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A high-performance electric field detector for space missions
Planetary and Space Science 153 (2018) 107–119
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A high-performance electric field detector for space missions D. Badoni a, b, *, R. Ammendola a, I. Bertello c, P. Cipollone a, L. Conti d, a, C. De Santis a, P. Diego c, G. Masciantonio a, P. Picozza b, a, R. Sparvoli b, a, P. Ubertini c, G. Vannaroni c, d a
INFN Sezione di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Rome, Italy Department of Physics, University of Rome ‘Tor Vergata’, Via della Ricerca Scientifica 1, I-00133 Rome, Italy c INAF-IAPS, Via del Fosso del Cavaliere 100, I-00133 Rome, Italy d Uninettuno University, Corso Vittorio Emanuele II, 39, I-00186 Rome, Italy b
A R T I C L E I N F O
A B S T R A C T
Keywords: Electric field detector Satellite instruments Magnetosphere Ionosphere Earthquake Electromagnetic emissions
We present the prototype of an Electric Field Detector (EFD) for space applications, that has been developed in the framework of the Chinese-Italian collaboration on the CSES (China Seismo-Electromagnetic Satellite) forthcoming missions. In particular CSES-1 will be placed in orbit in the early 2018. The detector consists of spherical probes designed to be installed at the tips of four booms deployed from a 3-axes stabilized satellite. The instrument has been conceived for space-borne measurements of electromagnetic phenomena such as ionospheric waves, lithosphere-atmosphere-ionosphere-magnetosphere coupling and anthropogenic electromagnetic emissions. The detector allows to measure electric fields in a wide band of frequencies extending from quasi-DC up to about 4 MHz, with a sensitivity of the order of 1μV=m in the ULF band. With these bandwidth and sensitivity, the described electric field detector represents a very performing and updated device for electric field measurements in space.
1. Introduction The lithosphere-atmosphere-ionosphere-magnetosphere coupling involves a lot of physical effects and interactions at all levels starting from underground up to the Earth's magnetosphere. Seismicity is a source of electromagnetic signals at ground and in the near-Earth space (Pulinets and Boyarchuk, 2005). Even not exhaustive, a review of some measurements and related analyses on ground and space based data of possible seismic precursors can be found in Rodger et al. (1999) and in Tables 2 and 3 of Sgrigna and Conti (2012). Early analyses have correlated some ULF anomalies detected on ground with earthquakes (such as the strong Loma Prieta event of 1989 (Fraser-Smith et al., 1990). Although these results have been intensely debated (Fraser-Smith et al. (2011); Campbell (2009) and Fraser-Smith et al. (2011)), the low frequency band is one of the most interesting for investigating seismo-electromagnetic signals due to the lower attenuation induced by the lithosphere-atmosphere-ionosphere layers. Nevertheless, in the case of very shallow and strong earthquakes, when the size of the preparation focal zone is greater than the hypo-central depth, also the higher frequency signals could be transmitted on ground and to the near space (Sgrigna et al., 2004).
Some slow electro-telluric and magnetic field variations (Varotsos and Alexopoulos, 1984a), (Varotsos and Alexopoulos, 1984b) as well as ULF emissions (Fraser-Smith et al., 1990); (Molchanov et al., 1995); (Kopytenko et al., 1993), ELF-VLF disturbances (Gokhberg et al., 1982), (Fujinawa and Takahashi, 1998), and HF emissions (Warwick et al., 1982) have been correlated with the occurrence of earthquakes. Some observations of electric and magnetic field fluctuations in the frequency range 10Hz 15kHz obtained by satellite AUREOL-3 (Parrot, 1989) have been claimed to be short-term seismic precursors. Perturbations in the ULF/ELF/VLF bands have been detected before earthquakes by the ITK1300 and the Intercosmos-24 satellites several hours before the main shock (Molchanov et al., 1993). The most recent satellite studies of seismo-associated electromagnetic and ionospheric phenomena have been performed by the French satellite Demeter (nearly Sun-synchronous orbit at an altitude of about 700 km). ICE, the Demeter electric field detector, has the sensitivity in ELF (at the frequencies above 100 Hz)/VLF/HF bands and resolution in ULF (quasi-DC) band of μVffiffiffiffi 0:1 m⋅p , 0:05 m⋅μpVffiffiffiffi , 0:1 m⋅μpVffiffiffiffi and 40 μmV respectively (Berthelier et al., Hz Hz Hz 2006). In this framework, several statistical analyses (Pisa et al. (2012); Nemec et al. (2008), Pisa et al. (2012, 2013)) of about 1 2 kHz electric field data (gathered by ICE), show that the normalized intensity of the
* Corresponding author. INFN Sezione di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Rome, Italy. E-mail address: [email protected] (D. Badoni). https://doi.org/10.1016/j.pss.2018.01.013 Received 26 July 2017; Received in revised form 25 January 2018; Accepted 29 January 2018 Available online 5 February 2018 0032-0633/© 2018 Elsevier Ltd. All rights reserved.
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Planetary and Space Science 153 (2018) 107–119
background noise and to asses their correlation with earthquakes occurrence (De Santis et al. (2015) and references therein). In order to infer the seismic signals in the electric field measurements, natural variations in the electric fields should be subtracted. In particular, it is relevant: the equatorial Electrojet (EEJ) (Alken and Maus, 2007), at the magnetic equator with an amplitude of the order of less than a mV=m, and the interplanetary electric field (IEF), a penetrating field that occurs especially when the interplanetary magnetic field turns negative with an amplitude of fraction of mV/m (Yamazaki and Maute, 2016). In addition, anthropogenic electromagnetic emissions as well as spurious fields induced by detector probe's intrinsic structure (Diego et al., 2017a), must be taken into account. In this framework, the study of precursors can take advantage from the continuous monitoring with repeated measurements in a short time period and high resolution performed by constellation of satellites such as the CSES missions and the three satellite of the Swarm mission (Olsen and Haagmansguest, 2006). 2. Space-borne measurements of the electric field
Fig. 1. Positioning of the EFD sensor (0, 1, 2 and 3) on the satellite booms. The x axis is parallel to S/C motion, z is to zenit and y completes the orthogonal reference system.
