Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES)

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ScienceDirect Advances in Space Research xxx (2017) xxx–xxx www.elsevier.com/locate/asr

Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES) P. Diego a,⇑, I. Bertello a, M. Candidi a, A. Mura a, I. Coco b, G. Vannaroni a, P. Ubertini a, D. Badoni c a

INAF/IAPS, Via Fosso del Cavaliere 100, 00133 Rome, Italy b INGV, Via Vigna Murata 605, 00143 Rome, Italy c INFN Sec. Rome2, Via Ricerca Scientifica 1, 00133 Rome, Italy Received 20 April 2017; received in revised form 4 August 2017; accepted 7 August 2017

Abstract The floating potential variability of the Electric Field Detector (EFD) probes, on board the Chinese Seismo-Electromagnetic Satellite (CSES), has been modeled, and the effects of several structural and environmental elements have been determined. The expected floating potentials of the probes are computed considering the ambient ionospheric plasma parameter variations. In addition, the ion collection variability, due to the different probe attitudes along the orbit, and its effect on each floating potential, are considered. Particular attention is given to the analysis of the shadow produced by the stubs, in order to determine the artificial electric field introduced by instrumental effects which has to be subtracted from the real measurements. The modulation of the altered electric field, due to the effect on shadowing of the ion drift, as measured by the ESA satellite Swarm A in a similar orbit, is also modeled. Such simulations are made in preparation of real EFD data analysis performed during the upcoming flight of CSES. Ó 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Ionosphere; Plasma; Electric field probes; Instrumental effects; Floating potential variability

1. Introduction The main objective of the China SeismicElectromagnetic Satellite (CSES) is the monitoring of electromagnetic emission in the ionosphere and the study of the ionospheric perturbations possibly associated with earthquakes (Fidani et al., 2012; Molchanov et al., 2006; Parrot et al., 1993). The final purpose is to consider new methods for providing reliable short term outlooks of ⇑ Corresponding author.

E-mail addresses: [email protected] (P. Diego), [email protected] (I. Bertello), [email protected] (M. Candidi), [email protected] (A. Mura), [email protected] (I. Coco), [email protected] (G. Vannaroni), pietro.ubertini@iaps. inaf.it (P. Ubertini), [email protected] (D. Badoni).

earthquake preparation mechanisms. As already performed by the DEMETER satellite (Berthelier et al., 2006), the CSES program will test the reliability of the proposed electromagnetic monitoring system, by evaluating electromagnetic fields, plasma conditions, and precipitating energetic particles at the same time (Fidani et al., 2012). The launch of CSES is currently planned for late 2017, the satellite will be placed in a Sun-synchronous circular orbit at an altitude of about 500 km, with descending node at 14:00 LT. The EFD is part of the payload of CSES, and is devoted to the measurement of the ambient electric field as described in Diego et al. (2017). In this paper we discuss the algorithms that will be used for the evaluation and the computation of several effects on the ionospheric electric field EFD measurements. These will include effects due to the presence of the conducting stubs (see Fig. 1), which

http://dx.doi.org/10.1016/j.asr.2017.08.005 0273-1177/Ó 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005

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Fig. 1. Left panel: schematic diagram of the CSES satellite indicating the deployed boom geometry with respect to the orbital motion. Right panel: sketch of EFD probe that shows the sensor current collecting surface (S), the upper stub (S1), and the lower stub (S2).

modulate the collection of ions from the ambient plasma for the different attitudes of the probes with respect to the ion flux direction. We also analyse the possible changes to the plasma collection as a consequence of the ambient plasma motion, which, combined with the ram velocity, determines different flux directions for the ions. 2. CSES orbital features and relevant plasma environment The measurements taken by the EFD reflect the external environment experienced by the instrument along the orbit. The CSES orbital parameters are defined in the document ‘‘CSES - Satellite System introduction” (Zhu, 2011) as reported below (see Table 1). Since the orbit is sun-synchronous, the inclination is not exactly 90° and the local time at sub satellite location is not always LT 14:00 but changes with latitude. Due to operational constraints, moreover, the satellite will not perform scientific measurements at poleward latitudes higher than 65°, therefore data will not be available in the polar regions. The external environment along the orbit is simulated by the IRI model (http://omniweb.gsfc.nasa.gov/vitmo/ iri_vitmo.html, Bilitza and Reinisch, 2008) for what concerns the plasma parameters, and by the IGRF model Table 1 Orbital parameters of CSES satellite. Orbit type

Sun-Synchronous

Local Time at Descending Node Semi-Major Axis Orbit Altitude Orbit Inclination Eccentricity Orbit Period Node Period Orbits Per Day

