Magnetosonic Research Articles

Global distribution of equatorial magnetosonic waves observed by THEMIS

We investigate the wave magnetic field data from three THEMIS spacecraft over the recent 31 months to perform a statistical study of equatorial magnetosonic (MS) wave properties and spatial distribution. The THEMIS spacecraft provide good data coverage in the dominant MS wave region near the equator and at 2≤L≤8. Our global survey shows that strong amplitudes and high occurrence of MS waves are generally observed near the equator, outside the plasmapause, on the dawnside during geomagnetically disturbed periods. In addition, increase of geomagnetic activity shifts the MS wave distribution toward earlier magnetic local time. Strong MS waves generally have RMS wave amplitudes ∼50 pT and an occurrence rate ∼20% on the dawnside outside the plasmapause and could therefore have an important influence on both ring current ion and energetic electron dynamics in the Earth's radiation belts.

Counter-Streaming Interaction between Fast Magnetosonic Wave and Radiation Belt Electrons

Fast magnetosonic (MS) waves have been suggested to be able to effectively accelerate radiation belt electrons. We present gyro-averaged test-particle simulations to investigate counter-streaming interaction between MS wave and radiation belt electrons. It is found that an MS wave can significantly scatter counter-streaming electrons through a non-resonant process. The corresponding energy diffusion coefficients for counter-streaming interaction are always comparable to those for co-streaming interaction, independent of wave normal angle. The pitch-angle and cross diffusion coefficients of counter-streaming interaction are much smaller than those of co-streaming interaction for small normal angles, while they become comparable for large normal angles. Moreover, the bounce-averaged diffusion coefficients exhibit quite different distribution from those for co-streaming or counter-streaming alone in the pitch-angle-energy space. These results suggest that the non-resonant effect associated with counter-streaming interaction is indispensable for the acceleration processes driven by an MS wave.

Temporal evolution of relativistic electrons induced by fast magnetosonic waves in the radiation belts

We study the influence of fast magnetosonic (MS) waves on the temporal evolution of the phase space density (PSD) of relativistic electrons in the outer radiation belts. We evaluate the bounce-averaged pitch angle, momentum and cross-diffusion rates, and find that all the diffusion rates are most pronounced within a limited region of pitch angles: ∼50°–70°. Consequently, numerical solutions of a 2D momentum–pitch angle diffusion equation show that significant increases in electron PSDs at energies of 1 MeV or above occur primarily within such a limited region of pitch angles. Moreover, neglecting cross-diffusion rates generally produces relatively large overestimates in the PSD evolution. The current results suggest that MS waves are primarily responsible for acceleration of relativistic (>1 MeV) electrons around relatively high pitch angles.

Gyro‐resonant scattering of radiation belt electrons during the solar minimum by fast magnetosonic waves

AbstractIn the current study, we perform statistical analysis of the magnetosonic (MS) waves (also often referred to as extremely low frequency (ELF) equatorial noise) in the range between the ion cyclotron frequency and the lower hybrid resonance frequency within 10° of the magnetic equator. Observations were made between 2 and 9 RE using THEMIS Filter Bank (FBK) data. ELF waves with spectral power exceeding 10−6 nT2/Hz are registered in ~3% of all samples in the inner magnetosphere. The survey has shown that, during the solar minimum, the average amplitude of equatorial ELF waves is less than 0.025 nT. Interpreting ELF events as MS waves, we have evaluated the corresponding wave‐induced resonant scattering coefficients of radiation belt energetic electrons. We also study the effect of heavy ions on the scattering rates. The analysis reveals that the scattering by magnetosonic waves for various plasma compositions during geomagnetically quiet times is by up to two orders of magnitude slower than was previously reported and cannot significantly contribute to the long‐term dynamics of the radiation belts. Computed electron scattering rates by magnetosonic waves extends to higher αeq when the fraction of H+ in the plasma decreases, while the range of pitch angles for which resonance occurs remains relatively insensitive to the plasma composition. While inclusion of multi‐ion species into the wave dispersion relation produces noticeable changes in bounce‐averaged scattering rates, the average rates are still significantly below typical scattering rates of chorus or hiss waves.