Measurements of electric fields in space plasma using the technique of double spherical probes installed on booms deployed from a spacecraft date back since 1960s. It was proposed from Aggson and Heppner (1964) for the ATS-4 satellite. The first successful results of this technique in a tenuous magnetospheric plasma was obtained onboard the S3-3 spacecraft in 1976 (Mozer et al., 1979), shortly followed by other experiments onboard GEOS-1 and ISEE-1 satellites in 1977. Afterwards, the use of crossed double probe systems in several satellite missions (such as GEOS-2 (Jones, 1978), Dynamic Explorer 2 (Maynard et al., 1981), Viking (Block et al., 1987), GEOTAIL (Tsuruda et al., 1994), Freja (Marklund et al., 1994), Polar (Harvey et al., 1995), Cluster (Gustafsson et al., 1997), C/NOFS (de La Beaujardiere, 2004), etc.) made possible measurements of the electric field vector components in the ionosphere and magnetosphere. These provided a direct knowledge of the electric field structure, also allowing a detailed correlation with the correspondent magnetic field configuration. Double probe sensors have been successfully used in the D, E, and F ionospheric regions, as well as in the
night-time electric field amplitude decreases below the mean background level few hours (0–4 h) before the shallow (depth < 40 km) of medium and strong (M > 5) earthquakes. The spatial scale of the affected area (about 350 km) is in good agreement with the size r of the preparation zone for earthquakes of magnitude M > 5 estimated with the Dobrovolsky et al. (1979) formula r ¼ 100:43 M km. A possible explanation of the observed perturbation is a local lowering of the ionosphere bottom layer induced by seismo-associated phenomena. Significantly, the largest decrease occurs at about 1:7 kHz, i.e. approximately near to the cut-off frequency of the first transverse magnetic mode of the Earth-ionosphere waveguide in night-time. An increase of this cut-off frequency would induce a decrease of the electric field power spectral density (as statistically observed by Demeter) and an increase of the plasma density in the lower ionosphere above the earthquakes preparation area. This will result into a local perturbation of the propagation of whistlers, which are the main natural electromagnetic waves propagating in the Earth-ionosphere wave-guide. Anyway, at present, there is not clear evidence of the precursors signature (i.e. in which range and condition a detected signal can be considered anomalous). The electric and magnetic components of seismo-associated electromagnetic fluctuation
Table 1 Typical plasma conditions as measured by Swarm A satellite and corresponding equivalent impedance values adopted for test measurements on the EFD prototype. Note that, the value of the plasma-probe capacitance, calculated for an spherical probe (3 cm radius) using Swarm plasma data, is 19 pF, but due to the presence of the stubs, which shade part of the sensor surface, its value is reduced to about 12 pF.
are estimated very faint, being some fraction of mV=mðHzÞ1=2 and some
Plasma conditions
fraction of nT=ðHzÞ1=2 or less, respectively (Parrot et al. (1993) and Sgrigna et al. (2007)). Moreover, the complexity of the lithosphere-atmosphere-magnetosphere coupling mechanism, asks for further studies in order to clearly distinguish anomalous signals from
Density 2:5 ⋅10
11
Temperature 3
m
2200 K
Current Injected 5 μA 10 μA
Impedance Zc Resistance
Capacitance
25 kΩ 14 kΩ
’ 12 pF
Fig. 2. EFD probe. The yellow sphere S is the sensor current collector; the first inner hollow shell C1 is bootstrapped to the potential of the probe in order to minimize the capacitive coupling of the outer sphere with the ground. The stubs St1 and St2 are also connected to the bootstrap circuit in order to reduce boom perturbations by preventing disturbances to the sphere potential. The cylinder C2 is the shell connected to the ground and contains the electronic board B. In the block diagram (c) the simplified schematic of the electronics is shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. EFD general architecture. The four Probes in the block diagram (left side) are denoted as S0, S1, S2 and S3. The Analog Processing Unit selects the probes, performs the signal filtering and sub-division in bands, generates the bias currents for the probes and carries out the analog/digital conversion (block in the center). The Digital Processing & Control Board (right block) provides further processing and implements the control and configuration features of the detector.
inner magnetosphere, where the electric field is a crucial quantity to be measured for understanding the non-linear processes that may produce plasma acceleration. Extensive investigations of the electromagnetic near-Earth space environment have been accomplished recently (i.e. Demeter (Parrot, 2006), THEMIS (Burch, 2008)), or are planned in future missions (such as TARANIS (Lefeuvre et al., 2008), CSES (Shen et al., 2011) etc. The payloads of these satellites include, in addition to an electric field detector, several instruments to measure magnetic field, plasma parameters and particle fluxes. In particular, the main objective of the CSES (China SeismoElectromagnetic Satellite) satellite program is the study of the electromagnetic, plasma and particle perturbations caused by seismicity in the ionosphere, magnetosphere and inner Van Allen belts, prosecuting the exploratory investigation performed by the Demeter satellite. CSES is a mission led by the Chinese (CNSA) and Italian (ASI) space Agencies that includes two satellite missions CSES-1 and CSES-2. The CSES satellite, 3-axis attitude stabilized, is based on the Chinese CAST2000 platform. It will be placed at a Sun-syncronous circular orbit at an altitude of about 500 km (descending node at 14 : 00 LT; orbit inclination 93∘ ; altitude 507 km). Nine instruments are installed on the satellite that will be launched in the early 2018 with an expected lifetime of 5 years (see website: cses.roma2.infn.it). The CSES-2 mission is designed as a twin version of CSES-1, with some adjustments and differences. CSES-2 is planned to be launched at the same orbit of CSES-1 within 2 years after the begin of the CSES-1 mission, in order to execute coordinated and synchronous measurements, aimed at measuring the background and to exploit the possibility to improve the observation of seismo-associated phenomena with two contemporary satellite missions.
second satellite. The instrument consists of four identical probes located at the tip of four booms deployed from a 3-axes stabilized spacecraft as shown in Fig. 1, and their associated electronics. The attitude information is acquired by star sensor together with gyroscope. There are three star sensors, two of which are guaranteed to be available in case one is illuminated by sunlight. The pointing accuracy of the satellite is better than 0:1∘ (3-Axis, 3σ), the knowledge accuracy is better than 0:03∘ (3-Axis, 3σ) and the stabilization accuracy is better than 0:001∘ =s (3-Axis, 3σ). Even though the design of CSES-2 has not been completed, the booms should be similar to those adopted for CSES-1, that are of the same type of the Kaleva ones installed on the Demeter and FAST missions (Dupuy et al., 2007), (Pankow et al., 2001). In CSES-1 the booms are 4.15 m long, with a minimum distance between probe pairs of 6741.4 mm. The difference of the electric potential between each pair of probes divided by their mutual distance determines the electric field component along the direction defined by the probe locations. The electric field is measured for an extended band of frequencies from quasi-DC up to about 4 MHz, with a sensitivity of the order of 1μV=m in the ULF band. The detector combines low intrinsic noise, high resolution and large bandwidth. The instrument has been conceived on the basis of the most recent technology in device development and digital signal processing. As observed by Demeter mission, perturbations on electric field detectors can be induced by other instruments and ancillary systems. For example, the ICE calibration mode introduced specific lines in the spectrum (625 Hz and 10 kHz). Disturbances up to several tens of mV=m at about 1 Hz due to sweeping voltages Langmuir probes (ISL) were detected in the ICE spectrograms (Lagoutte et al., 2005). At this purpose, the sweeping voltage of the Langmuir probe of CSES-1 has been strongly reduced to avoid significant loss of performance in the electric field detector. In order to minimize the electromagnetic interferences, and in general to reduce the electronic noise, the EFD electronic boards are installed inside a shielding metallic box and the connection between the central unit and the probes are implemented through highly shielded coaxial cables. For what concerns the satellite potential effect, CSES is very similar to Demeter where the spacecraft potential has been observed to be very stable and close (within 0 and 1 V) to the plasma potential, as recorded by ISL instrument. Thus, electrostatic perturbation due to S/C
3. The electrical field detector An electric field detector (EFD), suitable to investigate electromagnetic phenomena in the near Earth environment, such as ionomagnetosphere transition zone, has been developed in the context of the Chinese-Italian CSES collaboration in order to be installed on the 109
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Fig. 4. APU block diagram. The switch matrix permits a complete and independent reconfigurability of the sensor pairs selected for the measurements of the various electric field components. Four chains for LoF channels, three chains for the elaboration of the MeF bands and one for the HiF channel are available. The APU generates also the control voltage of the bias current for the probes through a DAC. Several temperature sensors are mounted in the board. The temperatures of the board as well as those coming from the sensors are digitized from the respective ADCs.
