Planetary Foreshocks Research Articles

Langmuir waves associated with magnetic holes in the solar wind

Context.Langmuir waves (electrostatic waves near the electron plasma frequency) are often observed in the solar wind and may play a role in the energy dissipation of electrons. The largest amplitude Langmuir waves are typically associated with type II and III solar radio bursts and planetary foreshocks. In addition, Langmuir waves not related to radio bursts occur in the solar wind, but their source is not well understood. Langmuir waves have been observed inside isolated magnetic holes, suggesting that magnetic holes play an important role in the generation of Langmuir waves.Aims.We provide the statistical distribution of Langmuir waves in the solar wind at different heliocentric distances. In particular, we investigate the relationship between magnetic holes and Langmuir waves. We identify possible source regions of Langmuir waves in the solar wind, other than radio bursts, by analyzing the local plasma conditions.Methods.We analyzed data from Solar Orbiter’s Radio and Plasma Waves (RPW) and Magnetometer (MAG) instruments. We used the triggered electric field snapshots and onboard statistical data (STAT) of the Time Domain Sampler (TDS) of RPW to identify Langmuir waves and investigate their properties. The plasma densities were derived from the spacecraft potential estimated by RPW. The MAG data were used to monitor the background magnetic field and detect magnetic holes, which are defined as regions with an isolated decrease in |B| of 50% or more compared to the background level. The statistical analysis was performed on data from 2020 to 2021, comprising heliocentric distances between 0.5 AU and 1 AU.Results.We show that 78% of the Langmuir waves in the solar wind not connected to radio bursts occur in regions of local magnetic field depletions, including the regions classified as isolated magnetic holes. We also show that the Langmuir waves occur more frequently inside magnetic holes than in any other region in the solar wind, which indicates that magnetic holes are important source regions of solar wind Langmuir waves. We find that Langmuir waves associated with magnetic holes in the solar wind typically have lower amplitudes than those associated with radio bursts.

Langmuir-Slow Extraordinary Mode Magnetic Signature Observations with Parker Solar Probe

Abstract Radio emission from interplanetary shocks, planetary foreshocks, and some solar flares occurs in the so-called “plasma emission” framework. The generally accepted scenario begins with electrostatic Langmuir waves that are driven by a suprathermal electron beam on the Landau resonance. These Langmuir waves then mode-convert to freely propagating electromagnetic emissions at the local plasma frequency f pe and/or its harmonic 2f pe . However, the details of the physics of mode conversion are unclear, and so far the magnetic component of the plasma waves has not been definitively measured. Several spacecraft have measured quasi-monochromatic Langmuir or slow extraordinary modes (sometimes called z-modes) in the solar wind. These coherent waves are expected to have a weak magnetic component, which has never been observed in an unambiguous way. Here we report on the direct measurement of the magnetic signature of these waves using the Search Coil Magnetometer sensor of the Parker Solar Probe/FIELDS instrument. Using simulations of wave propagation in an inhomogeneous plasma, we show that the appearance of the magnetic component of the slow extraordinary mode is highly influenced by the presence of density inhomogeneities that occasionally cause the refractive index to drop to low values where the wave has strong electromagnetic properties.

Occurrence rate of ultra-low frequency waves in the foreshock of Mercury increases with heliocentric distance

Studies of Mercury’s foreshock have analyzed in detail the properties of ultra-low frequency waves. However, an open question remains in regards to understanding favorable conditions for these planetary foreshocks waves. Here, we report that 0.05–0.41 Hz quasi-monochromatic waves are mostly present under quasi-radial and relatively low intensity Interplanetary Magnetic Field, based on 17 Mercury years of MESSENGER Magnetometer data. These conditions are consistent with larger foreshock size and reflection of solar wind protons, their most likely source. Consequently, we find that the wave occurrence rate increases with Mercury’s heliocentric distance. Detection of these waves throughout Mercury’s highly eccentric orbit suggests the conditions for backstreaming protons are potentially present for all of Mercury’s heliocentric distances, despite the relatively low solar wind Alfvén Mach number regime. These results are relevant for planetary magnetospheres throughout the solar system, and the magnetospheres of exoplanets, and provide knowledge of particle acceleration mechanisms occurring inside foreshocks.

