Plasma Science Experiment Research Articles

Updating the Jovian Electron Plasma Environment

Reanalysis of the plasma science (PLS) experiments on the Voyagers (~10 eV to ~5 keV) and Galileo (~1 to ~50 keV) has refined our understanding of the electron environment responsible for spacecraft charging at and near the Jovian moon Europa. The low-energy 1–50-keV environment measured by the Galileo PLS has previously been at best poorly defined as the PLS electron data were found to have been contaminated by high-energy electrons. To address this issue, the raw PLS data have been reanalyzed orbit by orbit and detector by detector. The results indicate that the Voyager and the Galileo spectra, previously fit by the Maxwell–Boltzmann distributions, may be better fit by a more complex combination of Maxwell–Boltzmann plus kappa or a polynomial. The new spectra derived from the Voyager and Galileo PLS instruments are presented for a range of applicability from ~8 to ~40 Rj.

The HelCat basic plasma science device

TheHelicon-Cathode(HelCat) device is a medium-size linear experiment suitable for a wide range of basic plasma science experiments in areas such as electrostatic turbulence and transport, magnetic relaxation, and high power microwave (HPM)-plasma interactions. The HelCat device is based on dual plasma sources located at opposite ends of the 4 m long vacuum chamber – an RF helicon source at one end and a thermionic cathode at the other. Thirteen coils provide an axial magnetic fieldB⩾ 0.220 T that can be configured individually to give various magnetic configurations (e.g. solenoid, mirror, cusp). Additional plasma sources, such as a compact coaxial plasma gun, are also utilized in some experiments, and can be located either along the chamber for perpendicular (to the background magnetic field) plasma injection, or at one of the ends for parallel injection. Using the multiple plasma sources, a wide range of plasma parameters can be obtained. Here, the HelCat device is described in detail and some examples of results from previous and ongoing experiments are given. Additionally, examples of planned experiments and device modifications are also discussed.

VOYAGER 2OBSERVATIONS OF PLASMAS AND FLOWS OUT TO 104 AU

Voyager 2 has crossed through 20 AU of the heliosheath; assuming the same heliosheath thickness as at Voyager 1, it is now two-thirds of the way to the heliopause. The plasma data are generally of good quality, although the increasing flow angle of the plasma makes analysis more difficult. The average plasma speed has remained constant but the flow angles have increased to almost 60° in the RT plane and to almost 30° in the RN plane. The average density and thermal speed have been constant since a density increase observed in 2011. Comparison of V2 plasma flows derived from plasma science experiment (PLS) data and Low Energy Charged Particle (LECP) proton anisotropies give good agreement except when heavy ion contributions or non-convective proton anisotropies are observed in the LECP data.

Low-Pressure Helicon-Plasma Discharge Initiation via Magnetic Field Ramping

Discharge initiation at low pressures and flow rates is investigated in the Madison helicon experiment flowing helicon source. At low pressures (below 14-sccm flow rate), a threshold magnetic field exists for discharge initiation which depends on RF power and gas flow rate. Above the threshold magnetic field, RF discharges start only after a significant delay (approximately seconds) and sometimes will not start at all. This threshold magnetic field is interpreted using electron multipactor arguments. A technique is described for initiating discharges at low flow rates and pressures (lambdaen,iz > Lsystem) and high magnetic fields (above the threshold value). Without a static magnetic field present, the RF power is turned on, and a lower density (< 1011 cm-3) unmagnetized discharge occurs. The magnetic field is then applied, and the discharge transitions to the higher density (up to 1013 cm-3) regime. Using this method, magnetized discharges can be started at flow rates as low as 1 seem (1.8 times 10-4 torr at z = -91 cm, 1.7 times 10-5 torr at z = 105 cm) at 500 W in a 1.04-kG magnetic field. This technique can be used to initiate low-pressure helicon discharges for basic plasma science experiments and other applications.