electronics block diagram of one of the EFD sensors. The probe consists of three concentric metal shells, an holder and two stubs, St-1 and St-2 in the figure, placed at the opposite sides of the sensor (aligned with the boom axis) to reduce the potential asymmetry caused by the conductive boom connected at the satellite ground. They are bootstrapped at the same potential of the probe in order to prevent disturbances to the sphere potential induced by the presence of the boom which is electrically connected to the satellite ground. As a drawback, however, the stubs may intercept the trajectories of ions and electrons altering the current collection by the probe, producing spurious electric fields in the range of mV=m that should be computed and removed (Diego
body could be neglected if compared to above mentioned effects. In addition, the booms are deployed in proper directions to prevent the probes to fall within the wake of S/C or other booms (Diego et al., 2017a). Although the EFD has been realized to be the candidate for the foreseen second CSES mission, it has been designed to be installed on broad class of satellites devoted to study electromagnetic phenomena of natural and anthropogenic origin. 4. The probe Fig. 2 shows a photograph, some details of mechanical layout and the 110
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Digital Processing & Control Board From/To Host
DSP SPI 31 bit SPI 31 bit SPI 31 bit
100 sps 100 sps
4 CH. LoF Filter Cores
Fig. 5. Digital Processing & Control Board block diagram which describes the functionalities implemented on the Xilinx. The DSP block (Digital Signal Processing) implements the digital filters for the digitized signals generated by the ADCs of the APU board and produces the FFT spectra for the VLF and HF bands. Furthermore, the board generates/controls and manages all the data from/to the DACs and the temperature readings from the APU board. Finally, the board manages all communication to/from a hostPC.
100 sps 100 sps
ULF Waveform - 0 Hz – 16 Hz
SPI 31 bit
21 Ksps 21 Ksps SPI 24 bit SPI 24 bit SPI 24 bit
3 CH. MeF Filter Cores
21 Ksps
ELF Waveform - 13 Hz – 2 KHz 125 Ksps 125 Ksps
3 CH. VLF FFT Cores
125 Ksps
ETH. UDP Interface
VLF Spectrum 1 KHz – 50 KHz Parallel 16 bit
HiF Filter Core
HF FFT Core
16 Msps
HF Spectrum 21 KHz – 4 MHz
Switch SPI/I2C IF
Switch Network State Machine
SPI 15 bit / SPI 12 bit
ADC SPI IF
Temperature reading State Machine
SPI 16 bit
DAC SPI IF
Bias current injecƟon State Machine
prescribed levels of current to the probe. The surface of the probe S and the internal shield C1 (bootstrapped at the sensor potential) form a capacitor between the output and the unity gain amplifier input. By using the Miller theorem, we can easily see that an instability could take place, due to a negative resistance that may rise at high frequency at the input of the amplifier. The low pass filter, reducing the high frequency response of the unity gain amplifier, prevents possible high frequency instability of the circuit. The EFD prototype is an evolution of the Demeter ICE sensor (Berthelier et al., 2006), opportunely updated to improve the measurement accuracy and to extend the bandwidth. The adopted technique for electric field measurements implies a quite efficient probe-plasma coupling, i.e. a low contact impedance (Zc ) with the surrounding plasma, which is, in turn, strongly dependent on the local plasma parameters (density and electron temperature) and possibly on the photoelectron current emitted by the probe surface. The Zc must be small compared to the input impedance of the sensor preamplifier. The value of this impedance is of the order of about 10 GΩ in DC, and decreases as frequency increases. The contact impedance exhibits its minimum close to the plasma potential point (Vpl ). A current injected on the probe, in
Table 2 EFD definition of frequencies bands. W: waveform, S: Spectrum. APU
DPCB
Name
ΔFreq: (Hz)
Name
ΔFreq: (Hz)
LoF
0 16
MeF
13 50k
HiF
21k 4M
ULF ELF VLF HF
0 16 13 2k 1k 50k 21k 4M
Type
Output Data rate
W W S S
100 sps 21 ksps 125 ksps 16 Msps
et al., 2017b). The outer shell, S, with a diameter of 6 cm and coated with conductive material DAG 213 from ACHESON, is the sensing surface. The second shell C1 is a hollow cylinder bootstrapped at the same potential of the probe to minimize the capacitive coupling of the sensor with the ground, thus increasing the frequency response of the readout circuit. The third shell, C2, is a conductive box connected to the ground which shields the electronic board B. The signal coming from the outer sphere is applied to the input of a voltage follower with a very high input impedance. Moreover, a voltage controlled current source injects
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Fig. 6. From upper to lower panel: ULF output voltage noise spectral density; ULF Transfer Function magnitude; ELF output noise voltage spectral density; ELF Transfer Function magnitude. All graphs have been obtained with the two different values of plasma impedance and injected current. The black line is the measured background noise obtained with the probes connected to ground. The small peaks at frequencies: 100 Hz, 500 Hz and 1 kHz are due to the MeF ADC.
the best value of the current to be injected for typical plasma conditions at CSES orbit, also by using data from Swarm satellite recorded in April 16, 2014 in an orbit similar to that of CSES. The obtained results are shown in Table 1. Despite at 10 μA the value of the resistance is lower than to that obtained at 5 μA, the latter value is assumed as the optimal one, since for 10 μA the probe potential could exceed the plasma potential (e.g. during exceptional plasma density minima). Anyway, during the in-flight operations, this bias current is adjustable through Telemetry & Telecommand control. Indeed, the contact plasma impedance can be measured during EFD calibration sessions by injecting of AC signals, superimposed to the DC bias current. Then, adjustments in the DC current bias permit to minimize the contact impedance. The AC signal adopted to determine the coupling resistance between probe and plasma consists of a sine current waveform at a frequency of 625 Hz, with an amplitude which can be adjusted from a few tens to a few hundreds of nA. Such a signal is applied to the sensors through the same electronics already used to apply the DC bias current. The AC current
contact with the plasma, modifies the balance among the various currents; in particular, a proper positive current can drive the probe potential close to the local plasma potential. The EFD probe behaves essentially as a floating electrode embedded in plasma. Three main currents determine the floating potential Vf of the conducting probe: the electron and ion collections and the photoelectron emission. The photoelectron emission has been evaluated to be negligible for the EFD probe at CSES orbit. In fact the intensity of the photoelectron current emitted from the EFD probes has been widely discussed in (Diego et al., 2017a), where it was concluded that at the CSES orbit the Iph is of the order of 50 nA. Therefore, if a bias current of 5 μA is applied (see the considerations discussed below), the photoelectron current is about a hundred times lower. Moreover, as the electron current collected by the probes dominates over the ion current, the sensors floats at negative voltages with respect to the local plasma potential. Therefore, the currents injected to the probes, which are the same for all the sensors, must be positive to move the probe voltage toward the plasma potential. A detailed study has been executed in Diego et al. (2017a) to estimate 112
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Fig. 7. From upper to lower panel: VLF output voltage noise spectral density; VLF Transfer Function magnitude; HF output noise voltage spectral density; HF Transfer Function magnitude. All graphs have been carried out with the two different values of plasma impedance and injected current. The black line is the measured background noise obtained with the probes connected to ground. In the HF band small peaks are visible at 2 MHz and 4 MHz as residual interference noise due to the clocks used in the digital electronics.
