Retarding Potential Analyzer Research Articles

Hot-Electron Production and Wave Structure in a Helicon Plasma Source

Energetic wave-trapped electrons of $\ensuremath{\sim}20\mathrm{eV}$, moving at the 13.56 MHz helicon wave phase velocity, are directly measured with a 3 ns response-time retarding-potential analyzer. The rf axial wavelength is measured with $B$-dot probes scanned axially. Changes in the axial magnetic field or the rf power cause order of magnitude changes in the energetic electron current. The current correlates with either a transition from partially propagating to nearly pure standing waves or quantized jumps in the number of half wavelengths between the two azimuthal antenna straps.

Electron emission at the surface of metal tritide films

Monte Carlo simulations of tritium decay electrons in three metal tritide films, LiT, MgT 2 and TiT 2, of effectively infinite thickness have been performed to determine distributions of the total electron energy and the normal energy E n at the surface of these materials. Monte Carlo calculations of electron emission at the surface of an effectively infinite `slab' of tritium gas (STP) have also been carried out. Using a planar retarding potential analyzer, measurement of the integrated electron flux and the distribution of normal energies E n from 50 eV to 7.4 keV have been carried out for lithium tritide films of effectively infinite thickness prepared with various tritium to lithium atomic ratios. A comparison of the measured and computed distributions shows that the measured flux is greater than the computed result for E n below approximately 1.5 keV, while for E n above 1.5 keV the two distributions are in good agreement. Calculations of high energy secondary and Auger electron contributions suggest that the observed discrepancy at lower energies can be largely accounted for by Auger electrons associated with impurities in the near-surface region of the lithium tritide films.

Energetic electrons in the dayside mantle of Venus

The purpose of the current study is to review the available and relevant experimental data in order to examine whether there is any evidence of accelerated electrons above the dayside ionosphere of Venus, where the shocked solar wind encounters the planetary plasma, that is in the dayside mantle. Such energetic electrons were seen around Mars in the corresponding region, and theoretical studies also predict their existence at Venus. Direct measurements, however, are not available, because the retarding potential analyzer (ORPA) onboard the Pioneer Venus Orbiter had an upper energy limit of 60 eV. Therefore we were forced to look for indirect evidences. We have reexamined the ORPA data and the magnetic field measurements in the dayside mantle. We have concluded, using these indirect indicators, that this energetic electron population is very likely to be present in the dayside mantle of Venus.

Lack of thermal equilibrium between H+ and O+ temperatures in the Venus nightside ionosphere

Analysis of orbiter retarding potential analyzer (ORPA) ion data recorded in the nightside Venus hydrogen bulge region has revealed that the temperature of the H+ ions is substantially cooler by approximately 2000°K than that of the O+ ions above 200 km altitude. Below 200 km altitude the H+ and O+ temperatures cross, with the H+ temperature becoming the hotter temperature. Frequently, the velocity distribution of the H+ ions appears to be non‐Maxwellian with too large a fraction of the ions having small velocities. We suggest that this may be the result of H+ ions, created from charge exchange collisions, not having completely thermalized with the ambient H+ gas. Charge exchange collisions between cold neutral hydrogen atoms at a temperature of 100°K and the hot O+ and H+ ion gases create H+ ions at a temperature of 100°K. The temporal and spatial extent of the cooled H+ gas has not been investigated in this study. We have attempted to understand the processes responsible for the observed temperature behavior by solving numerically the coupled electron, H+, and O+ energy equations in one‐dimensional form. Neglecting convective transport terms but including for the first time cooling processes resulting from charge exchange between H+ and O+ ions with cold neutral hydrogen atoms and using separate H+, O+, and electron thermal conductivities corrected for collisions, we have obtained temperature profiles for the H+ and O+ gases which are consistent with the observed profiles. The process primarily responsible for the observed behavior of the H+ and O+ temperatures appears to be the difference in the thermal conductivity of the two ion gases with the thermal conductivity of H+ being several times larger than that of O+given the same temperature and density. This result must be tempered with the fact that several processes or conditions, which may be important, have been neglected.

