Spacecraft Potential Measurements Research Articles

Simulating Secondary Electron and Ion Emission from the Cassini Spacecraft in Saturn’s Ionosphere

The Cassini spacecraft’s Grand Finale flybys through Saturn’s ionosphere provided unprecedented insight into the composition and dynamics of the gas giant’s upper atmosphere and a novel and complex spacecraft–plasma interaction. In this article, we further study Cassini’s interaction with Saturn’s ionosphere using three-dimensional particle-in-cell simulations. We focus on how electrons and ions, emitted from spacecraft surfaces due to the high-velocity impact of atmospheric water molecules, could have affected the spacecraft potential and low-energy plasma measurements. The simulations show emitted electrons extend upstream along the magnetic field, and for sufficiently high emission rates, charge the spacecraft to positive potentials. The lack of accurate emission rates and characteristics, however, makes differentiation between the prominence of secondary electron emission and ionospheric charged dust populations, which induce similar charging effects, difficult for Cassini. These results provide further context for Cassini’s final measurements and highlight the need for future laboratory studies to support high-velocity flyby missions through planetary and cometary ionospheres.

North‐south asymmetries in cold plasma density in the magnetotail lobes: Cluster observations

AbstractIn this paper, we present observations of cold (0–70 eV) plasma density in the magnetotail lobes. The observations and results are based on 16 years of Cluster observation of spacecraft potential measurements converted into local plasma densities. Measurements from all four Cluster spacecraft have been used, and the survey indicates a persistent asymmetry in lobe density, with consistently higher cold plasma densities in the northern lobe. External influences, such as daily and seasonal variations in the Earth's tilt angle, can introduce temporary north‐south asymmetries through asymmetric ionization of the two hemispheres. Likewise, external drivers, such as the orientation of the interplanetary magnetic field can set up additional spatial asymmetries in outflow and lobe filling. The persistent asymmetry reported in this paper is also influenced by these external factors but is mainly caused by differences in magnetic field configuration in the Northern and Southern Hemisphere ionospheres.

Evolution of the plasma environment of comet 67P from spacecraft potential measurements by the Rosetta Langmuir probe instrument

AbstractWe study the evolution of the plasma environment of comet 67P using measurements of the spacecraft potential from early September 2014 (heliocentric distance 3.5 AU) to late March 2015 (2.1 AU) obtained by the Langmuir probe instrument. The low collision rate keeps the electron temperature high (∼5 eV), resulting in a negative spacecraft potential whose magnitude depends on the electron density. This potential is more negative in the northern (summer) hemisphere, particularly over sunlit parts of the neck region on the nucleus, consistent with neutral gas measurements by the Cometary Pressure Sensor of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis. Assuming constant electron temperature, the spacecraft potential traces the electron density. This increases as the comet approaches the Sun, most clearly in the southern hemisphere by a factor possibly as high as 20–44 between September 2014 and January 2015. The northern hemisphere plasma density increase stays around or below a factor of 8–12, consistent with seasonal insolation change.

Deriving plasma densities in tenuous plasma regions, with the spacecraft potential under active control

AbstractVariations of the floating potential on spacecraft are often used to derive plasma densities, complementing the plasma instrument measurements. All spacecraft of the Cluster mission are equipped with Active Spacecraft POtential Control (ASPOC) instruments that control the spacecraft potential. Although the ASPOC operation increases the accuracy of electric field and plasma measurements, it prevents a direct use of the spacecraft potential variations to derive plasma densities. In this work we develop a new reconstruction method of uncontrolled spacecraft potential from controlled spacecraft potential data and a derived photoelectron curve, using multispacecraft observations. We assume that the same curve governs the plasma environment for the spacecraft with ASPOC on and off. The current work focuses on reconstructions in the magnetotail region. The period that was more extensively studied was the one from August to October 2003, when the spacecraft were close to each other, while results from the periods of August–October 2001, 2002, and 2004, when the Cluster spacecraft had their apogee in the magnetotail region, are also presented. In this way, we investigate how the photoelectron curve varies at different time intervals and also how the reconstruction results are modified when the distance between the spacecraft is increasing. Finally, by using the reconstructed results, we proceed by estimating the plasma densities and we discuss the uncertainties of such estimations. While, on average, spacecraft potential measurements are successfully reconstructed, plasma and solar activity variations on short timescales seem to affect the reconstructions, requiring in some cases more detailed analysis in order to achieve results with higher accuracy. Moreover, there are some limitations while deriving results in very tenuous plasma regions due to increased uncertainties in some of the scientific measurements used there. The results and the methods developed for this work can be applied for any other current and future mission with instruments that control the spacecraft potential, such as the recently launched Magnetospheric Multiscale Mission.

