Search

Context. Strong electron cooling on the neutral gas in cometary comae has been predicted for a long time, but actual measurements of low electron temperature are scarce. Aims. Our aim is to demonstrate the existence of cold electrons in the inner coma of comet 67P/Churyumov-Gerasimenko and show filamentation of this plasma. Methods. In situ measurements of plasma density, electron temperature and spacecraft potential were carried out by the Rosetta Langmuir probe instrument, LAP. We also performed analytical modelling of the expanding two-temperature electron gas. Results. LAP data acquired within a few hundred km from the nucleus are dominated by a warm component with electron temperature typically 5–10 eV at all heliocentric distances covered (1.25 to 3.83 AU). A cold component, with temperature no higher than about 0.1 eV, appears in the data as short (few to few tens of seconds) pulses of high probe current, indicating local enhancement of plasma density as well as a decrease in electron temperature. These pulses first appeared around 3 AU and were seen for longer periods close to perihelion. The general pattern of pulse appearance follows that of neutral gas and plasma density. We have not identified any periods with only cold electrons present. The electron flux to Rosetta was always dominated by higher energies, driving the spacecraft potential to order − 10 V. Conclusions. The warm (5–10 eV) electron population observed throughout the mission is interpreted as electrons retaining the energy they obtained when released in the ionisation process. The sometimes observed cold populations with electron temperatures below 0.1 eV verify collisional cooling in the coma. The cold electrons were only observed together with the warm population. The general appearance of the cold population appears to be consistent with a Haser-like model, implicitly supporting also the coupling of ions to the neutral gas. The expanding cold plasma is unstable, forming filaments that we observe as pulses.

The Rosetta dual Langmuir probe instrument, LAP, utilizes the multiple powers of a pair of spherical Langmuir probes for measurements of basic plasma parameters with the aim of providing detailed knowledge of the outgassing, ionization, and subsequent plasma processes around the Rosetta target comet. The fundamental plasma properties to be studied are the plasma density, the electron temperature, and the plasma flow velocity. However, study of electric fields up to 8 kHz, plasma density fluctuations, spacecraft potential, integrated UV flux, and dust impacts is also possible. LAP is fully integrated in the Rosetta Plasma Consortium (RPC), the instruments of which together provide a comprehensive characterization of the cometary plasma.

The Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe (LP) sensor observed the cold plasma environment around Titan during the first two flybys. The data show that conditions in Saturn's magnetosphere affect the structure and dynamics deep in the ionosphere of Titan. The maximum measured ionospheric electron number density reached 3800 per cubic centimeter near closest approach, and a complex chemistry was indicated. The electron temperature profiles are consistent with electron heat conduction from the hotter Titan wake. The ionospheric escape flux was estimated to be 10 25 ions per second.

The Rosetta spacecraft will rendez-vous with comet Wirtanen round 2011 and will study it for a period of nearly two years, thus providing a unique opportunity to monitor its behaviour over a wide range of distances from the Sun. A plasma and wave package, the Rosetta Plasma Consortium, RPC, will be part of the Rosetta orbiter payload. RPC is a highly integrated package that consists of five sensors: the Langmuir Probe, LAP, the Ion and Electron Sensor, IES, the Ion Composition Analyzer, ICA, the Fluxgate Magnetometer, MAG, and the Mutual Impedance Probe, MIP. RPC also includes the common instrument control, spacecraft interface, and power management unit, PIU, plus the Electrical Ground Support Equipment, EGSE. The prime objectives of RPC are to investigate: (1) the physical properties of the cometary nucleus and its surface, (2) the inner coma structure, dynamics, and aeronomy, (3) the development of cometary activity, and the microscopic and macroscopic structure of the solar-wind interaction region as well as the formation and development of the cometary tail. In addition, the planned asteroid, possibly Otawara and Siwa, flybys will provide an excellent opportunity to study in detail the physics of the solar wind-asteroid interaction. The magnetic and electric conductivity properties of the asteroid will also be studied.

