Double-probe Instrument Research Articles

The Spin-Plane Double Probe Electric Field Instrument for MMS

The Spin-plane double probe instrument (SDP) is part of the FIELDS instrument suite of the Magnetospheric Multiscale mission (MMS). Together with the Axial double probe instrument (ADP) and the Electron Drift Instrument (EDI), SDP will measure the 3-D electric field with an accuracy of 0.5 mV/m over the frequency range from DC to 100 kHz. SDP consists of 4 biased spherical probes extended on 60 m long wire booms 90∘ apart in the spin plane, giving a 120 m baseline for each of the two spin-plane electric field components. The mechanical and electrical design of SDP is described, together with results from ground tests and calibration of the instrument.

Experimental Analysis and Numerical Simulation of the Operation of the DEMETER Electric Field Instrument in Deep Equatorial Plasma Depletions

The development of deep plasma depletions in the equatorial ionosphere at time of magnetic storms is of particular interest in space and plasma physics as well as in the field of radio communications. Detailed plasma, dc electric field, and wave observations were performed by the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions microsatellite through several depletions where the large Debye length may lead to significant disturbances on the measurements of dc electric fields by the double probe instrument. After an overview of the data acquired in a well-developed plasma cavity, we present a numerical simulation study that was undertaken to model the operation of the electric field probes in their actual environment. The initial results indicate that, in low-density ionospheric plasma and depending on the potential difference between the probes and the boom, non negligible errors may arise from the influence of the boom on the probe current collection.

Double-Probe Measurements in Cold Tenuous Space Plasma Flows

Cold flowing tenuous plasmas are common in the terrestrial magnetosphere, particularly in the polar cap and tail lobe regions, which are filled by the supersonic plasma flow known as the polar wind. Electric field measurements with double-probe instruments in these regions suffer mainly from two error sources: 1) an apparent sunward electric field due to photoemission asymmetries in the probe-boom system and 2) an enhanced negatively charged wake forming behind the spacecraft, which will affect the probe measurements. The authors investigate these effects experimentally by Fourier analysis of the spin signature from the double-probe instrument Electric Fields and Waves (EFW) on the Cluster spacecraft. They show that while the signature due to photoemission asymmetry is very close to sinusoidal, the wake effect is characterized by a spectrum of spin harmonics. The Fourier decomposition can therefore be used for identifying wake effects in the data. As a spin-off, the analysis has also given information on the cold flowing ion population

Electric field measurements on Cluster: comparing the double-probe and electron drift techniques

Abstract. The four Cluster satellites each carry two instruments designed for measuring the electric field: a double-probe instrument (EFW) and an electron drift instrument (EDI). We compare data from the two instruments in a representative sample of plasma regions. The complementary merits and weaknesses of the two techniques are illustrated. EDI operations are confined to regions of magnetic fields above 30 nT and where wave activity and keV electron fluxes are not too high, while EFW can provide data everywhere, and can go far higher in sampling frequency than EDI. On the other hand, the EDI technique is immune to variations in the low energy plasma, while EFW sometimes detects significant nongeophysical electric fields, particularly in regions with drifting plasma, with ion energy (in eV) below the spacecraft potential (in volts). We show that the polar cap is a particularly intricate region for the double-probe technique, where large nongeophysical fields regularly contaminate EFW measurments of the DC electric field. We present a model explaining this in terms of enhanced cold plasma wake effects appearing when the ion flow energy is higher than the thermal energy but below the spacecraft potential multiplied by the ion charge. We suggest that these conditions, which are typical of the polar wind and occur sporadically in other regions containing a significant low energy ion population, cause a large cold plasma wake behind the spacecraft, resulting in spurious electric fields in EFW data. This interpretation is supported by an analysis of the direction of the spurious electric field, and by showing that use of active potential control alleviates the situation.

