Large-amplitude Magnetosonic Waves Research Articles

Interaction between Alternating Magnetic Fields and a Relativistic Collisionless Shock

We investigate the interaction between alternating magnetic fields with cold current sheets and a relativistic collisionless shock wave. Many kinds of high-energy astrophysical objects may involve such alternating magnetic fields and a relativistic shock. They can be potent sources for the generation of high-energy particles. We found that a precursor wave, propagating upstream from the shock front, accelerates a dense current sheet plasma upstream. In the case that the width of the respective magnetic field reversal is larger than the downstream gyroradius, the current sheet generates a large-amplitude magnetosonic wave downstream by the collision with a shock front. The motional electric field accompanied by the magnetosonic wave can further accelerate the preaccelerated particles, forming a nonthermal energy spectrum. In addition, the current sheet structure is stable against not only the collision but also compression by other current sheets. In the thin current sheet case, which means the case that the width of the alternating magnetic field reversal is smaller than the downstream gyroradius, the magnetic field dissipates and the magnetosonic wave excitation is absent. This result is applied to pulsar wind nebulae. The result of the dissipation could solve the σ problem.

Plasma waves driven by gravitational waves in an expanding universe

In a Friedmann–Robertson–Walker (FRW) cosmological model with zero spatial curvature, we consider the interaction of the gravitational waves with the plasma in the presence of a weak magnetic field. Using the relativistic hydromagnetic equations, it is verified that large-amplitude magnetosonic waves are excited, assuming that both the gravitational field and the weak magnetic field do not break the homogeneity and isotropy of the considered FRW spacetime.

Simple magnetohydrodynamic waves

Large-amplitude magnetic field fluctuations often accompanied by density variations are frequently observed in front of the earth's bow shock and in the vicinity of comets by extraterrestrial in situ measurements. They are identified as a manifestation of magnetohydrodynainic (MHD) waves in space plasmas. Because of their large amplitudes (i.e. because the magnetic field amplitude is of the order of the ambient magnetic field, for instance), these fluctuations cannot be satisfactorily described by linear wave theory. In this paper the properties of one-dimensional MHD waves of arbitrary amplitude, i.e. so-called simple MHD waves, are investigated, and a relationship is derived between the enhancement of the magnetic field and the density as well as the propagation velocity. Fast large-amplitude magnetosonic waves exhibit wave steepening. Here the dependence of the steepening time on the wave amplitude is derived and illustrated numerically.

Sources of magnetosheath waves and turbulence

In this paper the possible sources of waves and turbulence in the magnetosheath are discussed. In particular, three separate sources resulting in different types of waves are identified. One is the quasi-parallel portion of the bow shock which contains large amplitude magnetosonic waves generated at or upstream of the shock. Due to their small group velocity these waves are convected back into downstream. It is suggested that the convected wave energy results in the generation of Alfvén waves through a linear mode conversion process. Another source is the quasi-perpendicular portion of the shock which results in an anisotropic plasma downstream of the shock. This temperature anisotropy is unstable to the excitation of both Alfvén and mirror waves with the former having a larger growth rate. The competition between these two wave modes is discussed and it is shown that the mirror mode can be excited despite its lower growth rate. Finally, it is shown that the magnetopause itself can generate slow magnetosonic waves in the magnetosheath. It is suggested that these waves can be thought of as part of the overall magnetopause structure, as are dispersive whistlers associated with low Mach number shocks.

Hybrid simulations of the effects of interstellar pickup hydrogen on the solar wind termination shock

Hybrid (kinetic ions/fluid electrons) plasma simulations are used to study the effects of a population of energetic interstellar pickup hydrogen ions on the solar wind termination shock. The pickup hydrogen is treated as a second ion species in the simulations, and thus the effects of the pick‐ups on the shock, as well as the effects of the shock on the pickups, are treated in a fully self‐consistent manner. For quasi‐perpendicular shocks with 10‐20% pickup hydrogen the pickup ions manifest themselves in a small foot ahead of the shock ramp caused by pickup ion reflection. For oblique shocks with smaller angles between the field and the shock normal (θBn ≈ 40°‐60°) a large fraction of the pickup ions are reflected and move back upstream where they excite large amplitude magnetosonic waves which steepen into shocklets. These backstreaming pickup ions may provide advance warning of a spacecraft encounter with the termination shock.

