Electron Foreshock Research Articles

Generation of downshifted oscillations in the electron foreshock: A loss‐cone instability

Measurements performed aboard Cluster spacecraft near Earth's bow shock on 24 January 2001 provide convincing evidence of a loss‐cone feature within the electron foreshock region. This feature is formed by suprathermal electrons with energies 15–45 eV and pitch angles 130°–150° and is always accompanied by electrostatic waves with frequencies well below the local plasma frequency. An instability analysis shows that these downshifted oscillations can result from a loss‐cone instability of electron cyclotron modes rather than from the beam instability as previously suggested.

Planetary foreshock radio emissions

The electron foreshock regions upstream of Earth's bow shock and upstream of traveling interplanetary shocks are known to be propitious sites for a variety of energetic particle and plasma wave phenomena, including radio emissions. A quantitative theoretical model has been developed for radio emissions associated with the terrestrial foreshock and for type II radio bursts associated with interplanetary shocks. Here, we generalize this model and apply it to other planetary foreshocks. We present predictions for the levels of planetary foreshock radio emissions and compare these with observations by past and present space missions. One key result is that Mercury can be a strong source of foreshock radio emissions, and this prediction may be testable with the anticipated BepiColombo space mission. Although the terrestrial foreshock radio emissions are the most detectable with existing instruments, our results predict that they are the second strongest in absolute terms, following the Jovian foreshock emissions. Indeed, we predict that the radio instrument on board Ulysses should have detected Jovian foreshock radio emissions, and we suggest that there is some evidence in the data to support this. We also suggest that Cassini was potentially capable of detecting foreshock emissions from Venus during its gravity‐assist flybys and may possibly be capable of detecting foreshock emissions from Saturn under favorable solar wind conditions.

The Martian Electron Foreshock, Based on Mars Global Surveyor's Measurements

The Martian electron foreshock position and shape have been revealed using electron flux data obtained by the Electron Reflectometer (ER) experiment on board Mars Global Surveyor, in the time period between days 87 and 118 of 1998. From the electron flux measurements, 90 inbound and outbound electron foreshock crossings were detected. Apart from their spatial distribution, their temporal variability is also studied and investigated for possible correlation with the solar wind conditions, the solar radiation flux, and especially the interplanetary magnetic field configuration at the time. The outer boundary of the ion foreshock is identified as the inner limit of the electron foreshock. The energy distribution of the foreshock electrons is also discussed.

High order spectral analysis of electron plasma oscillations in the electron foreshock

The electron foreshock is the region in which electrons that have been reflected from the shock propagate back upstream along the magnetic field lines forming electron beams. The process of relaxation of these beams is one of the fundamental topics in plasma physics. It involves numerous nonlinear processes. High order spectral analysis is applied to Cluster measurements to assess the strength of various nonlinear processes involved in the dynamics of the turbulence.

Oblique “1‐Hz” whistler mode waves in an electron foreshock: The Cassini near‐Earth encounter

The Cassini spacecraft flew toward the Earth in a trajectory almost along the Sun‐Earth line, giving a unique perspective of low‐frequency plasma waves in the Earth's foreshock. At the time of the fly‐through, the angle between the magnetic field and Sun‐Earth direction was ∼70°. For this orientation, only the most energetic (≥12 keV) ions could reach the upstream region, thus Cassini passed through an almost “pure” electron foreshock. We demonstrate the presence of nonlinear foreshock “1‐Hz” whistler‐mode waves and their properties. Power spectra demonstrate that the waves span a frequency range from ∼0.3 to ∼10 Hz with a center frequency of ∼2 Hz. Close to the bow shock the waves had peak‐to‐peak transverse amplitudes as large as ∼14 nT in a 35‐nT ambient field. The waves were compressive with a maximum value of Δ|B|/B0 ≈ 0.25. The waves were propagating primarily along B0. Of the waves, 82% were right‐hand‐polarized, 9% were linearly polarized, and 9% were left‐hand‐polarized (in the spacecraft frame). The wave amplitudes were largest near the bow shock and decreased with increasing distance in the upstream direction. Further from the shock the waves are determined to be propagating obliquely to B0 (θkB≥20°). There is an equal mix of right‐hand, left‐hand, and linearly polarized (in the spacecraft frame) waves. Left‐hand‐polarized waves propagating parallel to B0 were not detected. Wave power changed dramatically as the interplanetary magnetic field rotated slightly. Arguments are presented for why low‐energy (tens of eV) electrons are responsible for wave generation, rather than cyclotron resonant energetic (>13 keV) electrons.