Nondissipative Saturation of the Magnetorotational Instability in Thin Disks

A new nondissipative mechanism is proposed for the saturation of the axisymmetric magnetorotational (MRI) instability in thin Keplerian disks that are subject to an axial magnetic field. That mechanism relies on the energy transfer from the MRI to stable magnetosonic waves. Such mode interaction is enabled due to the vertical stratification of the disk that results in the discretization of its MRI spectrum, as well as by applying the appropriate boundary conditions. A second order Duffing-like amplitude equation for the initially unstable MRI modes is derived. The solutions of that equation exhibit bursty nonlinear oscillations with a constant amplitude that signifies the saturation level of the MRI. Those results are verified by a direct numerical solution of the full nonlinear reduced set of thin disk magnetohydrodynamics equations.

Normal-Angle Dependence of the Interaction between Radiation Belt Electrons and Fast Magnetosonic Waves

Fast magnetosonic (MS) waves have been suggested to contribute significantly to radiation belt electron dynamics via Landau resonance and transit time scattering. The corresponding quasi-linear diffusion coefficients in pitch angle, energy and particularly cross-pitch-angle-energy are calculated from the gyro-averaged test-particle simulations. It is found that the cross diffusion coefficient is an effective indicator to differentiate between the contributions of resonant and non-resonant mechanisms. Furthermore, the dependence of diffusion coefficients on the normal angle of MS waves are parametrically investigated. Numerical results show that the increasing normal angle can lead to the shrinking of the Landau resonance region and the expansion of the transit-time scattering region. With the increasing normal angle, the pitch angle diffusion coefficients decrease significantly (by about two orders of magnitude), while the other two diffusion coefficients have a relatively limited decrease (within one order of magnitude). For arbitrary normal angles, the magnitude of cross diffusion coefficients is comparable to the other diffusion coefficients, suggesting that the cross diffusion is indispensable in the kinetic simulation works.

A ROLE OF MAGNETOSONIC PULSES ON VARIATIONS OF VOYAGER-1 MeV ELECTRON INTENSITY IN THE HELIOSHEATH

Voyager 1 (V1) spacecraft observed electrons of 6-14 MeV in the heliosheath which showed several enhancements of significant flux variation. We compare these temporal electron flux variations, from the time when V1 crossed the termination shock (TS) up to mid-2008, with dynamical phenomena in the heliosheath that are obtained from our MHD simulations which are based on Voyager 2 (V2) observed solar-wind data. Our simulations indicate that all electron flux enhancements, except for one, correspond fairly well to the times when a magnetosonic (MS) pulse was driven downstream of the TS due to collision of interplanetary shock (IPS) or shock-driven MS pulse and its reflection in the heliosheath that either passed V1, or collided with the TS or with the plasma sheet near the heliopause (HP). This result suggests that these enhancements in the electron flux should correspond to either direct or indirect effects of MS pulses in the heliosheath driven by IPSs. The scale of the heliosphere is estimated by comparing V1-observed magnetic field intensity with the simulated intensity which suggests that V1 is possibly located near the HP within 4-8 AU at the present time.

Three‐dimensional ray tracing of fast magnetosonic waves

A three dimensional ray tracing of fast magnetosonic (MS) waves is first performed by using a global core density model and a field‐aligned density model. Simulating results show that MS waves are primarily confined within a few degrees of the geomagnetic equator due to magnetospheric reflection. MS waves originating from different L‐shells on the dayside can propagate either into or out of the plasmasphere through the plasmapause. In particular, MS waves can propagate eastward (later MLT) or westward (earlier MLT) over a broad region of MLT. The current results further reveal a variety of propagation characteristics, particularly important for the MLT distribution of MS waves.

Regimes of the non-exponential temporal growth in thin Keplerian discs under toroidally dominated magnetic fields

The linear stability of thin vertically-isothermal density-stratified Keplerian discs in toroidally-dominated magnetic fields is treated by asymptotic expansions in the small aspect ratio of the discs. The discs are found to be spectrally stable. The great variety of possible initial conditions leads to three regimes of non-exponential growth of perturbations, which are classified according to different relative levels of the in-plane and axial perturbed velocities. The first two regimes of instability are characterized by the decoupling of the magneto-sonic (MS) and inertia-Coriolis (IC) modes, as well as by algebraic temporal growth of the perturbations, which are driven by either MS or IC modes (hereafter MS - and IC -regimes of instability, respectively). The third, mixed IC-MS regime of non-exponential, non-algebraic growth is due only to nonaxisymmetric perturbations. The latter regime is characterized by high radial and azimuthal wavenumbers, and growth time of the order of tens of rotating periods. The mixed IC-MS regime most likely exhibits the maximal growth as compared with the IC- and MS- regimes. In the first two regimes of instability the compressible MS mode plays a principal role either as the driver of the growth or the driven growing mode, while the mixed IC-MS regime is described by the Bousinesq approximation for incompressible fluid. The latter is obtained as a natural limit of the expansion scheme. The presence of magnetic field in the mixed IC- MS regime may drastically increase the growth rates of the perturbations as compared with the pure hydrodynamic system.