Interdependencies Between the Actively Controlled Cluster Spacecraft Potential, Ambient Plasma, and Electric Field Measurements

The payload of the four Cluster spacecraft dedicated to multipoint measurements in the earth’s magnetosphere includes ion emitters that can emit currents up to a few tens of microamperes to partially compensate for the photoemission current originating at the spacecraft surfaces and thereby achieve a reduction in the spacecraft potential from values often as high as several tens of volts positive to values well below 10 V. This effect is highly desirable to improve on-board particle measurements, especially of electrons, by reducing the photoelectron flux into the sensors and by reducing the distortions of particle trajectories in the spacecraft sheath. On the other hand, a perfectly stable spacecraft potential precludes the utilization of the spacecraft as a plasma probe, including the useful technique to estimate ambient plasma density from the spacecraft potential. This paper shows that the small residual variations of the potential still allow the determination of ambient plasma density, albeit at reduced accuracy. We present an outline of the applied method with examples. In spring 2002, in the cusp region, the Cluster interspacecraft distances were well below 1000 km. For selected intervals, the potentials of controlled and uncontrolled spacecraft are compared, taking advantage of the fact that the ion emitters were not operating on all spacecraft. Special considerations regarding the secondary electron production and its effect on the controlled spacecraft potential were found to be necessary, and examples of this aspect are presented as well. Finally, the effect of the potential control on the double-probe electric field measurements is assessed. It is found that the normal sun-aligned offset of the electric field caused by the asymmetric spatial distribution of photoelectrons around the spacecraft can be affected by the potential control. Nonetheless, the information about plasma density is not lost when active spacecraft potential control (ASPOC) is applied. This is also relevant for future missions with ASPOC, particularly the NASA Magnetospheric Multiscale mission launched in March 2015. Regular intercalibration of controlled and uncontrolled potentials is nonetheless advisable to increase the reliability of the method.

A critical overview of measurement techniques of spacecraft charging in space plasma

Under certain conditions in space there are physical processes which may cause misinterpretation of spacecraft potential measurements using some very common techniques. The use of long booms to measure the potential difference between a spacecraft and the tip of a boom has at least two deficiencies. First, when the spacecraft potential is high, the tip of the boom may be engulfed by the sheath of the spacecraft. Second, the tip of the boom itself may charge. Even if materials of high secondary emission coefficients are used for the boom tips, charging can still occur when the space plasma is energetic. Another common technique for measuring the energy shift of the incoming electron energy distribution function also has deficiencies. First, if ionization is abundant in the vicinity of the spacecraft (for example, during electron beam emissions), the energy gap resulting from the shift becomes blurred. Second, if the angular momentum of the incoming electrons is significant, the shift should not be attributed to potential energy only. The technique of using Langmuir probes, which are located at short distances from the spacecraft surface, is often deficient in measuring spacecraft potential. It may be too naive to apply either the attractive regime or the repulsive regime of Langmuir. For example, while the probe potential may be negative relative to that of the space plasma, it may be positive relative to that of the spacecraft. In that case, the probe is repelling electrons from the space plasma but attracting secondary electrons from the spacecraft.

A survey of the polar cap density based on Cluster EFW probe measurements: Solar wind and solar irradiation dependence

The plasma density above the Earth's polar caps provide crucial information about the state of the magnetosphere. This region of space is known for its tenuous plasma and extremely low plasma densities, thus making traditional measurements with particle and plasma instruments extremely difficult. A new method based on spacecraft potential measurements from the electric field instrument onboard the Cluster satellites has shown that more reliable density measurements can be obtained. In this paper, we utilize this method and present a survey of the polar cap densities and the response to changes in the solar irradiation, solar wind parameters as well as processes internal to the magnetosphere. Our observations spans a time interval of almost 10 years, thus covering almost a full solar cycle. The observations seem to confirm that solar irradiance, and thus ionization through UV absorption in the atmosphere is the most important mechanism controlling the polar cap cold plasma density. We also find positive correlations between polar cap density and solar wind density and solar wind dynamic pressure, as well as geomagnetic activity levels.