Long wave part of the plasma turbulence spectrum in the lower e-region

In the lower polar E-region electric field and plasma density fluctuations were measured with rocket-borne instrumentation (a floating double probe system and a retarding potential analyzer) during the ROSE project. The observational data are presented and compared with the recently developed theory (Gurevich et al. 1996) for long wave ionospheric plasma oscillations induced by the neutral turbulence. The measured shape and anisotropy of the plasma turbulence spectra at ionospheric heights below about 95 km are well described by this theory. Significant amplification of the low frequency plasma fluctuations in strongly disturbed ionospheric conditions is established.

Solar activity variations in the composition of the low‐latitude topside ionosphere

Ion composition data from the retarding potential analyzer onboard the Defense Meteorological Satellite Program (DMSP) F10 have been analyzed for the months of June and September for the years 1991–1994 when the solar F10.7 flux changed from near 200 to less than 100. Low‐latitude composition data have been averaged by dip latitude and geographic longitude for morning and evening passes to investigate variations attributable to solar variability. In 1991 the dominant ion is O+ in both the morning and the evening, but by 1994, O+ is the dominant ion only at certain locations in the morning. The O+ −H+ transition height is well above the DMSP altitude (800 km) in 1991, but the transition height is near or below 800 km in 1994. Neutral wind induced longitude variations in the topside are present under all levels of solar activity, but the differing role of interhemispheric plasma transport at different solar activity levels dramatically changes the latitude distributions resulting from the winds. At high solar activity the O+ −H+ transition height is well above 800 km, and interhemispheric transport of O+ in the flux tubes connecting the northern and southern hemispheric topside reduces the latitude asymmetry in O+ while producing the minimum observable H+ asymmetries at night. At lower solar activity levels, H+ dominates the topside ionosphere above 800 km, and larger latitudinal asymmetries in the O+ concentration are observed, while the H+ latitude distribution remains quite symmetric at all local times.

Magnetic field control of the dayside ion temperatures in the ionosphere of Venus

Ion temperatures measured by the retarding potential analyzer carried aboard the Pioneer Venus Orbiter were sorted according to the ionospheric magnetic field conditions. It was found that the dayside ion temperatures measured during the first couple of years of operations, corresponding to solar maximum conditions and while the periapsis was held at low altitudes (∼150 km), were basically independent of the magnetic conditions. This contrasts with earlier studies, which showed that electron temperatures are significantly elevated when the ionosphere is magnetized. Theoretical studies by Cravens et al. [1980] predicted that electron temperatures will depend on fluctuating magnetic fields, but not the ion temperatures. This is consistent with these findings.

Low energy (E

Low energy (<400 eV) ion fluxes measured by the hyperbolic retarding potential analyser (HARP), during the first two elliptic orbits of the Phobos 2 spacecraft, are presented. Combining these results with the overlapping higher energy observations of the TAUS instrument, carried on the same spacecraft, we show the presence of flowing and shocked/thermalized solar wind ions inside the magnetosheath and antisunward flowing heavy ion population below the upper boundary of the magnetic barrier. Our results are not consistent with the presence of a dense, stagnated planetary ion population there, in the region explored by Phobos-2. Our results do not indicate the existence of additional sharp plasma boundaries, such as the proposed ion composition (mass loading) boundary. The observed characteristics of the outer regions of the Venus mantle and the combined HARP and TAUS measurements close to and inside of the magnetic barrier at Mars do not indicate the presence of significant differences.

Incidence of extensive plasma density depletions in the lower F region

The Retarding Potential Analyser Payload onboard the Indian satellite SROSS-C recorded a few very steep plasma depletion occurrences near the magnetic equator in the pre-midnight hours during June–July, 1992. These plasma depletions that tantamount to an ionospheric hole in the altitude range from 230 to 300 km, always exist northward of the magnetic equator and extend to latitudes as high as 20°N. However, very adjacent to the magnetic equator on the southern side, a spectacular increase is seen in the ion density sometimes from less than 10 3 to almost 10 6 ions cm −3 within a latitude range of mere 5°, and then tapers off gradually towards southern latitudes. Thus, a peak in ionization which sometimes exceeds in magnitude even the daytime maximum value is seen during nighttime hours at south of the equator. These unusually large depletions and enhancements in plasma density distribution on either side of the magnetic equator, perhaps, can be explained in terms of the plasma transport due to transequatorial meridional winds and the F region electric fields over the equatorial and low latitudes.