14:00 6878 km 507 km 97° 0 95 min 95 min 15 + 1/5

(http://ccmc.gsfc.nasa.gov/modelweb/models/igrf_vitmo. php, Thebault et al., 2015) for the magnetic field. In addition, plasma data recorded by the Swarm satellite have been used to improve the simulations. The expected ranges of plasma parameters have been detailed in Diego et al. (2017) by using yearly values for the entire solar cycle n.23 (1996–2009) considering, in particular, the 14 values relevant to January 1st of each year. Seasonal values have been evaluated for the descending solar cycle phase simulating that of CSES launch in solar cycle n. 24. Inter-annual variation of density is several times lower than that due to the solar cycle. On the contrary, Te does not show any dependence on solar cycle but only a seasonal variation, which is negligible with respect to the local time change along the orbit. At the CSES orbit, the dataset retrieved from IRI shows a variability in the following ranges (Diego et al., 2017): Plasma density: 7
109–2
1012 m3; Electron temperature: 1030–3289 K; Ion temperature: 885–1709 K.

3. The Electric Field Detector (EFD) The EFD payload objectives and specifications are very similar to those of the ICE experiment (Instrument Champ Electrique), already flown on board the DEMETER satellite (Berthelier et al., 2006). Four spherical probes are located at the tips of different booms (about 4 m long), deployed from the spacecraft, in order to retrieve the three components of the ambient electric field. The potential difference, measured between a given pair of probes, provides the electric field component along the direction defined by their positions. Fig. 1 shows the boom layout on the CSES satellite. The boom directions are con-

Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005

P. Diego et al. / Advances in Space Research xxx (2017) xxx–xxx

ceived to avoid the possibility that sensors could fall in the satellite or other boom wakes. In this way the electric field measurements are not affected by the electrostatic perturbations which arise within the plasma wake regions (Miyake et al., 2013). The electric field components are determined in the non-orthogonal boom reference system by electing three couples of probes to be used for the potential difference evaluation. Thus, for example, *

Eij ¼

Vpj  Vpi

ð1Þ

*

d ðpi pj Þ *

where i j are the directions of the axes in the boom reference frame defined by the distance vector between probe *

pairs d ðpi pj Þ . The electric field components are then transformed to a more suitable X, Z, Y orthogonal reference frame, where X is perpendicular to the satellite face oriented nominally along the flight direction, Z is perpendicular to the satellite face pointing nominally towards the Earth center and Y completes the right handed coordinate system (Zhu, 2016). The coordinates of the EFD probes w.r.t. the satellite-rocket docking surface are shown in Table 2. Furthermore, the electric field components can be transformed to a geophysical reference system which uses Radial, Along-track, and Cross-track (RAC) components. The Radial component is parallel to the direction pointing to Nadir, the Along-track component is parallel to the satellite velocity vector and the Cross-track component is transverse and completes the orthogonal right handed frame. XYZ and RAC ideally coincide, but possible perturbations make it necessary to retain the distinction. As shown in Fig. 1 (right panel), each sensor consists of a sphere (6 cm diameter) containing a unity gain amplifier and a current generator (Badoni et al., 2015). The purpose of the bias current source is to bring the probe potential as close as possible to that of the local plasma, where the plasma sheath and the coupling resistance between probe and plasma are minimized. Such a condition improves substantially the EFD response in terms of sensitivity and frequency band (Diego et al., 2017). The probes floating potential (Vf) is measured in a wide band from DC to about 3.5 MHz. As shown in Fig. 1, the probes are also provided with cylindrical conducting stubs bootstrapped Table 2 Coordinates of EFD probes w.r.t. the satellite-rocket docking surface. X is perpendicular to the satellite surface nominally oriented along the satellite flight direction, Z is perpendicular to the satellite surface nominally directed towards the Earth center and Y completes the right handed coordinate system (Table 2 data from Diego et al., 2017).

Probe Probe Probe Probe

A B C D

X (mm)

Y (mm)

Z (mm)

35 2866 4811.8 1433.1

3881.2 49.4 525 4306

625 4020.6 3646 3111.1

3

at the potential of the electrodes. Their dimensions are: 2 cm diameter, 4.2 cm length for the outer stub, and 6.2 cm length for the inner stub. The interface ring at boom anchor point is 2.5 cm diameter. The inner stubs are needed to minimize possible perturbations due to the boom potential (which is that of satellite ground), while the outer ones have been added for symmetry, improving the measurement accuracy (Berthelier et al., 2006). Nonetheless, the stubs could produce shielded areas which reduce the plasma collection by the sensors, altering the electric field measurements, as will be shown in the following sections. When a probe is fully immersed in a plasma, its potential moves in order to attain a net collected current equal to zero. Such potential is usually called ‘‘floating potential” Vf. As amply discussed in Diego et al. (2017), given the plasma conditions encountered along the CSES orbit, only electron and ion collections are relevant for the EFD floating potential calculation (i.e. photoelectron emission for the EFD probe is negligible at CSES altitude). The floating potential Vf is determined through: X Ik ¼ 0 k ¼ 1; 2 ð2Þ k