THEMIS multipoint observations of Pi2 pulsations inside and outside the plasmasphere

[1] Much evidence indicates that Pi2 pulsations in the inner magnetosphere can be explained as radially trapped fast-mode waves in the plasmasphere. Most of these studies, however, are based on observations from a single spacecraft in the plasmasphere and low-latitude ground stations within several hours of local time. Here we present simultaneous observations of two Pi2 pulsations on 26 January 2009 by three THEMIS probes (P3, P4, and P5) and the Ascension Island (ASC) ground station (L ∼ 1.002, MLT ∼ 23). By using plasma density and spacecraft potential measurements, we determined that during this time P4 and P5 were located inside the plasmasphere and P3 was located outside it. It is found that fast-mode components (compressional component Bz, azimuthal component Ey) observed by P4 and P5 have a high correlation with the H component at ASC while those at P3 do not. The Bz oscillations at P4 are nearly in phase with ground Pi2 while those at P5 near the plasmapause are almost out of phase with the H component on the ground during the two Pi2 events. The Ey -H cross phases at P4 and P5 are nearly constant at −90° for both Pi2 events. These multipoint observations strongly suggest that fast-mode waves trapped in the plasmasphere are the source of the low-latitude Pi2 pulsations.

MULTI-SCALE ANTI-CORRELATION BETWEEN ELECTRON DENSITY AND MAGNETIC FIELD STRENGTH IN THE SOLAR WIND

This work focuses on the relation between the electron density and the magnetic field strength in the solar wind, and aims to reveal its compressive nature and to determine the level of compressibility. For this purpose, we choose a period of quiet solar wind data obtained at 1 AU by the Cluster C1 satellite. The electron density is derived with a sampling time as high as 0.2 s from the spacecraft-potential measurements made by the Electric Field and Waves instrument. We use the wavelet cross-coherence method to analyze the correlation between the electron density and the magnetic field strength on various scales. We find a dominant anti-correlation between them at different timescales ranging from 1000 s down to 10 s, a result which has never been reported before. This may indicate the existence of pressure-balanced structures (PBSs) with different sizes in the solar wind. The small (mini) PBSs appear to be embedded in the large PBSs, without affecting the pressure balance between the large structures. Thus, a nesting of these possible multi-scale PBSs is found. Moreover, we find for the first time that the relative fluctuation spectra of both the electron number density and the magnetic field strength look almost the same in the range from 0.01 Hz to 2.5 Hz, implying a similar cascading for these two types of fluctuations. Probable formation mechanisms for the multi-scale possible PBSs are discussed. The results of our work are believed to be helpful for understanding the compressive nature of solar wind turbulence as well as the connections between the solar wind streams and their coronal sources.

The THEMIS ESA Plasma Instrument and In-flight Calibration

The THEMIS plasma instrument is designed to measure the ion and electron distribution functions over the energy range from a few eV up to 30 keV for electrons and 25 keV for ions. The instrument consists of a pair of “top hat” electrostatic analyzers with common 180°×6° fields-of-view that sweep out 4π steradians each 3 s spin period. Particles are detected by microchannel plate detectors and binned into six distributions whose energy, angle, and time resolution depend upon instrument mode. On-board moments are calculated, and processing includes corrections for spacecraft potential. This paper focuses on the ground and in-flight calibrations of the 10 sensors on five spacecraft. Cross-calibrations were facilitated by having all the plasma measurements available with the same resolution and format, along with spacecraft potential and magnetic field measurements in the same data set. Lessons learned from this effort should be useful for future multi-satellite missions.

