Effects of magnetospheric turbulence on electron plasma waves

Our calculations indicate that high frequency plasma waves can be efficiently generated by electrostatic turbulence in the magnetosphere.     

Nonlinear low frequency electromagnetic waves in a relativistic electron-positron plasma

Using an exact solution of Maxwell's equations and the relativistic equations of motion for electrons and positrons we demonstrate the existence of an electromagnetic wave with a frequency considerably lower that the electron cyclotron frequency. For such a wave the cutoff frequency is determined by the Langmuir frequency and by the wave amplitude. For realistic pulsar parameters a remarkable reduction in the cutoff frequency is then controlled by nonlinear relativistic effects. Modulation instability of this wave is also investigated both analytically and numerically.     

Nonlinear theory of low frequency geomagnetic pulsations

The nonlinear equations for Alfvén waves in magnetospheric plasma are studied taking the influence of hot particles into account. It is shown that oscillations are localized in regions where the pressure of hot ions has local minima. The theoretical results are applied to the problem of generation of geomagnetic pulsations in the magnetospheric plasma.     

An agile very low frequency radio spectrum explorer

The very low frequency(VLF) regime below 30 MHz in the electromagnetic spectrum has presently been drawing global attention in radio astronomical research due to its potentially significant science outcomes exploring many unknown extragalactic sources,transients,and so on.However,the nontransparency of the Earth's ionosphere,ionospheric distortion and artificial radio frequency interference(RFI) have made it difficult to detect the VLF celestial radio emission with ground-based instruments.A straightforward solution to overcome these problems is a space-based VLF radio telescope,just like the VLF radio instruments onboard the Chang'E-4 spacecraft.But building such a space telescope would be inevitably costly and technically challenging.The alternative approach would be then a ground-based VLF radio telescope.Particularly,in the period of post 2020 when the solar and terrestrial ionospheric activities are expected to be in a 'calm' state,it will provide us a good chance to perform VLF ground-based radio observations.Anticipating such an opportunity,we built an agile VLF radio spectrum explorer co-located with the currently operational Mingantu Spectra Radio Heliograph(MUSER).The instrument includes four antennas operating in the VLF frequency range 1-70 MHz.Along with them,we employ an eight-channel analog and digital receivers to amplify,digitize and process the radio signals received by the antennas.We present in the paper this VLF radio spectrum explorer and the instrument will be useful for celestial studies of VLF radio emissions.     

The spectrum of Cosmic Microwave Background at low frequency

The spectrum of Cosmic Microwave Background at frequency close or below 1 GHz is currently known with a large uncertainty. The difficulties arise both from the calibration technique and the presence of large foreground signals. In this spectral region large deviations from the Planck distribution are still permitted. In this paper the current status of our knowledge is described. Experiments involved and techniques employed are also discussed.     

Observations of the optical spectrum of the dayside magnetospheric cleft aurora

Optical spectra of the cleft aurora in the region 5000–8500 Å were measured in December, 1977 at Cape Parry, N.W.T. A Michelson interferometer was used at a resolution of 10 cm^?1. The auroral features observed were OI (5577, 6300-64, 7774, 8446 Å), OII (7319-30 Å), NI (5200 Å), Hα, O₂ atm (1,1), some weak N₂1P bands and possibly some Meinel bands of N₂⁺. In addition, nightglow emissions of Na and OH were observed. Theoretical predictions of the OI and NI emission rates using the model of Link et al. (1980) fit the observed rates reasonably well if a 40 eV Maxwellian incident electron spectrum is assumed. The predicted rates for OII exceed the observed value by a factor of 4. It is suggested that the ionization cross-section may be over-estimated.     

Modifications of the magnetospheric plasma due to ponderomotive forces

By means of two simple examples it is demonstrated that the ponderomotive force induced by the geomagnetic pulsations significantly modifies the plasma distribution in the Earth's magnetosphere. The first example considers the quasi-static plasma equilibrium at closed magnetic shells. Ponderomotive forces, averaged over the geomagnetic pulsation period, increase the plasma density at the equator of the oscillating magnetic shell. If the oscillation amplitude exceeds a certain critical level, the quasi-static solution predicts a maximum of the plasma density at the equator of the magnetic shell. The second example considers the quasi-stationary plasma flow along the open field lines stretching into the geomagnetic tail (the polar wind). The inclusion of the ponderomotive force may shift the critical point for the flow transition through the supersonic barrier closer to the Earth.     

