Plasma Wave System Research Articles

An In Situ Study of Turbulence near Stellar Bow Shocks

Abstract Stellar bow shocks are observed in a variety of interstellar environments and shaped by the conditions of gas in the interstellar medium (ISM). In situ measurements of turbulent density fluctuations near stellar bow shocks are only achievable with a few observational probes, including Hα-emitting bow shocks and the Voyager Interstellar Mission (VIM). In this paper, we examine density variations around the Guitar Nebula, an Hα bow shock associated with PSR B2224+65, in tandem with density variations probed by VIM near the boundary of the solar wind and ISM. High-resolution Hubble Space Telescope observations of the Guitar Nebula taken between 1994 and 2006 trace density variations over scales from hundreds to thousands of au, while VIM density measurements made with the Voyager 1 Plasma Wave System constrain variations from thousands of meters to tens of au. The power spectrum of density fluctuations constrains the amplitude of the turbulence wavenumber spectrum near the Guitar Nebula to log 10 C n 2 = − 0.8 ± 0.2 m−20/3 and for the very local ISM probed by Voyager to log 10 C n 2 = − 1.57 ± 0.02 m−20/3. Spectral amplitudes obtained from multiepoch observations of four other Hα bow shocks also show significant enhancements from values that are considered typical for the diffuse, warm ionized medium, suggesting that density fluctuations near these bow shocks may be amplified by shock interactions with the surrounding medium or selection effects that favor Hα emission from bow shocks embedded in denser media.

Persistent plasma waves in interstellar space detected by Voyager 1.

In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium1. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 au of interstellar space, revealing a large-scale density gradient2,3 and turbulence4 outside the heliopause. Previous studies of the plasma density relied on the detection of discrete plasma oscillation events triggered ahead of shocks propagating outwards from the Sun and used to infer the plasma frequency and hence density5,6. We present the detection of a class of very weak, narrowband plasma wave emission in the Voyager 1 data that persists from 2017 onwards and enables the first steadily sampled measurement of the interstellar plasma density over about 10 au with an average sampling distance of 0.03 au. We find au-scale density fluctuations that trace interstellar turbulence between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and could be clarified by new findings from Voyager or a future interstellar mission. The emission's persistence suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events.

Terminator Double Layer Explorer (TerDLE): Examining the Near-Moon Lunar Wake

Abstract As the solar wind flows by the Moon, an antisunward-directed low-density wake forms as the plasma expands to fill in the trailing void in the plasma flow. Analytical modeling and modern plasma simulations suggest that plasma quasi-neutrality could possibly be broken close to the terminator obstruction as solar wind electrons expand into the wake ahead of the ions, leading to the formation of a standing (time-stationary) double layer. The objective of the Terminator Double Layer Explorer is to extend the fundamental understanding of the plasma expansion into the trailing near-vacuum wake region by (1) identifying any plasma expansion density anomalies at low altitudes near the terminator wake initiation region, (2) assessing the highly variable solar wind’s effect on the low-altitude wake region, and (3) determining if plasma neutrality is maintained or lost during passages through the low-altitude expansion region. The mission concept uses a propulsion-driven CubeSat with ion spectrometer and plasma wave system in elliptical orbit about the Moon with periselene near the terminator. Over the course of the mission, the periselene decreases, placing the CubeSat ever closer to the terminator wake initiation location and the possible nonneutral region.

Saturn's rings and associated ring plasma cavity: Evidence for slow ring erosion

We re-examine the radio and plasma wave observations obtained during the Cassini Saturn orbit insertion period, as the spacecraft flew over the northern ring surface into a radial distance of 1.3 Rs (over the C-ring). Voyager era studies suggest the rings are a source of micro-meteoroid generated plasma and dust, with theorized peak impact-created plasma outflows over the densest portion of the rings (central B-ring). In sharp contrast, the Cassini Radio and Plasma Wave System (RPWS) observations identify the presence of a ring-plasma cavity located in the central portion of the B-ring, with little evidence of impact-related plasma. While previous Voyager era studies have predicted unstable ion orbits over the C-ring, leading to field-aligned plasma transport to Saturn's ionosphere, the Cassini RPWS observations do not reveal evidence for such instability-created plasma ‘fountains’. Given the passive ring loss processes observed by Cassini, we find that the ring lifetimes should extend >109 years, and that there is limited evidence for prompt destruction (loss in <100Myrs).