The HelCat dual-source plasma device

The HelCat (Helicon-Cathode) device has been constructed to support a broad range of basic plasma science experiments relevant to the areas of solar physics, laboratory astrophysics, plasma nonlinear dynamics, and turbulence. These research topics require a relatively large plasma source capable of operating over a broad region of parameter space with a plasma duration up to at least several milliseconds. To achieve these parameters a novel dual-source system was developed utilizing both helicon and thermionic cathode sources. Plasma parameters of n(e) approximately 0.5-50 x 10(18) m(-3) and T(e) approximately 3-12 eV allow access to a wide range of collisionalities important to the research. The HelCat device and initial characterization of plasma behavior during dual-source operation are described.

Voyager 2 observations of helium abundance enhancements from 1–60 AU

The Plasma Science Experiment on the Voyager 2 spacecraft recently detected helium abundance enhancements (HAEs) near 60 AU in which the He++ ions comprise more than 10% of the solar wind density. These large values may result from solar events related to the rising phase of solar cycle 23. Large He++/H+ density ratios are probably the best signature for tracking coronal mass ejections (CMEs) through the outer heliosphere for the subset of CMEs with this property. We analyze the high‐resolution spectra for the Voyager 2 mission (from 1977 to 1999, 1–60 AU) and find that HAE events are observed ∼1.1% of the time with He++/H+ abundance ratios as high as 0.35. The long‐term variation of HAEs is related to the solar cycle. HAEs occur in all solar wind regimes but are associated with higher speeds, lower proton temperatures, and higher magnetic fields than the ambient solar wind. HAEs are generally not associated with interplanetary shocks. The He++/H+ temperature ratio increases with distance from the Sun, is larger when He++/H+ density ratios are large, and is smaller when proton densities are high.

Developments in Jovian Radio Emissions Tomography and Observations Techniques

Jupiter radio emission is known to be the most powerful nonthermal planetary radiation. In recent years specifically space-based observations allow us to permanently cover a large frequency band (from 100 kHz up to 40 MHz combined with ground-based telescopes) of the Jovian spectrum. The Plasma and Wave Science experiment onboard Galileo enables the observation of Jovian kilo-metric and hectometric emissions; Wind/WAVES and ground-based telescopes (mainly Decametric Array in Nancay, France, and UTR-2 in Kharkov, Ukraine) cover also hectometric and mainly decametric emissions. Specific geometrical configurations between Cassini approaching Jupiter and Wind spacecraft orbiting Earth, with Galileo orbiting Jupiter and Wind, in combination with ground-based observations provide a new approach to perform Jovian radio tomography. The tomography technique is used to analyze ray paths of Jovian radio emission observed in different directions (e.g. solar and anti-solar direction) and for different declination of Earth. The developments of Jovian radio emission tomography in recent years treated refraction effects and its connection to the local magnetic field in the radio source as well as the radio wave propagation through the Io torus and the terrestrial ionosphere. Most recently ground-based multi-site and simultaneous Jupiter decametric radio observations by means of digital spectropolarimeter and waveform receiver provide the basis of a new data analysis treatment. The above addressed topics are without exemption deeply connected to the plasma structures the radio waves are generated in and propagating through.

Comprehensive analysis of electron observations at Saturn: Voyager 1 and 2

We present a comprehensive analysis of Voyager 1 and 2 electron observations within Saturn's magnetosphere. This analysis entails the merging of electron observations from the Plasma Science (PLS) experiment, the Low Energy Charged Particle (LECP) experiment and the Cosmic Ray System (CRS) experiment. For each encounter, the three instruments combined allow us to compute the electron energy spectra over a wide range of energies from 10 eV to ∼ 2MeV between the closest approaches and L = 18.5. The instruments use different technologies, different sensitivities, and different fields of view; however, we observe a surprisingly good matching of the data sets on a 15‐min timescale. The PLS‐LECP‐CRS spectra include the low‐energy thermal component of the magnetospheric plasma, the keV suprathermal electrons, and the high‐energy tail extending into the MeV energy range. From the combined spectra, we compute a comprehensive set of macroscopic parameters (electron density, pressure, beta factor, and electron current at the spacecraft): the analysis reveals a variety of radial gradients for these quantities and the corresponding electron populations. We also compute phase space densities over a wide range in energy and radial distances, analyzing local time symmetries, electron source distributions, and temporal variations of Saturn's magnetosphere. The ultimate goal of this study is to provide a comprehensive empirical model of the charged particle population within Saturn's magnetosphere. It will be used to support the development of the Cassini mission and to allow detailed planning of the tour design with regard to charged particle science and radiation hazards.