Solitary electrostatic waves (SWs) were frequently observed by the Viking satellite in the auroral plasma. Previously, Malkki et al. [1993] reported occurrence statistics and voltage data for these structures. In this paper, we make a detailed study of the measurement process in order to establish what further knowledge can be extracted from the data. The possibilities that SWs are spacecraft-generated pulses or signatures of rectification of hypothetical wave packets are investigated and rejected. We analyze Langmuir probe data from several SW events and show that densities are at least as low as previously reported (a few cm−3). In order to assess the quality of the SW observations in this very tenuous plasma, we use a semiempirical model of the complete probe-spacecraft-plasma system, including nonlinearities in the probe sheaths. The usefulness of the model is shown by its ability to model details in the measured fluctuations. It is found that the errors in the voltage (electric field) measurements of solitary waves are very small, while the density fluctuation estimates may be severely affected by spurious signals. Therefore details in the shape of the observed probe current signature cannot be used to study the detailed shape of the density depletion. This limits the possibility of identifying weak double layer (WDL) characteristics in SWs, as effects of time evolution result in signatures similar to those of net potential drops.

Lower hybrid cavities (LHC), i. e., small-scale density depletions coinciding with enhanced lower hybrid waves, are frequent in the upper ionosphere. It is found that they exist in all regions the Freja satellite has scanned, as long as lower hybrid waves are present. They are found most frequently at magnetic latitudes between 55° to 65°, preferably on the morning side. In the auroral oval, the LHCs do not occur as densely as at lower latitudes but they usually form larger structures.

The electric-field and wave experiment (EFW) on Cluster is designed to measure the electric-field and density fluctuations with sampling rates up to 36000 samples s-1. Langmuir probe sweeps can also be made to determine the electron density and temperature. The instrument has several important capabilities. These include (1) measurements of quasi-static electric fields of amplitudes up to 700 mV m-1 with high amplitude and time resolution, (2) measurements over short periods of time of up to five simualtaneous waveforms (two electric signals and three magnetic signals from the seach coil magnetometer sensors) of a bandwidth of 4 kHz with high time resolution, (3) measurements of density fluctuations in four points with high time resolution. Among the more interesting scientific objectives of the experiment are studies of nonlinear wave phenomena that result in acceleration of plasma as well as large- and small-scale interferometric measurements. By using four spacecraft for large-scale differential measurements and several Langmuir probes on one spacecraft for small-scale interferometry, it will be possible to study motion and shape of plasma structures on a wide range of spatial and temporal scales. This paper describes the primary scientific objectives of the EFW experiment and the technical capabilities of the instrument.

We have analyzed complete spectrum of waves (0-4 MHz) and electron distributions during events of modulated Langmuir waves observed by Freja in the topside polar ionosphere. Modulated Langmuir wave ...

Various electrostatic solitary structures are observed by the Freja satellite in the upper ionosphere in the auroral zone. The use of multiple probes makes the determination of their propagation characteristics possible. Some of them are possibly similar to the solitary waves and weak double layers previously observed in the magnetosphere, but the Freja examples move much slower. Their size is less than 100 meters. Another type is the lower hybrid cavities, density depletions with a lower hybrid wave packet inside. The depletions are about 5%, and they have little or no speed relative to the ambient plasma in the direction perpendicular to the geomagnetic field. A third type is related to the lower hybrid cavities, but inside the density depletion there is a solitary electrostatic structure, as well as lower hybrid waves.

Localized density depletions filled with electrostatic waves in the lower hybrid frequency range are commonly observed by the wave instrument on the Freja satellite. We refer to these phenomena by the phenomenological name Lower Hybrid Cavities (LHCs). Typically, the amplitude of the density depletion is a few per cent of the ambient plasma density, and its width is around 50 m. The structures are identified at all magnetic latitudes we have searched (60–75 degrees), and at all local times at the satellite altitude (around 1,700 km). Clear examples are found in regions with fairly low or moderate wave activity, but not where the highest wave amplitudes are encountered. We do not investigate the detailed small-scale correlation between LHCs and ion heating in this letter, but note that up to now, we have no LHC observations in intense ion heating regions.

