V. V. Krasnoselskikh

Nonstationarity of strong collisionless quasiperpendicular shocks: Theory and full particle numerical simulations

Whistler waves are an intrinsic feature of the oblique quasiperpendicular collisionless shock waves. For supercritical shock waves, the ramp region, where an abrupt increase of the magnetic field occurs, can be treated as a nonlinear whistler wave of large amplitude. In addition, oblique shock waves can possess a linear whistler precursor. There exist two critical Mach numbers related to the whistler components of the shock wave, the first is known as a whistler critical Mach number and the second can be referred to as a nonlinear whistler critical Mach number. When the whistler critical Much number is exceeded, a stationary linear wave train cannot stand ahead of the ramp. Above the nonlinear whistler critical Mach number, the stationary nonlinear wave train cannot exist anymore within the shock front. This happens when the nonlinear wave steepening cannot be balanced by the effects of the dispersion and dissipation. In this case nonlinear wave train becomes unstable with respect to overturning. In the p...

Determination of dispersion relations in quasi-stationary plasma turbulence using dual satellite data

The joint frequency-wavenumber spectrum is one of the basic quantities for analyzing plasma turbulence. It is shown how the full spectrum can be recovered from wavefields measured by two or more satellites via spectral methods based on wavelet transforms. Compared to standard cross-correlation techniques, different branches in the dispersion relation can be resolved and quasi-stationary wavefields can be accessed. Using this new approach, low frequency magnetic field data from the AMPTE-UKS and AMPTE-IRM spacecraft are investigated and the impact of nonlinear processes on wave propagation at the Earths foreshock is revealed.

Non-diffusive resonant acceleration of electrons in the radiation belts

We describe a mechanism of resonant electron acceleration by oblique high-amplitude whistler waves under conditions typical for the Earth radiation belts. We use statistics of spacecraft observations of whistlers in the Earth radiation belts to obtain the dependence of the angle θ between the wave-normal and the background magnetic field on magnetic latitude λ. According to this statistics, the angle θ already approaches the resonance cone at λ∼15° and remains close to it up to λ∼30°–40° on the dayside. The parallel component of the electrostatic field of whistler waves often increases around λ∼15° up to one hundred of mV/m. We show that due to this increase of the electric field, the whistler waves can trap electrons into the potential well via wave particle resonant interaction corresponding to Landau resonance. Trapped electrons then move with the wave to higher latitudes where they escape from the resonance. Strong acceleration is favored by adiabatic invariance along the increasing magnetic field, wh...

Experimental determination of the dispersion of waves observed upstream of a quasi‐perpendicular shock

Highly-coherent waves in the frequency range 1-15 Hz are usually observed upstream of the ramp of supercritical quasi-perpendicular shocks. A few models have been proposed to explain their origin. In the present paper the wave vectors of these waves are determined using AMPTE UKS and AMPTE IRM data in order to differentiate between theoretical models.

A statistical study of the propagation characteristics of whistler waves observed by Cluster

[1] VLF waves play a crucial role in the dynamics of radiation belts, and are responsible for the loss and the acceleration of energetic electrons. Modeling wave‐particle interactions requires the best possible knowledge for how wave energy and wave‐normal directions are distributed in L‐shells and for the magnetic latitudes of different magnetic activity conditions. In this work, we performed a statistical study for VLF emissions using a whistler frequency range for nine years (2001–2009) of Cluster measurements. We utilized data from the STAFF‐SA experiment, which spans the frequency range from 8.8 Hz to 3.56 kHz. We show that the wave energy distribution has two maxima around L ∼ 4.5 − 6 and L ∼ 2, and that wave‐normals are directed approximately along the magnetic field in the vicinity of the geomagnetic equator. The distribution changes with magnetic latitude, and so that at latitudes of ∼30°, wave‐normals become nearly perpendicular to the magnetic field. The observed angular distribution is significantly different from Gaussian and the width of the distribution increases with latitude. Since the resonance condition for wave‐particle interactions depends on the wave normal orientation, our results indicate that, due to the observed change in the wave‐normal direction with latitude, the most efficient particle diffusion due to wave‐particle interaction should occur in a limited region surrounding the geomagnetic equator.

