Earth's Electron Research Articles

On the amplitude of intense Langmuir waves in the terrestrial electron foreshock

Waveforms of large‐amplitude Langmuir oscillations were recorded by the Wind spacecraft in the Earth's upstream electron foreshock region. We present some statistics of the waveforms and discuss them in the context of various saturation mechanisms. In particular, it is found that the value of Epeak/Erms is not large, as previously suggested, and that the largest‐amplitude Langmuir waveforms are generally somewhat sinusoidal and lack structure on small spatial scales. The measured probability distribution of electric field amplitude and dimensionless energy suggest that some stochastic process may play a role in wave generation. The values of dimensionless energy needed to arrest Langmuir wave collapse occur with very small probability and the value of Epeak/Erms for large fields suggests that, statistically, Langmuir wave collapse is not an important process in the terrestrial foreshock.

Magnetic storm modeling in the Earth's electron belt by the Salammbô code

The Salammbô code, which solves the three‐dimensional phase‐space diffusion equation for the electron radiation belt, was used to explain the dynamic conditions present during a geomagnetic storm. With a simple injection model, we have characterized the dynamic behavior for relativistic electrons in the outer belt. The particles in the range 100 keV – 500 keV are diffused throughout the belt, with a shape essentially dependent on the radial diffusion coefficients. Particles with higher energies are “created” by acceleration of slower particles near the plasmapause location. The calculated shape of the fluxes in an L versus time grid for the 1 MeV electrons looks globally like those measured aboard the CRRES satellite.

Spatial distribution of electron plasma oscillations in the Earth's foreshock at ISEE 3

Electric field oscillations recorded by the 10–56 kHz channels of TRW's plasma wave detector during parts of two of the ISEE 3 circumterrestrial orbits in 1983 have been used to make the first mapping of Earth's electron plasma wave foreshock. By combining data from the two trajectory segments, each of which provided relatively meager spatial sampling outside the bow shock, but high variation of interplanetary magnetic field (IMF) direction, a first‐order pattern of occurrence of electron plasma waves, hence also backstreaming electrons, has been determined. We depict the pattern with an adaptation of the mapping program previously used for the Venus electron foreshock. As at Venus, plasma wave activity was concentrated most densely along the IMF line tangent to the bow shock where energized electrons stream against the solar wind from the quasi‐perpendicular part of the shock. The size of the Earth's electron plasma wave foreshock, however, is vastly greater than that of Venus, implying that a foreshock's dimension scales with the size of its diamagnetic obstacle and associated bow shock. Our mappings with three additional ISEE 3 channels surrounding the local electron plasma frequency indicate a richer distribution of waves in the foreshock than the single electron frequency channel of Pioneer Venus Orbiter could detect around Venus.

Radiation-induced voltage on spacecraft internal surfaces

The charging of the surfaces of insulators beneath the thermal control blankets on spacecraft has been numerically simulated. Such insulators are found on wiring harnesses, bulkhead feed-throughs, component mounting hardware, tiewraps, thermal sensors, etc. These insulators are shielded from the low-energy space plasma by the grounded solar blankets. It is determined that the surfaces of these insulators rise to many tens of kV in the Earth's electron belts unless sufficient conduction to the spacecraft frame is established. Secondary electron emission is not sufficient to hold the potential to low values. The dependence of charging on insulator thickness, electron energy spectra, and material properties is investigated. This phenomenon is posited as an explanation for the anomalies seen on many spacecraft, and on the CRRES (Combined Release and Radiation Effects Satellite) in particular.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Observations of electron plasma waves upstream of the Jovian bow shock

The Ulysses spacecraft encountered the planet Jupiter in February 1992, on its journey towards high heliospheric latitude. During the approach to the planet, as well as on the outbound pass, while receding from the Jovian bow shock, the Plasma Frequency Receiver that is part of the Unified Radio and Plasma Wave experiment (URAP) recorded bursts of plasma waves in the frequency range of a few kHz. These emissions, first observed by the PWS experiment onboard the Voyager spacecraft, have been identified as upstream electron plasma waves. In this paper, we present the first analysis of the characteristics of these emissions, which are very similar to those found in the Earth's electron foreshock, upstream of the Earth's bow shock. These bursty emissions, with a peak frequency very close to the local electron plasma frequency F pe, have a typical electric field amplitude in the range 0.01–0.1 mV m −1, with some bursts above 1 mV m −1. The frequency bandwidth over which significant power can be found above the instrument background noise ranges from below 0.2 F pc to about 2 F pc. On the basis of our present knowledge of similar emissions observed at Earth, we suggest that the broadband emissions are triggered by suprathermal (a few tens of eV) electrons, streaming back from Jupiter's bow shock.