One‐ and two‐dimensional simulations of electron beam instability: Generation of electrostatic and electromagnetic 2fp waves

We have performed computer simulations of the self‐consistent nonlinear evolution of electrostatic and electromagnetic 2fp waves excited by electron beams with electromagnetic particle code. In both one‐ and two‐dimensional periodic systems an electrostatic 2fp wave is generated at twice the wave number of forward propagating Langmuir waves by wave‐wave coupling. This wave grows with the forward propagating Langmuir wave in the nonlinear stage of the simulations. The electrostatic 2fp wave in the simulations is saturated at about −20 ∼ −30 dB of that of the Langmuir waves. It is larger than the value expected from observations in the terrestrial electron foreshock. The electromagnetic 2fp wave is only excited in two‐dimensional systems. The magnitude of the electromagnetic 2fp wave is correlated with the backward propagating Langmuir wave, not with the electrostatic 2fp wave. This result suggests that the electromagnetic 2fp wave is excited by the wave‐wave coupling of forward and backward propagating Langmuir waves. The typical power density estimated from a reasonable amplitude of Langmuir wave is of the same order or much weaker than the value typically observed around the electron foreshock.

Two‐dimensional simulations of a curved shock: Self‐consistent formation of the electron foreshock

A collisionless curved shock is analyzed in a supercritical regime with the help of a two‐dimensional electromagnetic full particle code. Curvature effects are included self‐consistently and allow one to follow continuously the transition from a narrow and step‐like strictly perpendicular shock to a wider and more turbulent oblique shock within the quasi‐perpendicular range 65° < θBn < 90°. Present results reproduce the formation of the electron foreshock without any simplifying assumptions. In agreement with experimental data, local bump‐on‐tail parallel distribution functions are well recovered in the foreshock region and correspond to electrons backstreaming along the magnetic field lines. Present detailed analysis shows that local back‐streaming distributions have two components: (i) a high parallel energy component corresponding to back‐streaming electrons characterized by a field‐aligned bump‐in‐tail or beam signature, and (ii) a low‐energy parallel component characterized by a loss cone signature (mirrored electron). Two types of bump‐in‐tail patterns, broad and narrow, are identified at short and large distances from the curved shock, respectively, and are due to different contributions of these two components according to the local impact of the time‐of‐flight effects. Present results allow one to identify more clearly the nature of the bump‐in‐tail pattern evidenced experimentally (narrow type). These also confirm that mirroring electrons make the dominant contribution to the bump‐in‐tail pattern in the total distribution in agreement with previous studies. Results suggest that low and high parallel energy populations are intimately related and may contribute together to the upstream wave turbulence.

Excitation of electron acoustic waves near the electron plasma frequency and at twice the plasma frequency

In this paper we investigate the nonlinear development of the electron acoustic instability that can lead to the transfer of wave energy to frequencies just above the electron plasma frequency (ωpe) and to waves with approximately twice the electron plasma frequency (2ωpe). Using plasma conditions in the upstream electron foreshock region based on data from the AMPTE‐UKS spacecraft, an electron beam is considered in plasma containing a background of hot and cold electrons. This leads to the linear excitation of large‐amplitude electron acoustic waves at frequencies between about 0.8 and 1.0 ωpe. A modified decay instability then excites waves in the spectrum just above ωpe. This is followed by a second nonlinear coalescence process that causes the excitation of waves at frequencies just below 2ωpe. The linear and nonlinear properties of the electron acoustic instability are examined for observed conditions using analytical theory, particle‐in‐cell simulations, and Vlasov simulations. These results have application to observations made inside the electron foreshock region, as well as the polar cap and auroral zone, where plasma oscillations and waves at 2ωpe are observed.