First demonstration of HF-driven ionospheric currents

[1] The first experimental demonstration of HF driven currents in the ionosphere at low ELF/ULF frequencies without relying in the presence of electrojets is presented. The effect was predicted by theoretical/computational means in a recent letter and given the name Ionospheric Current Drive (ICD). The effect relies on modulated F-region HF heating to generate Magneto-Sonic (MS) waves that drive Hall currents when they reach the E-region. The Hall currents inject ELF waves into the Earth-Ionosphere waveguide and helicon and Shear Alfven (SA) waves in the magnetosphere. The proof-of-concept experiments were conducted using the HAARP heater in Alaska under the BRIOCHE program. Waves between 0.1–70 Hz were measured at both near and far sites. The letter discusses the differences between ICD generated waves and those relying on modulation of electrojets.

HF-driven currents in the polar ionosphere

[1] Polar ionospheric heaters have generated ULF/ELF/VLF waves by modulating the auroral electrojet at D/E region altitudes. We present theoretical/computational results indicating that modulated F-region HF heating can generate ionospheric currents even in the absence of electrojet currents. The ELF currents are driven in a two-step process. First, the pressure gradient associated with F-region electron heating drives a local diamagnetic current. This acts as an antenna to inject Magneto-Sonic (MS) waves in the ionospheric plasma. Second, the electric field of the magneto-sonic wave drives Hall currents when it reaches the E region of the ionosphere. The Hall currents act as a secondary antenna that injects waves in the Earth-Ionosphere Waveguide below and Shear Alfven waves upwards to the conjugate regions. The paper examines the scaling and limitations of the concept and suggests proof-of-principle experiments using the HAARP ionospheric heater.

Transit time scattering of energetic electrons due to equatorially confined magnetosonic waves

Recent analysis using quasilinear theory (QLT) has shown that magnetosonic (MS) waves are able to accelerate electrons to relativistic energies on fast time scales (∼1 day). However, the large obliquity of the wave and typical equatorial confinement of the MS wave power create conditions that bring into question the fundamental applicability of QLT to this problem. In this paper, a test particle code is used to model the interaction of energetic electrons with fast MS waves, to test the results of QLT analysis, and to investigate any potential nonlinear effects. It is found that in the expected Landau‐resonant region, test particle results show good agreement with QLT, but outside this region, the spatial confinement of the low‐frequency waves introduces a new source of scattering which we call “transit time diffusion.” Although this mechanism is weaker than resonant scattering, it is nevertheless able to interfere with the Landau resonance to create nulls in the energy‐pitch angle diffusion map, and the scattering persists even when resonant diffusion is completely removed.

Magnetosonic solitons in space plasmas: dark or bright solitons?

AbstractThe nonlinear theory of large-amplitude magnetosonic (MS) waves in highβ space plasmas is revisited. It is shown that solitary waves can exist in the form of ‘bright’ or ‘dark’ solitons in which the magnetic field is increased or decreased relative to the background magnetic field. This depends on the shape of the equilibrium ion distribution function. The basic parameter that controls the nonlinear structure is the wave dispersion, which can be either positive or negative. A general dispersion relation for MS waves propagating perpendicularly to the external magnetic field in a plasma with an arbitrary velocity distribution function is derived.It takes into account general plasma equilibria, such as the Dory–Guest–Harris (DGH) or Kennel–Ashour-Abdalla (KA) loss-cone equilibria, as well as distributions with a power-law velocity dependence that can be modelled by κdistributions. It is shown that in a bi-Maxwellian plasma the dispersion is negative, i.e. the phase velocity decreases with an increase of the wavenumber. This means that the solitary solution in this case has the form of a ‘bright’ soliton with the magnetic field increased. On the contrary, in some non-Maxwellian plasmas, such as those with ring-type ion distributions or DGH plasmas, the solitary solution may have the form of a magnetic hole. The results of similar investigations based on nonlinear Hall–MHD equations are reviewed. The relevance of our theoretical results to existing satellite wave observations is outlined.