where Ik indicates the equations for ion and electron collected currents, expressed as a function of the probe potential. A bias current generated within the probe can be added to these, carrying the probe potential away from Vf. In a plasma at thermal equilibrium, the velocities of the electrons are characterized by a Maxwellian distribution function. For the EFD the expected thermal speed is in the range 2
105 –3:6
105 m=s (for Te between 1030 and 3289 K as stated in Section 2). In this case, the satellite velocity ð7:5
103 m=sÞ is much lower than the electron thermal speed, and can be neglected. The current collection from plasma and the floating potential of the EFD probes, along the CSES orbit, have been widely discussed in Diego et al. (2017). In that paper one of the main objectives was the evaluation of the EFD response vs. the different bias currents to evaluate the current source settings capable of minimizing the plasma coupling resistance. The optimum bias current level was determined between 5 and 10 lA which could be maintained for entire orbits, unless exceptional plasma density minima (not modeled by IRI) take place. With such bias currents, the potential of a single probe, along an entire orbit, exhibits variations of about ±300 mV about the local plasma potential. This operating range has been considered acceptable to attain a satisfactory electric field accuracy if, in particular, we consider the differential readings scheme used between different pairs of sensors for the electric field measurements, which virtually cancels the common mode offset. In Diego et al. (2017), the current collection by the EFD was based on the assumption that the ions were collected only from the ram direction while the electrons showed an isotropic distribution due to the non-negligible

Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005

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electron gyration radius w.r.t. the probe dimensions. Indeed, considering the maximum and minimum values of magnetic field and electron thermal velocities expected along the CSES orbit, the ratio of electron gyroradius to the probe radius varies in the range Rce/Rp  0.63–3. The crude isotropic assumption for electron distribution was in turn based on the Rubinstein and Laframboise (1982) paper which calculated the electron current collected by a spherical probe in a magnetic field under a wide range of probe potentials and electron thermal velocities. In particular Fig. 8 of their paper shows that, at nearly zero probe potential (vp  0, a condition that roughly applies to the EFD sensors), and considering the Rce/Rp values indicated above, the electron current collection is very close to the random thermal current which would be collected in absence of the field (see the results of the dimensionless current i given in Fig. 8 of the cited paper, at vp  0 and with 1 b ¼ ðRce =Rp Þ in the range 0.33–1.59). However, in the present paper, a better accuracy for the electron collection modeling is required as the main task here is the evaluation of the probe potential variations caused by the shadows projected by the stubs on the collecting surface of the various EFD sensors. Since the four probes are differently oriented with respect to the ram direction (see Fig. 1), the shadows produced by the stubs are a little different from each other, thus implying small variations (up to 10%) in the ion current collection (and consequently in the measured potential). Being different for the various probes, these effects cannot be cancelled simply using the differential reading scheme for the electric field measurement. The modeling is additionally complicated by the fact that in the probe wake region the lack of ions, due to the satellite motion, induces a corresponding lack of electrons which cannot enter into the wake due to the electric field arising for plasma neutrality violation. However, it is to be taken into account that plasma neutrality can be violated within a distance of the order of one Debye length, which in the CSES case (given the electron temperature and density estimated in Section 2) ranges between 1.6 and 47 mm. This would allow electron collection, to a small amount, even in the probe area within the wake region, but such effect may be considered marginal and has not been factored into this paper. The simultaneous presence of magnetic field and probe motion in the plasma has been the subject of several papers based on both numerical simulations (see for example Singh et al., 1997; Imtiaz et al., 2013) and laboratory experiments (Stenzel and Urrutia, 1990). All numerical simulations show that in the wake probe region a significant lowering of both ion and electron densities takes place, which rejects the validity of isotropic electron current collection models. On the other hand, the experiments performed by Stenzel and Urrutia (1990) showed that the motion of the electrode in the magnetized plasma is capable of producing waves which modify the plasma dynamics in the wake region of the probe, possibly increasing the

