Field In Space Plasma Research Articles

Electron firehose instability in kappa-Maxwellian-distributed space plasmas

Temperatures of charge species are observed to be constrained due to collisional and collective processes. In collision-poor plasma from space, microinstabilities appeared as a major candidate to regulate the observed temperature anisotropies. This manuscript investigates the electron parallel electron firehose instability, which might be excited by the temperature anisotropy in a direction parallel to the background magnetic field in space plasmas. Based on the observed data, we employ a kappa-Maxwellian model distribution function to unlock the dispersion characteristics of the electron firehose mode. We find that in the regions where there are large populations of suprathermal electron, the real mode propagation, growth rates, and associated domains of the unstable wave numbers tend to increase. Maximum growth levels of the instability get enhanced with an increase in the temperature anisotropy in direction parallel to the background magnetic field and electron plasma beta. Furthermore, we decoded the instability threshold and maximum growth rate for the choice of input parameters suitable to excite the electron firehose instability.

Space\u2013time structure and wavevector anisotropy in space plasma turbulence

Space and astrophysical plasmas often develop into a turbulent state and exhibit nearly random and stochastic motions. While earlier studies emphasize more on understanding the energy spectrum of turbulence in the one-dimensional context (either in the frequency or the wavenumber domain), recent achievements in plasma turbulence studies provide an increasing amount of evidence that plasma turbulence is essentially a spatially and temporally evolving phenomenon. This review presents various models for the space–time structure and anisotropy of the turbulent fields in space plasmas, or equivalently the energy spectra in the wavenumber–frequency domain for the space–time structures and that in the wavevector domain for the anisotropies. The turbulence energy spectra are evaluated in different one-dimensional spectral domains; one speaks of the frequency spectra in the spacecraft observations and the wavenumber spectra in the numerical simulation studies. The notion of the wavenumber–frequency spectrum offers a more comprehensive picture of the turbulent fields, and good models can explain the one-dimensional spectra in the both domains at the same time. To achieve this goal, the Doppler shift, the Doppler broadening, linear-mode dispersion relations, and sideband waves are reviewed. The energy spectra are then extended to the wavevector domain spanning the directions parallel and perpendicular to the large-scale magnetic field. By doing so, the change in the spectral index at different projections onto the one-dimensional spectral domain can be explained in a simpler way.

DC and low‐frequency double probe electric field measurements in space

AbstractThe double probe technique for measuring DC and low‐frequency electric fields in space plasmas is described with an emphasis on uncertainties in the measurements, data analysis techniques, and methods for improving the electric field measurements in future missions.

RELAXATION PROCESSES WITHIN FLUX ROPES IN SOLAR WIND

Flux ropes are localized structures in space plasma whose tube-like organized magnetic configuration can be well approximated by a force-free field model. Both numerical simulations and simple models suggest that the ideal magnetohydrodynamics (MHD) can relax toward a minimum energy state, where magnetic helicity is conserved, characterized by force-free magnetic fields (Taylor relaxation). In this paper, we evaluate MHD rugged invariants within more than 100 flux ropes identified in the solar wind at 1 AU, showing that the magnetic and cross-helicity content carried out by these structures tend to be attracted toward a particular subphase in the parameter plane. The final configuration of the MHD rugged invariants in the parameter plane suggests indeed that flux ropes represent well-organized structures coming from the dynamical evolution of MHD turbulent cascade. These observational results, along with a simple model based on a truncated set of nonlinear ordinary differential equations for both the velocity and magnetic field Fourier coefficients, thus, support a scenario in which the flux ropes naturally come out from the ideal MHD decay to large-scale magnetic field in space plasmas, probably governed by relaxation processes similar to those observed in laboratory plasmas.

Analysis of the dependence of surfatron acceleration of electrons by an electromagnetic wave in space plasma on the particle momentum along the wave front

Based on the numerical solution of the nonlinear nonstationary second-order equation for the wave phase on the particle trajectory, the dynamics of surfatron acceleration of electrons by an electromagnetic wave propagating across the external magnetic field in space plasma is analyzed as a function of the electron momentum along the wave front. Numerical calculations show that, for strongly relativistic initial values of the electron momentum component along the wave front gy(0) (the other parameters of the problem being the same), electrons are trapped into the regime of ultrarelativistic surfatron acceleration within a certain interval of the initial wave phase Ψ(0) on the particle trajectory. It is assumed in the calculations that |Ψ(0)| ≤ π. For strongly relativistic values of gy(0), electrons are immediately trapped by the wave for 19% of the initial values of the phase Ψ(0) (favorable phases). For the rest of the values of Ψ(0), trapping does not occur even at long times. This circumstance substantially simplifies estimations of the wave damping due to particle acceleration in subsequent calculations. The dynamics of the relativistic factor and the components of the electron velocity and momentum under surfatron acceleration is also analyzed. The obtained results are of interest for the development of modern concepts of possible mechanisms of generation of ultrarelativistic particle fluxes in relatively calm space plasma, as well as for correct interpretation of observational data on the fluxes of such particles and explanation of possible reasons for the deviation of ultrarelativistic particle spectra detected in the heliosphere from the standard power-law scalings and the relation of these variations to space weather and large-scale atmospheric processes similar to tropical cyclones.