Comparing a spherical harmonic model of the global electric field distribution with Astrid‐2 observations

Electric field measurements provided by the double probe instrument on the Astrid‐2 satellite are compared with the empirical Weimer electric field model for all magnetic local times, except between 11 and 13 MLT, and poleward of 55° corrected geomagnetic latitude (CGLat). We focus the model evaluation on its ability to predict the latitudinal locations of the convection reversal boundaries for two‐cell convection patterns and to estimate the magnitude of the electric field above 55° CGLat. A total number of 780 polar cap passes are employed from the Northern Hemisphere between January and July 1999. The measured average electric field magnitude in the dawn‐dusk meridian plane above 55° CGLat is generally 25% larger than the predicted field independent of the interplanetary magnetic field (IMF) direction. The model shows a better correspondence with the observed electric field for southward IMF than for northward IMF, with most cases centered around Bz = −1.5 nT and r = 0.88. However, the agreement for northward IMF is promising, and a few examples are shown to corroborate this fact. The observed and predicted convection reversal boundary locations along the satellite track for southward IMF are on the average found 2–3° CGLat apart in the dawn‐dusk meridian plane but may be as far apart as 9° CGLat. An initial investigation of the relative timing of a 20‐min averaging window for the IMF along the 20–25 min polar cap crossing suggests that a time‐dependent transfer function may be found that applies a higher weight to the input solar wind data early in the pass and a lower weight later in the pass for an IMF window that corresponds to the first half of the crossing and the opposite weight versus time dependence for an IMF window corresponding to the last half of the crossing.

Low‐frequency electromagnetic turbulence observed near the substorm onset site

On the basis of wave and plasma observations of the Geotail satellite, the instability mode of low‐frequency (1–10 Hz) electromagnetic turbulence observed at the neutral sheet during substorms has been examined. Quantitative estimation has also been made for the anomalous heating and resistivity resulting from the electromagnetic turbulence. Four possible candidates of substorm onset sites, characterized by the near‐Earth neutral line, are found in the data sets obtained at substorm onset times. In these events, wave spectra obtained by the search‐coil magnetometer and the spherical double‐probe instrument clearly show the existence of electromagnetic wave activity in the lower hybrid frequency range at and near the neutral sheet. The linear and quasi‐linear calculations of the lower hybrid drift instability well explain the observed electromagnetic turbulence quantitatively. The calculated characteristic electron heating time is comparable to the timescale of the expansion onset, while that of ion heating time is much longer. The estimated anomalous resistivity fails to supply enough dissipation for the resistive tearing mode instability.

Comparability and reproducibility of the results of water loss measurements: a study of 4 evaporimeters.

A study was made of the comparability and reproducibility of the results of measurements of water loss, both in vitro and in vivo, using 4 ServoMed Evaporimeters (3 single-probe and 1 double-probe instrument). The optimum time for recording transepidermal water loss (TEWL) after the initial application of the probe to the skin (in vivo), and the best technique for optimizing the accuracy of measuring TEWL were determined. An evaporation device with a constant level of water loss was constructed for in vitro studies. The volar aspect of the right forearm skin of one subject was used in vivo. Both in vitro and in vivo measurements showed that there were some differences between the results of 4 of the 5 probes. The other probe was distinctly out of range. For all probes, the reproducibility of results of successive measurements was high. Stabilization of TEWL values was reached for all probes from 30-45 s after their initial application to the skin. It is recommended that TEWL be recorded for a further 30-s period, after the initial stabilization (45 s), and that this be taken as the true value. The manufacturer's recommended calibration procedure is based only on adjustments for the standard specified humidities and zero water loss. The importance of incorporating an additional calibration procedure which includes adjustments for an actual standard constant water loss is thus strongly stressed.

Long-term behaviour of photo-electron emission from the electric field double probe sensors on GEOS-2

A double probe instrument with a probe separation of 42 m was used aboard the geostationary spacecraft GEOS-2 to measure the ambient electric field. The probes contained built-in pre-amplifiers and were current biased in order to clamp their surface potential near to the local plasma potential while the spacecraft surfaces floated at a more positive potential. Depending on the plasma density, the potential difference ranges between 1 V (dense plasma, N e > 10 cm −3) and more than 10 V (tenuous plasma, N e < 3 cm −3). In the case of the more tenuous plasma, photoelectrons emitted from the probes tend to be picked up by the more positive potential of the wire booms. The net current of this electron flow depends on the spin orientation of the wire booms with respect to the Sun. It occurrs in the data as a spurious signal and was already earlier found to be a function of the spacecraft potential. With data from quiet magnetospheric conditions ( K p ⩽ 2) the long-term variation of the spurious signal was investigated. It turned out to be constant during the initial 3 y of the mission. Afterwards, as from 1981, it became smaller and did so for the following 2 y. In 1983, the end of the 5-y period of available data, it had vanished. The cause of the drop is suggested to be a consequence of the partial removal of the conductive surface layer of the wire booms by sputter effects. Expressions were derived that yield more accurate correction of the data. A valuable by-product of this work is the statistical relation of spacecraft potential and total electron flux.

