Electromagnetic Whistler Waves Research Articles

Electromagnetic waves and electron anisotropies downstream of supercritical interplanetary shocks

We present waveform observations of electromagnetic lower hybrid and whistler waves with fci ≪ f < fce downstream of four supercritical interplanetary shocks using the Wind search coil magnetometer. The whistler waves were observed to have a weak positive correlation between δB and normalized heat flux magnitude and an inverse correlation with Teh/Tec. All were observed simultaneous with electron distributions satisfying the whistler heat flux instability threshold and most with T⊥ h/T∥ h > 1.01. Thus, the whistler mode waves appear to be driven by a heat flux instability and cause perpendicular heating of the halo electrons. The lower hybrid waves show a much weaker correlation between δB and normalized heat flux magnitude and are often observed near magnetic field gradients. A third type of event shows fluctuations consistent with a mixture of both lower hybrid and whistler mode waves. These results suggest that whistler waves may indeed be regulating the electron heat flux and the halo temperature anisotropy, which is important for theories and simulations of electron distribution evolution from the Sun to the Earth.

Possibility of magnetospheric VLF response to atmospheric infrasonic waves

In this paper, we consider a model of the influence of atmospheric infrasonic waves on VLF magnetospheric whistler wave excitation. This excitation occurs as a result of a succession of processes: a modulation of the plasma density by acoustic-gravity waves in the ionosphere, a reflection of the whistlers by ionosphere modulation, and a modification of whistler wave generation in the magnetospheric resonator. A variation of the magnetospheric resonator Q-factor has an influence on the operation of the plasma magnetospheric maser, where the active substances are radiation belt particles, and the working modes are electromagnetic whistler waves. The magnetospheric maser is an oscillating system which can be responsible for the excitation of self-oscillations. These self-oscillations are frequently characterized by alternating stages of accumulation and precipitation of energetic particles into the ionosphere during a pulse of whistler emissions. Numerical and analytical investigations of the response of self-oscillations to harmonic oscillations of the whistler reflection coefficient shows that even a small modulation rate can significantly change magnetospheric VLF emissions. Our results can explain the causes of the modulation of energetic electron fluxes and whistler wave intensity with a time scale from 10 to 150 s in the day-side magnetosphere. Such quasi-periodic VLF emissions are often observed in the sub-auroral and auroral magnetosphere and have a noticeable effect on the formation of space weather phenomena.

Microinstabilities at perpendicular collisionless shocks: A comparison of full particle simulations with different ion to electron mass ratio

A full particle simulation study is carried out for studying microinstabilities generated at the shock front of perpendicular collisionless shocks. The structure and dynamics of shock waves are determined by Alfvén Mach number and plasma beta, while microinstabilities are controlled by the ratio of the upstream bulk velocity to the electron thermal velocity and the plasma-to-cyclotron frequency. Thus, growth rates of microinstabilities are changed by the ion-to-electron mass ratio, even with the same Mach number and plasma beta. The present two-dimensional simulations show that the electron cyclotron drift instability is dominant for a lower mass ratio, and electrostatic electron cyclotron harmonic waves are excited. For a higher mass ratio, the modified two-stream instability is dominant and oblique electromagnetic whistler waves are excited, which can affect the structure and dynamics of collisionless shocks by modifying shock magnetic fields.

Inner magnetosphere plasma characteristics in response to interplanetary shock impacts

[1] In order to characterize plasma (0.03–45 keV) properties in response to interplanetary (IP) shock impact at the geosynchronous orbit, we have examined 95 shock events from 1997 to 2004. These shock events have been categorized into two groups: shock fronts associated with southward interplanetary magnetic field (IMF; 47 cases) and with northward IMF (48 cases). Our results show that under southward IMF, the plasma becomes denser and hotter following the IP shock arrival. The proton (0.1–45 keV) and electron (0.03–45 keV) number densities have peaks of 1.8 and of 2.5 cm−3 at the duskside, respectively (the typical tail plasma sheet density is about 0.7 cm−3). After the IP shock impact, the plasma (proton and electron) temperature anisotropy increases remarkably at the noon sector, decreases toward dawn and dusk, and minimizes at midnight, suggesting that both electromagnetic ion cyclotron and whistler waves can be stimulated mainly at the dayside magnetosphere. In addition, there are more oxygen ions injecting into the inner magnetosphere, and the density of ionospheric oxygen ions is comparable to proton density. However, for IP shocks associated with northward IMF, the plasma density and temperature increases are insignificant, while slight enhancements of the plasma temperature anisotropy are distributed globally.