The outer boundary of the Earth's electron radiation belt: Dependence upon L, energy, and equatorial pitch angle

From near‐equatorial pitch angle measurements made with fine angular resolution on the CRRES and SCATHA satellites an investigation has been made of the location of the nightside outer boundary of the Earth's electron radiation belt and its dependence upon energy and mirror point altitude. The energy versus L dependence of the position of the trapping boundary is important for comparisons with models of the mechanisms to explain the loss of trapping at the outer edge of the radiation belt. Often the satellites passed through the trapping boundary very slowly, permitting detailed fine‐scale measurements. With fine spatial resolution the cutoff L values were frequently found to be almost independent of energy for E ≤ 300 keV and were nearly independent of equatorial pitch angle up to 30°. In the radiation belt near the outer edge sometimes the flux dropped to only intermediate values or to background level just for short times or equivalently narrow L intervals, suggesting time variations or the irregular onset of the mechanism(s) responsible for the trapping boundary.

Electron density in the Martian foreshock as a by‐product of the electron plasma oscillation observations

Electron plasma oscillations have been observed in the solar wind upstream of the Martian bow shock by the plasma wave system (PWS) on the Phobos 2 spacecraft. These waves occurred when the interplanetary magnetic field line through the spacecraft position was connected to the shock, i.e., when the spacecraft went through the electron foreshock region in which suprathermal electrons accelerated at the bow shock were streaming back from it. The PWS observations are compared to the expectations one might have in light of the results obtained in the Earth's electron foreshock, which are briefly reviewed. The spectrum of the electron plasma oscillations peaks at or near the ambient solar wind plasma frequency. We then have an estimate of the solar wind plasma density upstream of the Martian bow shock. The 12‐min averages of the plasma density are shown to vary between less than 0.5 cm−3 and 19 cm−3 from February 1 to March 26, 1989. This corresponds to an electron plasma frequency range from 6.4 to 39 kHz. Moreover, some structures were observed in the electron foreshock region of Mars. They are presented and tentatively interpreted.

Strong plasma turbulence in the Earth's electron foreshock

A quantitative model is developed to account for the distribution in magnitude and location of the intense plasma waves observed in the Earth's electron foreshock given the observed rms levels of waves. In this model, nonlinear strong‐turbulence effects cause solitonlike coherent wave packets to form and decouple from incoherent background beam‐excited weak turbulence, after which they convect downstream with the solar wind while collapsing to scales as short as 100 m and fields as high as 2 V m−1. The existence of waves with energy densities above the strong‐turbulence wave‐collapse threshold is inferred from observations from IMP 6 and ISEE 1 and quantitative agreement is found between the predicted distribution of fields in an ensemble of such wave packets and the actual field distribution observed in situ by IMP 6. Predictions for the polarization of plasma waves and the bandwidth of ion‐sound waves are also consistent with the observations. It is shown that strong‐turbulence effects must be incorporated in any comprehensive theory of the propagation and evolution of electron beams in the foreshock. Previous arguments against the existence of strong turbulence in the foreshock are refuted.

Trapping saturation of the bump‐on‐tail instability and electrostatic harmonic excitation in Earth's foreshock

Trapping saturation of the bump‐on‐tail instability is discussed using electron plasma Vlasov simulation results. The role of electrostatic harmonic excitation is considered in detail and shown to play a decisive role in the saturation of the instability. An extensive discussion of the simulation results is given to show that the results are not significantly limited by the finite number of Fourier modes used nor by the discrete distribution of those modes in wave number. It is argued that in the leading edge of Earth's electron foreshock a narrow wave number band of unstable field modes leads to trapping saturation of the bump‐on‐tail instability while simultaneously exciting electrostatic plasma waves at harmonics of the plasma frequency in similar narrow bands of shorter wavelengths. The argument is based (1) on the observations of Lacombe et al. (1985), who found intense plasma waves at the leading edge of the foreshock with a spectral distribution sufficiently narrow to trap particles in resonance with the waves, and (2) on numerical simulations of the foreshock electron plasma which indicate that trapping saturation of the bump‐on‐tail instability leads to phase space vortex formation with consequent excitation of electrostatic harmonics. Thus it is suggested that observations of electrostatic harmonics in the leading edge of the foreshock would strongly implicate trapping as the saturation mechanism for the bump‐on‐tail instability in that region.