Evolution of lower hybrid turbulence in the ionosphere

Three-dimensional evolution of the lower hybrid turbulence driven by a spatially localized ion ring beam perpendicular to the ambient magnetic field in space plasmas is analyzed. It is shown that the quasi-linear saturation model breaks down when the nonlinear rate of scattering by thermal electron is larger than linear damping rates, which can occur even for low wave amplitudes. The evolution is found to be essentially a three-dimensional phenomenon, which cannot be accurately explained by two-dimensional simulations. An important feature missed in previous studies of this phenomenon is the nonlinear conversion of electrostatic lower hybrid waves into electromagnetic whistler and magnetosonic waves and the consequent energy loss due to radiation from the source region. This can result in unique low-amplitude saturation with extended saturation time. It is shown that when the nonlinear effects are considered the net energy that can be permanently extracted from the ring beam is larger. The results are applied to anticipate the outcome of a planned experiment that will seed lower hybrid turbulence in the ionosphere and monitor its evolution.

Effect of the longitudinal momentum of electrons on their surfatron acceleration by an electromagnetic wave in space plasma

By numerically calculating the second-order nonlinear time-dependent equation for the wave phase on a particle trajectory, the effect of the longitudinal (with respect to the external magnetic field) momentum of electrons on the dynamics of their surfatron acceleration by an electromagnetic wave propagating across the external magnetic field in space plasma is analyzed. It is shown that, for strongly relativistic initial values of the longitudinal component of the electron momentum (the other parameters of the problem being fixed), the electrons are trapped into the ultrarelativistic regime of surfatron acceleration within a definite interval of the initial wave phase Ψ(0) on the particle trajectory. It was assumed in the calculations that Ψ(0) ≤ π. For the initial wave phases lying within the interval of 0 < Ψ(0) ≤ π, the electrons are immediately trapped by the wave, whereas at π ≤ Ψ(0) ≤ 0, no electron trapping is observed even at long computation times. This result substantially simplifies estimates of the wave damping caused by particle acceleration. The dynamics of the velocity components, momentum, and relativistic factor of electrons in the course of their ultrarelativistic acceleration are considered. The obtained results present interest for the development of modern concepts of the mechanisms for the generation of ultrarelativistic particles in space plasma, correct interpretation of experimental data on the flows of such particles, explanation of possible reasons for the deviation of the fast particle spectra observed in the heliosphere from the standard power-law scaling, and analysis of the relation between such deviations and the space weather.

Electric Field Instrument Using Radiated Electrons (E‐FIRE): An Innovative Approach to the Measurement of Electric Fields in the Earth's Magnetosphere

We are proposing a novel compact sensor system for measuring electric fields in space plasmas. The measurement of electric fields from earth orbiting satellites has been a difficult challenge for the large scale DC component of this field. Traditional techniques using double-probe style instruments are troubled by contact potentials to the plasma environment and photoelectric effects due to solar radiation. Newer techniques using low energy electron beams put requirements on costly spacecraft electric cleanliness, in order to avoid beam deflection by spacecraft potentials. E-FIRE uses energetic electrons of approximately one MeV in connection with recently developed ion-implanted solid-state detectors which have good position resolution and energy determination. The electrons are produced by a radioactive source with a line spectrum that tags the particles as being emitted from the satellite. These electrons will return to the spacecraft within a few microseconds having displacements of millimeters to centimeters due to the ambient electric (and in some cases magnetic) field, independent of any electric potentials that may have been produced by solar radiation or the interaction of the spacecraft with the local plasma environment.

Numerical simulations of MHD dynamos

The generation of magnetic fields in space plasmas and in astrophysics is usually described within the framework of magnetohydrodynamics. Turbulent helical flows produce magnetic fields very efficiently, with correlation length scales larger than those characterizing the flow. Within the context of the solar magnetic cycle, a turbulent dynamo is responsible for the so-called alpha effect, while the Omega effect is associated to the differential rotation of the Sun. We present direct numerical simulations of turbulent magnetohydrodynamic dynamos including two-fluid effects such as the Hall current. More specifically, we study the evolution of an initially weak and small-scale magnetic field in a system maintained in a stationary regime of hydrodynamic turbulence, and explore the conditions for exponential growth of the magnetic energy. In all the cases considered, we find that the dynamo saturates at the equipartition level between kinetic and magnetic energy, and the total energy reaches a Kolmogorov power spectrum.