Parallel electric field-induced instability in the magnetospheric ion-electron plasma

A complete dispersion relation for a whistler mode wave propagation in an anisotropic warm ion-electron magnetoplasma in the presence of parallel electric field using the dispersion relation for a circularly polarized wave has been derived. The dispersion relation includes the effect of anisotropy for the ion and electron velocity distribution functions. The growth rate of electron-ion cyclotron waves for different plasma parameters observed atL = 6.6R _E has been computed and the results have been discussed in detail in the light of the observed features of VLF emissions and whistlers. The role of the combination of ion-cyclotron and whistler mode electromagnetic wave propagation along the magnetic field in an anisotropic Maxwellian weakly-ionized magnetoplasma has been studied.     

Quasilongitudinal approximation for whistler-mode waves in the magnetospheric plasma

The range of applicability of an improved quasilongitudinal approximation for whistler-mode waves in the equatorial magnetosphere (4 L 6.6) is specified based on the direct comparison between numerical solutions of the hot electromagnetic dispersion equation with the corresponding analytical quasilongitudinal solutions. It is pointed out that this approximation can be used at frequencies ω less than but not close to the electron gyrofrequency Ω (ω 0.6 Ω) and wave normal angles θ less than but not close to the resonance cone angle θ_R. At ω = 0.8 Ω the analytical results deviate considerably from numerical ones due to the strong damping of the waves, and so the quasilongitudinal solution becomes no longer valid.     

Whistler wave amplitude oscillation and frequency modulation in the magnetospheric cavity

A magnetospheric cavity model is presented to explain self-pulsing and frequency modulation of ducted whistler waves associated with triggered emissions. Partial reflection of these waves at the ends of the duct suggests such a magnetospheric cavity analogous to a laser cavity.     

Interpretation of no radio pulsar inside supernova 1987A: The high plasma cut-off frequency by the remnant media

Three decades have passed since the supernova SN 1987A was observed in the Large Magellanic Cloud, inside which the product is most likely a neutron star (NS) formed in the core collapse explosion.Although lots of observations with sensitive radio telescopes have taken place, astronomers have not yet detected any evidence for a radio pulsar around the remnant of 1987A. To investigate pulsars inside the SN remnants, we calculate the cut-off oscillation frequency of the plasma around the presumed NS inside SN1987A, as shown to be about 33 GHz at present (2018 CE), which is much higher than the favorite “searching window” (e.g. L-band ∼ 1.4 GHz) of radio pulsar surveys that have been commonly exploited by astronomers. Since radio waves with frequencies lower than the plasma cut-off frequency cannot penetrate the SN remnant media, we suggest that astronomers use higher frequency bands to search for a pulsar in SN 1987A.Furthermore, with the expansion of SN remnant media, we find that the plasma cut-off frequency can decay to the L-band (1.4 GHz) in the future. The strategy of finding a pulsar of SN 1987A is that either the high frequency bands of radio telescopes, or the high energy detections at Gamma-ray and X-ray bands by space satellites are applied.     

Drift anisotropy instability of a finite-β magnetospheric plasma

A unified theory of low frequency instabilities in a two component (cold and hot) finite-β magnetospheric plasma is suggested. It is shown that the low frequency oscillations comprise two wave modes : compressional Alfvén and drift mirror mode. No significant coupling between them is found in the long-wave approximation. Instabilities due to spontaneous excitation of these oscillations are considered. It is found that the temperature anisotropy significantly influences the instability growth rate at low frequency. A new instability due to the temperature anisotropy and density gradient appears when the frequency of compressional Alfvén waves is close to the drift mirror mode frequency. The theoretical predictions are compared in detail with the Pc5 event of 27 October 1978 observed simultaneously by the GEOS 2 satellite and the STARE radar facility. It is shown that the experimental results can be interpreted in terms of a compressional Alfvén wave driven by the drift anisotropy instability.     

On the convective mechanism for formation of the plasma sheet in the magnetospheric tail

Calculation of stationary distributions of the most important plasma parameters (particle energy, density, field-aligned and transversal pressure) is performed for a model magnetotail plasma sheet which is formed by convecting plasma mantle particles injected into the closed geomagnetic field line tubes. Computations have been done for two convection models: (i) a model of completely adiabatic particle motion with conservation of the first two invariants and (ii) a model with a strong pitch-angle diffusion which maintains isotropy. It is found that in both cases the heating and compression of the plasma are somewhat more effective than is necessary to account for the observed gradients of magnetic field in the magnetospheric tail. A leakage of accelerated particles through the dawn and dusk edges of the plasma sheet is proposed as a possible mechanism for maintenance of stationary convection in the magnetotail. The question of the dependence of the stationary magnetotail parameters on the solar wind state is discussed briefly.     