Comparison of spherical double probe electric field measurements with plasma bulk flows in plasmas having densities less than 1 cm−3

Point‐by‐point comparisons of ISEE‐1 spherical double probe dawn‐to‐dusk electric field measurements and ISEE‐2 plasma flows, converted to electric fields, are presented during a three hour time interval in the magnetotail when the plasma density varied from less than 0.1 to about 1 cm−3. The zero lag cross‐correlation coefficient between 768 second averages of the two data sets was 0.93. The cross‐correlation coefficient decreased with shorter averaging intervals due to the increasing magnitude of statistical uncertainties, becoming 0.49 for 9 second averages. A statistical relative uncertainty between pairs of points in the two data sets, estimated by inclusion only of counting statistics in the plasma measurement and time variations of the observed electric field during the measurement interval, is able to account for at least 75% of the deviations between the two data sets. During this three hour interval, the spacecraft potential (measured by the double probe instrument) varied as the logarithm of the product of the plasma density and square root of electron temperature (measured by the fast plasma analyzer), in agreement with expectations from simple Langmuir probe theory.

Midlatitude measurements of the ionospheric electric field during the ALADDIN programme

Three measurements of ionospheric electric field were made during the 24 h ALADDIN rocket programme at Wallops Island (37°50′N, 75°29′W) on June 29–30, 1974. The first of these used a double probe instrument, flown at 1500 Local Solar Time, and the second and third measurements were made by barium cloud releases at evening and morning twilight. These three electric field vectors have been compared with the predictions of a number of models of electric field due to the dynamo effects of various atmospheric tides, and also of a possible magnetospheric origin. On the assumption that the measurements were made at a location equatorward of the afternoon convergence and poleward of the morning divergence in the electric field patterns related to the Sq current cystem, Stening's model of the diurnal variation of the electric field induced by the (1, −2) tidal model at the time of the Summer solstice correctly predicts the directions of the observed electric field. Forbes and Lindzen's model, incorporating the three major propagating tidal modes as well as the evanescent (1, −2) mode, also bears an acceptable relationship to the ALADDIN electric field directions. The ALADDIN E-field magnitudes are comparable with those obtained by ground-based observations (incoherent scatter) from Millstone Hill and from Saint Santin but are about half of Stening's model values, and three times those of Forbes and Lindzen. While the Millstone Hill E-field directions are compatible with the ALADDIN observations, Saint Santin E-field directions, at the same latitude but 75° difference in longitude, are distinctly different from ALADDIN, implying that longitudinal differences are significant.

The plasmapause revisited

Saturation of the dc double-probe instrument on Explorer 45 has been used to identify the plasmapause. Fifteen months of these data resulted in a data base of sufficient size to statistically study the average position of the plasmapause over 14.5 hours of magnetic local time (MLT) under differing magnetic conditions. The afternoon-evening bulge in the L coordinate of the plasmapause versus local time was found to be centered between 20 and 21 hours MLT during magnetically quiet periods and shifted toward dusk as activity increased, but always postdusk. During quiet periods a bulge in the L coordinate near noon was also seen, which disappeared as activity increased. The average local time distribution plasmapause position during high magnetic activity was irregular in the afternoon region, where large-scale convection models predict the creation of plasmatails or detached plasma regions from increases in the solar wind induced convection. The results suggest that solar wind induced convection is partially shielded from the dayside. As the intensity of the convection is increased, it more effectively penetrates the dayside, which shifts the postdusk bulge nearer to dusk and eliminates the quiet time bulge near noon (created by Sq dynamo ionospheric electric fields projecting into the magnetosphere when dayside convection is weak). Correlations of the plasmapause L coordinate with Dst were more ordered than corresponding correlations with Kp or with the maximum Kp over the previous 12 hours.