Statistical studies of plasma waves and backstreaming electrons in the terrestrial electron foreshock observed by Geotail

We present statistical studies of the direction finding of 2ƒp radiation and the spatial distribution of plasma waves and energetic particles in the terrestrial electron foreshock observed by Geotail. First, we investigate the geometry of the electron foreshock which is assumed to be the “2ƒp radio source.” The 2ƒp radio source is likely to be in the leading region of the electron foreshock where the most intense Langmuir waves are observed. The Langmuir wave activities and the population of energetic electrons gradually decrease in the region beyond 10 RE from the contact point. The decreasing rate of Langmuir wave activity is very small, about 10−0.008/RE. We also find that in the region around the contact point of the tangential interplanetary magnetic field (IMF) lines and the bow shock, the observed amplitude of the 2ƒp radiation seems to become weak. We think that it is due to the weak activities of Langmuir waves in the region close to the contact point and/or the directivity of 2ƒp radiation along the tangential IMF line. Next, we investigate the influence of the solar wind conditions on the activities in the electron foreshock. We confirm a positive correlation of the 2ƒp radio activity with the solar wind kinetic energy flux and a decrease of 2ƒp radio activity with decreasing IMF cone angle resulting in IMF lines tangent to the far flank of the bow shock. The 2ƒp radio activity is more affected by both parameters than the amplitude of Langmuir waves is affected. This suggests that the 2ƒp radio emissivity is very sensitive to the energy of Langmuir waves as expected from the generation process of 2ƒp radiation. Such high sensitivity also supports the concentration of the radio emissivity in the leading region of the electron foreshock and the limitation of the radio source extension along the magnetic field line. We also reinvestigate the comparison between the terrestrial and Venusian foreshocks. The differences between them suggest the nonsimilarity of shocks with different sizes.

Electron plasma waves beyond the Mars' bow shock: The PWS/Phobos-2 observations

Abstract Electron plasma waves are commonly observed at or near the ambient solar wind plasma frequencies beyond planetary bow shocks. They are correlated with times when the interplanetary magnetic field is connected to the shock front, i.e., when entering the electron foreshock region in which suprathermal electrons accelerated at the bow shock are streaming back from it. Such electron plasma oscillations have been observed upstream of the Martian bow shock by the plasma wave system, PWS, on the Phobos 2 spacecraft. Phobos 2 has been operational in Martian orbits for almost two months. From January 29 to March 27, 1989, the Martian shock was crossed at least a hundred times and the upstream region extensively probed. The aim of this paper is to present the characteristics of the Martian electron plasma waves and to compare them with the Earth's and Venus'ones.

Evidence for acceleration of ions to ∼ 1 Mev by adiabatic‐like reflection at the quasi‐perpendicular Earth's bow shock

On December 6, 1994, during a CIR (Corotating Interaction Region) event, the WIND 3D‐Plasma and energetic particle experiment observed a burst (≤24 s) of 238–676 keV protons close to the electron foreshock boundary, followed by a 156–236 keV, 101–156 keV and 33–101 keV proton bursts about ∼40 s, 65 s and 85 s later, respectively. Similar dispersed bursts of helium with energies between ∼50 keV and ∼1 MeV followed the proton bursts. During this time, the IMF direction varied slowly with an almost monotonic decrease in θBn. The proton energy spectrum is initially peaked at ∼350 keV, and progresses to lower energies with time. The proton 3D angular distributions are peaked at ∼30° pitch‐angle, propagating away from the shock, but they are nongyrotropic. Finally, we show that the proton energy‐spectra as well as the pitch angle distribution agree quantitatively with a model of a single adiabatic reflection of the incident energetic interplanetary ions by the quasi‐perpendicular shock.

The source region of an interplanetary type II radio burst

We present the first observation of the source region of an interplanetary type II radio burst, using instruments on the Wind spacecraft. Type II radio emission tracks the motion of a CME‐driven interplanetary (IP) shock which encounters the spacecraft. Upstream of the IP shock backstreaming electrons are observed, first antiparallel to the interplanetary magnetic field (IMF), and then later parallel as well. Langmuir waves are observed concomitant with the shock‐accelerated electrons. The electron energy spectrum and Langmuir wave amplitudes are very similar to those observed in the terrestrial electron foreshock. From the connection times to the shock, we infer the existence and characteristic size of large scale structure on the shock front. The type II radio emission seems to be generated in a small bay upstream of the shock, and this may account for some splitting structure observed in the frequency spectrum of many type II bursts.