Cluster observations of near-Earth magnetospheric lobe plasma densities – a statistical study

Abstract. The Cluster-mission has enabled a study of the near-Earth magnetospheric lobes throughout the waning part of solar cycle 23. During the first seven years of the mission the satellites crossed this region of space regularly from about July to October. We have obtained new and more accurate plasma densities in this region based on spacecraft potential measurements from the EFW-instrument. The plasma density measurements are found by converting the potential measurements using a functional relationship between these two parameters. Our observations have shown that throughout this period a full two thirds of the measurements were contained in the range 0.007–0.092 cm−3 irrespective of solar wind conditions or geomagnetic activity. In fact, the most probable density encountered was 0.047 cm−3, staying roughly constant throughout the entire observation period. The plasma population in this region seems to reflect an equilibrium situation in which the density is independent of the solar wind condition or geomagnetic activity. However, the high density tail of the population (ne>0.2 cm−3) seemed to decrease with the waning solar cycle. This points to a source region influenced by the diminishing solar UV/EUV-intensity. Noting that the quiet time polar wind has just such a development and that it is magnetically coupled to the lobes, it seems likely to assume that this is a prominent source for the lobe plasma.

Correction to “Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions”

Correction to “Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions”

Electron density estimations derived from spacecraft potential measurements on Cluster in tenuous plasma regions

Spacecraft potential measurements by the EFW electric field experiment on the Cluster satellites can be used to obtain plasma density estimates in regions barely accessible to other type of plasma experiments. Direct calibrations of the plasma density as a function of the measured potential difference between the spacecraft and the probes can be carried out in the solar wind, the magnetosheath, and the plasmashere by the use of CIS ion density and WHISPER electron density measurements. The spacecraft photoelectron characteristic (photoelectrons escaping to the plasma in current balance with collected ambient electrons) can be calculated from knowledge of the electron current to the spacecraft based on plasma density and electron temperature data from the above mentioned experiments and can be extended to more positive spacecraft potentials by CIS ion and the PEACE electron experiments in the plasma sheet. This characteristic enables determination of the electron density as a function of spacecraft potential over the polar caps and in the lobes of the magnetosphere, regions where other experiments on Cluster have intrinsic limitations. Data from 2001 to 2006 reveal that the photoelectron characteristics of the Cluster spacecraft as well as the electric field probes vary with the solar cycle and solar activity. The consequences for plasma density measurements are addressed. Typical examples are presented to demonstrate the use of this technique in a polar cap/lobe plasma.

Optimal reconstruction of magnetopause structures from Cluster data

Abstract. The Grad-Shafranov (GS) reconstruction technique, a single-spacecraft based data analysis method for recovering approximately two-dimensional (2-D) magnetohydrostatic plasma/field structures in space, is improved to become a multi-spacecraft technique that produces a single field map by ingesting data from all four Cluster spacecraft into the calculation. The plasma pressure, required for the technique, is measured in high time resolution by only two of the spacecraft, C1 and C3, but, with the help of spacecraft potential measurements available from all four spacecraft, the pressure can be estimated at the other spacecraft as well via a relationship, established from C1 and C3 data, between the pressure and the electron density deduced from the potentials. Consequently, four independent field maps, one for each spacecraft, can be reconstructed and then merged into a single map. The resulting map appears more accurate than the individual single-spacecraft based ones, in the sense that agreement between magnetic field variations predicted from the map to occur at each of the four spacecraft and those actually measured is significantly better. Such a composite map does not satisfy the GS equation any more, but is optimal under the constraints that the structures are 2-D and time-independent. Based on the reconstruction results, we show that, even on a scale of a few thousand km, the magnetopause surface is usually not planar, but has significant curvature, often with intriguing meso-scale structures embedded in the current layer, and that the thickness of both the current layer and the boundary layer attached to its earthward side can occasionally be larger than 3000km.

Comparison of interball-2 spacecraft potential from IESP and KM-7 experiments in high-latitude regions of the magnetosphere at altitudes of 2–3 R E