Suprathermal electrons associated with a plasma discharge on an active sounding rocket experiment

Electrons with energies up to 600 eV are observed with the retarding potential analyzer (RPA) instrument aboard the Several Compatible Experiments (SCEX) III sounding rocket. The electrons are concomitant with high‐energy (2–6 keV) electron gun injections and also evidence themselves by luminosity observed with 3805 Å and 3914 Å photometers. Both the collected electron flux and luminosity measurements are strongly nonlinear with gun injection current. For a typical event, the electron distribution is similar to laboratory beam‐plasma discharge (BPD) distributions reported by Sharp (1982) and when backed by HF electric field observations (Goerke et al., 1992; Llobet et al., 1985), the BPD mechanism becomes a most likely explanation. Strong turbulence theories of BPD predict a power law tail in the electron distribution, and we compare our spectral index with some previous observations.

Adaptive identification and characterization of polar ionization patches

Dynamics Explorer 2 (DE 2) spacecraft data are used to detect and characterize polar cap “ionization patches”, loosely defined as large‐scale (&gt;100 km) regions where the F region plasma density is significantly enhanced (≳ 100%) above the background level. These patches are generally believed to develop in or equatorward of the dayside cusp region and then drift in an antisunward direction over the polar cap. We have developed a flexible algorithm for the identification and characterization of these structures, as a function of scale‐size and density enhancement, using data from the retarding potential analyzer, the ion drift meter, and the langmuir probe on board the DE 2 satellite. This algorithm was used to study the structure and evolution of ionization patches as they cross the polar cap. The results indicate that in the altitude region from 240 to 950 km ion density enhancements greater than a factor of 3 above the background level are relatively rare. Further, the ionization patches show a preferred horizontal scale size of 300–400 km. There exists a clear seasonal and universal time dependence to the occurrence frequency of patches with a northern hemisphere maximum centered on the winter solstice and the 1200–2000 UT interval.

Ion density, temperature, and composition of the Venus nightside ionosphere during a period of moderate solar activity: Implications for maintaining the central nightside

Ion density, temperature, and the partial densities of the main constituents of the Venusian nightside ionosphere during moderate solar activity are presented. The data were measured by the retarding potential analyzer of the Pioneer Venus Orbiter during the entry phase of the mission in 1992. The highly variable plasma density is, on average, clearly reduced relative to that measured at high solar activity. A significant dawn‐dusk asymmetry, an excess of H+ at dawn and O+ at dusk, occurs in the postterminator sector. In the central nightside sector, beyond 150° solar zenith angle, the ionosphere is strongly depleted. The ion temperature distribution is similar to that measured during high solar activity, but the values are slightly smaller. The measurements indicate that the ion flux across the terminator, which is the dominant maintenance source for the nightside ionosphere at high solar activity, decreases with decreasing solar activity. At moderate solar activity we find that plasma transport and particle precipitation contribute approximately equally to the ionization of the central sector.

Energy deposition in the upper atmosphere in the EXCEDE III experiment

The EXCEDE III sounding rocket flight of April 27, 1990 used a ∼18 Ampere ∼2.5 keV electron beam to produce an artificial aurora in the region ∼90 to ∼115 km. A “daughter” sensor payload remotely monitored the low-energy X-ray spectrum while scanning photometers measured the spatial profile of prompt emissions of N 2 + (1N) and N 2 (2P) transitions (3914Å and 3805Å, respectively). Two Ebert-Fastie spectrometers measured the spectral region from 1800 to 8000Å. On the “mother” accelerator payload, the return current electron differential energy spectra were monitored by an electrostatic analyzer (up to 10 keV) and by a retarding potential analyzer (0 eV to 100 eV). We present an overview of the results from this experiment.