Citation: Cairns, R. A., A. A. Mamun, R. Bingham, R. Bostrom, R. O. Dendy, C. M. C. Nairn, and P. K. Shukla (2011), Correction to “Electrostatic solitary structures in non‐thermal plasmas,” Geophys. Res. Lett., 38, L13309, doi:10.1029/2011GL048422. [1] In the paper “Electrostatic solitary structures in non‐ thermal plasmas” by R. A. Cairns et al. (Geophysical Research Letters, 22(20), 2709–2712, 1995) the second author’s name was misspelled. The correct spelling appears above.

The Titan's plasma wake has been investigated using observations from the Radio and Plasma Wave Science (RPWS) instrument onboard the Cassini spacecraft during one Titan flyby on December 26, 2005. The Langmuir Probe and the wideband receiver suggest a strong asymmetry of the plasma wake, which is displaced from the ideal wake. Two distinct structures are identified inbound and outbound of the flyby with significantly different electron number densities (ne). The maximum electron number density reached 14 cm−3 on the Saturn side, connected to the sunlit ionosphere, while on the opposite side of Saturn observations indicate a density smaller than 2 cm−3. Other derived parameters of the Langmuir probe analysis suggest also a difference in plasma composition between the two structures, where heavy and light ions dominate the Saturn and anti-Saturn side respectively. The total ion outflow is estimated at 2–7 × 1025 ions/s assuming a cylindrical geometry for the plasma wake.

International audience; We present new results from the inner magnetosphere of Saturn obtained by the Radio and Plasma Wave Science (RPWS) investigation onboard Cassini around the period of the Saturn orbit injection (July 1, 2004). Plasma wave electric field emissions, voltage sweeps by the Langmuir probe (LP) and radio sounder data were used to infer the cold plasma (

[1] The distribution of current-driven electrostatic potentials along auroral magnetospheric flux tubes is studied analytically, based on the principles of quasi-neutrality and kinetic orbital motion of electrons and ions governed by inertia, electric forces, and magnetic mirror forces. When certain particle accessibility conditions are fulfilled, the kinetics determines the electron and ion number densities as unique functions of position along the flux tube and local potential. These functions are used to define potential profiles for which electron and ion number densities everywhere match, or alternatively positions where potential jumps (double layers) may occur. Magnetospheric and ionospheric particle sources in the form of Maxwellians with loss cones depending on the total voltage drop are considered, as well as trapped particles. The potential profile generally comprises a potential jump that accounts for a sizeable fraction of the total voltage drop (only by assuming unrealistic velocity distributions of the magnetospheric electron source can solutions without a jump be found). The jump may typically occur around an altitude of 1 RE, an altitude that increases with density of the ionospheric source and that decreases with increasing density of trapped, secondary, and backscattered particles and total voltage drop along the flux tube. Below the potential jump, only small potential variations occur. Above the potential jump, the potential falls off gradually, asymptotically linear in magnetic field intensity. Only a small percentage of the potential drop occurs above an altitude of a few RE, but the weak field extending to higher altitudes is essential for quasi-neutrality and for the current-voltage relationship (discussed in a previous paper). Above the potential jump, the number density of particles is reduced as compared to regions without current flow and potential drops. Depending on the density of the ionospheric source, solutions are possible for total potential drops much larger than the temperature of the magnetospheric electron source. While full accessibility for the mirroring electrons tends to be locally violated, accessibility for all particles within the source cone is fulfilled in these solutions, so the current-voltage relationship does not depend on the shape of the potential profile. Some comments are also made on transient conditions when there is no time for transport of neutralizing ions from the ionosphere.