Voyager observations of O+6 and other minor ions in the solar wind

The plasma science (PLS) experiments on the Voyager 1 and 2 spacecraft began making measurements of the solar wind shortly after the two launches in the fall of 1977. In reviewing the data obtained prior to the Jupiter encounters in 1979, we have found that the large dynamic range of the PLS instrument generally allows a clean separation of signatures of minor ions (about 2.5% of the time) during a single instrument scan in energy per charge. The minor ions, most notably O+6, are well separated from the protons and alpha particles during times when the solar wind Mach number (ratio of streaming speed to thermal speed) is greater than ∼15. During the Earth to Jupiter cruise we find that that the average ratio of alpha particle number density to that of oxygen is 66 ± 7 (Voyager 1) and 71 ± 17 (Voyager 2). These values are consistent with the value 75 ± 20 inferred from the Ion Composition Instrument on ISEE 3 during the period spanning 1978 and 1982. We have inferred an average coronal temperature of (1.7 ± 0.1) × 106 K based on the ratio of O+7 to O+6 number densities. Our observations cover a period of increasing solar activity. During this time we have found that the alpha particle to proton number density ratio is increasing with the solar cycle, the oxygen to proton ratio increases, and the alpha particle to oxygen ratio remains relatively constant in time.

Solar wind conditions in the outer heliosphere and the distance to the termination shock

The Plasma Science experiment on the Voyager 2 spacecraft has measured to date the properties of solar wind protons from 1 to 40.4 AU. We use these observations to discuss the probable location and motion of the termination shock of the solar wind. A least squares fit of proton ram pressure to heliocentric distance R over this distance yields a ram pressure equal to (1.67 × 10−8 dynes cm−2) R−2.00 ± 0.02, where R is measured in astronomical units. Assuming that the interstellar pressure is due to a 5 µG magnetic field draped over the upstream face of the heliopause, this radial variation of ram pressure implies that the termination shock will be located at an average distance near 89 AU. This distance scales inversely as the assumed field strength, i.e., for a 7 µG field, the termination shock will be located on average at 64 AU. In addition to the global falloff with distance, there are large variations in ram pressure on relatively short time scales (tens of days), due primarily to large variations in solar wind density at a given radius. Such rapid changes in the solar wind ram pressure can cause large perturbations in the location of the termination shock. Using a simple kinematic model, we study the nonequilibrium location of the termination shock as it responds to these ram pressure changes. The results of this study suggest that the position of the termination shock can vary by as much as 10 AU in a single year, depending on the nature of variations in the ram pressure, and that multiple crossings of the termination shock by a given outer heliosphere spacecraft are likely. After the first crossing, such models of shock motion will be useful for predicting the timing of subsequent crossings.

Large-scale density structures in the outer heliosphere

The Plasma Science experiment on the Voyager 2 spacecraft has measured the solar wind density from 1 to 38 AU. Over this distance, the solar wind density decreases as the inverse square of the heliocentric distance. However, there are large variations in this density at a given radius. Such changes in density are the dominant cause of changes in the solar wind ram pressure in the outer heliosphere and can cause large perturbations in the location of the termination shock of the solar wind. Following a simple model suggested by Suess, we study the non-equilibrium, dynamic location of the termination shock as it responds to these pressure changes. The results of this study suggest that the termination shock is rarely if ever at its equilibrium distance and may depart from that distance by as much as 50 AU at times.