electron current. Such a turbulence however arises only for a highly charged probe which is not applicable to the EFD probe situation, as in our case the potential of the electrode is nearly at local plasma potential. To circumvent the uncertainties related to the various modeling approaches, we have decided to limit both the ion and electron collections to the portion of spherical probe oriented towards the ram direction. Thus the collecting area for electrons, which in the plasma are assumed to have an isotropic distribution, is S e ¼ pð2R2p  R2S Þ (Rp being the probe radius and Rs the stub radius). Similar considerations are applicable for the stub shadowed areas behind each stub. In this case the electron shielding can be neglected as a result of partial ions penetration into the shaded area (due to their temperature) combined with the effect of electron penetration within one Debye length. The electron current collected by an electrode embedded in the plasma under a retarding potential (i.e. with q (V  Vpl) < 0) (Medicus, 1961; Schott, 1968) has been parametrized in our model, by considering the probe surface occlusions caused by the inner and outer stubs, as follows: sffiffiffiffiffiffiffiffiffiffi pl Þ 1 8kTe qðVV Ie ¼ qNe ð3Þ Se e kTe 4 pme In Eq. (3), Ne is the electron density, Se is the crosssection area of the probe defined above for electron collection, V denotes the probe potential, and Vpl is the local plasma potential. The expression which describes the collected electron current Ie as a function of the electrode potential V (for V > Vpl) is based on the ‘‘thick sheath approximation” (Medicus, 1961; Schott, 1968; Hershkowitz, 1989) and is: sffiffiffiffiffiffiffiffiffiffi   1 8kTe qðV  Vpl Þ Ie ¼ qNe Se 1 þ ð4Þ 4 kTe pme Differently from the electrons, the ions, in the reference system moving with the satellite, are seen as a flux of particles coming from a defined direction (that we now consider, as an initial assumption, coincident with the ram direction) with a velocity close to the satellite orbital speed (i.e. vi  7.5
103 m/s). This is due to the ions mass, which implies a thermal velocity of the order of 1 km/s (significantly lower than the orbital speed), at the ion temperatures given by IRI. Therefore the space distribution of ion velocity implies that the probe cross section for ion collection is that of a flux tube, aligned with the satellite velocity vector, which for a spherical shape probe is, (for an ideal sphere without stubs), S i ¼ pR2p . The ion mass is assumed to be mi = 3.3
1026 kg (as discussed in Diego et al., 2017), and the thick sheath approximation is considered for plasma conditions along the CSES orbit.

Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005

P. Diego et al. / Advances in Space Research xxx (2017) xxx–xxx

Then, the collected current can be expressed as:   qðV  Vpl Þ 2 Ii ¼ ð1  Sh ÞpRp qNi vorb
1  Kion

5

ð5Þ

where Sh is a parameter relevant to the shadow, i.e. the fraction of the probe disk surface that does not collect ions (for the full disk Sh = 0 while Sh = 1 for full shadow, even if both extreme conditions are never encountered). The variability of Sh depends on the probe attitude with reference to the ion flux direction, as discussed in the following sections. Note that, differently from the electron collection discussed in the previous section, Eq. (5) is valid for both retarding and accelerating potentials. 4. Shadowing and spurious electric fields As mentioned before, the probe is provided with two stubs on the sphere along the boom direction (see Fig. 1 right panel). The probe particle collecting area is therefore partially reduced by the stubs. Due to the boom position and direction of the incoming ions, such stubs will provide a shadowing effect on the collection of ions by the probe, varying as a function of the different positions along the orbit. In order to take into account such collecting surface variations, the formulas of Section 3 have been parametrized by introducing the stub radius RS in the Se equation of electron current collection and the factor Sh for the ion current. Because of the boom configuration and the ion temperature that tends to refill the shaded flux tube within few meters, the shadowing on each probe by other booms is absolutely negligible for ions coming from the ram direction (Diego et al., 2017). Moreover, on the basis of geometrical considerations, we observe that even for probe B (the one placed in the anti-ram direction), the ion flux divergence from the ram never reaches values that imply shadowing by its own boom (about 30°, in comparison to the angular deviation lower than about 15° computed along the entire studied orbit as shown in Fig. 2). Therefore, for each probe, the expression of Sh basically takes into account the shadow only due to its own stubs. The actual arrival direction of the ion flux is obtained by adding the satellite velocity to the plasma drift which is known to be present in the ionosphere at the CSES orbit. At the low and mid latitudes, ionospheric particles drift is mainly generated by heating and tidal atmospheric phenomena known as Ionospheric Dynamo (Richmond, 1989). The Sun heats the ionosphere to high temperatures and causes it to flow away from noon toward midnight. The flow moves both neutral atoms and charged particles across the Earth magnetic field lines. The Lorentz force causes the charges to be deflected in opposite directions perpendicular to the velocity of the charges and also to the local field. This charge separation creates an electric field that also exerts a force on the charged particles. The form of the resulting electric field distribution is strongly

Fig. 2. Ion velocity (as sum of vsat and vdrift) angular deflection as a function of satellite orbital angle along the selected Swarm orbit (April 16, 2014).