Nonlinear electron acceleration by oblique whistler waves: Landau resonance vs. cyclotron resonance

This paper is devoted to the study of the nonlinear interaction of relativistic electrons and high amplitude strongly oblique whistler waves in the Earths radiation belts. We consider electron trapping into Landau and fundamental cyclotron resonances in a simplified model of dipolar magnetic field. Trapping into the Landau resonance corresponds to a decrease of electron equatorial pitch-angles, while trapping into the first cyclotron resonance increases electron equatorial pitch-angles. For 100 keV electrons, the energy gained due to trapping is similar for both resonances. For electrons with smaller energy, acceleration is more effective when considering the Landau resonance. Moreover, trapping into the Landau resonance is accessible for a wider range of initial pitch-angles and initial energies in comparison with the fundamental resonance. Thus, we can conclude that for intense and strongly oblique waves propagating in the quasi-electrostatic mode, the Landau resonance is generally more important than the fundamental one.

Determination of the electron anomalous mobility through measurements of turbulent magnetic field in Hall thrusters

Measurements of the turbulent magnetic field in a Hall thruster have been carried out between 1kHz and 30MHz with the aim of understanding electron transport through the magnetic field. Small detecting coils at the exit of the accelerating channel and outside of the ionic plume were used to characterize various instabilities. The characteristic frequencies of the observed power spectral densities correspond to known classes of instabilities: low frequency (20–40kHz), transit time (100–500kHz), and high frequency (5–10MHz). A model of the localized electron currents through a magnetic barrier is proposed for the high-frequency instability, and is found to be in good quantitative agreement with the observations. Based on the measured high-frequency turbulent magnetic field, the turbulent electric field is estimated to be about 1V∕cm outside of the plume and ranges from 10to102V∕cm at the channel midradius at the exit of the thruster. The “anomalous” electron collision frequency, related to the high-frequenc...

On the nature of low frequency turbulence in the foot of strong quasi-perpendicular shocks

Abstract Low frequency fluctuations of the magnetic field in the vicinity of the Earths bow shock are presented. Observations from the Prognoz-10 (or Intershock) and AMPTE UK satellites are analysed using cospectral techniques. For high Mach number quasi-perpendicular shocks the main low frequency fluctuations seem to be due to whistler waves which are generated by non-linear evolution of the shock front. This is in contradiction to previous proposals that these waves are generated by an instability driven by the reflected ions.

A study of the dispersion of the electron distribution in the presence of E and B gradients: Application to electron heating at quasi-perpendicular shocks

The dynamics of electrons in the presence of E and B field gradients is considered. The results are applied to electron heating in quasi-perpendicular shocks. The main focus is on the transitions between adiabatic and overadiabatic heating and from perpendicular to oblique geometry. Contrary to previous results, it is shown that the divergence of initially close electron trajectories always takes place in the quasi-perpendicular collisionless shock front. In the case of negligible electric field gradients at the shock front, the divergence rate depends only upon the magnetic field gradient and results in adiabatic heating. If the electric field gradient is not negligible, divergence rate depends on increasing electric field gradients. In this case, the divergence rate in the first part of the ramp will exceed the “adiabatic” divergence rate. This enhancement is significant for scales of the electric field of about 4–6 c/ωpe. Under the conditions of the Earths bow shock the magnetic field and particularly its gradients affect the electron trajectory divergence rate and thus the process of thermalization of electrons.

The FIELDS Instrument Suite for Solar Probe Plus

NASA’s Solar Probe Plus (SPP) mission will make the first in situ measurements of the solar corona and the birthplace of the solar wind. The FIELDS instrument suite on SPP will make direct measurements of electric and magnetic fields, the properties of in situ plasma waves, electron density and temperature profiles, and interplanetary radio emissions, amongst other things. Here, we describe the scientific objectives targeted by the SPP/FIELDS instrument, the instrument design itself, and the instrument concept of operations and planned data products.