Upstream Ultra‐Low Frequency Waves Observed by MESSENGER's Magnetometer: Implications for Particle Acceleration at Mercury's Bow Shock

AbstractWe perform the first statistical analysis of the main properties of waves observed in the 0.05–0.41 Hz frequency range in the Hermean foreshock by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Magnetometer. Although we find similar polarization properties to the “30 s” waves observed at the Earth's foreshock, the normalized wave amplitude (δB/|B0|∼0.2) and occurrence rate (∼0.5%) are much smaller. This could be associated with relatively lower backstreaming proton fluxes, the smaller foreshock size and/or less stable solar wind (SW) conditions around Mercury. Furthermore, we estimate that the speed of resonant backstreaming protons in the SW reference frame (likely source for these waves) ranges between 0.95 and 2.6 times the SW speed. The closeness between this range and what is observed at other planetary foreshocks suggests that similar acceleration processes are responsible for this energetic population and might be present in the shocks of exoplanets.

Nonlinear Wave‐Particle Interaction: Implications for Newborn Planetary and Backstreaming Proton Velocity Distribution Functions

AbstractSeen from the solar wind (SW) reference frame, the presence of newborn planetary protons upstream from the Martian and Venusian bow shocks and SW protons reflected from each of them constitutes two sources of nonthermal proton populations. In both cases, the resulting proton velocity distribution function is highly unstable and capable of giving rise to ultralow frequency quasi‐monochromatic electromagnetic plasma waves. When these instabilities take place, the resulting nonlinear waves are convected by the SW and interact with nonthermal protons located downstream from the wave generation region (upstream from the bow shock), playing a predominant role in their dynamics. To improve our understanding of these phenomena, we study the interaction between a charged particle and a large‐amplitude monochromatic circularly polarized electromagnetic wave propagating parallel to a background magnetic field, from first principles. We determine the number of fix points in velocity space, their stability, and their dependence on different wave‐particle parameters. Particularly, we determine the temporal evolution of a charged particle in the pitch angle‐gyrophase velocity plane under nominal conditions expected for backstreaming protons in planetary foreshocks and for newborn planetary protons in the upstream regions of Venus and Mars. In addition, the inclusion of wave ellipticity effects provides an explanation for pitch angle distributions of suprathermal protons observed at the Earth's foreshock, reported in previous studies. These analyses constitute a mean to evaluate if nonthermal proton velocity distribution functions observed at these plasma environments present signatures that can be understood in terms of nonlinear wave‐particle processes.

Applying bicoherence analysis to spacecraft observations of Langmuir waves

In type II and type III solar radio bursts and planetary foreshocks, the processes which convert Langmuir waves (LWs) to transverse waves are not well understood. One of the proposed mechanisms for generating transverse waves involves electrostatic (ES) decay followed by coalescence of two LWs. One of the tests used to identify this process is bicoherence analysis. Bicoherence has been applied to spacecraft observations of LWs to yield results consistent with ES decay and coalescence. However, recent work has shown that the harmonic fields produced by LWs are more consistent with sheath rectification and nonlinear currents. It is shown here that sheath rectification and nonlinear currents yield bicoherences similar to those expected for ES decay and coalescence, explaining the bicoherences associated with spacecraft observations of LWs. These results show that bicoherence alone cannot be used to identify ES decay and coalescence and emphasize the importance of sheath rectification.