Measurements of Interball-2 spacecraft potential by two instruments, IESP and KM-7, are reviewed and simultaneous measurements are compared. Unacceptable discrepancy between results of spacecraft potential measurements, including opposite signs, was found. Actually, both experiments are methodically identical: they used the same type of sensor - spherical Langmuir probes operating in the “floating” mode, and they have measured the same parameter - the voltage difference between the probe and the satellite structure. The IESP instrument measured one value of this parameter at fixed bias current to the probe. The KM-7 measured the whole current-voltage characteristic (the probe current as a function of the probe potential), which consists of 11 IESP-type measurements at different values of bias current. The difference lies only in the way of technical implementation, as the probes were operating in different ambient conditions. The IESP probes were mounted at the ends of long booms and thus were affected by the solar UV emission, while the KM-7 probe was rather well protected against UV emission by proper mounting and screening the head of the sensor from both direct UV emission and those reflected from spacecraft elements. The comparison of two data sets and variations along the orbit has shown, that KM-7 correctly measures the spacecraft potential. In high-latitude inner magnetosphere (in auroral region and polar cap) at altitudes 2–3 R E the spacecraft potential was predominantly negative down to −10 V, increasing up to about +5 V in some locations at rather extended parts of the satellite orbit. Reasonably good agreement between two experiments was obtained only at spacecraft potential ≤ −2 V and after inversion the sign of available calibration curve of IESP experiment; in this case both experiments very accurately reflect variation of potential data even in the small details. The operation mode of IESP experiment was not optimal and requires further analysis. The wave results obtained with IESP may be considered as reliable only when both experiments show, after inversion of available IESP calibration curve, coincident data, but the phase relations, if any, will be inversed in any case. The method of spacecraft potential measurements adopted in IESP should be used only for positive potential; if realspacecraft potential is negative, the method gives wrong and misleading results.

Seasonal variations along auroral field lines: Measurements from the Polar spacecraft

[1] Measurements from the Polar electric field instrument are used to study large electric fields and the ambient plasma density as a function of altitude (1.8–6.0 RE geocentric). Results from the premidnight sector (1800–2400 MLT) along auroral field lines show roughly a fourfold increase in the occurrence of small scale size, large amplitude electric fields (>100 mV m−1) at altitudes from 1.9 to 2.5 RE for dark compared to sunlit ionospheric conditions. Density values inferred from spacecraft potential measurements show these electric fields to be correlated with low plasma densities (0.2 to 3.0 cm−3). An increase in the average plasma density from 10 to 60 cm−3 is also observed for sunlit compared to dark conditions for the same altitude range. In addition, the distribution of density measurements from 1.9 to 2.2 RE also show evidence for an increase in cold ionospheric plasma (from ∼30 to 60 cm−3) for sunlit compared to dark ionospheric conditions.

A statistical study of large‐amplitude parallel electric fields in the upward current region of the auroral acceleration region

We present a survey of 64 direct observations of large‐amplitude parallel electric fields E∥ in the upward current region of the southern auroral acceleration zone, obtained by the three‐axis electric field experiment on Polar. These E∥ events range in amplitude from about 25 to 300 mV/m and represent a significant fraction of the total electric field strength (E∥/E⟂ ranges from ∼0.25 to O(10)). The E∥ structures, which tend to occur at the edges of oppositely directed (converging) pairs of perpendicular electric field structures (electrostatic shocks), have transverse (to the magnetic field) widths of ∼1.0–20 km at altitudes ranging from 0.8RE to 1.5RE, assuming the structures are stationary. The parallel potential drops associated with these large‐amplitude parallel electric fields are highly localized in altitude (e.g., tens of kilometers as opposed to thousands of kilometers). The amplitude of the parallel electric field shows a strong anticorrelation with the plasma density inferred from spacecraft potential measurements. We find no apparent correlation between the amplitude of the parallel electric field and altitudes sampled (between 0.8RE and 1.5RE), current density, and Kp, though there is a suggestion that the largest E∥/E⟂ ratios are confined to lower altitudes. Taking sampling biases into consideration, we find that the large parallel electric fields occur preferentially at higher values of Kp and within a thin layer centered about 1.28RE. A detailed analysis favors ambipolar effects over electron inertial effects as an explanation for the parallel electric field signatures.