The Cassini radio and plasma wave investigation is designed to study radio emissions, plasma waves, thermal plasma, and dust in the vicinity of Saturn. Three nearly orthogonal electric field antennas are used to detect electric fields over a frequency range from 1 Hz to 16 MHz, and three orthogonal search coil magnetic antennas are used to detect magnetic fields over a frequency range from 1 Hz to 12 kHz. A Langmuir probe is used to measure the electron density and temperature. Signals from the electric and magnetic antennas are processed by five receiver systems: a high frequency receiver that covers the frequency range from 3.5 kHz to 16 MHz, a medium frequency receiver that covers the frequency range from 24 Hz to 12 kHz, a low frequency receiver that covers the frequency range from 1 Hz to 26 Hz, a five-channel waveform receiver that covers the frequency range from 1 Hz to 2.5 kHz in two bands, 1 Hz to 26 Hz and 3 Hz to 2.5 kHz, and a wideband receiver that has two frequency bands, 60 Hz to 10.5 kHz and 800 Hz to 75 kHz. In addition, a sounder transmitter can be used to stimulate plasma resonances over a frequency range from 3.6 kHz to 115.2 kHz. Fluxes of micron-sized dust particles can be counted and approximate masses of the dust particles can be determined using the same techniques as Voyager. Compared to Voyagers 1 and 2, which are the only spacecraft that have made radio and plasma wave measurements in the vicinity of Saturn, the Cassini radio and plasma wave instrument has several new capabilities. These include (1) greatly improved sensitivity and dynamic range, (2) the ability to perform direction-finding measurements of remotely generated radio emissions and wave normal measurements of plasma waves, (3) both active and passive measurements of plasma resonances in order to give precise measurements of the local electron density, and (4) Langmuir probe measurements of the local electron density and temperature. With these new capabilities, it will be possible to perform a broad range of studies of radio emissions, wave-particle interactions, thermal plasmas and dust in the vicinity of Saturn.

[1] The current–voltage characteristic of magnetospheric magnetic flux tubes is studied under rather general conditions, assuming the current to be carried by collision-free particles experiencing inertia, electric forces, and magnetic mirror forces. The approach is that of the powerful kinetic “orbital motion” theory of current collectors in plasma as developed by H. M. Mott-Smith and I. Langmuir in 1926. When C. Davisson's 1925 condition on particle access is fulfilled, the total voltage drop is uniquely determined from the current by kinetic/geometric considerations only, without considerations of the actual space charge and electrostatic potential distribution. For any velocity distribution that is isotropic within the source cone, the current–voltage characteristic is linear for sufficiently small currents. The nature of the corresponding global flux tube electric resistance is discussed. The current–voltage characteristic is given for a variety of distribution functions (where the Maxwellian case corresponds to that studied by S. Knight in 1973) including distributions with a reduced (nonisotropic) flux in the source cone. The current–voltage characteristic is also discussed for cases where the Davisson condition is not fulfilled, when the solution method provides a lower bound on the voltage for a given current. Making assumptions on the altitude extent of the voltage drop, we can also define an upper bound on the voltage for a particular current. The magnetospheric source of particles should be taken to be located at the top of the acceleration region in order to fulfill the Davisson condition. This may limit the effective magnetic mirror ratio, such that the applicability of the linear approximation to the current–voltage characteristic is more restricted, and the saturation current is moderately small, which may lead to overvoltages, a redistribution of the current, or an increased extent of the acceleration region. Joining solutions that piecewise fulfill the Davisson condition, we can derive the current flow for a rather wide range of prescribed potential distributions. Earlier works on the Knight mechanism and related theories for determining the voltage drop along a flux tube are discussed. Determination of the electrostatic potential distribution, and verification of the Davisson condition, requires considerations of the space charge (Poisson equation), to be discussed in a second paper.

The authors present an analysis using statistical methods of a large number of observations of weak double layers and solitary waves in auroral regions, made from Viking spacecraft. Such structures, supporting non-neutral plasma regions, exhibit voltage drops along field lines, and are a possible explanation for ion acceleration mechanisms in the auroral region. The authors show that these events are found in conjunction with beams of upward flowing ions. They are primarily seen in the altitude range 7 to 11,000 km. They are seen in groups which may have perpendicular extents of a few km.

When the Viking satellite crossed the auroral acceleration region, where the electron density is supposed to be less than a few particles per cubic centimeter, a strong spin modulation of the signal of a positively biased Langmuir probe was observed. This spin modulation is better correlated with the angle of the antenna to the geomagnetic field than with the angle to the Sun. A simple model based on the photoelectron orbits in a spherically symmetric electric field, in the presence of a uniform magnetic field, gives the right phase and amplitude of the extrema. In addition, it shows that the perturbation could be negligible when the antenna is perpendicular to the magnetic field, provided that the photoelectron sources are far enough from the magnetic field lines connected to the probe (about 2 Larmor radii). As a consequence, we deduce important information for the design of experiments devoted to very low density measurements, and we show that the magnetic field may be a crucial parameter for the study of the exchange of photoelectrons between parts of a satellite. The most important result of this study is to facilitate the probe density measurements inside the acceleration structures. It is shown that in these regions the cold plasma density is depleted, with a density of thermal electrons below 5 cm−3 at altitudes between 6500 km and 8500 km. When auroral kilometric radiation (AKR) is generated inside such structures, the density of thermal electrons can be below 1 cm−3. The edges of the structures are sharply defined, and the gradient scale can be as low as 1.4 km. This is the first time that density measurements of thermal electrons have been realized inside clearly identified AKR sources, with appropriate resolution.