Electron temperature and density of magnetospheric plasma from GEOTAIL spacecraft potentials

The spacecraft potential is an index giving the density and temperature of electrons surrounding spacecraft, as understood from the single probe theory. We investigate the correlation between electron density derived from the plasma wave and spacecraft potential observations of the GEOTAIL spacecraft in the solar wind and magnetosphere. We have obtained the good correlation between them within the voltage range of spacecraft potential less than 35V which may mean that electron density is in the range above 0.02 cm −3. We could estimate the electron temperature in the regions of solar wind and electron foreshock, and we found the cases that the electron temperature is higher when the Langmuir wave appears than when no Langmuir wave appears. Then, accumulating many observations of electron density along the GEOTAIL trajectory, we obtained a map showing the global plasma population, in which the bowshock and magnetopause positions are found to be nearly same with the their average positions during the magnetically quiet times.

Full curvature effects of a collisionless shock

Curvature effects of a quasi-perpendicular collisionless shock in a super-critical regime are studied with the help of a 2 - D full particle electromagnetic simulation code, where both ions and electrons are treated as particles moving within self-consistent fields. The curved shock is initiated by using a cylindrical magnetic piston. Main characteristics of quasi-perpendicular collisionless shocks in the angular range 90° to 65° are fully recovered. Full curvature effects allow to follow continously the transition from a narrow and step-like strictly perpendicular shock to wider and more turbulent oblique shock waves. Present preliminary results emphasize the differences between planar and curved shock. For the first time to our knowledge, the formation of electron foreshock is fully recovered without any simplifying assumptions; moreover, loss cone distribution is also reproduced for high energy reflected electrons which is the signature of a dominant fast Fermi acceleration type process.

Plasma waves in the Earth's electron foreshock: 1. Time‐of‐flight electron distributions in a generalized Lorentzian plasma and dispersion solutions

The correlated studies discussed here used high‐quality and frequent in situ measurements provided by the Wind spacecraft to investigate the properties of the foreshock plasma (measured by Wind/3DPlasma) and the enhanced plasma waves (detected by Wind/WAVES). A time‐of‐flight electron distribution model was developed in a generalized Lorentzian plasma using experimental data. The Wind/3DP results show that typical foreshock electron distributions are well approximated by this model, although discernible “bumps” in distributions that may be unstable are not often observed or resolved by the 3DP instrument. We used unstable distributions described by the model to initialize our study. In addition to verifying the interdependence of dispersion, cutoff velocity and beam density that was previously found in a Maxwellian beam‐plasma system, our dispersion solutions demonstrate how the dispersion topology and wave spectral bandwidths evolve as a consequence of wave modifications of time‐of‐flight distributions. By identifying sections of distributions that interact with each of the growing and damping branches, we analyzed the role played by nonthermal electrons in the evolution of the instability. Our results complement those from previous investigations and, more importantly, provide information about the linear behavior of the system that will enable the simulation studies discussed in our companion paper [Yin et al., this issue] to examine possible nonlinear interactions and to address the issue of whether nonlinear wave‐wave interactions in the foreshock nonthermal plasma are a significant factor in producing the observed electromagnetic emissions.

Bursts of whistler mode waves in the upstream of the bow shock: Geotail observations

Bursts of narrowband and short‐lived electromagnetic waves are frequently observed in the upstream (the electron foreshock and the pure solar wind) by the waveform capture instrument on board Geotail satellite. The electromagnetic waves with frequencies above 10 Hz are all right‐hand circularly polarized and identified as whistler mode waves. We call the waves narrowband and short‐lived whistlers (NSW). Nearly all the NSW in the electron foreshock propagate in a downstream direction parallel to the ambient magnetic field (Bo) with an average θkB of 16°, where θkB is an angle between NSW wave vector and the Bo. Their frequencies cover a range from 0.05 to 1.0Ωe with an average of 0.35Ωe, where Ωe is a local electron cyclotron frequency. Their amplitudes range from a few picoteslas to 100 pT with an average of 23 pT. Because of their parallel propagation, the NSW must be excited by electron cyclotron resonance. These features of the NSW suggest existence of electron beams which travel in an upstream direction parallel to the Bo and which have a temperature anisotropy. Kinetic energies of the beams range from a few eV to about 200 eV (28 eV on average). All these characteristics of the electron beams revealed from the NSW are consistent with ISEE and WIND particle observations. The competition between electrostatic and whistler instabilities and the finite size of the beams are very likely the reasons why the NSW are short‐lived. These NSW can be well explained by a modeled electron beam with a losscone distribution in the electron foreshock. The NSW in solar wind are very similar to those in the electron foreshock. However, they have larger amplitudes (34 pT on average), lower frequencies (0.24Ωe on average), and higher cyclotron resonant energy (100 eV on average). They are very likely excited by halo electrons or nonlocal sources in the solar wind.