On the persistence of unstable bump‐on‐tail electron velocity distributions in the Earth's foreshock

Further evidence for the persistence of bump‐on‐tail unstable reduced velocity distributions in the Earth's electron foreshock is presented. This persistence contradicts our understanding of quasi‐linear saturation of the bump‐on‐tail instability; the distributions should be stabilized through velocity space diffusion too quickly to allow an observation of their unstable form. A modified theory for the saturation of the bump‐on‐tail instability in the Earth's foreshock is proposed and examined using numerical simulation and quasi‐linear theoretical techniques. It is argued that the mechanism due to Filbert and Kellogg and to Cairns which is responsible for the creation of the bump‐on‐tail velocity distribution in the foreshock is still operative during the evolution of the bump‐on‐tail instability. The saturated state of the plasma must represent a balance between this creation mechanism and velocity space diffusion; the saturated state is not determined by velocity space diffusion alone. Thus the velocity distribution of the saturated state may still appear bump‐on‐tail unstable to standard linear analysis which does not take the creation mechanism into account. The bump‐on‐tail velocity distributions observed in the foreshock would then represent the state of the plasma after saturation of the bump‐on‐tail instability, not before. Numerical simulations of two types are discussed. In the first, the particle distribution function and the field are assumed periodic over a spatial interval of several tens or hundreds of Debye lengths along the local magnetic field. In the second, nonperiodic boundary conditions are imposed on the distribution function to model the spatial gradients which are responsible for the creation of the bump‐on‐tail unstable distribution. This mechanism for creating the unstable distribution continues to drive the particle distribution unstable during the evolution of the bump‐on‐tail instability. The saturated velocity distributions of the periodic and nonperiodic simulated plasmas are compared. At saturation the periodic velocity distribution exhibits plateau formation while the nonperiodic distribution does not. A new quasi‐linear theory for the space‐averaged velocity distribution function is constructed. This theory includes the effects of nonperiodic boundary conditions; it can be used to compare the evolution of periodic and nonperiodic bump‐on‐tail unstable particle distributions. In addition, this theory is based on the filtered Vlasov‐Maxwell system of equations; velocity space filamentation has been removed from the distribution function. It is shown that this theory is applicable on a finite spatial interval within which the field has been expressed in terms of a discrete Fourier mode expansion. Thus direct comparisons between theory and simulation are possible. Some predictions of this new quasi‐linear theory are compared favorably to the results of the bump‐on‐tail simulations.

Electron plasma waves in the solar wind: AMPTE/IRM and UKS observations

Selected events of plasma wave and electromagnetic emissions in the earth's electron fore-shock region have been studied. Strong emissions are observed in the plasma-wave band when the site of the satellite is magnetically connected to the bow shock. These emissions are generally highly fluctuating. Under certain conditions one observes electromagnetic radiation at the second harmonic produced locally. Electromagnetic emission generated at a position far away from the site of the spacecraft is occasionally detected giving rise to remote sensing of the bow shock. These emissions are related to energetic electron fluxes.

The downshift of electron plasma oscillations in the electron foreshock region

Electron plasma oscillations in the earth's electron foreshock region are observed to shift above and below the local electron plasma frequency. As plasma oscillations shift downward from the plasma frequency, their bandwidth increases and their wavelength decreases. Observations of plasma oscillations well below the plasma frequency are correlated with times when ISEE 1 is far downstream of the electron foreshock boundary. Although wavelengths of plasma oscillations below the plasma frequency satisfy kλDe ≈ 1, the Doppler shift due to the motion of the solar wind is not sufficient to produce the observed frequency shifts. A beam‐plasma interaction with beam velocities on the order of the electron thermal velocity is suggested as an explanation for plasma oscillations above and below the plasma frequency. Frequency, bandwidth, and wavelength changes predicted from the beam‐plasma interaction are in good agreement with the observed characteristics of plasma oscillations in the foreshock region.

Plasma electron signature of magnetic connection to the Earth's bow shock: ISEE 3

Enhanced fluxes of low‐energy electrons backstreaming from the earth's bow shock have been identified at ISEE 3. When present, these fluxes modify ambient solar wind electron velocity distributions f(υ) in characteristic ways that depends on whether ISEE 3 is near the edge of, or within the interior of the earth's electron foreshock. Near the edge, energy peaks in f(υ) are observed. Such distributions should be locally unstable to electron plasma oscillations. Well within the interior of the foreshock, enhanced fluxes of electrons with energies up to the maximum detected by the Los Alamos electron analyzer (∼1 keV) are observed over the full backward hemisphere. These electrons can be modelled with an asymptotic power law distribution having index in the range 4 ≲ pbs ≲ 6. At intermediate energies (∼20–50 eV), twin angular peaks are observed centered on the magnetic field direction . Also observed at these times are depressions in f(υ) at energies less than ∼20 eV that are centered on . Such distributions having a perpendicular temperature greater than their parallel temperature may be locally unstable to the generation of whistler waves. Analysis of a particularly clean example of connection to the bow shock is consistent with the possibility that the observed electron fluxes emerge from the forward foot of the electron heating region within the bow shock where the electron density and temperature are larger than that of the unperturbed upstream solar wind by a factor of ∼1.2. This analysis also indicates that the electrostatic potential within the forward foot of the shock is between ∼5 and 50 V more positive than that within plasma far upstream at ISEE 3. However, these interpretations depend on the assumption of nearly scatter‐free propagation, which may not hold. Using parameters derived from model fits to measured velocity distributions, we estimate ∼10−4 of the incident solar wind electrons are accelerated by the bow shock back into the upstream region with energies greater than 1 keV.