Harmonic waves and sheath rectification in type III solar radio bursts

AbstractIn type III solar radio bursts and planetary foreshocks, Langmuir waves are produced by electron beams and converted partially to radio waves by linear and nonlinear processes. Lower amplitude second harmonic electric fields are observed simultaneously during the most intense Langmuir wave events in type III source regions. The electric fields at the harmonic frequencies can arise from various mechanisms, such as radio wave emission by either coalescence or antenna mechanisms, nonlinear currents, harmonics of Langmuir waves, electron trapping in Langmuir wave potentials, and Langmuir wave rectification at the sheath surrounding the spacecraft, or they can result from instrumental harmonics. In this paper the relative powers and electric field vectors of Langmuir waves and the harmonic fields are compared for multiple events. The structure of the harmonic field is shown to be determined by the Langmuir waveform, but the harmonic field direction is typically closely aligned with the solar wind flow. The magnitude, structure, and orientation of the harmonic fields is used to determine which processes are responsible. It is shown that the dominant process generating the observed harmonic fields is Langmuir wave rectification at the sheath surrounding the spacecraft.

Langmuir wave harmonics due to driven nonlinear currents

The conversion of Langmuir waves into electromagnetic radiation near the local plasma frequency (fpe) and twice the local plasma frequency (2fpe) occurs in diverse heliospheric environments including along the path of type III radio bursts, at interplanetary shocks, and in planetary foreshocks. This radiation has the potential to act as a probe of remote plasma conditions, provided that the conversion mechanism is well understood. One candidate conversion mechanism is the antenna radiation of localized Langmuir waves. Antenna radiation near 2fpe requires the presence of nonlinear currents at 2fpe. In this work, properties of these currents are predicted from theory and compared with observations of Langmuir wave electric fields made using the WAVES instrument on the STEREO spacecraft. It is found that the observed frequency structure, polarization, and wave number ratio are consistent with nonlinear current predictions, once electric fields near 2fpeconsistent with sheath effects are taken into account.

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.

Electric field distributions for Langmuir waves in planetary foreshocks

This paper addresses several key issues regarding the statistics of Langmuir wave fields E in planetary foreshocks by presenting (1) a detailed theory for the electric field strength probability distributions averaged over Earth's foreshock and (2) the first analyses of the distributions observed in the outer planets. First, existing theory is extended and generalized, describing in detail how lognormal distributions of P(log E) for a specific location can be aggregated over foreshock location to give power law distributions (log E). The extended theory is in good agreement with the observations of power law distributions (log E) for Earth's foreshock, and implies that stochastic growth theory (SGT) is likely to be relevant. The statistics of the Langmuir wave fields of the outer planets Saturn, Uranus, and Neptune are then analyzed and compared. The characteristics of the observed distributions (logE) agree qualitatively with each other and Earth, and these distributions collapse onto a single curve when scaled on the assumption that the ratio of the wave energy density to thermal plasma energy density is constant with heliocentric distance. The extended theory developed for Earth is also found to agree well with the scaled (log E) distributions for each outer planet. The evidence is thus that the physics of electrons and Langmuir waves in all planetary foreshocks is essentially identical and that the distributions (log E) are consistent with SGT.

Electron beam interaction with space plasmas

Active space experiments involving the controlled injection of electron beams and the formation of artificially generated currents can provide in many cases a calibration of natural phenomena connected with the dynamic interaction of charged particles with fields. They have a long history beginning from the launches of small rockets with electron guns in order to map magnetic fields lines in the Earth's magnetosphere or to excite artificial auroras. Moreover, natural beams of charged particles exist in many space and astrophysical plasmas and were identified in situ by several satellites; a few examples are beams connected with solar bursts, planetary foreshocks or suprathermal fluxes traveling in planetary magnetospheres. Many experimental and theoretical works have been performed in order to interpret or plan space experiments involving beam injection as well as to understand the physics of wave-particle interaction, as wave radiation, beam dynamics and background plasma modification. Recently, theoretical studies of the nonlinear evolution of a thin monoenergetic electron beam injected in a magnetized plasma and interacting with a whistler wave packet have led to new results. The influence of an effective dissipation process connected with whistler wave field leakage out of the beam volume to infinity (that is, effective radiation outside the beam) on the nonlinear evolution of beam electrons distribution in phase space has been studied under conditions relevant to active space experiments and related laboratory modelling. The beam-waves system's evolution reveals the formation of stable nonlinear structures continuously decelerated due to the effective friction imposed by the strongly dissipated waves. The nonlinear interaction between the electron bunches and the wave packet are discussed in terms of dynamic energy exchange, particle trapping, slowing down of the beam, wave dissipation and quasi-linear diffusion.