Plasma observations near Neptune: Results from Voyager 2

Results from observations made by the Plasma Science experiment on Voyager 2 at Neptune are reviewed. The magnetosphere of Neptune is filled with a tenuous plasma, which consists of at least two components: a light ion, probably H + and a heavy ion, probably N +. Triton's atmosphere or ionosphere is thought to be the source of both heavy and light ions. Much of the low energy plasma in the inner magnetosphere is concentrated near the magnetic equator and near closest approach to the planet. The large tilt of the magnetic dipole axis from the rotation axis produces a dynamic magnetosphere which goes from an “earth-like” configuration to a “pole-on” configuration and back every 16 hours. The polar cusp regions change location and size as the planet rotates; at the time of the inbound magnetopause crossing, the phase of Neptune's rotation was such that the spacecraft entered the magnetosphere through the southern polar cusp region. Outbound from Neptune observations made in the magnetosheath show a possible signature of diurnal oscillation of the plasma mantle that grows, shrinks, and rocks, in a diurnal cycle. After the encounter with Neptune's magnetosphere, an upstream wave event was observed when the interplanetary magnetic field connected to the bow shock. The low frequency waves observed appear to be a mixture of Alfvénic and/or fast mode waves propagating away from the planet.

Voyager 2 electron observations in the magnetosphere of Neptune

This paper describes electron data obtained during the Neptune encounter by the Voyager 2 plasma science experiment. We derive the densities and temperatures of low‐energy (10–5950 eV) electrons and the electrostatic potential of the spacecraft near Neptune. The data indicate that the escape of charged particles from Triton and the local ionization of atoms in the neutral torus originating from Triton are the major plasma sources. We infer that this neutral torus of hydrogen atoms has a density of about 300 cm‐3 and an inner boundary at 8RN. The data near Neptune exhibit signatures suggesting that both precipitation into Neptune's atmosphere and ring absorption are important plasma loss mechanisms. Plasma transport in the magnetosphere appears to be very fast; the diffusion coefficient is xl0−7L3 RN2 s−1.

Atomic and molecular hydrogen from Titan in the Kronian magnetosphere

We consider the question of the neutral gas and plasma population in the region of the Kronian magnetosphere, outside the plasma mantle and inside the magnetopause, dominated by the efflux of Titan's atmosphere. A model that has been used successfully to describe the inner magnetosphere is applied to this region under constraints derived from the Plasma Science (PLS) and Ultraviolet Spectrometer (UVS) experiments on Voyager. We find that it is not possible to reconcile the results of these two experiments simultaneously with the values of the atomic and molecular source strengths of the atmosphere of Titan extant in the literature. The possible sources of the discrepancy are delineated.

A plasma density model for Saturn based on Voyager observations

The density, temperature, and velocity of plasma in the inner magnetosphere (L&lt;12) of Saturn are well determined from observations by the Voyager plasma science experiment. We combine the ion and electron data sets from both Voyager flybys to produce an overview of the Saturnian plasma environment. A small spacecraft potential of −10 to −20 V is found to be a plausible mechanism for reconciling differences between the observed ion and electron densities. Electron and ion measurements from the Voyager 1 and 2 encounters are combined to produce a map of density contours inside L = 12. This contour map incorporates all the available Voyager thermal plasma data in this region under the assumption that the inner magnetosphere was stable during the nine months between encounters. In the model, heavy ions are confined to a narrow region near the equator because of their large temperature anisotropy and their low thermal velocity parallel to the magnetic field. The protons are spread much more widely in latitude. The model does not satisfactorily reproduce the densities of the suprathermal electrons observed inside L =7 , but these constitute only a few percent of the total electron density. The oxygen flux tube content decreases rapidly inside L = 5, indicating that losses occur in this region. The lifetime of neutral atoms in the inner magnetosphere is a strong function of latitude, with lifetimes varying from weeks to years.

Influence of the solar wind/interplanetary medium on saturnian kilometric radiation.

Previous studies on the periodicities of the Saturnian kilometric radiation (SKR) suggested a considerable solar wind influence on the occurrence of SKR, so it was obvious to investigate the relationship between parameters of the solar wind/interplanetary medium and this Saturnian radio component. Voyager 2 data from the Plasma Science experiment, the Magnetometer experiment and the Planetary Radio Astronomy experiment were used to analyze the external control of SKR. Out of the examined quantities known to be important in controlling magnetospheric processes this investigation yielded a dominance of the solar wind momentum, ram pressure and kinetic energy flux, in stimulating SKR and controlling its activity and emitted energy, and confirmed the results of the Voyager 1 analysis.

