loading, the average amplitude of the waves decreases, while the skewness increases. Using a modified 1D magnetohydrodynamic (MHD) model, we investigated if the waves can be described by the combination of nonlinear and dissipative effects. By combining the model with observations of amplitude, width and skewness, we obtain an estimate of the effective plasma diffusivity in the comet–solar wind interaction region and compare it with suitable reference values as a consistency check. At 67P/Churyumov–Gerasimenko, these steepened waves are of particular importance as they dominate the innermost interaction region for intermediate to high activity.

Using Compressibility and Electric Field to Characterize Circularly-Polarized Waves Near the Proton Cyclotron Frequency Observed by Solar Orbiter

2021 ◽  
Author(s):  
Yuri Khotyaintsev ◽  
Daniel B Graham ◽  
Konrad Steinvall ◽  
Andris Vaivads ◽  
Milan Maksimovic ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Electric Field ◽  
Electromagnetic Waves ◽  
Cyclotron Frequency ◽  
Density Fluctuations ◽  
Background Magnetic Field ◽  
The Waves ◽  
The Magnetic Field ◽  
Proton Cyclotron

<p>We report Solar Orbiter observations of electromagnetic waves near the proton cyclotron frequency during the first perihelion. The waves have polarization close to circular and have wave vectors closely aligned with the background magnetic field. Such waves are potentially important for heating of the solar wind as their frequency and polarization allows effective energy exchange with solar wind protons. The Radio and Plasma Waves (RPW) instrument provides a high-cadence measurement of plasma density and electric field which we use together with the magnetic field measured by MAG to characterize these waves. In particular we compute the compressibility and the phase between the density fluctuations and the parallel component of the magnetic field, and show that these have a distinct behavior for the waves compared to the Alfvénic turbulence. We compare the observations to multi-fluid plasma dispersion and identify the waves modes corresponding to the observed waves. We discuss the importance of the waves for solar wind heating.</p>

scholarly journals Steepening of magnetosonic waves in the inner coma of comet 67P/Churyumov-Gerasimenko

2020 ◽  
Author(s):  
Katharina Ostaszewski ◽  
Karl-Heinz Glassmeier ◽  
Charlotte Goetz ◽  
Philip Heinisch ◽  
Pierre Henri ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Interaction Region ◽  
Occurrence Rate ◽  
Solar Wind Interaction ◽  
Plasma Properties ◽  
Background Magnetic Field ◽  
The Waves ◽  
Comet 67P ◽  
The Magnetic Field

Abstract. We present a statistical survey of large amplitude, asymmetric plasma, and magnetic field enhancements at comet 67P/Churyumov-Gerasimenko from December 2014 to June 2016. The aim is to provide a general overview of these structures' properties over the mission duration. At comets, nonlinear wave evolution plays an integral part in the development of turbulence and in particular facilitates the transfer of energy and momentum. As the first mission of its kind, the ESA Rosetta mission was able to study the plasma properties of the inner coma for a prolonged time and during different stages of activity. This enables us to study the temporal evolution of steepened waves and their characteristics. In total, we identified ~70000 events in the magnetic field data by means of machine learning. We observe that the occurrence of wave events is linked to the activity of the comet, where events are primarily observed at high outgassing rates. No clear indications of a relationship between the occurrence rate and solar wind conditions were found. The waves are found to propagate predominantly perpendicular to the background magnetic field, which indicates their compressive nature. Characteristics like amplitude, skewness, and width of the waves were extracted by fitting a skew normal distribution to the magnetic field magnitude of individual events. With increasing massloading the average amplitude of steepened waves decreases while the skewness increases. Using a modified 1D MHD model it was possible to show that such solitary structures can be described by the combination of nonlinear, dispersive, and dissipative effects. By combining the model with observations of amplitude, width, and skewness we obtain an estimate of the effective plasma viscosity in the comet-solar wind interaction region. At 67P/Churyumov-Gerasimenko steepened waves are of particular importance as they dominate the innermost interaction region for intermediate to high activity.