Observation of a Sharp Negative Dipolarization Front in the Reconnection Outflow Region

A sharp dipolarization front (DF) has recently been detected in the Earth's magnetotail and is associated with complex kinetic effects. We present one event where a tailward propagating negative DF (with Bz decreasing sharply to negative value) was observed near a reconnection region. The thickness of the negative DF is comparable with the local ion gyro-radius/inertial length. There is a strong field-aligned current at the front. Electromagnetic whistler wave enhancements are observed around the front, associated with counter-streaming electron beams. We further compare the features of the observed negative DF with the recent kinetic simulation results, as well as the Earthward propagating DFs observed by the THEMIS spacecraft.

Density cavity in magnetic reconnection diffusion region in the presence of guide field

[1] Understanding the structure of the diffusion region of magnetic reconnection is crucial to pinpoint the mechanism of energy conversion from magnetic field to plasma. Characteristics of a diffusion region with guide field (i.e., component reconnection) may be significantly different from those of a diffusion region without guide field (i.e., antiparallel reconnection). In this study, we attempt to understand the structure of a diffusion region with guide field by studying the density cavity along separatrix. We present an event in which a density cavity was detected by the Cluster spacecraft in a diffusion region in the presence of guide field. The cavity was located around the separatrix region on the southern hemisphere of the neutral sheet and earthward of the X-line and was coincident with strong magnetic field compression. The width of the cavity was on the ion inertial scale. This cavity contained a relatively strong antiparallel current, which was mainly contributed by parallel streaming electrons with energy of 1–10 keV. Enhancements of lower hybrid wave and electromagnetic whistler wave were observed inside the cavity. These waves are probably excited by parallel streaming electrons along separatrix via electron beam instability. Two-dimensional electromagnetic particle-in-cell simulation was employed to study the structure of the density cavity. The location and scale of the cavity and the signature of electric current and electron velocity are consistent with our observations. It is found that there was displacement between the position of electron density minimum and out-of-plane magnetic field maximum in reconnection with guide field. However, this displacement is much less than that in reconnection without guide field. There was no significant acceleration for electrons to reach energy larger than 30 keV at the cavity.

Response of the magnetic field and plasmas at the geosynchronous orbit to interplanetary shock

Interplanetary shock can greatly disturb the Earth’s magnetosphere and ionosphere, causing the temporal and spatial changes of the magnetic field and plasmas at the geosynchronous orbit. In this paper, we use the magnetic field data of GOES satellites from 1997 to 2007 and the plasma data of MPA on the LANL satellites from 1997 to 2004 to study the properties of magnetic field and plasma (0.03–45 keV) at the geosynchronous orbit (6.6 RE) within 3 hours before and after the arrival of shock front at the geosynchronous orbit through both case study and superposed epoch analysis. It is found that following the arrival of shock front at the geosynchronous orbit, the magnetic field magnitude, as well as GSM BZ component increases significantly on the dayside (8–16 LT), while the BY component has almost no change before and after shock impacts. In response to the interplanetary shock, the proton becomes much denser with a peak number density of 1.2 cm−3, compared to the typical number density of 0.7 cm−3. The proton temperature increases sharply, predominantly on the dusk and night side. The electron, density increases dramatically on the night side with a peak number density of 2.0 cm−3. The inferred ionospheric O+ density after the interplanetary shock impact reaches the maximum value of 1.2 cm−3 on the dusk side and exhibits the clear dawn-dusk asymmetry. The peak of the anisotropy of proton’s temperature is located at the noon sector, and the anisotropy decreases towards the dawn and dusk side. The minimum of temperature anisotropy is on the night side. It is suggested that the electromagnetic ion cyclotron (EMIC) wave and whistler wave can be stimulated by the proton and electron temperature anisotropy respectively. The computed electromagnetic ion cyclotron wave (EMIC) intense on the day side (8–16 LT) with a frequency value of 0.8 Hz, and the wave intensity decreases towards the dawn and dusk side, the minimum value can be found on the night side. The computed electron whistler wave locates on the day side (8–16 LT) with a value of 2 kHz.