dependent on the distribution of ionospheric conductivity and magnetic field. For example, a lower ionospheric conductivity is generally assumed on the nightside and hence no current can flow there. As for the magnetic field, it points upward in the Southern Hemisphere, horizontally northward at the Equator, and downward in the Northern Hemisphere. The horizontal component of the magnetic field exerts a vertical force on charges that move as a result of winds. At the equator this causes the positive and negative charges to be deflected vertically and produces a strong vertical electric field that impedes further separation of the charges. At higher magnetic latitudes the magnetic field is primarily vertical and the deflections are horizontal, producing horizontal electric fields. The pattern of the electric currents flowing in the ionosphere has been deduced from ground observations of daily variations in the magnetic field. On magnetically quiet days the field is observed to change in a systematic manner dependent primarily on local time and latitude. This variation is called the solar quiet-day variation, Sq. The magnetic variations can be used to deduce an equivalent electric current system, which, if flowing in the ionosphere, would produce the observed changes. As a consequence of the Sq there is an electrostatic field directed east-west (dawn-dusk) in the equatorial day side of the ionosphere. At the magnetic dip equator, where the geomagnetic field is horizontal, this electric field results in an enhanced eastward current flow close to the magnetic equator, known as the equatorial electrojet (EEJ). The amplitude of the electric field associated with the EEJ is of the order of mV/m. In addition, penetration of the interplanetary electric field (IEF) can occur. The penetrating field is still of fraction of mV/m (Alken and Maus, 2007). In order to infer the seismic signals natural electric fields should be subtracted from the actual measurements as well as the

Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005

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P. Diego et al. / Advances in Space Research xxx (2017) xxx–xxx

spurious fields induced by the probe’s intrinsic structure (Diego et al., 2017). To estimate the plasma drift velocity at CSES orbit, we retrieved Swarm A plasma data (electron density and temperature, satellite and ion drift velocities) for April 16, 2014 since on that date its orbit was very close to the envisaged orbit of CSES (about 14 LT descending node and about 500 km altitude). The ESA Swarm mission, consisting of three low Earth orbiting spacecraft, is flying since November 2013, and its concept is fully described in FriisChristensen et al. (2006). The Swarm plasma data come from the Electric Field Instrument (EFI), detailed in Knudsen et al. (2017). In order to evaluate the stubs and main s/c body shadowing, also when plasma drift is present, we simulated the particles arrival limited to a cone with angular aperture of 50° and with axis coincident with the satellite velocity direction. This angle is sufficient to cover all possible ion incoming directions, even if the s/c velocity is combined with the ion drift velocities giving rise to a bulk velocity v. Fig. 2 shows the angular spread between the ion actual direction and the ram satellite direction for the selected orbit. Values are always lower than 15°, therefore the 50° of cone aperture assumed in our simulation is a satisfactory choice for further analysis. For each sphere, the actual surface exposed to the ion flux is evaluated for all possible flux directions. Thus, for * each sphere and each direction m of ion velocity, in the satellite reference frame, a Montecarlo ray-tracing simulation launched some test-particles from that sphere back* ward in the  m direction and checked whether the trajectory is occluded or not by any other s/c element. * The bi-dimensional space of the flux direction m is explored with a resolution of 1 deg by 1 deg (polar and azimuth). The actual surface is assumed as a fraction of the full disk (pR2p). The result is shown in Fig. 3. The high number of launched test-particles (106 for each sphere and for each direction) ensures a sufficient confidence in the results (and good homogeneity of the figure). The Montecarlo method is alternative to pure geometrical integration of the occluded area, but since it is computationally fast, easier to be implemented, and sufficiently flexible to permit future consideration of further s/c details (if needed), it has been preferred. 4.1. Ions collecting surface reduction A first step in the analysis of the shadowing effects consists in evaluating the reduction of the exposed area to the influx of ions. Ions are more massive than electrons, thus, we can start our analysis by considering a cold ion flux (ions thermal velocity is of the order of 1 km/s). The ions contribution to the collected current will be therefore reduced proportionally to the modification of the collecting area due to shadowing, according to the geometrical position of each individual probe. Electrons are dispersed in