Table 3 Total output voltage noise VRMS in ULF end ELF bands. Band
ULF
ELF
Table 4 EFD sensitivities.
Current Injected; Plasma Impedance
Noise (μVRMS )
0 μA ; Direct coupling 5 μA ; 25 kΩ//12 pF 10 μA ; 14 kΩ//12 pF 0 μA ; Direct coupling 5 μA ; 25 kΩ//12 pF 10 μA ; 14 kΩ//12 pF
3.3 3.0 3.2 5.6 5.8 5.8
ULF nV= pffiffiffiffiffiffi m⋅ Hz
ELF nV= pffiffiffiffiffiffi m⋅ Hz
VLF nV= pffiffiffiffiffiffi m⋅ Hz
310
60
15
HF nV= pffiffiffiffiffiffi m⋅ Hz
(@ 30 40kHz) (@ 4 MHz)
30 900
5. EFD signal acquisition and data processing system The block diagram of the EFD instrument is shown in Fig. 3. It consists of 4 probes and a complete DAQ system for signal filtering, acquisition and data processing. The four probes are denoted as S0, S1, S2, S3. The data acquisition chain consists of two main units:
signal with amplitude ΔI produces a corresponding variation of potential ΔV in the sensors (according to the probe current-voltage characteristic). The coupling resistance is thus determined from the ratio ΔV=ΔI, that is expected to be of several tens of kΩ. Such a method is similar to that adopted on ICE (Berthelier et al., 2006).
The Analog Processing Unit is a signal conditioning unit which provides the first analog filtering of the signals that performs a 113
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VLF and HF. The digital filters were designed to obtain a stopband attenuation of at least 120=150 dB with a multirate architecture to further reduce the ADC data output. The digital decimation filter on the LoF decreases the output data rate to 100 sps, that is a trade-off between data budget constrains of a satellite mission (storage & download) and a good signal reconstruction. The acquired MeF data undergo to a multirate low-pass filtering that reduces the data rate to 125 ksps. The resulting signal is then fed to a pair of different processing flows. The first path provides a multirate filtering with 2 kHz cutoff frequency and a decimation ratio of 6, necessary to obtain the waveform for the ELF band. The second path provides modules for the spectral analysis in the VLF band. The HiF processing unit has a structure similar to the VLF one, with a first stage composed by a cascade of multirate low-pass filters, to reduce the data rate to 16 Msps, followed by a spectral analysis module. The spectral analysis section execute a 2048 points FFT for both the VLF and the HF bands, with a spectral resolutions of 61:04 Hz and 7:81 kHz respectively. Table 2 shows a summary of the various bands implemented by the various filters of APU and DPCB. In addition, the DPCB implements the control and configuration of the APU switch matrix and the setting of the voltages to be applied to the current generators embedded into each Electric Field Probe (EFP). This section also allows the managing of the calibration mode and performs the temperature reading of the EFPs and of critical APU areas. Finally, the DPCB provides the management of all communication between the EFD and a host-PC through a UDP/IP Ethernet interface. Note that, as already introduced at the beginning of this section, this work provides the description of the maximum data acquisition and transfer rate. Nonetheless, a strong data flow reduction will be imposed by the satellite bandwidth downlink.
preliminary sub-division of the signal into frequency bands; generates all the necessary control voltages for the EFD (as the bias current injection of the probe) and, last, performs the Analog/Digital conversion. The Digital Processing & Control Board is a digital unit for data sampling and processing that execute further elaborations of the signals and the Fast Fourier Transform (FFT). Moreover, it implements the control and configuration features of the detector. After the experience of Demeter, and waiting for the results of CSES-1 mission, we have adopted the most flexible data acquisition strategy according to the state-of-art of the updated electronic components. Such strategy is conceived to minimizing intrinsic noise, to optimize resolution, dynamic range and sampling acquisition modes. Considering the maximum expected voltage dynamic (about 2:5V at low frequencies), mainly due to the v x B and to the probe Vfloating as reported in Diego et al. (2017), the ADCs input ranges are consequently chosen as the trade-off among the relevant requirements as: low noise, high S=N ratio, linearity and dynamic ranges. 5.1. The Analog Processing Unit (APU) The block diagram of the APU is shown in Fig. 4. The signals coming from the 4 probes are multiplexed to select couples Si Sj (i 6¼ j ¼ 1; …; 4) needed to evaluate three electric field components. The switch matrix scheme allows the choice of all the possible couples Si Sj , thus ensuring the measurement of at least one or two electric field components in case of failure of one EFD sensor. The signal spectrum is divided via analog filters into three different bands (low, medium and high frequency channels) denoted LoF, MeF and HiF, which are further digitally filtered. The LoF band includes signals up to 16 Hz, the MeF band selects frequencies from 13 Hz to 50 kHz and the HiF band selects frequencies from 21 kHz to 4 MHz. The LoF channel uses a ADC with extremely high-performance and high-accuracy instrumentation having a S=N ratio of 120 dB. The LoF, MeF and HiF ADCs have a voltage input full scale of 2:5 V, 3:0 V and 1:5 V respectively. In each chain the signal is amplified with a gain slightly different from the unity to optimize their effective range as shown in Table 5. In the LoF bands, the acquired signals are the 4 potentials directly measured between each probe and the satellite body which, in turn, is considered as ground reference. These LoF signals are acquired through 4 dedicated ADCs with independent amplification chains. The acquisition in the MeF band is carried out by measuring three potential differences among the selected sensors, while in the HiF band only one potential difference is measured between a selected pair of EFD sensors. All the acquired signals are over-sampled to minimize the intrinsic analog noise, lowering the order of the anti-aliasing filters and reducing the number of the operational amplifiers. The LoF filter is a low-pass third order Butterworth filter implemented with Sallen-Key architecture plus a single pole filter. The signal is sampled by a 31-bit ADC at 1 ksps output data rate. The MeF signal undergoes a pass band filtering, with a second order Butterworth implemented with a Multiple-Feedback (MFB) architecture. The signal is sampled by a 24-bit ADC at 1 Msps output data rate. The HiF signal is elaborated by a band pass network composed by a cascade of a fourth-order Butterworth high-pass and a fourth-order Butterworth low-pass filters with Sallen-Key architectures. The HiF signal is then sampled by a 16-bit ADC operating at 128 Msps output data rate.