The influence of the magnetospheric convection on the current and wind systems in the lower ionosphere

The distributions of the current and the neutral winds driven by the electric field of convection are calculated in the dynamo-region of the ionosphere. At high latitudes the convection field drives the current and wind systems which consist of two cells with the centres at about 6 and 18 hours LT. In the northern hemisphere in the dawn cells winds and currents are clockwise, in the dusk cells they are counterclockwise. The appearance of the wind system shows that the upper atmosphere moves in the direction inverse to the displacement of the ionospheric ends of the magnetic flux tubes taking part in the convection. In the disturbed conditions the calculated wind system has the directions and velocities of the winds which are in a satisfactory agreement with the data of the irregularity drifts in the lower ionosphere in the winter season.     

The magnetospheric plasma as a source of energy for space instrumentation

It is proposed to convert the thermal motion of a plasma into electrical power: energetic electrons collected by a plate dissipate their energy into a load, and are re-injected into the medium by means of an electron source. This concept may find applications in the magnetospheres of the outer planets, but present knowledge does not allow one to assess whether the energy fluxes are sufficient for practical applications. It is therefore neccessary to perform in situ preliminary investigations with electron emitters. It is pointed out that electron sources can be simultaneously used for additional tasks: spacecraft potential clamping, plasma diagnostics and detection of electromagnetic and electrostatic waves.     

Nonlinear frequency shift and self-modulation of the quasi- monochromatic whistlers in the inhomogeneous plasma (magnetosphere)

The nonlinear frequency shift arising from the interaction of the quasimono- chromatic whistler-mode wave with resonant particles in an inhomogeneous plasma is derived. The modulational instability caused by this shift is investigated. The results are applied to the propagation of long-duration VLF whistler-mode signals along the magnetic field in the magnetosphere. It is shown that the modulational instability of these waves in the equatorial region leads to pulsations very similar to those observed experimentally     

Whistler observations of magnetospheric electric field in the night side plasma-sphere at low latitude

Whistlers recorded at low latitude ground stations of Gulmarg, Nainital and Varanasi were used to infer the east-west component of electric field on the nightside plasmasphere atL=1.2, 1.12, and 1.07 during magnetic storm periods. The method of measuring electric field from the observed cross-L motions of whistler ducts within the plasma-sphere, indicated by changes in nose frequency of whistlers has been outlined. The nose frequencies of the non-nose whistlers under consideration have been deduced from Dowden-Allocock linear Q-technique. The results show eastward electric fields of 0.7 mVm^–1 in the equatorial plane of Gulmarg and 0.3 mVm^–1 in the equatorial plane of Nainital in the premidnight local time sector. Near midnight, there is a sharp transition from eastward field to a westward electric field of 0.2–0.7 mV m^–1 for Gulmarg, 0.3–0.5 mV m^–1 for Nainital and 0.1–0.3 mVm^–1 for Varanasi.     

Synchrotron X-ray emission from supernova remnants. Exponential cut-off in the electron spectrum

We present a model which describes the evolution of the energy spectrum of relativistic electrons in supernova remnants, with radiation losses of electrons taken into account. The model can be used to calculate the synchrotron X-ray emission from supernova remnants in the uniform interstellar medium and in the uniform interstellar magnetic field. The importance of various factors in the variations of spatial distributions of nonthermal electrons and their synchrotron emissive capacity is demonstrated. We analyze the errors which arise in the magnetic field strength when it is estimated with the use of the models which ignore the detailed pattern of the evolution of the magnetic field and the electron spectrum behind the shock front in the remnant. The evolution of synchrotron emission spectrum and the ratio between the synchrotron radio and X-ray fluxes from supernova remnants are calculated.     

Nonlinear properties of electron acoustic wave turbulence in the geomagnetic tail

The nonlinear properties of electron acoustic waves in a magnetized plasma consisting of hot electrons, hot ions, and cold electrons are investigated. Using a fluid-guiding center model for the cold electrons and Boltzmann distributions for the hot species, a set of nonlinear mode-coupling equations is derived. Monopole and dipole-vortex solutions are shown to exist for the system of nonlinear equations. Spectrum cascade by mode-coupling in the electron acoustic wave turbulence is investigated. Relevance of our investigation to broadband electrostatic noise (BEN) in the geomagnetic tail is discussed.