Conditions for Ion Reflection in a Large-Amplitude Magnetosonic Wave

Ion orbits in a large-amplitude magnetosonic wave are studied numerically and analytically. The orbits are classified into three basic types. In the first type, ions pass through the wave without reflection and do not gain much energy. In the second type, they are reflected once and are accelerated in the direction of the wave normal. In the third type, they are reflected several times and are accelerated in the direction parallel to the wave front. Conditions for these types of reflection are discussed and the energies of accelerated ions are evaluated.

Prompt ion acceleration to relativistic energies by a large-amplitude magnetosonic wave

The rapid acceleration of ions by a large-amplitude magnetosonic wave is discussed. Theory and relativistic particle simulation show that a large-amplitude magnetosonic wave (such as a shock wave) can trap some fraction of ions and resonantly accelerate them in the direction perpendicular to the ambient magnetic field. In a rather strong magnetic field, those resonant ions gain relativistic energies (above 1 GeV) in a short time period (much shorter than 1 s). The condition for the ion trapping is also studied; a few percent of ions can be strongly accelerated, and more than 10 percent of ions can be mildly accelerated for the shock with the Alfven Mach number of about 2. 30 refs.

Relativistic particle dynamics in a steepening magnetosonic wave

Acceleration of both electrons and ions to relativistic energy by large amplitude magnetosonic waves is investigated by use of numerical simulation. Nonlinear effects are shown to form the saturation mechanism and limit the amplitude below the level where a particle specie can undergo unlimited acceleration, which is expected theoretically. Spiky structures appear both in density and field waveforms that are characteristics of the relativistic regime. Both electrons and ions are strongly accelerated by Elx×Bz drift and Ety field, but their resonance features versus fields are strongly different. Around the trapping time, relativistic electron solitonlike wavelets are triggered from the main wave ramp; a few mechanisms are proposed for their interpretation. Both electrons and ions are strongly heated at the expense of the wave energy. This damping in association with the large space charge effects resulting from the spiky structures is the origin of some observed saturation level in the field energy.

Solar flares and particle acceleration.

The theory and observations of solar flares are reviewed with emphasis on particle acceleration. Hard X-ray and γ-ray measurements of solar flares have revealed that the acceleration of particles to relativistic energies can occur promptly (within-1s). This can be explained by the theory of resonant particle acceleration in a large-amplitude magnetosonic wave. Some models of magnetic reconnection and loop-loop interactions are also discussed which are believed to play an essential role in initiating the flare and/or releasing the magnetic energy.

Simulation of magnetic phenomena driven by parallel ion motion

In this paper a one-dimensional Darwin quasineutral hybrid simulation code is used to investigate the large spatial scale (λ∼c/ωpi) low-frequency (ω∼ωci) collisionless evolution of a plasma slug of finite width traveling parallel to an ambient magnetic field in the presence of a uniform background plasma. Such a system may cause excitation of large amplitude magnetosonic waves and mixing of the background and slug ions. Under certain conditions, background ions and slug ions are driven upstream of the interaction region. Particular attention is paid to the width of the plasma slug, thermal effects, the anomalous resistivity, and the relative velocity of the two plasmas.

Ion Heating and Acceleration by Strong Magnetosonic Waves

Large-amplitude magnetosonic waves ($\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ensuremath{\perp}\stackrel{\ensuremath{\rightarrow}}{\mathrm{B}}$) with ${\ensuremath{\omega}}_{\mathrm{ci}}<\ensuremath{\omega}<{\ensuremath{\omega}}_{\mathrm{lh}}$ (${\ensuremath{\omega}}_{\mathrm{ci}}$ and ${\ensuremath{\omega}}_{\mathrm{lh}}$ are the ioncyclotron and lower-hybrid frequencies) are investigated by particle simulation. Some ions are accelerated and trapped in the potential troughs of the waves; the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}\ifmmode\times\else\texttimes\fi{}\frac{\stackrel{\ensuremath{\rightarrow}}{\mathrm{B}}}{c}$ force accelerates these particles parallel to the phase fronts. Ultimately, the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}\ifmmode\times\else\texttimes\fi{}\frac{\stackrel{\ensuremath{\rightarrow}}{\mathrm{B}}}{c}$ force detraps the ions but only when $\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}$ is much larger than the Alfv\'en velocity; the wave is strongly damped by this effect.