6. Test measurements on the EFD prototype in Faraday Cage The intrinsic noise, the dynamic range and the transfer function of the EFD instrument has been characterized end to end (probes, APU and DPCB) installing a pair of probes in a Faraday Cage to obtain an efficient shielding from the external electromagnetic environment. Due to the very high input impedance of the probes, a special care has been taken in the design of the harness in order to reduce the external noise. The intrinsic noise, in the various bands, is assumed to be the sensitivity of the instrument, since it represents the bottleneck of the system being in general larger than the LSB (Less Significant Bit) of the ADC: 2VðnFSbit Þ ADC
where VFS is the value of the full-scale sine wave and nbit is the bit numbers of the ADC.
6.1. Test set-up and methods The EFD tests have been performed as a function of the bias current applied to the probes. The bias current is applied directly at the input of the sensors through the built-in constant current source present in each probe. Tests have been carried out by taking into account the estimated average plasma conditions described in previous section (Table 1); thus the two probes have been connected to the measurement setup through a parallel Rpl ==Cpl element which emulates the probe-plasma Zc coupling impedance. The used values have been 25 kΩ==12 pF with the optimal bias current of 5 μA and 14 kΩ==12 pF with the bias current of 10 μA, as previously defined. The Rpl ==Cpl element at each probe input is connected in two different configurations: (i) to perform noise measurements the Rpl ==Cpl are connected to ground; (ii) to perform measurement the dynamic range and transfer function tests the two Rpl ==Cpl elements are connected one at ground and the other to an external signal generator. In the ELF, VLF and HF bands, each signal at the output of the measurement chain is obtained directly as potential difference between the EFD probe pairs, while for the ULF band, an off-line manipulation of data is required
5.2. The Digital Processing & Control Board (DPCB) The Digital Processing & Control Board shown in Fig. 5 is based on a FPGA Xilinx Virtex 6. The LoF, MeF and HiF signals are processed inside a digital signal processing (DSP) block to obtain the four bands ULF, ELF, 114
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from the background noise, thus a local approach (narrow bandwidth) for the VLF and HF dynamic range estimation has been adopted. The frequency bandwidth depends on the frequency resolution δf and on the type of windowing used in the FFT, therefore the correspondent noise is rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn Vi 2 pffiffiffiffi VNOISERMS ¼ ⋅δf where n is the number of points in the i¼1 Hz i 2 i is the power noise spectral narrowband frequency window and pVffiffiffiffi Hz
to obtain the potential difference. The output noise measured at the end of the digital processing chain has to be normalized at the input in order to define the sensitivities and dynamic range. The Transfer Function is obtained by applying a white noise input signal (0-44 MHz band, 1 VRMS ). 6.2. The output voltage noise and the transfer functions
i
The noise and Transfer Function measurements in ULF/ELF and VLF/HF bands are presented in Figs. 6 and 7 respectively. The resulting voltage noise spectral densities in all bands (except HF), starting from 0:2 Hz are essentially independent from both bias current and plasma conditions.
density of the i-th frequency point. Along with these considerations, an additional constraint is represented by the dynamic ranges quoted in the manufacturer data-sheet for the specific ADCs used: 120 dB for the LoF ADC, 110 dB for the MeF ðELF and VLFÞ ADC and 78 dB for the HiF ADC. Concerning the VLF, the good flatness of the Transfer Function implies that the dynamic range is extended in the whole band. For the HF, the dynamic range is 78 dB from lower frequencies up to about 200 kHz where the Transfer Function exhibits few dB of variation, while it is 58 dB at 4 MHz. The results are summarized in the Table 5.
μV Vnoise < 2:1 pffiffiffiffiffiffi ðULFÞ Hz nV Vnoise < 400 pffiffiffiffiffiffi ðELFÞ Hz nV Vnoise < 100 pffiffiffiffiffiffi ðVLFÞ Hz
7. Test measurements of the EFD prototype in plasma chamber The INAF-IAPS plasma chamber is a facility capable to reproduce at
The total noise in the time-domain for the ULF and ELF bands has been determined as standard deviation from the correspondent waveform data. The results are shown in Table 3. The cut-off frequencies are clearly visible around 16 Hz; 2 kHz and 50 kHz for ULF; ELF and VLF respectively. The HF Transfer Function, differently from all other bands, exhibits a not negligible dependence on the plasma impedance. Such a high frequency dependence has its origin mainly in the sensor input circuit, due to the unavoidable presence of parasitic capacitances at the input node of the voltage follower with respect to ground. At the high frequencies the resulting gain of the chain is lower than unity, and this has to be considered for the overall instrument calibration. Taking into account the Transfer Function, the HF intrinsic noise (i.e. voltage noise spectral density) measured at the two band limits (normalized to the EFD input), is:
Table 5 Dynamic Range in all bands. Band
VFSRMS (VRMS )
VNOISERMS (μVRMS )
DR (dB)
ULF ELF VLF
3.1 1.9 1.9
3 5.8 ADC limited ADC limited 1100
120 110 110 78 58
HF
20 200 kHz up to 4 MHz
0.88
nV Vnoise < 200 pffiffiffiffiffiffi @ð30 40 kHzÞ Hz
μV Vnoise < 6:2 pffiffiffiffiffiffi considering all the band up to 4 MHz Hz 6.3. Electric field sensitivities and resolution Starting from all the voltage noise spectral densities and VRMS values determined above and considering a minimum nominal distance of 6:8 m between probe pairs (see section 3), we can compute the sensitivities of the EFD instrument in terms of electric field. The results are summarized in Table 4. For the DC-ULF band, the resolution in terms of electric field is 0:5 μVmRMS . 6.4. Dynamic range evaluation The dynamic range (DR) is defined as the ratio of the root-meansquare (RMS) of a full-scale sine wave (VFSRMS ) to the noise (VNOISERMS ) which is assumed as minimum discernible signal. DR ¼ 20⋅log10
VFSRMS VNOISERMS
Fig. 8. Upper panel shows the floating potential measured by the EFD for three different bias current levels (0, 0:5 μA, and 5 μA). Measurements are obtained for three electron temperatures: 2800 K (blue markers), 3700 K (green markers), 4400 K (red markers). Lower panel shows the correspondent plasma/probe coupling impedance Zc . The Zc has been computed by applying a sinusoidal signal at 625 Hz. Plasma potential in the chamber is þ2.7 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
(1)
According to Eq. (1), the dynamic ranges for ULF and ELF bands are directly determined using the couple of values: VFSRMS , relevant to ADCs characteristics, and VNOISERMS , previously determined for the two bands (see Table 3). A different criterion has been applied for the VLF and HF spectra. A signal is discerned at a specific frequency when it emerges 115
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Fig. 9. Upper panel left side shows the rotary stage used to change the probe attitude w.r.t. the plasma beam direction. Upper panel right side shows the attitude of the sensors set to perform plasma tests in the INAF plasma chamber. The lower panel shows the I-V curves obtained for probe set perpendicular (black line), parallel (green line), and 45∘ diagonally oriented w.r.t the plasma beam. Measurements have been performed at B ¼ 0 G. Plasma potential in the chamber is þ2.7 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
7.1. Bias current effects on Vf and Zc
ground the ionospheric environment. The plasma source produces an Argon plasma with parameters very close to the values encountered in the daytime ionosphere at F layer altitudes (e.g. plasma density in the range 1011 1012 m3 , electron temperature in the range of 2000 4000 K). The plasma generated by the source is accelerated into the chamber at a velocity that can be tuned to simulate the relative motion between an object orbiting in space and the ionosphere ( 7:5 km=s). The EFD has been fully tested in the INAF-IAPS Plasma Chamber which, in particular, allowed the measurement of the EFD floating potential in a real ionospheric plasma environment. The tests were performed under various bias currents and for different probe attitudes with respect to the ion beam (assumed parallel to the satellite velocity vector). In the following subsection we summarize the main results relevant to the EFD performance: floating potential Vf and dependence of plasma/probe coupling impedance Zc on plasma parameters, probe attitude and bias current level.