Solid State Plasma in the Pulse Modulated Magnetic Field

Radio frequency (RF) magnetoplasmic waves known as helicons will propagate in solid-state plasma of semiconductors when a strong magnetic field is applied. Helicons have an exact analogy with an electromagnetic whistler wave which is frequently propagated in the low density plasma of the Earth ionosphere. In our experiments the modulated magnetic field is being used for excitation of helicons. It is shown that in the case of pulse modulated field along with the RF helicon waves the transient cyclotron frequency oscillations exist in the semiconductor plasma. For observation of the cyclotron radiation frequency ωc the modulation depth about one percent is sufficient. The measurement of ωc provides an opportunity to determine the masses of electrons and holes in solid-state plasma of semiconductors. In already existing method the absorption of electromagnetic radiation on the cyclotron frequency is being used. For this case it is necessary to have an external infrared generator. In our method the cyclotron radiation is being generated by electrons (or holes) directly and additional external excitation is not needed.

Lightning-induced plasma turbulence and ion heating in equatorial ionospheric depletions

The occurrence of lower-hybrid solitary structures in the ionoshere are triggered by lightning-induced whistlers, revealing a new coupling process between the troposphere and the ionosphere. A wide range of plasma instabilities exist in various regions of the terrestrial ionosphere, leading to the development of plasma turbulence, in particular close to the lower-hybrid frequency—the frequency of a longitudinal oscillation of ions and electrons in a magnetized plasma that must be near perpendicular to the magnetic field. Most observations have been carried out in the auroral regions, where intense lower-hybrid emissions are frequently observed, possibly producing solitary structures1 and ion heating2,3,4. Lower-hybrid turbulence with a smaller intensity has also been observed at mid- and low latitudes above thunderstorms5,6 and was shown to be triggered by the electromagnetic whistler wave generated by the lightning current. Here we present observations of equatorial plasma waves that demonstrate the existence of lower-hybrid solitary structures and the simultaneous occurrence of ion heating in deep, large-scale equatorial plasma depletions that form at night during disturbed geomagnetic conditions. These phenomena follow the development of lower-hybrid turbulence triggered by lightning-induced whistlers, revealing a new coupling process between the troposphere and the ionosphere. Since the energy source of the equatorial solitary structures is different from that involved in the auroral processes, our findings support the idea that the formation of lower-hybrid solitary structures may be a universal mechanism operating in inhomogeneous, magnetized plasma and possibly leading to ion heating and acceleration.

COLD PLASMA INJECTION ON VLF WAVE MODE FOR RELATIVISTIC MAGNETOPLASMA WITH A.C. ELECTRIC FIELD

The effect of cold plasma beam on electromagnetic whistler wave with perpendicular AC electric field has been studied by using the unperturbed Lorentzian (Kappa) distribution in the Earth's atmosphere for relativistic plasma. The cold plasma has been described by a simple Maxwellian distribution where as Lorentzian (Kappa) distribution function has been derived for relativistic plasma with temperature anisotropy in the presence of a perpendicular AC electric field to form a hot/warm background. The dispersion relation is obtained by using the method of characteristic solutions and kinetic approach. An expression for the growth rate of a system with added cold plasma injection has been calculated. Results for representative values of parameters suited to the Earth's magnetosphere has been obtained. It is inferred that in addition to the other factors, the relativistic plasma modifies the growth rate and it also shifts the wave band significantly. The relativistic electrons by increasing the growth rate and widening the bandwidth may explain a wide frequency range of whistler emissions in the Earth's magnetosphere.

Particle‐in‐cell simulation of Maxwellian ring velocity distribution

Electrostatic electron cyclotron harmonic (ECH) and electromagnetic whistler wave emissions are sometimes observed simultaneously in the near‐Earth nightside equatorial magnetotail region. The excitation mechanism for these two wave emissions, however, is quite different with ECH waves excited by a positive slope in the velocity distribution function perpendicular to the ambient magnetic field (such as due to a loss cone or ring velocity distribution), while whistler waves are excited by a temperature anisotropy whereby the perpendicular temperature is larger than the parallel temperature. Here we examine, using a two‐dimensional electromagnetic particle‐in‐cell computer simulation, the excitation of both waves as a consequence of a warm electron ring distribution in the presence of cold background electrons. Initially, ECH waves are excited by the ring distribution in the linear stage, which results in heating of the cold electrons and smearing of the ring distribution. During a nonlinear consequence of the ECH excitation and ensuing electron wave‐particle interactions, whistler waves are also excited by the different instability. Thus both waves are excited during the same simulation run, which may account for the magnetospheric observations of both ECH and whistler waves at the same location where electron loss cone distributions form in the equatorial magnetotail at about 6–9 RE from the Earth.