Non-Maxwellian effects in magnetosonic solitons

The role of non-Maxwellian effects on magnetosonic (MS) solitons propagating perpendicular to the external magnetic field in high- β plasmas is analysed. It is shown that they can exist in the form of either humps or holes in the magnetic field in which the field is either increased or decreased relative to the background magnetic field. The shape of the solitary structure depends upon both the form of the ion velocity distribution function and the wave dispersion. A nonlinear equation describing the propagation of MS solitons in high- β plasmas with an arbitrary particle velocity distribution function is derived. It is shown that for Maxwellian and bi-Maxwellian plasmas MS solitons can only exist in the form of the magnetic humps. The same is true for plasmas possessing either a kappa distribution or Kennel–Ashour–Abdalla equilibria. However, plasmas with a ring type ion velocity distribution or a Dory–Guest–Harris distribution with large loss-cone index can support the formation of magnetic holes. The theoretical results obtained are then compared with recent satellite observations.

Magnetosheath interaction with high latitude magnetopause: Dynamic flow chaotization

Exploration of plasma–plasma interactions at the high-latitude magnetopause versus a simulated sheared current sheet with strong fluctuations of realistic spectral shape, revealed a new type of dynamic equilibrium, in which nonlinear disturbances serve as an effective obstacle for 80% of the incident magnetosheath ions, providing also the exchange by ∼10% of plasma particles with the stagnant high-beta boundary layer in the minimum field region over the polar cusps. The measured waves, reflected upstream by the boundary, interact in the 3-wave manner with the magnetosonic (MS) fluctuations of the incident flow, resulting in their amplification and then decay into accelerated MS-jets and Alfven waves, driving decelerated flows at the Alfven speed. This impulsive momentum loss via the MS-jets contributes in the average flow bend around the magnetosphere. The leading jet appearance is suggested to be phase-synchronized with both the initial MS fluctuations and nonlinear cascades upstream at the magnetopause, which constitutes the wavy obstacle with multiple decays into the smaller MS-jets and Alfvenic flows. High dynamic pressure in the MS-jets does not fit their acceleration by a reconnection; instead the jets are able to initiate the driven reconnection in the process of interaction with a downstream magnetopause. The acceleration of the MS-jets is consistent with a Fermi-type mechanism, in which electric wave-trains play the role of a moving non-continuous ‘wall’. Estimations of the jet scales from the approach of a nonlinear Cherenkov resonance conforms 2–3 reflections of the jet from the ‘wall’ before overcoming the ‘wall’ potential barrier. We demonstrate quantitative agreement of the acceleration of the leading MS-jet in the process of inertial ion drift in variable electric fields. Current sheets, generated due to opposite sign of the ion and electron inertial drift, can account for the intermittency of the TBL fluctuations.

Particle interactions with obliquely propagating magnetosonic waves

Four cases of nonlinear obliquely propagating magnetosonic (MS) waves are considered using a test particle approach for particles interaction with (1) monochromatic waves propagating both sunward and antisunward, (2) monochromatic waves propagating unidirectionally toward the Sun, (3) a broad band spectrum, propagating both sunward and antisunward, and (4) a broadband spectrum, propagating sunward only. As the solar wind decelerates rapidly inside the bow shock, the interplanetary magnetic field (IMF) increases. Calculations have been performed taking into account such a spatially dependent IMF (based on observations). We find that significant particle acceleration is achieved for both the monochromatic wave and the “turbulent” MS waves, for oblique propagation (angles between the interplanetary magnetic field and the propagation vector larger than 30°). This is due to the fact that at oblique angles resonance width increases. We have compared the relative acceleration of particles for the four cases. Pitch angle scattering and acceleration of particles are found to be much larger in the case of sunward and antisunward propagating waves than those for the sunward waves only. This is due to larger relative phase velocities in the two cases. Also, acceleration of particles is less in the case of a uniform magnetic field than that in the case of spatially increasing magnetic field. The particles get more pitch angle scattered and accelerated in the latter case.