The nightward ion flow scenario at Venus revisited

In this paper, we indicate how existing Pioneer Venus Orbiter (PVO) data might be used to gain a better understanding of nightward ion flow in the Venusian ionosphere. Calculations based on PVO measurements made at solar maximum suggest that the global nightward flow of O + may be significantly greater than is required to maintain the observed nightside ionosphere densities. The validity of this conclusion depends upon (1) the accuracy with which the flow can be determined from the PVO ion density and velocity measurements and (2) the validity of the ionosphere theory used to estimate the required downward O + flux on the night side. If the measurements and theory are assumed to be accurate, the excess nightward flow implies a significant rate of ion escape from the planet, particularly at times of low solar wind dynamic pressure, Psw, when the ionopause rises to allow increased nightward flow. To illustrate a potentially important mechanism for ion escape from Venus, we present Orbiter Electron Temperature Probe (OETP) observations of plasma clouds and scavenged ionospheric plasma observed above the ionopause. We then employ OETP and Orbiter Retarding Potential Analyzer (ORPA) data to reexamine the global ion flow for average Psw conditions, and we find the net flow to be in agreement with previous estimates. We also find that the net transterminator flow at average Psw approaches the limiting upward ion flow available from the dayside ionosphere, a limit which is imposed by O + collisions with O 2 +. OETP measurements in the terminator ionosphere show that N e declines at times of low Psw. This suggests that the dayside ionosphere is unable to supply the increased demand when the ionopause rises above its average height, thus requiring an upward diversion of some of the nightward flow to populate the overlying, newly-created ionosphere. We predict that the ion velocities will also be found to decrease at times of low Psw, in part due to the source-limited flow, and in part because the expansion of the ionosphere diminishes the “nozzle effect” that has been identified as a source of nightward ion acceleration. Collectively, these effects act to limit the increase in the net nightward ion flow at times of low Psw. Since O +-O 2 + collisions will impart an upward velocity to the O 2 + ions, the O + diffusion limit may be a soft one, so the nightward flow can continue to increase somewhat as Psw declines. The collision process should also enhance the O 2 + concentration in the dayside upper ionosphere and, to the extent that the O 2 + participates in the nightward flow, should produce O 2 + enhancements on the night side as well. These expectations could be verified by an examination of the orbit-to-orbit changes in ion composition and velocity in response to Psw variations.

Spacecraft potential effects on the Dynamics Explorer 2 satellite

The relationship between the plasma environment and spacecraft potential is examined for the Dynamics Explorer 2 (DE 2) spacecraft in an attempt to improve the accuracy of ion drift measurements by the retarding potential analyzer (RPA). Because of the DE 2 orbit characteristics (apogee near 1000 km and perigee near 300 km) and the configuration of conducting surfaces on the spacecraft, thermal electrons and ions constituted the only significant contributions to the charging currents to the spacecraft surface for the majority of geophysical conditions encountered. The geomagnetic field had considerable effect on the spacecraft potential due to magnetic field confinement of the electrons as well as to the V × B electric field resulting from the movement of the spacecraft across magnetic field lines. Using a database of inferred spacecraft potentials from the RPA, measured electron temperatures from the Langmuir probe (LANG), and calculated V × B electric fields, we derive an algorithm for determining the spacecraft potential (at the location of the RPA on the spacecraft) for any point of the DE 2 orbit. Knowledge of the spacecraft potential subsequently allows us to retrieve relatively accurate ion drifts from the RPA data.

DMSP F8 observations of the mid‐latitude and low‐latitude topside ionosphere near solar minimum

The retarding potential analyzer on the DMSP F8 satellite measured ion density, composition, temperature, and ram flow velocity at 840‐km altitude near the dawn and dusk meridians close to solar minimum. Nine days of data were selected for study to represent the summer and winter solstices and the autumnal equinox under quiet, moderately active, and disturbed geomagnetic conditions. The observations revealed extensive regions of light‐ion dominance along both the dawn and dusk legs of the DMSP F8 orbit. These regions showed seasonal, longitudinal, and geomagnetic control, with light ions commonly predominating in places where the subsatellite ionosphere was relatively cold. Field‐aligned plasma flows also were detected. In the morning, ions flowed toward the equator from both sides. In the evening, DMSP F8 detected flows that either diverged away from the equator or were directed toward the northern hemisphere. The effects of diurnal variations in plasma pressure gradients in the ionosphere and plasmasphere, momentum coupling between neutral winds and ions at the feet of field lines, and E × B drifts qualitatively explain most features of these composition and velocity measurements.