VLF/ELF wave activity in the vicinity of the polar cusp: Cluster observations

Abstract. Observations by the Cluster spacecraft of VLF/ELF wave activity show distinct signatures for different regions in the vicinity of high altitude polar cusps, which are identified by using magnetic field and plasma data along spacecraft trajectories. These waves include: (1) Broad band magnetic noise observed in the polar cusp at frequencies from several Hz to ~100 Hz, below the local electron cyclotron frequency, fce. Similar magnetic noise is also observed in the high latitude magnetosheath and the magnetopause boundary layer. (2) Strong broad band electrostatic emissions observed in the cusp, in the magnetosheath, and in the high latitude magnetopause boundary layer, at frequencies extending from several Hz to tens of kHz, with maximum intensities below ~100 Hz. (3) Narrow-band electromagnetic whistler waves at frequencies ~0.2–0.6 fce, frequently observed in the closed boundary layer (CBL) adjacent to the polar cusp. These waves are for the first time observed in this region to be accompanied by counter-streaming electron beams of ~100 eV, which suggests that the waves are excited by these electrons through wave-particle interaction. (4) Narrow-band electrostatic waves observed slightly above the local fce in the CBL. (5) Lion roars, observed in the high latitude magnetosheath, often in magnetic troughs of mirror mode oscillations. The above wave signatures can serve as indicators of the regions in the vicinity of the magnetospheric cusp.

Transitions from electrostatic to electromagnetic whistler wave excitation

At frequencies below the electron cyclotron and above the lower hybrid frequency in a magnetoplasma, the refractive index is anisotropic and shows resonances at certain angles caused by electron inertia. For a radiating antenna in a plasma, the short wavelengths near this resonance angle may contribute to the radiation pattern of the antenna. A series of experiments is reported, in which waves were excited using a small 1-cm-diam electrostatically coupled antenna into a preformed plasma, densities (ne) from 1015 to 1018 m−3 and magnetic fields from 30 to 60 G. Maps of the wave amplitude and phase were made within the plasma by scanning the position of a b-dot probe. At low densities (e.g., ne<5×1016 m−3 at 50 G and 3 mTorr), a single amplitude maximum along the group velocity resonance cone angle was measured that decayed as the distance from the antenna increased. The observed radiation pattern in all cases was consistent with that of a point source in an unbounded plasma, and no global eigenmode resonances were found. As the density was increased, the apparent attenuation of the resonance cone waves increased and they appeared to withdraw into the antenna. At high densities (ne>5×1016 m−3) the radiation pattern was characterized by a monotonic increase in wave phase in the axial direction, a central maximum for Bz, and off-axis maxima for Bx and By which are consistent with the propagation of m=0 helicon waves, and no evidence of resonance cone structure. This change in the radiation pattern is reproduced numerically in a homogeneous plasma model including an electrostatically coupled antenna with the same geometry as that used in the experiment.

Variations in planetary radiation belt fluxes and their radiative observations

The variations in the fluxes of the relativistic electrons in the planetary radiation belts are due to a set of different physical processes which violate one or more of the adiabatic invariants. We survey the mechanisms which break down these invariants and investigate the time scales for the processes and the resulting effects on the observed fluxes. The mechanisms include (a) sudden deformation of the magnetic field configuration, (b) radial diffusion due to low-frequency electromagnetic oscillations, (c) transit-time damping due to fast waves and (d) diffusion due to electromagnetic ion-cyclotron (emic) or whistler waves. It is indicated how the waves which interact resonantly with the relativistic electrons are responsible for enhancement in the radiative spectra of the gyrosynchrotron emissions in the GHz frequency range and the X-ray bremsstrahlung emissions at the MeV energy range.

Generation of circularly polarized radio waves within magnetic holes of the solar wind

The possibility of nonlinear interaction between Langmuir waves and ion–acoustic waves in the solar wind has received considerable interest in the last years. However, little attention has been paid to the possibility of nonlinear coupling between Langmuir waves and whistler waves in the interplanetary medium. Close temporal correlation between high-frequency Langmuir waves and low-frequency electromagnetic whistler waves has been observed recently within magnetic holes of the solar wind. In order to account for these observations within magnetic holes of the solar wind, we develop a nonlinear three-wave theory describing the parametric interaction of Langmuir waves, electromagnetic whistler waves with either right- or left-hand circularly polarized electromagnetic waves. We suggest that the nonlinear coupling of Langmuir waves and whistler waves may lead to the formation of modulated Langmuir wave packets as well as the generation of circularly polarized radio waves at the plasma frequency in the solar wind.