Birkeland (field-aligned) sheet currents flowing into and out of the auroral oval as reported by Zmuda and Armstrong (1974) are integrally associated with convective motion of plasma in the magnetotail. It is demonstrated that these currents can be driven by energy supplied by the braking of this convective motion of the plasma sheet particles as they drift toward the flanks of the magnetosphere. In the ionosphere the sheet currents close as Pedersen currents, resulting in the dissipation of power, while far from the earth the closure currents, which provide the braking force for the plasma, flow in the plasma sheet approximately normal to the neutral sheet out to radial distances of about 80 RE. During periods of moderate magnetospheric activity the Birkeland currents result in a rate of dissipation of convective energy of the order of 10 GW.

The theories concerning solitary waves and weak double layers in the auroral plasma are discussed. We make comparisons with Viking satellite observations and computer simulation experiment results of similar phenomena. The lack of velocity vs. amplitude relation in the Viking data is not consistent with the predictions of ion acoustic soliton theories and neither are the measured speeds of the structures. Numerical simulations show results, which are somewhat more consistent with Viking measurements but not with the ion acoustic soliton theory predictions. The nonlinear phase space ion hole instability offers another explanation for simulation results and space observations. Even there we still are quite uncertain, since the theory so far is only one-dimensional whereas the observations indicate that the solitary structures have a two- or three-dimensional shape with such scale sizes that the ions cannot be considered as one-dimensional.

The third flight of the Trigger experiment is described here. Band-limited electric field and density waves were observed shortly after the ion cloud was detected. Since the electric field and density components had similar spectral shape and the wave electric field was oriented parallel to the ambient magnetic field, the waves were identified as collisionless ion acoustic waves. The linear theory of ion-ion streaming is considered a likely generation mechanism for the waves.

The Viking low frequency wave experiment, with unique two point measurements of plasma density fluctuations and measurements of potential fluctuations, provided detailed diagnostics of small scale, large amplitude solitary structures in the auroral acceleration region. The structures have a spatial scale of the order of 100 m, density depletions up to 50%, negative potentials up to about 5V, and propagate upwards along the magnetic field lines with velocities from about 5 to more than 50 km s−1. Some structures have a net potential drop of up to a few volts, in general directed upwards. The structures are characterized as solitary waves (no net potential drop) and weak double layers (a net potential drop comparable to kTe/e), or holes in the thermal ion (and electron) phase space distribution. Their occurrence is correlated with that of beams of upward flowing energetic ions, dominated by protons, and electrostatic ion cyclotron waves. The field-aligned currents seem to be relatively weak. A great number of structures with a net potential may account for a large scale potential drop along the geomagnetic field lines and the observed acceleration of auroral particles up to kilovolt energies.

Plasma and Field Signatures of Poleward Propagating Auroral Precipitation Observed at the Foot of the Geos 2 Field Line

Instruments onboard the Trigger payload detected a large-amplitude, low-frequency, electric field pulse which was observed with a time delay consistent only with an electromagnetic wave. A model for this perturbation is constructed, and the associated field-aligned current is calculated as a function of altitude. This experiment may simulate the acceleration mechanism which results in the formation of auroral arcs, and possibly even other events in cosmic plasmas.