Ulysses observations of a discrete wavepacket upstream of an interplanetary shock

We present a single example of a previously unidentified type of upstream wave structure, found several minutes ahead of a forward propagating, oblique interplanetary shock observed on day 6, 1992, using Ulysses magnetometer data. The wave appears as an isolated high frequency wavepacket associated with a larger scale compressive structure. In the region between the wavepacket and the shock the magnetic field is moderately, but significantly, compressed and deflected suggesting the existence of a foreshock. The wavepacket is dispersive and wavelet analysis shows a near linear decrease in frequency from 0.3 to 0.1 Hz, continuing across the large scale structure which is assumed to be the source of these waves. We suggest that the feature is similar in nature to the shocklet and associated discrete wavepacket commonly found in planetary foreshocks, but further examples are required before a definite conclusion can be reached.

On the formation of wave packets in planetary foreshocks

Kinetic localization of beam‐driven Langmuir waves is studied for parameters relevant to the electron foreshock environment. Particle‐in‐cell simulations of the interaction occurring between energetic electrons and Langmuir waves are performed. It is shown that the nonlinearities in the resonant electrons lead to the formation of packets growing out of the noise. The characteristic spatial scale depends upon the beam velocity and has a weak dependence on the wave amplitude. The mechanism takes place within shorter timescales and for smaller amplitudes than the classic nonlinearities due to bulk electrons and ions. As an example, we apply the results to observations made in the Earth and Jovian foreshocks.

Maximum Langmuir fields in planetary foreshocks determined from the electrostatic decay threshold

Maximum electric fields of Langmuir waves at planetary foreshocks are estimated from the threshold for electrostatic decay, assuming it saturates beam‐driven growth, and incorporating heliospheric variation of plasma density and temperature. Comparison with spacecraft observations yields good quantitative agreement. Observations in type III radio sources are also in accord with this interpretation. A single mechanism can thus account for the highest fields of beam driven waves in both contexts.

Comparison of upstream phenomena at Venus and Earth

The region upstream of a planetary bow shock, known as the foreshock, contains a variety of phenomena. Electrons and ions are reflected and energized at the shock. As these stream back upstream they generate both VLF and ULF waves. Studies of the terrestrial foreshock have provided most of our understanding of these phenomena. However, comparisons with other planetary foreshocks are beneficial, even though the instrumentation used to provide the data may be less sophisticated than that flown on Earth orbiting spacecraft. In particular maps of the VLF emissions upstream of the Venus bow shock, using data acquired by the Pioneer Venus Orbiter are particularly illuminating. These maps show that the tangent field line is clearly marked by the presence of plasma oscillations. Of additional interest is evidence that the emissions only extend some 15 Venus radii away from the shock, indicating that the emissions are controlled by the shock scale size. Lower frequency ion acoustic waves are observed deep in the ion foreshock. Only close to the shock do both the ion acoustic waves and ULF waves occur simultaneously. The ULF waves mark the ion foreshock boundary where ion beams should be present. The ion acoustic waves tend to be observed further downstream, where diffuse ion distributions are expected to occur. A similar mapping of the terrestrial foreshock, using data from the ISEE-3 spacecraft shows similar results for the electron foreshock. An extension of this study to include ULF and ion acoustic waves would be helpful.