High-resolution retarding potential analyzer

A simple electrostatic retarding potential analyzer (RPA) configuration is described that gives a true measure of charged particle energy irrespective of the angle of incidence of the particles. The device has an inherently high energy resolution (ΔE/E&amp;lt;0.01). The device eliminates errors in particle energy measurement usually associated with planar gridded RPAs. Electrostatic models and results from an experimental prototype are presented.

A Comparison of in situ measurements of and from Dynamics Explorer 2

Dynamics Explorer‐2 provided the first opportunity to make a direct comparison of in situ measurements of the high‐latitude convection electric field by two distinctly different techniques. The vector electric field instrument (VEFI) used antennae to measure the intrinsic electric fields and the ion drift meter (IDM) and retarding potential analyzer (RPA) measured the ion drift velocity vector, from which the convection electric field can be deduced. The data from three orbits having large electric fields at high latitude are presented, one at high, one at medium, and one at low altitudes. The general agreement between the two measurements of electric field is very good, with typical differences at high latitudes of the order of a few millivolts per meter, but there are some regions where the particle fluxes are extremely large (e.g., the cusp) and the disagreement is worse, probably because of IDM difficulties. The auroral zone potential patterns derived from the two devices are in excellent agreement for two of the cases, but not in the third, where bad attitude data may be the problem. At low latitudes there are persistent differences in the measurements of a few millivolts per meter, though these differences are quite constant from orbit to orbit. This problem seems to arise from some shortcoming in the VEFI measurements. Overall, however, these measurements confirm the concept of “frozen‐in” plasma that drifts with velocity within the measurement errors of the two techniques.

Equatorial spacecraft‐plasma interaction phenomenon observed with DE 2

Data from the retarding potential analyzer and ion drift meter on Dynamics Explorer 2 indicate that an unusual spacecraft‐plasma interaction phenomenon occurs at times when the spacecraft velocity vector becomes nearly aligned with the local geomagnetic field lines. The primary signature of the interaction is a transient increase in the ion collection currents obtained with these instruments, up to approximately 15% in magnitude and typically a few tens of seconds duration. This signature is indicative of an increase in the net ion and electron currents to the satellite and is accompanied by a small positive increase in the spacecraft potential relative to the plasma. We present here a case study covering six of the strongest such events observed with DE 2 and discuss a possible physical mechanism. We suggest in particular that what might be called a collisional electron “snowplow” effect may be occurring, and we derive a simple numerical model based on this scenario. Least squares fitting is employed to test the model and to derive new estimates of the ambient ion concentration at times when the measurements are being perturbed by the interaction.

Critical Ionization Velocity (CIV) experiment XANI onboard the INTERCOSMOS 24 — “ACTIVE” satellite

Satellite INTERCOSMOS-24 “ACTIVE” was launched on September 30 1989, with an initial apogee 2500 km, perigee 530 km and an orbital inclination of 82.2°. Under “ACTIVE” project, a wide number of plasma diagnostics instruments were especially designed to study the ionospheric response to high power VLF transmitter emissions, by means of satellite-subsatellite system during this mission. As a separate part of this project, a Critical Ionization Velocity (CIV) experiment called XANI (Xenon ANomalous Ionization) had been carried out to study neutral gas/plasma interaction processes in the ionosphere using neutral Xe gas injection. Seven Xe gas release experiments had been realized at daytime high latitude ionosphere. In this paper an Ion Drift Meter (IDM) and Retarding Potential Analyzer (RPA) data concerning transverse ion drift velocity vector measurements and total ion density variations during gas release experiment are shown. Two examples of the RPA data, taken after the beginning of Xe injection, show some prompt ionization at the ionization front about 30% in comparison with the background ion concentration. Horizontal ion drift of few hundred meters per second, transversal to the satellite velocity vector, had been observed at the ionization front.