Electron density dropout near Enceladus in the context of water‐vapor and water‐ice

On 12 March 2008, the Cassini spacecraft made a close encounter with the Saturnian moon Enceladus, passing within 52 km of the moon. The spacecraft trajectory was intentionally‐oriented in a southerly direction to create a close alignment with the intense water‐dominated plumes emitted from the south polar region. During the passage, the Cassini Radio and Plasma Wave System (RPWS) detected two distinct radio signatures: 1) Impulses associated with small water‐ice dust grain impacts and 2) an upper hybrid (UH) resonance emission that both intensified and displayed a sharp frequency decrease in the near‐vicinity of the moon. The frequency decrease of the UH emission is associated with an unexpectedly sharp decrease in electron density from ∼90 el/cm3 to below 20 el/cm3 that occurs on a time scale of a minute near the closest encounter with the moon. In this work, we consider a number of scenarios to explain this sharp electron dropout, but surmise that electron absorption by ice grains is the most likely process.

An empirical model of Saturn's bow shock: Cassini observations of shock location and shape

We present a new empirical model of Saturn's bow shock that utilizes observations from the Cassini spacecraft. Shock crossings are identified in magnetic field and plasma observations made by Cassini between June 2004 and August 2005. The Cassini crossings are then combined with the crossings made during the Saturn flybys of Pioneer 11, Voyager 1, and Voyager 2. Solar wind dynamic pressures for the Cassini crossings are estimated using upstream electron densities determined from Langmuir wave observations made by the Radio and Plasma Wave System. The crossing positions are rotated into aberrated coordinates to correct for the effect of the planet's orbital motion. In the case of Saturn this rotation is by ∼1°. To correct for solar wind dynamic pressure variations, the crossing positions are normalized to the average pressure 〈PSW〉 = 0.048 nPa. The model is then obtained by fitting a conic section to the crossings using a nonlinear least squares technique. To validate the assumptions made in constructing the model, we treat the parameters previously assumed to be constants as variables and fit their values using an optimization routine; this leads to a conic section that is within the positional uncertainty of the model. The spacecraft trajectories are considered, and we conclude that they do not significantly bias the model. The new model is compared to the existing models, and the similarities and differences are discussed. We suggest that the new model gives the most accurate empirical representation of the shape and location of Saturn's bow shock.

Nonlinear drift-kinetic equation in the presence of a circularly polarized wave

This paper presents a theoretical description of plasma motion in the presence of a circularly polarized wave of arbitrary frequency with its wave vector directed along the ambient magnetic field. Both the equations of motion for a single particle and the nonlinear kinetic equation in the drift approximation are given. The nonstationarity and inhomogeneity of the plasma-wave system are taken into account. The distribution function is expressed with the help of initial, non-averaged variables, which is a transformation that greatly simplifies the calculation of the moments. The time-dependent part of the ponderomotive force is discussed, and it is shown that this component can be on the order of the other terms in the particle motion equation. The examples of ion acceleration in the auroral zone and the effects of lightning discharge are discussed to show that this theoretical study has applications to near-Earth space plasmas.

A multi‐instrument study of a Jovian magnetospheric disturbance

Using observations from different Galileo experiments (plasma wave system, magnetometer and energetic particle detector), we analyze a strong magnetospheric disturbance that occurs on day 311 of 1996 as Galileo was close to Jupiter (less than 15 Jovian radii). This perturbation is characterized by multiple injections of energetic particles in the inner magnetosphere and has been described as a possible analog of the terrestrial magnetic storm by Mauk et al. [1999]. We show here that it also corresponds to a large‐scale magnetospheric perturbation similar to the “energetic events” described by Louarn et al., [1998, 2000]. It is associated with the development of a particular magnetic activity in the outermost part of the Io torus, over periods of 2–4 hours and in sectors of longitude with a typical 30°–80° longitudinal extension. At distances ranging from 10 to 13 Rj, the activity itself is characterized by the generation of low‐frequency magnetic oscillations (18 min periodicity in the present case) that correlate with dispersionless energetic electron injections and modulations of the auroral radio flux. When they are observed a few hours after their formation, these injections present a weak energy‐time dispersion and are still periodic. They then progressively mix and finally define a region of limited longitudinal extension where the density of energetic particles is particularly large. We show that this region corresponds to the source of the narrowband kilometric radiation (n‐KOM). By combining remote sensing radio observations, in situ particle, and magnetic field measurements, we show that the active zone where the large scale disturbance initially develops most probably does not corotate and would even be almost fixed in local time. In the present case, the magnetospheric event is the consequence of two activations separated by a few hours. They occur in two separated longitude sectors and give rise to two different n‐KOM sources. During the event, some 1012 W are transferred to the electron population. It is proposed that this set of phenomena is the manifestation of a sporadic dissipation of a part of the Io torus rotational energy and would be thus associated with the development of a large‐scale instability in the external part of the Io torus.