Polar observations of electron density distribution in the Earth’s magnetosphere. 2. Density profiles

Abstract. Using spacecraft potential measurements of the Polar electric field experiment, we investigate electron density variations of key plasma regions within the magnetosphere, including the polar cap, cusp, trough, plasmapause, and auroral zone. The statistical results were presented in the first part of this study, and the present paper reports detailed structures revealed by individual satellite passes. The high-altitude (> 3 RE) polar cap is generally one of the most tenuous regions in the magnetosphere, but surprisingly, the polar cap boundary does not appear as a steep density decline. At low altitudes (1 RE) in summer, the polar densities are very high, several 100 cm-3 , and interestingly, the density peaks at the central polar cap. On the noonside of the polar cap, the cusp appears as a dense, 1–3° wide region. A typical cusp density above 4 RE distance is between several 10 cm-3 and a few 100 cm-3 . On some occasions the cusp is crossed multiple times in a single pass, simultaneously with the occurrence of IMF excursions, as the cusp can instantly shift its position under varying solar wind conditions, similar to the magnetopause. On the nightside, the auroral zone is not always detected as a simple density cavity. Cavities are observed but their locations, strengths, and sizes vary. Also, the electric field perturbations do not necessarily overlap with the cavities: there are cavities with no field disturbances, as well as electric field disturbances observed with no clear cavitation. In the inner magnetosphere, the density distributions clearly show that the plasmapause and trough densities are well correlated with geomagnetic activity. Data from individual orbits near noon and midnight demonstrate that at the beginning of geomagnetic disturbances, the retreat speed of the plasmapause can be one L-shell per hour, while during quiet intervals the plasmapause can expand anti-earthward at the same speed. For the trough region, it is found that the density tends to be an order of magnitude higher on the day-side (~1 cm-3) than on the nightside (~0.1–1 cm-3), particularly during low Kp.Key words. Magnetospheric physics (auroral phenomena; plasmasphere; polar cap phenomena)

Polar observations of electron density distribution in the Earth’s magnetosphere. 1. Statistical results

Abstract. Forty-five months of continuous spacecraft potential measurements from the Polar satellite are used to study the average electron density in the magnetosphere and its dependence on geomagnetic activity and season. These measurements offer a straightforward, passive method for monitoring the total electron density in the magnetosphere, with high time resolution and a density range that covers many orders of magnitude. Within its polar orbit with geocentric perigee and apogee of 1.8 and 9.0 RE, respectively, Polar encounters a number of key plasma regions of the magnetosphere, such as the polar cap, cusp, plasmapause, and auroral zone that are clearly identified in the statistical averages presented here. The polar cap density behaves quite systematically with season. At low distance (~2 RE), the density is an order of magnitude higher in summer than in winter; at high distance (>4 RE), the variation is somewhat smaller. Along a magnetic field line the density declines between these two altitudes by a factor of 10–20 in winter and by a factor of 200–1000 in summer. A likely explanation for the large gradient in the summer is a high density of heavy ions that are gravitationally bound in the low-altitude polar cap. The geomagnetic effects are also significant in the polar cap, with the average density being an order of magnitude larger for high Kp; for an individual case, the polar cap density may increase even more dramatically. The plasma density in the cusp is controlled primarily by the solar wind variables, but nevertheless, they can be characterized to some extent in terms of the Kp index. We also investigate the local time variation of the average density at the geosynchronous distance that appears to be in accordance with previous geostationary observations. The average density decreases with increasing Kp at all MLT sectors, except at 14–17 MLT, where the average density remains constant. At all MLT sectors the range of the density varies by more than 3 orders of magnitude, since the geostationary orbit may cut through different plasma regions, such as the plasma sheet, trough, and plasmasphere.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; plasmasphere; polar cap phenomena)

Variation of the spacecraft potential in the magnetosphere

This paper deals with the surface charging of the Polar satellite, using the spacecraft potential measurements of the electric field instrument (EFI) gathered during 48 months in 1996–1999. For most of the time, the satellite floats at a positive potential V sc in the magnetosphere because of a high flux of photoelectrons emitted from the satellite surface; typically V sc varies between 0 and 50 V so that it is 0– 1 V in the plasmasphere, at the plasmapause it exhibits a steep increase by 3– 5 V , and outside the plasmasphere V sc is usually more than 5 V . On the dayside magnetosphere V sc is 7– 8 V in average, whereas on the nightside it lies in the 10– 20 V range. In the cusp, however, V sc is only about 3 V , similar to spacecraft potentials observed in the magnetosheath. Highest V sc's occur in the high-altitude (>4 R E) polar cap, where V sc is usually between 20 and 30 V , and on auroral field lines where it is frequently in the 30– 50 V range and occasionally, approximately 1.5% of time, above 50 V .