ULF waves in the foreshock

We review our current knowledge of ULF waves in planetary foreshocks. Most of this knowledge comes from observations taken within a few Earth radii of the terrestrial bow shock. Terrestrial foreshock ULF waves can be divided into three types, large amplitude low frequency waves (∼ 30-s period), upstream propagating whistlers (1-Hz waves), and 3-s waves. The 30-s waves are apparently generated by back-streaming ion beams, while the 1-Hz waves are generated at the bow shock. The source of the 3-s waves has yet to be determined. In addition to issues concerning the source of ULF waves in the foreshock, the waves present a number of challenges, both in terms of data acquisition, and comparison with theory. The various waves have different coherence scales, from ∼ 100 km to ∼ 1 Earth radius. Thus multi-spacecraft separation strategies must be tailored to the phenomenon of interest. From a theoretical point of view, the ULF waves are observed in a plasma in which the thermal pressure is comparable to the magnetic pressure, and the rest-frame wave frequency can be a moderate fraction of the proton gyro-frequency. This requires the use of kinetic plasma wave dispersion relations, rather than multi-fluid MHD. Lastly, and perhaps most significantly, ULF waves are used to probe the ambient plasma, with inferences being drawn concerning the types of energetic ion distributions within the foreshock. However, since most of the data were acquired close to the bow shock, the properties of the more distant foreshock have to be deduced mainly through extrapolation of the near-shock results. A general understanding of the wave and plasma populations within the foreshock, their interrelation, and evolution, requires additional data from the more distant foreshock.

Outer heliospheric radio emissions: 1. Constraints on emission processes and the source region

The Voyager 1 and 2 spacecraft observed low‐frequency radio emissions near 2 and 3 kHz during the interval 1983–1987 while at heliocentric distances from 15 to 27 AU and 11 to 20 AU, respectively. We consider the detailed theoretical and observational requirements for this radiation to be produced near multiples of the plasma frequency ƒp by nonlinear, weak turbulence, wave‐wave processes involving electrostatic Langmuir waves. Constraints on the emission processes and source characteristics are discussed. The minimum brightness temperature of the radiation, 3×1014K, requires the Langmuir waves to be generated by a plasma instability, most plausibly by an electron beam instability. This requires electron acceleration and beam production in the near vicinity of the source. Minimum Langmuir wave electric fields in the source region, based solely on the kinematics of the radiation processes, lie in the range 1–100 μV m−1 for nominal source and electron beam parameters. Path lengths for fundamental and harmonic emission processes are calculated: the observed levels of radiation can be produced in path lengths (and source dimensions) smaller than 1 AU provided the participating Langmuir waves have effective temperatures TL ∼ 1017K. These Langmuir wave levels are plausible, based on observations in planetary foreshocks and theoretical calculations. Previous source models for the radiation are discussed. Suggestions that the radiation is generated on the downstream side of the inner heliospheric shock or in the vicinity of the heliopause are shown to face severe theoretical and observational problems, based on present knowledge of the outer heliosphere. However, the upstream (sunward) side of the inner heliospheric shock is predicted theoretically to have a foreshock region containing electron beams and associated Langmuir waves. Theoretically this source region can plausibly produce radiation at multiples of ƒp with the observed brightness temperatures.

On the excitation of cyclotron harmonic waves by newborn heavy ions

Wave measurements in planetary foreshocks and cometary environments show the sporadic occurrence of magnetic spectra with harmonic structure related to ion cyclotron frequencies. Dilute populations of anisotropic and/or drifting charged particles can excite obliquely propagating modes with spacecraft frequencies close to the observed harmonics. We extend previous analyses of this generation mechanism to drifting and nondrifting loss‐cone type distributions of heavy ions in a dense hydrogen magnetoplasma, characterizing the complex (real frequency and growthrate) dispersion, polarization and compressibility of the unstable cyclotron harmonic waves. Solution of the full kinetic dispersion equation shows that it is possible to attain harmonic excitation, both in the drifting and nondrifting regimes. However, the bandwidth inherent to frequency Doppler shifts of obliquely propagating waves might preclude the observation of spectral structure in the spacecraft frame. The Giotto observations in the upstream region of comet Halley provide a reference to discuss the results.