As already introduced in section 4, the EFD payload is a conducting body immersed in plasma and, therefore, moves its potential (floating potential Vf ) in order to have a total collected current equal to zero. An important parameter, which needs to be computed and measured is the contact impedance (Zc ¼ Rpl ==XCpl ) between probe and plasma. At the frequencies used in this test XCpl is very large, therefore in the following the Zc is assumed as Rpl . Measurements of Vf have been performed in Plasma Chamber for three different bias current levels, 0, 0.5 and 5 μA. In addition, the electron temperature has been varied to evaluate the probe response to different plasma conditions encountered by the CSES satellite along its orbit. Vf has been measured for three electron temperature levels: 2800, 3700 and 4400 K, as shown in Fig. 8. The measurements show that the floating potential decreases (i.e. the difference between Vf and Vpl increases) for higher electron temperature because their thermal velocity enhances the electron current collection moving the probe potential more negative. This temperature effect be116
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electron temperature Te (i.e. at constant Vf Vp difference). Note that the Kaufman plasma source, in use at INAF-IAPS plasma chamber, has a separate neutralizer filament for electron emission which can be used to control plasma potential in the chamber. The filament is characterized by a relatively long thermal time constant, therefore there is an intrinsic limitation in following the ion potential at high frequencies (i.e. greater than 0:1 Hz). This test has been performed by applying a sinusoidal signal of 100 mV in the range 1 1000 mHz. Such a signal is directly applied to the neutralizer filament which, in turn, produces a correspondent potential variation of the entire volume of plasma in the chamber. Fig. 10 shows the induced Vf variation by the applied sine signal as well as the attenuation as a function of frequency. The variations are lower than 3 dB for frequencies lower than 100 mHz.
comes smaller for higher bias current. On the other hand, the values of Zc show a dependence on the electron temperature greater than that induced by the bias current. In fact, the Zc reduction obtained by applying 5 μA is about three times in a plasma with Te ¼ 2800 K with respect to the case with Te ¼ 4400 K. 7.2. Probe attitude effect on Vf In order to check the effect of the stubs on the ion collecting surface of EFD probes, a specific set of tests have been performed by injecting in the probe a bias current in the range 0 30 μA (much larger than that typically foreseen in the CSES satellite operation) for different probe attitude w.r.t. the plasma beam direction. Since the thermal velocity of electron (for Te of about 2000 K) is about 105 m=s, while the satellite moves at 7:5⋅103 m=s, we may consider the electrons isotropic collected, and thus, not dependent on the probe attitude. On the contrary, the ion thermal velocity is about 103 m=s for Ti of 1500K (according to data from IRI), which is lower than that of satellite, therefore these are collected only on the probe surface portion perpendicular to the ram direction (i.e. a disk with surface of π ⋅R2 ). For this reason, the effective ion collecting surface of the probe depends on the presence of stubs (conducting cylinders extending in the boom direction on both inner and outer side of the each probe). The stub dimensions are: radius 1:7 cm, outer stub length 2:8 cm and inner stub length 5:15 cm (see Fig. 9 right panel). For different boom attitudes w.r.t. the flight directions such stubs produce a shadowing effect that reduces the ion collection. In order to verify the resulting Vf variation, a dual axis rotary mechanism has been developed to change the polar and azimuth angles, while maintaining the center of the probe in the same point of the plasma chamber (i.e. at the same distance from plasma source and, consequently, at the same plasma conditions). Fig. 9 (top panel left side) shows a sketch of the rotary stage with the EFD probe installed. Top panel right side shows the probe attitudes set to perform tests in the plasma chamber. The system is remote controlled and designed for obtain an accuracy of 1∘ . Fig. 9 lower panel shows the results of the tests to determine the floating potential variation induced by the EFD sensor orientation w.r.t. the satellite orbital direction. The measurements have been performed at B ¼ 0 G (obtained applied a proper current in the Helmholtz coil system of the IAPS Plasma Chamber) to avoid spurious effects due to the plasma beam bending. Lower panel of Fig. 9 shows three I V curves obtained for three different attitudes. The curve coloured in black refers to the configuration (a) with the boom axis perpendicular to the plasma beam. This represents the case of maximum ion collecting surface being almost null the stub shadow. In fact, the black I V curve exhibits the largest positive values of the probe floating potential Vf , irrespective of the bias current level. The other positions (b and c) have been obtained by setting the boom parallel (green line) and 45∘ diagonally oriented (red line) w.r.t. the plasma beam, respectively. In these two cases we have an ion collecting surface reduction of about 10% w.r.t. of the full disk obtained in the first configuration. The probe moves its potential to balance the total current of about 10% more negative w.r.t. the full disk collecting area, as expected. These measurements confirm early suggestions about a spurious electric field that will be sensed by EFD probes during the flight due to the different probes attitudes w.r.t. the satellite ram direction (i.e. the ion flux direction).
8. Conclusions An electric field detector able to measure electric field in a wide band, from DC up to about 4 MHz, has been developed, realized and fully tested for space missions. Primarily, tests have been performed in a Faraday Cage, in order to verify accurately all the electrical characteristics and specifications. Further tests took place in a Plasma Chamber to evaluate plasma-probe coupling effects in a real ionospheric environment. The instrument consists of four identical spherical sensors with embedded preamplifier and a controlled current source in order to obtain accurate measurements of the three components of the electric field. Compared with its predecessor ICE experiment onboard Demeter, important and substantial improvements have been achieved. The improved performance of EFD have been obtained by adopting electronic components (ADC and amplifiers) with low noise and a higher bit resolution, and with a careful design of the electronic board (reduced cross-talk, shielding, etc.) conceived for reducing the electronic noise of the whole signal processing chain. The introduction of a switch matrix permits a complete reconfigurability of the selected sensor pairs through telecommand, useful also in case of failure of one or more EFD sensors. Expected plasma conditions at CSES have been studied in order to
7.3. Plasma potential variation effect on Vf
Fig. 10. Upper panel shows the induced plasma potential sinusoidal oscillation (black line) with 100 mV amplitude at 1 Hz. The red line represents the corresponding oscillation of the probe floating potential. Bottom panel shows attenuation vs frequency in the range 1 1000 mHz. Though Vf oscillation is largely attenuated, it maintains the sine shape of the input up to 1 Hz. In the upper panel the phase between the two signals is arbitrary being independently acquired vs time. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
As already introduced in sect. 7.1, for a probe immersed in a plasma the difference between Vp and Vf depends on the electron temperature Te . On the other hand, the Te variation time scale is widely larger than that of plasma fluctuation we want to detect. For this reason, we have tested the EFD ability to follow plasma potential fluctuations specifically induced in the plasma chamber during the tests, maintaining constant the 117
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characterize the EFD measurements. In particular, the analysis of ion and electron current collection along the orbit allowed the identification of the best value of the bias current to be injected on the sensor during the flight to reduce the plasma-coupling impedance.