Generation of electromagnetic fpe and 2fpe waves in the Earth's electron foreshock via linear mode conversion

This study examines the mode conversion from electrostatic to electromagnetic waves near the plasma frequency in the Earth's electron foreshock. The conversion and reflection coefficients are obtained by solving coupled differential equations in a weakly magnetized warm plasma with a longitudinal linear density gradient. Results indicate that the foreshock first harmonic electromagnetic emissions and the backward‐propagating Langmuir waves required for the generation of the second harmonic electromagnetic waves may be efficiently produced by the linear conversion process in an inhomogeneous plasma.

Whistler instability enhanced by suprathermal electrons within the Earth's foreshock

The electron temperature‐anisotropy driven whistler instability in the presence of a suprathermal power law tail on the electron population, such as is observed in the Earth's electron foreshock, is investigated. The enhancement in energetic electrons significantly accentuates whistler growth at wavenumbers smaller than those expected for a bi‐Maxwellian electron distribution of similar temperature anisotropy and brings the unstable frequencies and wavelengths closer to those expected for the 1‐Hz (in the spacecraft frame) whistler waves observed in the electron foreshock. The presence of a power law tail, furthermore, causes a rapid increase in growth rate with increasing electron temperature anisotropy. This means that even very modest temperature anisotropies, like those that are frequently present in the magnetically connected foreshock, can strongly destabilize parallel whistlers. This instability could be responsible for the nearly field aligned so‐called “1 Hz” whistlers previously observed in the Earth's foreshock, which are more difficult to account for in models of the oblique whistler instability.

Statistical imaging of the Venus foreshock using VLF wave emissions

Strong VLF wave emissions have been shown to be an intrinsic property of the Venus foreshock and foreshocks in general. In this work, we use these measured wave emissions to construct statistical “images” of the Venus foreshock. This analysis method allows us to develop a macroscopic picture of the wave properties, the inferred particle distributions, and their evolution as a function of position within the foreshock as well as compare our observations to Earth. The electron foreshock emissions at Venus are parallel polarized Langmuir mode waves with the same peak amplitude at the foreshock boundary as terrestrial emissions (10 mV/m). However, the wave characteristics differ markedly between the upstream and downstream foreshocks for near Parker spiral interplanetary magnetic field (IMF) orientations (∼35° for Venus). Additionally, there is a dramatic decrease in wave intensity for distances beyond ∼15 Rv from the point of tangency along the foreshock boundary. These characteristics in the wave emissions provide strong observational evidence supporting reflection and energization of solar wind electrons at the shock as the dominant source for providing upstream electrons, not leakage. In the ion foreshock the wave emissions consist of parallel polarized ion acoustic‐like waves with similar intensities and spectral characteristics to terrestrial emissions. However, these waves are situated much deeper in the foreshock than expected from terrestrial observations. Surprisingly, no emissions are observed in regions where field aligned ion distributions are expected. Rather, the emissions are confined to a region where diffuse ion distributions are expected.

Wave observations at the foreshock boundary in the near-Mars space

Wave and plasma measurements carried out by the Phobos-2 spacecraft in the Martian environment indicate that a part of solar wind electrons and ions are reflected and accelerated at the Martian bow shock. These particles streaming back into the solar wind lead to the formation of the foreshock region upstream the planetary bow shock. It is shown that the general structure of the foreshock at Mars is similar to that of the foreshock region upstream the Earth’s bow shock. However, the electric emissions observed at frequencies around 100 Hz at the leading edge of electron foreshock at Mars have been never reported for observations in the similar region at Earth. The burst of these waves occurs before the spacecraft enters the electron foreshock identified with the onset of electron plasma waves and calculations of the magnetic connection to the planetary bow shock.