Plasma Observations Near Neptune: Initial Results from Voyager 2

The plasma science experiment on Voyager 2 made observations of the plasma environment in Neptune's magnetosphere and in the surrounding solar wind. Because of the large tilt of the magnetic dipole and fortuitous timing, Voyager entered Neptune's magnetosphere through the cusp region, the first cusp observations at an outer planet. Thus the transition from the magnetosheath to the magnetosphere observed by Voyager 2 was not sharp but rather appeared as a gradual decrease in plasma density and temperature. The maximum plasma density observed in the magnetosphere is inferred to be 1.4 per cubic centimeter (the exact value depends on the composition), the smallest observed by Voyager in any magnetosphere. The plasma has at least two components; light ions (mass, 1 to 5) and heavy ions (mass, 10 to 40), but more precise species identification is not yet available. Most of the plasma is concentrated in a plasma sheet or plasma torus and near closest approach to the planet. A likely source of the heavy ions is Triton's atmosphere or ionosphere, whereas the light ions probably escape from Neptune. The large tilt of Neptune's magnetic dipole produces a dynamic magnetosphere that changes configuration every 16 hours as the planet rotates.

Remote sensing of the termination of the solar wind via in situ plasma measurements

Since 1983 the Plasma Wave experiments on the two Voyager spacecraft have detected low frequency radio noise in the outer heliosphere which has been postulated to emanate from the terminal shock of the solar wind or, possibly, from the heliopause itself. We have examined the solar wind data from the Plasma Science experiment on the Voyager spacecraft to search for correlations with these radio emissions. We find that two anomalous high speed streams passed Voyager 2 in late 1982 and early 1983 and suggest that the interaction of the streams with the heliospheric terminal shock is responsible for the generation of the most intense radio noise observed later in the same year. If the stream speeds did not decrease in traveling to the interaction region, that region is ∼135 A.U. from the sun. This is consistent with previous estimates of the distance to the inner heliospheric shock.

A solar‐wind “trigger” for the outer heliosphere radio emissions and the distance to the terminal shock

For roughly five years the Plasma Wave experiments on the two Voyager spacecraft have detected 2 to 3 kHz radio noise in the outer heliosphere. It has been suggested that this noise originates at the terminal shock of the solar wind, and this idea has been used to estimate the distance to the shock. We have examined the solar wind data from the Plasma Science experiment on the Voyager spacecraft to search for correlations with these radio emissions. We find that two anomalous high speed streams passed Voyager 2 before the noise was initially observed. We suggest that the interaction of these streams with the terminal shock may be responsible for the more intense emission. We estimate the distance to the shock using a time‐of‐flight argument as ∼70 A.U. ‐ 140 A.U.; the larger value is consistent with an estimate using Voyager data and a standard pressure balance argument.

Plasma bulk flow in Jupiter's dayside middle magnetosphere

Using the plasma data obtained during the Voyager 1 encounter and the full response function of the Plasma Science (PLS) experiment, convective plasma velocities have been determined in the dayside middle magnetosphere of Jupiter (10 RJ &lt; r &lt; 25 RJ). We find that temperature anisotropies have very little effect on plasma velocity determination and that the plasma data are well approximated by converted, isotropic Maxwellian ion distribution functions. The insensitivity of the analysis to any thermal anisotropies which may exist allows a good determination of the bulk plasma flow velocity. In addition to the subcorotational azimuthal flow there exists a substantial nonazimuthal component of plasma flow. This nonazimuthal flow is mostly aligned (antialigned) with the local magnetic field but also exhibits a cross‐field component. The velocity pattern is inconsistent with enhanced plasma outflow in the active sector, as suggested by the corotating convection model of plasma transport. The contribution of field‐aligned flow along the curved magnetic field lines to the stress on the magnetic field is evaluated. In the region studied such flow contributes up to one half the stress produced by the azimuthal plasma flow.