Ion isotropy and ion resonant waves in the solar wind: Cassini observations

Electric fields in the solar wind, in the range of one Hertz, are reported for the first time from a 3‐axis stabilized spacecraft. The measurements are made with the Radio and Plasma Wave System (RPWS) experiment on the Cassini spacecraft [Gurnett et al., 2000]. Kellogg [2000] suggested that such waves could be important in maintaining the near‐isotropy of solar wind ions and the validity of MHD for the description of the solar wind. The amplitudes found are larger than those estimated by Kellogg from other measurements, and are due to quasi‐electrostatic waves. These amplitudes are quite sufficient to maintain isotropy of the solar wind ions.

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.

Martian planetopause as seen by the plasma wave system onboard Phobos 2

The existence of a plasma boundary between the bow shock and the atmosphere of Mars has been confirmed by the Phobos 2 observations. This boundary is called planetopause, magnetopause, ion‐composition boundary or protonopause, depending upon the authors. A careful examination of plasma wave system (PWS) data has revealed that planetopause signatures are generally clear and not questionable, enabling the calculation of the mean planetopause position up to 16 Martian radii in the distant tail. The planetopause model derived from PWS measurements is compared with models calculated from data sets obtained with other instruments on Phobos 2. With the exception of the transition plasma layer, called mass‐loading boundary by the Automatic Space Plasma Experiment with a rotating analyzer (ASPERA) investigators, the published planetopause, magnetopause/areomagnetopause and ion‐composition boundary locations are similar to the planetopause locations deduced from PWS data. This would suggest that, despite their different names, these boundaries correspond to the same plasma transition region. The PWS observations show that the planetopause position varies in the same way as does the bow shock position and is not significantly affected by the solar wind ram pressure. The absence of correlation between the planetopause position and solar wind ram pressure raises serious questions about the role that possible intrinsic magnetic field may play at the Martian planetopause. Finally, the planetopause is believed to be an ion‐composition discontinuity rather than the “obstacle” predicted by gasdynamic models.

Correlation between terrestrial myriametric and kilometric radio bursts observed with Galileo

Data from the Galileo plasma wave system recorded during an investigation of the Earth magnetotail on December 8, 1990, are analyzed in both the myriametric and the kilometric wavelength range. In the far tail, between 80 and 30 Earth radii, two components are observed in the myriametric range (frequencies between 6 and about 30 kHz): (1) the “classical” trapped continuum radiation with a smoothly varying intensity in time and (2), superposed on this component, bursts of waves with typical spectral densities of 10−15 V²m−2Hz−1. These bursts of myriametric waves are shown B to correlate with bursts of intense auroral kilometric radiation (AKR) with typical spectral density of 10−12 V²m−2Hz−1. This bursty myriametric radiation is the analog of the low frequency radiation (LFR) reported by Filbert and Kellog (1989). Its close correlation with the kilometric radiation suggests the existence of a direct relationship between these two radiations. A generation mechanism that explains this correlation and the frequency gap between the AKR and the LFR is proposed.

Position and shape of the Martian bow shock: the Phobos 2 plasma wave system observations

We have derived a Martian bow shock model using all the data that are available from the plasma wave system on the Phobos 2 spacecraft. In contrast with a previous study, performed with a limited data set of 26 shock encounters, we have considered about 120 shock crossings, including four new distant downstream shocks. Thus, the shock position is determined accurately at the nose (four inbound shocks with the elliptical orbits), at the terminator (more than 110 inbound and outbound shocks in the circular orbits), and far downstream (four outbound shocks with the elliptical orbits). Emphasis is placed upon comparisons with the results of earlier studies derived from smaller data sets, data sets that come from other experiments and/or other missions. The location of the bow shock of Mars seems to respond to variations in solar activity, as is the case for Venus. On the other hand, the subsolar stand-off distance appears to vary slightly with the solar wind dynamic pressure: the greater the subsolar shock distance is from the planet, the lower the solar wind dynamic pressure. Assuming a sixth-root dependence. an upper limit of a possible Martian magnetic moment of 2.2 × 10 12Tm 3 has been obtained. Finally, the shock is closer to Mars on the dawn side than on the dusk side at the terminator: the angle between the local bow shock normal and the upstream magnetic field controls this asymmetry.