Ionospheric Diagnostics Using Resonances of an Electric Dipole Impedance

A technique of using the driving‐point impedance of an electric antenna in the ionosphere to diagnose the ionospheric plasma is presented. Specifically, the locations of the resonances in the driving‐point impedance of a center‐fed cylindrical electric dipole imbedded in the ionosphere are analyzed. The ionospheric model is a lossy, anisotropic, homogeneous neutral plasma containing various ionic constituents. It is found that there are zeros in the reactance at the upper and lower hybrid frequencies and poles at the electron and ion gyrofrequencies. These critical frequencies are independent of antenna length and orientation (parallel or perpendicular to the earth's magnetic field) to the first‐order approximation. It is shown that pole‐zero frequencies could be used to determine the earth's magnetic field strength, electron density, and ion masses and densities at any point in the ionosphere at which the impedance was measured. Consequently, this technique offers the possibility of measuring several ionospheric plasma characteristics with one instrument aboard rockets and satellites.

Exact Ray‐Path Solutions in a Quasi‐Linear Ionosphere

Exact ray solutions are helpful in determining the accuracy of general purpose ray‐tracing techniques. In this paper such solutions are presented for range, group path, phase path and reflection height of radio rays entering the ionospheric layer from the bottom and from the top. The ionospheric model used to produce the exact solutions is a function which allows closed‐form integration of the ray parameter expressions. In this ionosphere, first introduced by Muldrew, electron density is almost a linear function of height, but it does not vary in the horizontal direction. The earth's magnetic field and electron collisions are neglected but curved earth‐ionosphere geometry is included.

Refraction of short radio waves in the upper atmosphere

The paper shows that the striking phenomena of short-wave radio transmission (i.e., below 60 meters) can be quantitatively accounted for on a simple electron refraction theory in which the effect of the earth's magnetic field and electron collisions may be neglected as a first approximation. The distribution and number of electrons per unit volume in the upper atmosphere required on this theory to account for the meager experimental data appear to be in general accord with the values required in the explanation of the diurnal variations of the earth's magnetic field, aurorae and long-wave radio transmission. The paths taken by the waves from an antenna to distant points on the surface of the earth are calculated. The path calculations give a definite picture of the now familiar skip distance effects. Ideal signal intensity curves (i.e., neglecting absorption and scattering) are given, which show how the energy sent out by a transmitter is distributed over the surface of the earth. A focussing of energy just beyond the skip distance, and again just inside the point where the ray tangent to the ground at the transmitter comes back to earth is clearly shown. The reflection of waves at the surface of the earth is also considered. The results of these calculations make it possible to estimate the most suitable wave lengths for night and day communication between any two points on the earth's surface. It is also pointed out that there will be a minimum wave length, in the vicinity of 10 meters, below which long distance communication becomes impossible. It is shown that from the point of view of long distance communication low angle radiation is most effective. The ray paths and energy flux density in the wave front of the sky waves are independent of the plane of polarization of the transmitter. The effects of polarization on the reception problem are not discussed.

Refraction of Short Radio Waves in the Upper Atmosphere

The paper shows that the striking phenomena of short-wave radio transmission (i. e., below 60 meters) can be quantitatively accounted for on a simnple electron refraction theory in which the effect of the earth's magnetic field and electron collisions may be neglected as a first approximation. The distribution and number of electrons per unit volume in the upper atmosphere required on this theory to account for the meager experimental data appear to be in general accord with the values required in the explanation of the diurnal variations of the earth's magnetic field, aurorae and long-wave radio transmission. The paths taken by the waves from an antenna to distant points on the surface of the earth are calculated. The path calculations give a definite picture of the now familiar skip distance effects. Ideal signal intensity curves (i. e., neglecting absorption and scattering) are given, which show how the energy sent out by a transmitter is distributed over the surface of the earth. A focussing of energy just beyond the skip distance, and again just inside the point where the ray tangent to the ground at the transmitter comes back to earth, is clearly shown. The reflection of waves at the surface of the earth is also considered. The results of these calculations make it possible to estimate the most sutitable wave lengths for night and day communication between any two points on the earth's surface.