On Gravity Induced Electric Field in Space Plasmas

An analysis of ambipolar, gravity-induced, electric field in ordinary and dusty space plasmas is presented. It is shown that the ambipolar field modifies density distribution of light ions in such a way that, in a part of space occupied by plasma, it increases with the decrease of the gravity field. The presence of dust changes the characteristic scale length and the amplitude of the ambipolar field by several orders of magnitude.

Trapping of Solar Energetic Particles by the Small-Scale Topology of Solar Wind Turbulence

The transport of energetic particles perpendicular to the mean magnetic field in space plasmas has long been viewed as a diffusive process. However, there is an apparent conflict between recent observations of solar energetic particles (SEPs): (1) Impulsive solar flares can exhibit dropouts in which the SEP intensity near Earth repeatedly disappears and reappears, indicating a filamentary distribution of SEPs and little diffusion across these boundaries. (2) Observations by the IMP-8 and Ulysses spacecraft, while they were on opposite sides of the Sun, showed similar time-intensity profiles for many SEP events, indicating a rapid lateral diffusion of particles throughout the inner solar system within a few days. We explain these seemingly contradictory observations using a theoretical model, supported by computer simulations, in which many particles are temporarily trapped within topological structures in statistically homogeneous magnetic turbulence and ultimately escape to diffuse at a much faster rate.

Calculation of the electric field in space plasma near a conducting sphere injecting an electron beam

The problem is considered of determining the electric field induced in the vicinity of a conducting spherical body that is at rest in a collisionless plasma and at the surface of which there is a prescribed sink of negative charge. The problem is solved for the general relativistic case under the assumption that the electron velocity in the neutralizing current is comparable with the speed of light. An integrodifferential equation is derived that describes the radial behavior of the electric field potential in the vicinity of the injector. A simplified method for determining the potential in the perturbed region is developed. The method implies that the problems of the potentials in a space charge region of radius R* (with a prescribed boundary potential ϕ*) adjacent the body and in the outer region r>R* are solved separately and then the solutions obtained are matched at boundary between these regions.

Debye-Scale Plasma Structures Associated with Magnetic-Field-Aligned Electric Fields

We report a new type of spatially coherent plasma structure that is associated with quasistatic, magnetic-field-aligned electric fields in space plasmas. The solitary structures form in a magnetized plasma, are multidimensional, and are highly supersonic. The size along ${\mathbf{B}}_{0}$ is a few ${\ensuremath{\lambda}}_{D}$ and increases with increasing amplitude, unlike a classical soliton. The perpendicular size appears to be influenced by ion motion. We show that the structures facilitate ion-electron momentum exchange and suggest that an aggregate of structures may play a role supporting large-scale, parallel electric fields.

The application of the generalized Capon method to the analysis of a turbulent field in space plasma : experimental constraints

A high-resolution method is used to characterize homogeneous and stationary electromagnetic turbulence in space plasma by the field-energy distribution (FED) that specifies how the field energy is distributed with regards to the angular frequency ω and to the wave vector k. It is based on the simultaneous measurement of several field components at several points in space. The analysis of synthetical data have made it possible to study the effects of experimental constraints for multisatellite missions such as CLUSTER on the field-energy distribution estimation. The constraints tested are: the geometry of the satellite configurations, the measured field components, the accuracy in the measurements (distance between satellites, time synchronization).

Space physics program

The National Aeronautics and Space Administration solicits proposals for Fiscal Year 1990 for the support of basic research in the general area of space physics, including cosmic and heliospheric physics, solar physics, magnetospheric physics, and ionospheric physics.The Space Physics Division of NASA supports investigations of the origin, evolution, and interactions of particles and electromagnetic fields in space plasmas. Focus of their studies is on interactions between solar wind and solar system bodies, including studies of the ionosphere and magnetospheres of the Earth, other planets, comets, primordial bodies, and of magnetosphere‐ionosphere‐atmosphere interactions; on the Sun as a star and source of solar system energy, plasma, and energetic particles; and on the origin, acceleration, and transport of solar and galactic cosmic rays.

Plasma Instabilities Generated by Streaming Particles

It is well known that particles streaming along the ambient magnetic field in space plasmas may generate waves with frequencies of the order of the local ion gyrofrequency (ion waves), (Kindel and Kennel 1971). In this study we analyze the dispersion relation of these waves numerically and discuss mechanisms for damping and instability. All numerical results in this report are obtained with the computer code WHAMP (Rönnmark 1982a,b), which solves the dispersion relation of linear waves in a homogeneous plasma for a complex frequency as a function of a real wavevector. As a specific example we consider the S3-3 satellite observations of banded electrostatic ion waves, associated with streaming particles (Kintner et. al. 1979, Cattell 1981).