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Acknowledgements This work was supported by the Italian Space Agency in the framework of the ”Accordo Attuativo n. 2016-16-H0 Progetto Limadou Fase E/ Scienza” (CUP F12F1600011005). The authors want to thank the Chinese colleagues from Lanzhou Institute of Physics (LIP China) who partecipate to the definition of the EFD Project and in particular the P.I. of CSES-1 EFD experiment Dr. Lei Jungang. References Aggson, T.L., Heppner, J.P., 1964. A Proposal for Electric Field Measurements on the ATS4 Satellite submitted to NASA, August 1964. Alken, P., Maus, S., 2007. Spatio-Temporal characterization of the equatorial electrojet from CHAMP, Ørsted and SAC-C Satellite magnetic measurements. J. Geophys. Res. 112, 2007. https://doi.org/10.1029/2007JA012524. 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. https://doi.org/10.1016/j.pss.2005.10.016. Block, L.P., F€ althammar, C.-G., Lindqvist, P.-A., Marklund, G., Mozer, F.S., Pedersen, A., Potemra, T.A., Zanetti, L.J., 1987. Electric field measurements on Viking: first results. Geophys. Res. Lett. 14.4, 435–438. https://doi.org/10.1029/GL014i004p00435. Burch, J.L., 2008. In: Angelopoulos, V. (Ed.), The THEMIS Mission, 141/vols. 1-4. Reprinted from Space Science Reviews journal. Campbell, W.H., 2009. Natural magnetic disturbance fields, not precursors, preceding the Loma Prieta earthquake. J. Geophys. Res. 114 https://doi.org/10.1029/ 2008JA013932. A05307. de La Beaujardiere, O.C.S.D.T., 2004. C/NOFS: a mission to forecast scintillations. J. Atmos. Sol. Terr. Phys. 66.17, 1573–1591. De Santis, A., De Franceschi, G., Spogli, L., Perrone, L., Alfonsi, L., Qamili, E., ~ 3n-Carrasco, F.J., Cianchini, G., Di Giovambattista, R., Salvi, S., Filippi, E., PavA Monna, S., Piscini, A., Battiston, R., Vitale, V., Picozza, P.G., Conti, L., Parrot, M., ~ PinAxon, J.-L., Balasis, G., Tavani, M., Argan, A., Piano, G., Rainone, M.L., Liu, W., Tao, D., 2015. Geospace Perturbations Induced by the Earth: the State of the Art and Future Trends. Physics and Chemistry of the Earth. Parts A/B/C, vol. 2015. Elsevier, pp. 17–33, 85–86. Diego, P., Bertello, I., Candidi, M., Mura, A., Vannaroni, G., Badoni, D., 2017a. Plasma and fields evaluation at the Chinese seismo-electromagnetic satellite for electric field detector measurements. IEEE-Geoscience and Remote Sensing 5, 3824–3833. https:// doi.org/10.1109/ACCESS.2017.2674019, 2017. Diego, P., Bertello, I., Candidi, M., Mura, A., Coco, I., Vannaroni, Ubertini, P., G, ., Badoni, D., 2017b. 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A high-performance electric field detector for space missions D. Badoni a, b, *, R. Ammendola a, I. Bertello c, P. Cipollone a, L. Conti d, a, C. De Santis a, P. Diego c, G. Masciantonio a, P. Picozza b, a, R. Sparvoli b, a, P. Ubertini c, G. Vannaroni c, d a
INFN Sezione di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Rome, Italy Department of Physics, University of Rome ‘Tor Vergata’, Via della Ricerca Scientifica 1, I-00133 Rome, Italy c INAF-IAPS, Via del Fosso del Cavaliere 100, I-00133 Rome, Italy d Uninettuno University, Corso Vittorio Emanuele II, 39, I-00186 Rome, Italy b
A R T I C L E I N F O
A B S T R A C T
Keywords: Electric field detector Satellite instruments Magnetosphere Ionosphere Earthquake Electromagnetic emissions
We present the prototype of an Electric Field Detector (EFD) for space applications, that has been developed in the framework of the Chinese-Italian collaboration on the CSES (China Seismo-Electromagnetic Satellite) forthcoming missions. In particular CSES-1 will be placed in orbit in the early 2018. The detector consists of spherical probes designed to be installed at the tips of four booms deployed from a 3-axes stabilized satellite. The instrument has been conceived for space-borne measurements of electromagnetic phenomena such as ionospheric waves, lithosphere-atmosphere-ionosphere-magnetosphere coupling and anthropogenic electromagnetic emissions. The detector allows to measure electric fields in a wide band of frequencies extending from quasi-DC up to about 4 MHz, with a sensitivity of the order of 1μV=m in the ULF band. With these bandwidth and sensitivity, the described electric field detector represents a very performing and updated device for electric field measurements in space.
1. Introduction The lithosphere-atmosphere-ionosphere-magnetosphere coupling involves a lot of physical effects and interactions at all levels starting from underground up to the Earth's magnetosphere. Seismicity is a source of electromagnetic signals at ground and in the near-Earth space (Pulinets and Boyarchuk, 2005). Even not exhaustive, a review of some measurements and related analyses on ground and space based data of possible seismic precursors can be found in Rodger et al. (1999) and in Tables 2 and 3 of Sgrigna and Conti (2012). Early analyses have correlated some ULF anomalies detected on ground with earthquakes (such as the strong Loma Prieta event of 1989 (Fraser-Smith et al., 1990). Although these results have been intensely debated (Fraser-Smith et al. (2011); Campbell (2009) and Fraser-Smith et al. (2011)), the low frequency band is one of the most interesting for investigating seismo-electromagnetic signals due to the lower attenuation induced by the lithosphere-atmosphere-ionosphere layers. Nevertheless, in the case of very shallow and strong earthquakes, when the size of the preparation focal zone is greater than the hypo-central depth, also the higher frequency signals could be transmitted on ground and to the near space (Sgrigna et al., 2004).