A note on the westward drift of the earth's magnetic field

1977 ◽  
Vol 82 (2) ◽  
pp. 389-400 ◽  
Author(s):  
W. W. Wood
Keyword(s):  
Magnetic Field ◽  
Liquid Core ◽  
Alfven Waves ◽  
Alfven Wave ◽  
Stratified Medium ◽  
Alfvén Waves ◽  
Axially Symmetric ◽  
Basic States ◽  
The Waves ◽  
Westward Drift

A model of the earth's liquid core is assumed in which the underlying magnetic field and velocity are zonal and axially symmetric. Alfvén waves that vary as ei(kϕ−σt) are considered, where ϕ is the angle of longitude. Buoyancy and Coriolis forces Ω × U are included.For a wide class of basic states and regions of flow, it is shown that roughly as many of the waves with a given k [ges ] 2 propagate eastwards as propagate westwards. All these waves are neutrally stable. The class of basic states is restricted by certain inequalities involving their velocity, magnetic field and entropy gradient.it is observed that the known equivalence (Malkus 1967) between Alfvén waves with frequencies σ [Lt ] Ω and inertial waves with frequencies σp which are O(Ω) still holds when buoyancy forces are present. The equivalence requires σp2 to be real. If σp is pure imaginary, as is possible (though perhaps uncommon) in an unstably stratified medium, then the corresponding Alfvén wave is not neutrally stable and travels westwards.

Nonlinear plasma waves with steady-state d.c. current

1994 ◽  
Vol 52 (3) ◽  
pp. 373-390 ◽  
Author(s):  
S. Rauf ◽  
J. A. Tataronis
Keyword(s):  
Magnetic Field ◽  
Steady State ◽  
Low Frequency ◽  
Travelling Wave ◽  
Magnetized Plasma ◽  
Two Dimensional ◽  
Antenna Structure ◽  
Background Magnetic Field ◽  
The Waves ◽  
Dimensional Wave

Using a low-frequency, low-β model of a magnetized plasma, several nonlinear waves are studied. All the waves share the property that they have a self- consistent time-averaged current in the steady state. The first wave is a one- dimensional solitary wave that propagates obliquely to a static magnetic field. The wave magnetic field has a kink structure, which results in the production of the time-averaged current. A two-dimensional wave that propagates parallel to the background magnetic field is also discussed. This is a forced wave, requiring an antenna structure to support it in the steady state. The characteristics of the two-dimensional wave are dependent upon the wave speed, which in turn is determined by the speed of the travelling wave on the external antenna.

On the existence of compressional MHD oscillations in an inhomogeneous magnetoplasma

1990 ◽  
Vol 44 (2) ◽  
pp. 361-375 ◽  
Author(s):  
Andrew N. Wright
Keyword(s):  
Magnetic Field ◽  
Wave Equation ◽  
Density Distribution ◽  
Cold Plasma ◽  
Wave Equations ◽  
Perturbation Field ◽  
Plasma Density Distribution ◽  
Background Magnetic Field ◽  
Transverse Fields ◽  
The Magnetic Field

In a cold plasma the wave equation for solely compressional magnetic field perturbations appears to decouple in any surface orthogonal to the background magnetic field. However, the compressional fields in any two of these surfaces are related to each other by the condition that the perturbation field b be divergence-free. Hence the wave equations in these surfaces are not truly decoupled from one another. If the two solutions happen to be ‘matched’ (i.e. V.b = 0) then the medium may execute a solely compressional oscillation. If the two solutions are unmatched then transverse fields must evolve. We consider two classes of compressional solutions and derive a set of criteria for when the medium will be able to support pure compressional field oscillations. These criteria relate to the geometry of the magnetic field and the plasma density distribution. We present the conditions in such a manner that it is easy to see if a given magnetoplasma is able to executive either of the compressional solutions we investigate.

Schwinger effect in an instanton background with electric and magnetic fields via holography

2018 ◽  
Vol 33 (25) ◽  
pp. 1850144
Author(s):  
Maryam Gholizadeh Arashti ◽  
Majid Dehghani
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Production Rate ◽  
Pair Creation ◽  
Zero Temperature ◽  
Expectation Value ◽  
Background Magnetic Field ◽  
Electric And Magnetic Fields ◽  
Schwinger Effect ◽  
Horizon Limit

The Schwinger effect in the presence of instantons and background magnetic field was considered to study the dependence of critical electric field on instanton density and magnetic field using AdS/CFT conjecture. The gravity side is the near horizon limit of D3[Formula: see text]D(−[Formula: see text]1) background with electric and magnetic fields on the brane. Our approach is based on the potential analysis for particle–antiparticle pair at zero and finite temperatures, where the zero temperature case is a semi-confining theory. We find that presence of instantons suppresses the pair creation effect, similar to a background magnetic field. Then, the production rate will be obtained numerically using the expectation value of circular Wilson loop. The obtained production rate in a magnetic field is in agreement with previous results.

Sign in / Sign up

Export Citation Format

Share Document