direction due to thermal spread (see Section 3), since their thermal velocity is much larger than the s/c ram velocity and therefore their collection will not depend on probe attitude, and will be reduced by shadowing of the stub for a constant value. While the reduction in electron current is identical for the four probes and independent of probe attitudes (about 4.4% of the whole probe surface due to the stub crosssections of radius Rs in Eq. (3) and (4)), the reductions in ion current depend on attitude, and thus, these are smaller for probes A and D (almost perpendicular to the ion flux) and larger for probes B and D. The ion collecting surface reductions are shown in Table 3; they are individually shown for the four different probes, and are computed assuming that ions are ramming in from the flight direction, and thus implying a stationary plasma and no drift velocity. The values in Table 3 refer to the central points in Fig. 3. Table 3 details the stub effect on the ion collecting areas for the different sensors only depending on the attitude of each boom with respect to the satellite motion direction. In the fourth column the floating potential values are given for an ideal sphere without any stub (i.e. 100% of ions and electrons collecting areas), just as an example, at the day side equator of the selected orbit. The fifth column shows the Vf values computed taking into account the stub effects. Note that the stub presence reduces both electron and ion collection but only the latter depends on the probe attitude. Shadowing reduces the electron current by 4.4%, therefore the reduction of about 10% (probes B and C) in ion current moves the Vf more negative w.r.t. the ideal case (no stubs). When instead the ion shadow is about 1% (probes A and D) a positive variation of Vf is necessary to balance the currents. The Vf variations with respect to the ideal case are reported in the last column of Table 3. The electric field components obtained by subtracting the floating potentials over couples of probes, and taking into account the geometrical distances among them, will therefore result in a measurement affected by an artificial electric field actually detected. Fig. 4 shows the expected values of these spurious electric fields due to the different shadowing. The ambient plasma parameters are measured by Swarm A along a typical orbit. The orbital position is plotted in the upper panel of Fig. 4, the observed plasma parameters are plotted in the second panel from the top showing the typical day to night asymmetry, and the equatorial trough signature on the plasma density profile at the crossing of the dayside equator. The electron temperature shows large fluctuation only in the polar regions where CSES will not generally be in operation. The three lower panels show the spurious electric fields due to the different shadowing and, thus, the different floating potentials of the pairs of probes. We notice that the effect of the large density fluctuation does not appear in electric field traces, since both electron and ions currents are identically modulated by density.

Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005

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Fig. 3. Shadow surface computation for different values of polar and azimuth angles is shown. The values of the colour-map (Sh) are expressed as a fraction of the full disk for each sphere while polar and azimuth angles are expressed in deg.

Electron temperature, instead, produces large variations in the electron collection and results in Vf changes. The values of the spurious electric field on the three satellite axes (X, Y, Z system in Fig. 4) show different orders of magnitude, since the X component mainly results from the difference between probes B and C which are similarly shadowed; the Y and Z components, instead, result from the combination of the four probes, and hence reflect different shadowing. 4.2. Ions drift contribution The computation of the shadowing effect depends on the direction of the ambient plasma flow; this velocity is affected in the real world by the motion of plasma, if any. We know that the plasma may be flowing and we intend to take the measured flow, as observed by Swarm, to analyse the effect on the spurious electric field. To analyse the behaviour of the incoming ions under the effect of the drift, the simulation described in Section 4.1 was used to evaluate the shadow variations with polar and azimuth angles for the incoming ions, with reference to the ram direction (see Fig. 2). The resultant shadow computation under various values of polar and azimuth angles is shown in Fig. 3. We have taken into account the measured plasma flow as observed in a real Swarm orbit, and modified the direc-

tion of the ions flow relevant to the four EFD probes accordingly (Fig. 2 shows the angular variations induced along the orbit w.r.t. the ram), and consequently evaluated the different shadows. In this way, we finally obtained the new expected spurious electric field components, as shown in Fig. 5. The computations have been carried out for various levels of bias current. We notice that higher bias current injection enhances the electron current, modifying the ratio between electron and ion currents in Eq. (2). This reduces the weight of ion current variations on the biased probe potentials balance and, therefore, on the spurious electric fields values. The imbalance of electron and ion currents mimics the profile of the plasma density as we note in the day side equatorial region. In order to highlight the effect of the plasma drift, we have plotted in Fig. 6 the spurious field simulated only with the drift velocities measured by Swarm (neglecting the satellite velocity). 4.3. Ions temperature contribution In the previous sections we have studied an ideal cold ion flux; therefore, the stub shadow is determined only based on the satellite and drift velocity measurements. Actually, the ion temperature produces a spread in the distribution of their arrival direction around the ram direc-

Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005

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P. Diego et al. / Advances in Space Research xxx (2017) xxx–xxx

Table 3 For each sensor the ion collecting surfaces are expressed in cm2 and % of full disk in columns n. 2 and 3, respectively. In the fourth column the values of an ideal sphere without any stubs are reported (i.e. 100% of ions and electrons collecting areas) while crossing the day side equator of the selected orbit. The fifth column reports the Vf values computed taking into account the stub effects. The last column displays the variation between fourth and fifth column data. Ion collecting surface: case Vion = Vsat Sensor

Actual ion collecting area (cm2)

Stub shadow Sh (% of probe cross-section)

Floating Potential without stubsa (mV)

Floating potential with stub shadowa (mV)

DVf (mV)

EFD EFD EFD EFD

27.8 25.3 25.3 27.8

1.6 10.6 10.4 1.6

536.03 536.03 536.03 536.03

530.81 548.21 547.81 530.81

+5.22 12.18 11.78 +5.22

a

A B C D

Values computed at the day side equator crossing of the considered orbit for a selected Swarm orbit (April 16, 2014).