Acceleration of cometary H2O group pickup ions by obliquely propagating nonlinear magnetosonic waves

The observations made during the encounter with comet Giacobini Zinner show that the character of MHD turbulence is governed by the magnetosonic (MS) waves generated by the pickup ions via a resonant cyclotron instability. Close to the bow shock these waves are highly nonlinear, |ΔB/B0| ∼ 1, and are propagating obliquely to the magnetic field. The interaction of cometary ions in the mass loaded solar wind with MS waves propagating away from the comet and oblique to the interplanetary magnetic field (IMF) is investigated using the test particle approach. Ion trajectories, distribution functions, widths of pitch angle scattering and energy diffusion are obtained. Because of the MS “turbulence”, the particle velocity and acceleration are found to increase with increasing wave amplitude, inclination of the wave vector to the background magnetic field, and the range of resonant mode numbers. It is found that the interaction of water group pickup ions with MS waves propagating obliquely to the IMF gives larger pitch angle scattering and acceleration than that in the case of parallel and antiparallel propagating waves. In particular, obliquely propagating MS waves at angles greater than 40° to the ambient magnetic field are very effective at accelerating particles because of a high phase velocity along the magnetic field. In the case of monochromatic MS waves, Landau damping is found to play an important role; the particles get electromagnetically trapped in a potential well due to Landau damping. In the case of MS turbulence, the particles are stochastically heated and the temperature continues to grow linearly with time. We have also investigated the relationship between pitch angle scattering and the three parameters, namely, α, the angle between the solar wind flow direction and the ambient magnetic field, θBk, the angle between the ambient magnetic field and the wave propagation vector, and the ion injection velocity. The pitch angle scattering rates are obtained using a monochromatic nonlinear MS wave as well as MS turbulence in both the quasi‐parallel (0 < α < 60°) and quasi‐perpendicular (60° < α < 90°) regimes for various values of θBk. In the case of a monochromatic MS wave, pitch angle scattering rates are found to change very slightly with respect to θBk, α and the mode number of the MS wave due to their Landau damping. In the case of MHD turbulence the pitch angle scattering rates decrease with increasing α for θBk < 40° and increase with increasing α for θBk > 40°. They are found to be independent of the injection velocity. The results are in agreement with the observations.

Contribution to hydrodynamic equations from interaction of oblique magnetosonic waves with plasmaspheric thermal electrons

The contribution to hydrodynamic equations from thermal electron interactions on Cerenkov resonance with magnetosonic (MS) waves propagating at “large” angles to the Earth's magnetic field. The Grad 13-moment approximation is used to calculate the moments of quasilinear collision integrals. The moments describe the variations of the macroscopic parameters of electron components in the respective hydrodynamic equations due to resonance interaction. The dependence of the collision moments on plasma concentration and electron temperature for a set of MS-wave spectra measured in the outer plasmasphere is studied. The MS-waves are shown to give rise to the particle and energy transport from the interaction region, thereby markedly affecting the spatial distribution of plasma parameters. In turn, this leads to self-adjustment of the process of resonance interactions of MS-waves with thermal electrons. One of the consequences of the feature discussed is that the inhomogeneities of the electron number density and temperature may be formed in the Earth's outer plasmasphere.

Test particle simulation study of whistler wave packets observed near comet Giacobini‐Zinner

In order to study nonlinear interactions of water group ions with large amplitude whistler wave packets detected at the leading edge of steepened magnetosonic waves observed near comet Giacobini‐Zinner, we carried out test particle simulations of water ion interactions with a model wave based on the G‐Z data. As the model wave, we adopted a linearly polarized magnetosonic (MS) wave as the trailing portion of the wave, and circularly polarized whistler waves in the leading edge of the wave. Both the MS and whistler waves are a priori assumed to have large amplitudes. We found that some of the water ions are decelerated in the steepened portion of the magnetosonic wave to the resonance velocity with the whistler wave packets. Through resonance and related nonlinear interaction with the large amplitude whistler waves, the water ions become trapped by the whistler wave packet. An energy balance calculation demonstrates that the trapped ions do lose their kinetic energy during the trapped motion in the whistler wave packet. It is thus demonstrated that the nonlinear trapping motion in the wave structure leads to the effective energy transfer from the water group ions to the whistler wave packets in the leading edge of the steepened MHD waves.

Enhanced scintillations associated with high speed streams in the solar winds

The long wavelength (λ > ρi ion‐gyro radius) density homogeneities in the solar wind that raise the stellar radio scintillation index during the onset of a high‐velocity stream, may be generated by a low‐frequency compressional electromagnetic mode such as the magnetosonic (MS) wave. Our investigation shows that obliquely propagating MS waves are driven unstable by the double‐peaked proton distributions observed in the solar wind. We propose this as a reason for the correlation of high speed streams with enhanced scintillation. The stability at 1 AU of the observed nonequilibrium, double‐peaked particle distributions emerges as a natural consequence of the suggested mechanism.