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.

The Martian bow shock: Wave observations in the upstream region

Distinct electric field oscillations in the frequency range from 4 to 40 kHz are detected by the plasma wave system in the upstream region of the bow shock during the operation of the Phobos 2 spacecraft around Mars. The upstream electron foreshock boundary approximately coincides with the magnetic field line tangential to the bow shock surface. A polarization analysis shows clearly that the electric field vector of the waves is aligned with the local magnetic field. The features of these high‐frequency oscillations appear to be very similar to those of the emissions seen near the electron plasma frequency in the electron foreshock regions upstream of the bow shocks of Venus and Earth.

Plasma wave system measurements of the Martian bow shock from the Phobos 2 spacecraft

The high‐resolution data of the electric field observations performed by the plasma wave system (PWS) during some of the Martian bow shock intersections by Phobos 2 were analyzed. Plasma and wave detectors are very useful instruments for locating the shock transition region and studying structures in the upstream region, such as the foot or the electron foreshock. The electron plasma oscillations that develop in the latter give access to the plasma density of the solar wind. Shock surface models derived from the PWS data are compared to those obtained by other authors, and attention is paid to similarities and differences between the electric field measurements obtained for Mars, Venus, and Earth.

Plasma and wave observations in the night sector of Mars

The Phobos 2 spacecraft, initially injected into an elliptical orbit around Mars on January 29, 1989, was subsequently transferred, on February 18, 1989, to a nearly circular orbit, close to that of the Phobos moon, with an areocentric radius of the order of 9600 km. The spacecraft remained in this orbit until the end of the mission, on March 27, 1989. This paper summarizes the plasma and wave observations carried out in the night sector of Mars with the plasma wave system (PWS). Embedded in the magnetic field structure of the Martian tail, cold electron enhancements (“tail rays”) with densities in the range 10–65 cm−3 are observed in association with broadband wave activity extending from a few hertz up to several kilohertz; these enhancements appear to have characteristics analogous to enhancements observed at Venus. The ion outflow through the Martian eclipse region is estimated from Langmuir probe measurements to be of the order of 1025 ions/s.

Location of the Martian bow shock measurements by the plasma wave system on Phobos 2

During the early months of 1989, the Plasma Wave System, PWS, onboard the Soviet spacecraft Phobos‐2 made plasma and electric‐field spectrum measurements near the Martian bow shock. A total of 26 shock boundaries were unambiguously identified in the PWS data coming from the high‐resolution mode. The location of these boundaries was modeled and the results compared with other studies. Despite the limited coverage of the Phobos‐2 observations, the previous shock‐surface models and ours show a relatively good agreement.

Plasma and waves around Mars

The Plasma Wave System (PWS) on board the Soviet Spacecraft Phobos 2 has performed electron density and, for the first time, electric field measurements in the environment of the planet Mars. Electron plasma oscillations are observed upstream of the bow shock and yield a solar wind density of the order of 2 cm −3; the shock foot and/or ion foreshock are detected on each orbit. The altitude of the bow shock in the noon sector fluctuates between 0.45 and 0.75 Mars radii ( R ms) above the planetary surface. The downstream solar wind, in the planetosheath, is characterized by increased plasma density and broadband electrostatic noise. The planetopause is crossed at altitudes of the order of 0.28 R ms. Electromagnetic waves with frequencies below the local gyrofrequency, propagating in the whistler mode, are recorded within the planetosphere, where the electron plasma density reaches unexpectedly large values, of up to 700 cm −3 at an altitude of 0.25 R ms. Intense electrostatic emissions generated by heavy planetary ions are observed upstream of the shock ; these waves are linked with the erosion process of the Martian atmosphere by the solar wind.

Waves and cold plasma observations near Mars

The Phobos-2 spacecraft, initially injected into an elliptical orbit around Mars on 29 January 1989, was subsequently transferred, on 18 February 1989 to a nearly circular orbit, close to that of the Phobos moon, with an areocentric radius of the order of 9600 km. The spacecraft remained in this orbit until the end of the mission, on 27 March 1989. This paper summarizes the plasma and wave observations performed in the night sector of Mars with the Plasma Wave System (PWS), on this circular orbit. Embedded in the magnetic field structure of the Martian tail, cold plasmas with densities in the range 10–65 cm −3 are observed in association with broadband wave activity extending from a few Hz up to several kHz; these plasmas have characteristics analogous to the plasma clouds observed at Venus.