Nonlinear Coupling of Langmuir Waves with Whistler Waves in the Solar Wind

Close temporal correlation between high-frequency Langmuir waves and low-frequency electromagnetic whistler waves has been observed recently within magnetic holes of the solar wind. In order to account for these observations, a theory is formulated to describe the nonlinear coupling of Langmuir waves and whistler waves. It is shown that a Langmuir wave can interact nonlinearly with a whistler wave to produce either right-hand or left-hand circularly polarized electromagnetic waves. Nonlinear coupling of Langmuir waves and whistler waves may lead to the formation of modulated Langmuir wave packets as well as the generation of circularly polarized radio waves at the plasma frequency in the solar wind. Numerical examples of whistler frequency, nonlinear growth rate and modulation frequency for solar wind parameters are calculated.

Magnetic field pulses produced via whistler mode wave decay in the ionosphere

The nonlinear ionospheric effects of very low-frequency (VLF) electromagnetic radiation produced by lightning discharges are considered. Electric and magnetic field measurements by a rocket in the ionosphere reported by Kelly et al. [Geophys. Res. Lett. 17, 2221 (1990)] show the simultaneous presence of powerful electromagnetic whistler wave bursts along with electrostatic waves and a magnetic pulse with duration about 20 ms. The parametric decay of whistler mode waves into electrostatic waves and ultralow-frequency (ULF) electromagnetic waves is proposed as a possible explanation for the observed phenomenon. The evolution of whistler mode wave decay instabilities in time and two spatial dimensions is studied. The nonlinear dispersion equation is obtained, and solved numerically. Two-fluid magnetohydrodynamics is used to describe the three-wave interaction. The theoretical results are compared with observational data.

Laboratory studies of field-aligned density striations and their relationship to auroral processes

Magnetic field-aligned structures in current, density, and temperature are common features of the auroral ionospheric plasma. These structures both generate and transform low frequency waves in the plasma. The results of laboratory studies of two processes involving magnetic field-aligned density depletions (striations) that play a role in auroral plasma dynamics are presented. The first process involves the spontaneous generation of density and magnetic fluctuations at the striation edge. The nature of the fluctuations depends upon the electron plasma beta. At high beta (greater than the electron to ion mass ratio, m/M) the drift Alfvén wave is excited. At lower beta the density and magnetic field fluctuations separate and the shear Alfvén wave dominates. This process creates an environment conducive to electron acceleration along the magnetic field when the striation size is on the order of the electron skin depth because the shear Alfvén wave then has a substantial field-aligned electric field. The second process is the direct conversion of electromagnetic whistler waves to electrostatic lower hybrid waves at the striation edge. This process provides a mechanism for concentrating lower hybrid wave energy in the vicinity of striations where it may play a role in electron and ion heating.

New aspects of whistler waves driven by an electron beam studied by a 3-D electromagnetic code

We have restudied electron beam driven whistler waves with a 3-D electromagnetic particle code. In the initialisation of the beam-plasma system, “quiet start” conditions were approached by including the poloidal magnetic field due to the current carried by beam electrons streaming along a background magnetic field. The simulation results show electromagnetic whistler wave emissions and electrostatic beam modes like those observed in the Spacelab 2 electron beam experiment. It has been suggested in the past that the spatial bunching of beam electrons associated with the beam mode may directly generate whistler waves. However, the simulation results indicate several inconsistencies with this picture: (1) the parallel (to the background magnetic field) wavelength of the whistler wave is longer than that of the beam instability, (2) the parallel phase velocity of the whistler wave is smaller than that of the beam mode, and (3) whistler waves continue to be generated even after the beam mode space charge modulation looses its coherence. The complex structure of the whistler waves in the vicinity of the beam suggest that the transverse motion (gyration) of the beam and background electrons is involved in the generation of the whistler waves.

Nonlinear Alfvén wave phenomena in the planetary magnetosphere

Three nonlinear Alfvén wave phenomena in the planetary magnetosphere are discussed: (1) magnetohydrodynamic parametric instabilities induced by a nonlinear standing Alfvén wave, (2) parametric excitation of Alfvén and Langmuir waves by a nonlinear electromagnetic whistler wave, and (3) parametric generation of electromagnetic waves in the vicinity of electron plasma frequency via nonlinear coupling of Langmuir and Alfvén waves. Observational evidence in support of these nonlinear Alfvén wave phenomena, such as the auroral Alfvén-acoustic turbulence and the auroral Langmuir-Alfvén-whistler (LAW) events, in the planetary magnetosphere is presented.