Fig. 4. Results of the computation of floating potentials on the different probes, after the correction due to shadowing, and the resultant spurious electric fields, for a selected Swarm orbit (April 16, 2014).

Fig. 5. Spurious Electric field components obtained by adding Swarm ion drift data to the satellite velocity for a selected Swarm orbit (April 16, 2014). The three lines correspond to various bias current levels.

tion and, therefore, we cannot precisely detect the ion collecting surface but we can only consider its upper and lower values. This causes unpredictability of Vf within upper and lower value range. Based on the IRI model data, we considered the maximum ion temperature of the studied orbit (1500 K at the CSES operating latitude, as described in Section 2) to evaluate its effect on the probe Vf by calculating, at each

orbital position, the contribution of 100 particles at the average velocity pertinent to such temperature, coming from 100 random directions. The result is then compared with the cold ion case in order to determine the computation uncertainties on Vf and electric field. We may assume the values reported in Table 4 as an intrinsic limit in the computation of the shadow effect on Vf and electric field.

Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005

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Fig. 7. For a dayside near equatorial portion of the orbit, the plots show values of the total spurious electric fields due to, respectively from top to bottom, v  B, shadowing without ion drift (Vion = Vsat), shadowing due to ion drift (vion = vdrift), ion temperature (vion = vdrift  vsat + vterm). Fig. 6. The figure shows the spurious electric field computed only taking into account the plasma drift velocity measured by Swarm. Red lines represent the drift velocities recorded by the TII instrument of Swarm A (in the North-East-Center (NEC) coordinate system) for the selected Swarm orbit (April 16, 2014). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 4 Uncertainties induced by the ion temperature (1500 K) on collecting surface, floating potential, and electric field. Ion collecting surface uncertainty

Floating potential uncertainty

Electric field uncertainty

2%

<1.2 mV

<0.2 mV/m

5. Discussion We have outlined here the procedures to account for some effects induced by the structural spacecraft layout and ionospheric environment on the electric field measurements performed by the EFD sensors onboard CSES. Indeed, the removal of the artificial electric fields has to be considered one of the main tasks to be pursued on the flight data analysis. An important element, which alters the probe potential measurements (and consequently the !

!

retrieved electric field), is the induced v  B field, where !

!

v is the orbital velocity and B the terrestrial magnetic field. !

!

However the effect of the v  B has not been treated in the

present paper as it was already widely discussed in Diego et al. (2017). On the other hand, the effects specifically evaluated in this paper are those caused by the presence of the stubs on the four probes which modify the current collection on the EFD sensors. Such stubs prevent electrons and ions to access the surface of the probe, in different ways on each probe. In particular, the stubs affect the four probes identically for electrons, due to the electron high thermal velocity. The access of ions is instead affected differently for the various probe, due to the fact that the ion thermal velocity is low with respect to their bulk velocity. Thus, each probe exhibits different areas to such (nearly collimated) flow of ions, consequently reaching a different potential. Indeed, the floating condition expressed by Eq. (2) requires that the electron and ion currents are selfbalanced through a variation of the probe voltage, which moves up to the ‘‘floating” value. The different ‘‘shadowing”, produced by the stubs on the four probes for ion collection, implies the appearance of a spurious electric field among the various pairs of probes. Such a spurious field has to be determined and subtracted from the measured values, in order to determine the underlying true ambient electric field. Another important effect which has to be considered is that the ‘‘shadow” on each probe may be modulated by a plasma drift which could change the bulk velocity of the plasma, and hence the amount of probe shaded areas. The ion temperature will introduce another additional difference on each probe, since the thermal spread of the

Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005

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P. Diego et al. / Advances in Space Research xxx (2017) xxx–xxx

Fig. 8. The equatorial ionospheric eastward electric field (EEF) from Cooperative Institute for Research in Environmental Sciences (CIRES).

ions around the bulk velocity will differently affect the shaded area, again producing variations on the ion collected current and consequently of the relevant floating potential. The spurious electric fields due to all the effects listed above has been evaluated individually, and the corresponding traces along a real Swarm orbit (April 16, 2014), which substantially mimics a CSES orbit, are given in Fig. 7. The environmental data, i.e. plasma density, temperature, and the plasma drift used to estimate the perturbing electric field are those actually measured by the Swarm satellite along the real orbit. We notice a trend in the magnitude of each effect, decreasing from vxB (which exhibits the maximum electric field perturbation), down to stub shadow, plasma drift and ion temperature, in the order. The intrinsic expected values for the ambient ‘‘geophysical” electric field are as shown in Fig. 8, taken from CIRES (NOAA) (http://geomag.colorado.edu/real-timemodel-of-the-ionospheric-electric-fields, Alken and Maus, 2007).