The low frequency wave experiment on Viking comprises waveform as well as power spectrum analysis of high temporal and spectral resolution and with a large dynamic range. As sensors any two of a set of triaxial electric field probes and two Langmuir probes can be used simultaneously, the latter providing measurements of plasma density fluctuations at two spatially separated points, which is a unique and very useful feature of the Viking low frequency wave experiment. Three interesting low frequency wave phenomena with characteristics different from those reported from other spacecraft experiments are reported here. Solitary structures of a vertical thickness of less than 80 m have been found to propagate at about 10 kms−1 along the fieldlines away from the Earth. Ion cyclotron harmonic waves have been observed at frequencies slightly above high harmonics of the proton gyrofrequency. Low frequency harmonic waves have also been observed where the frequency of the harmonics are multiples of the fundamental rather than being separated by the proton gyrofrequency as usually reported, and where the direction of propagation is closely perpendicular to the Earth's magnetic field.

The wave experiment of the Viking satellite frequently detected dynamic small-scale (\ensuremath{\simeq} 100 m), large-amplitude, rarefactive ($|\frac{\ensuremath{\Delta}n}{n}|\ensuremath{\lesssim}50%$) solitary waves of negative potential ($|\ensuremath{\varphi}|\ensuremath{\lesssim}2$ V) moving upwards along the magnetic field lines ($\ensuremath{\upsilon}=5 \mathrm{to} g50$ km/s). The structures, which resemble ion holes, often have an upward-directed net potential drop (\ensuremath{\lesssim} 1 V) and are then interpreted as weak double layers.

Models of auroral processes have been advanced in which the ionosphere plays an active role in stimulating auroral particle precipitation. The validity of these suggestions can be investigated by releasing an artificial plasma cloud into the ionosphere and studying the effects. To this end, the sounding rocket experiment Trigger, comprising a diagnostic and a chemical release payload, was conducted. As a consequence of the release, a drastic increase of the field-aligned charged particle flux was observed over the approximate energy range 10 eV to more than 300 keV, starting about 150 ms after the release and lasting about 1 s. There is also evidence of a second particle burst, starting 1 s after the release and lasting for tens of seconds. In addition, there is evidence for a periodic train of particle bursts occurring with a 7.7-s period from 40 to 130 s after the release. A transient electric field pulse of 200 mV/m appeared just before the particle flux increase started. Electrostatic wave emissions around 2 kHz, as well as a delayed perturbation of the E region below the plasma cloud were also observed. Some of the particle observations are interpreted in terms of field-aligned electrostatic acceleration a few hundred kilometers above the injected plasma cloud. It is suggested that the accelerating electric field was created by an instability driven by field-aligned currents caused by the plasma cloud release. This paper gives an overview of the experiment design and the general result. Particular observations and their interpretation are discussed in more detail in companion papers.

The hot magnetospheric plasma interacts electrodynamically with the cold ionospheric plasma by means of Birkeland currents flowing along geomagnetic field lines connecting the two plasmas. The fully ionized, collision free plasma of the magnetosphere and the partially ionized, collisionally dominated plasma of the ionosphere behave differently and in general there will be an electric mismatch between the two regions resulting in Birkeland current flow. The set of relations governing the system is summarized in Figure 1. We have a closed system of equations that in principle can be solved given appropriate boundary conditions (cf. Vasyliunas 1970). Most studies thus far, including this one, consider only some links of the framework. Here we will not use the approach of a boundary value problem. Rather, we will assume that some of the physical parameters are known from measurements and we will ask what information about the other quantities can be extracted from this knowledge using the governing equations. In particular we will consider possible conclusions regarding mechanisms for driving the Birkeland currents that can be obtained from the characteristics of these currents as observed by Zmuda and Armstrong (1974).

A cesium-doped high explosion was detonated at 165 km altitude in the auroral ionosphere during quiet conditions. An Alfven wave pulse with a 200-mV/m electric field was observed, with the peak occurring 135 ms after the explosion at a distance of about 1 km. The count rate of fixed energy 2-keV electron detectors abruptly increased at 140 ms, peaked at 415 ms, and indicated a downward field-aligned beam of accelerated electrons. An anomalously high-field aligned beam of backscattered electrons was also detected. The acceleration is interpreted as due to production of an electrostatic shock or double layer between 300 and 800 km altitude. The structure was probably formed by an instability of the intense field-aligned currents in the Alfven wave launched by the charge-separation electric field due to the explosion.