6. Conclusion The measurement of natural ionospheric electric fields performed by the CSES EFD instrument needs correction of several spurious effects that appear superimposed on the natural signal. We have analysed various effects that influence the probe voltages due to both environmental and structural causes, which are measured by the EFD probes, and appear therefore as spurious voltages. The spurious electric field appears to be of different orders of magnitude larger than the natural fields (compare the electric fields shown in Fig. 7 to that of Fig. 8), as simulated using real data from the Swarm A satellite. The relevance of all the spurious effects, with respect to the expected ‘‘geophysical” signals, makes it necessary to remove the calculated perturbing fields from the measurements; a strict control on the precision with which all

effects are evaluated will have to be maintained by using the real measured values for all parameters during flight. The three main effects to be accounted for are, as shown in Fig. 7, the vxB field, the spurious field due to differential shadowing of the probes, and the variation of the latter due to the plasma drift. The respective values are 100’s of mV/ m for vxB, and several mV/m for the other two. The signal to be measured is expected to be in the range of fractions of mV/m. This poses rigid limits to the precision required for the calculation of the three effects. v  B can be calculated on the basis of v and B real time measurements with accuracy much better than the 104; the other two will need to be determined with a precision of the order of 10%, which seems to be attainable easily, based on real time measurements of drift, and on refined computations of the shadowing effect. Hardware modifications of probe structure for future missions may be recommended to minimise the shadowing effect (e.g. varying the sensor orientations, and/or the stub’s layout), which seems to be the most intriguing effect. Acknowledgements This work is financially supported by the Italian Space Agency (ASI) in the frame of the ‘‘Progetto Premiale Limadou” phase E (CUP F12F1600011005). The authors would like to thank the P.I. of the EFD experiment Dr. Lei Jungang (Lanzhou Institute of Physics), the P.I. of CSES mission Prof. Shen Xuhui (China Earthquake Administration), and Dr. Zhu Xinghong (DFH Satellite Co., LTD) for their support to this work. References Alken, P., Maus, S., 2007. Spatio-Temporal characterization of the equatorial electrojet from CHAMP, Oersted and SAC-C Satellite magnetic measurements. J. Geophys. Res. 112. http://dx.doi.org/ 10.1029/2007JA012524. Badoni, D., Masciantonio, G., Cipollone, P., Vannaroni, G., Diego, P., Ammendola, R., Balyaev, V.A., 2015. An electric field detector for high-performance measurements of the electric field in the ionosphere. In: Proceedings of 34th International Cosmic Ray Conference, 30 July6 August, The Hague, The Netherlands, 588. Berthelier, J.J., Godefroy, M., Leblanc, F., Malingre, M., Menvielle, M., Lagoutte, D., Brochot, J.Y., Colin, F., Elie, F., Legendre, C., Zamora, P., Benoist, D., Chapuis, Y., Artru, J., Pfaff, R., 2006. ICE, the electric field experiment on DEMETER. Planet. Space Sci. 54, 456–471. Bilitza, D., Reinisch, B.W., 2008. International Reference Ionosphere 2007: improvements and new parameters. Adv. Space Res. 42, 599– 609. http://dx.doi.org/10.1016/j.asr.2007.07.048. CIRES BOULDER data from . Diego, P., Bertello, I., Candidi, M., Mura, A., Vannaroni, G., Badoni, D., 2017. Plasma and fields evaluation at the Chinese seismo-electromagnetic satellite for electric field detector measurements. IEEE – Geosci. Rem. Sens. http://dx.doi.org/10.1109/ACCESS.2017.2674019. Fidani, C., Battiston, R., Burger, W.J., Conti, L., 2012. A study of NOAA particle flux sensitivity to solar activity and strategies to search for correlations among satellite data and earthquake phenomena. Int. J. Remote Sens. 33, 4796–4814. http://dx.doi.org/10.1080/ 01431161.2011.638337.

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Please cite this article in press as: Diego, P., et al. Electric field computation analysis for the Electric Field Detector (EFD) on board the China Seismic-Electromagnetic Satellite (CSES). Adv. Space Res. (2017), http://dx.doi.org/10.1016/j.asr.2017.08.005