Some slow electro-telluric and magnetic field variations (Varotsos and Alexopoulos, 1984a), (Varotsos and Alexopoulos, 1984b) as well as ULF emissions (Fraser-Smith et al., 1990); (Molchanov et al., 1995); (Kopytenko et al., 1993), ELF-VLF disturbances (Gokhberg et al., 1982), (Fujinawa and Takahashi, 1998), and HF emissions (Warwick et al., 1982) have been correlated with the occurrence of earthquakes. Some observations of electric and magnetic field fluctuations in the frequency range 10Hz 15kHz obtained by satellite AUREOL-3 (Parrot, 1989) have been claimed to be short-term seismic precursors. Perturbations in the ULF/ELF/VLF bands have been detected before earthquakes by the ITK1300 and the Intercosmos-24 satellites several hours before the main shock (Molchanov et al., 1993). The most recent satellite studies of seismo-associated electromagnetic and ionospheric phenomena have been performed by the French satellite Demeter (nearly Sun-synchronous orbit at an altitude of about 700 km). ICE, the Demeter electric field detector, has the sensitivity in ELF (at the frequencies above 100 Hz)/VLF/HF bands and resolution in ULF (quasi-DC) band of μVffiffiffiffi 0:1 m⋅p , 0:05 m⋅μpVffiffiffiffi , 0:1 m⋅μpVffiffiffiffi and 40 μmV respectively (Berthelier et al., Hz Hz Hz 2006). In this framework, several statistical analyses (Pisa et al. (2012); Nemec et al. (2008), Pisa et al. (2012, 2013)) of about 1 2 kHz electric field data (gathered by ICE), show that the normalized intensity of the
* Corresponding author. INFN Sezione di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Rome, Italy. E-mail address: [email protected] (D. Badoni). https://doi.org/10.1016/j.pss.2018.01.013 Received 26 July 2017; Received in revised form 25 January 2018; Accepted 29 January 2018 Available online 5 February 2018 0032-0633/© 2018 Elsevier Ltd. All rights reserved.
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background noise and to asses their correlation with earthquakes occurrence (De Santis et al. (2015) and references therein). In order to infer the seismic signals in the electric field measurements, natural variations in the electric fields should be subtracted. In particular, it is relevant: the equatorial Electrojet (EEJ) (Alken and Maus, 2007), at the magnetic equator with an amplitude of the order of less than a mV=m, and the interplanetary electric field (IEF), a penetrating field that occurs especially when the interplanetary magnetic field turns negative with an amplitude of fraction of mV/m (Yamazaki and Maute, 2016). In addition, anthropogenic electromagnetic emissions as well as spurious fields induced by detector probe's intrinsic structure (Diego et al., 2017a), must be taken into account. In this framework, the study of precursors can take advantage from the continuous monitoring with repeated measurements in a short time period and high resolution performed by constellation of satellites such as the CSES missions and the three satellite of the Swarm mission (Olsen and Haagmansguest, 2006). 2. Space-borne measurements of the electric field
Fig. 1. Positioning of the EFD sensor (0, 1, 2 and 3) on the satellite booms. The x axis is parallel to S/C motion, z is to zenit and y completes the orthogonal reference system.
Measurements of electric fields in space plasma using the technique of double spherical probes installed on booms deployed from a spacecraft date back since 1960s. It was proposed from Aggson and Heppner (1964) for the ATS-4 satellite. The first successful results of this technique in a tenuous magnetospheric plasma was obtained onboard the S3-3 spacecraft in 1976 (Mozer et al., 1979), shortly followed by other experiments onboard GEOS-1 and ISEE-1 satellites in 1977. Afterwards, the use of crossed double probe systems in several satellite missions (such as GEOS-2 (Jones, 1978), Dynamic Explorer 2 (Maynard et al., 1981), Viking (Block et al., 1987), GEOTAIL (Tsuruda et al., 1994), Freja (Marklund et al., 1994), Polar (Harvey et al., 1995), Cluster (Gustafsson et al., 1997), C/NOFS (de La Beaujardiere, 2004), etc.) made possible measurements of the electric field vector components in the ionosphere and magnetosphere. These provided a direct knowledge of the electric field structure, also allowing a detailed correlation with the correspondent magnetic field configuration. Double probe sensors have been successfully used in the D, E, and F ionospheric regions, as well as in the
night-time electric field amplitude decreases below the mean background level few hours (0–4 h) before the shallow (depth < 40 km) of medium and strong (M > 5) earthquakes. The spatial scale of the affected area (about 350 km) is in good agreement with the size r of the preparation zone for earthquakes of magnitude M > 5 estimated with the Dobrovolsky et al. (1979) formula r ¼ 100:43 M km. A possible explanation of the observed perturbation is a local lowering of the ionosphere bottom layer induced by seismo-associated phenomena. Significantly, the largest decrease occurs at about 1:7 kHz, i.e. approximately near to the cut-off frequency of the first transverse magnetic mode of the Earth-ionosphere waveguide in night-time. An increase of this cut-off frequency would induce a decrease of the electric field power spectral density (as statistically observed by Demeter) and an increase of the plasma density in the lower ionosphere above the earthquakes preparation area. This will result into a local perturbation of the propagation of whistlers, which are the main natural electromagnetic waves propagating in the Earth-ionosphere wave-guide. Anyway, at present, there is not clear evidence of the precursors signature (i.e. in which range and condition a detected signal can be considered anomalous). The electric and magnetic components of seismo-associated electromagnetic fluctuation
Table 1 Typical plasma conditions as measured by Swarm A satellite and corresponding equivalent impedance values adopted for test measurements on the EFD prototype. Note that, the value of the plasma-probe capacitance, calculated for an spherical probe (3 cm radius) using Swarm plasma data, is 19 pF, but due to the presence of the stubs, which shade part of the sensor surface, its value is reduced to about 12 pF.
are estimated very faint, being some fraction of mV=mðHzÞ1=2 and some
Plasma conditions
fraction of nT=ðHzÞ1=2 or less, respectively (Parrot et al. (1993) and Sgrigna et al. (2007)). Moreover, the complexity of the lithosphere-atmosphere-magnetosphere coupling mechanism, asks for further studies in order to clearly distinguish anomalous signals from
Density 2:5 ⋅10
11
Temperature 3
m
2200 K
Current Injected 5 μA 10 μA
Impedance Zc Resistance
Capacitance
25 kΩ 14 kΩ
’ 12 pF
Fig. 2. EFD probe. The yellow sphere S is the sensor current collector; the first inner hollow shell C1 is bootstrapped to the potential of the probe in order to minimize the capacitive coupling of the outer sphere with the ground. The stubs St1 and St2 are also connected to the bootstrap circuit in order to reduce boom perturbations by preventing disturbances to the sphere potential. The cylinder C2 is the shell connected to the ground and contains the electronic board B. In the block diagram (c) the simplified schematic of the electronics is shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. EFD general architecture. The four Probes in the block diagram (left side) are denoted as S0, S1, S2 and S3. The Analog Processing Unit selects the probes, performs the signal filtering and sub-division in bands, generates the bias currents for the probes and carries out the analog/digital conversion (block in the center). The Digital Processing & Control Board (right block) provides further processing and implements the control and configuration features of the detector.
inner magnetosphere, where the electric field is a crucial quantity to be measured for understanding the non-linear processes that may produce plasma acceleration. Extensive investigations of the electromagnetic near-Earth space environment have been accomplished recently (i.e. Demeter (Parrot, 2006), THEMIS (Burch, 2008)), or are planned in future missions (such as TARANIS (Lefeuvre et al., 2008), CSES (Shen et al., 2011) etc. The payloads of these satellites include, in addition to an electric field detector, several instruments to measure magnetic field, plasma parameters and particle fluxes. In particular, the main objective of the CSES (China SeismoElectromagnetic Satellite) satellite program is the study of the electromagnetic, plasma and particle perturbations caused by seismicity in the ionosphere, magnetosphere and inner Van Allen belts, prosecuting the exploratory investigation performed by the Demeter satellite. CSES is a mission led by the Chinese (CNSA) and Italian (ASI) space Agencies that includes two satellite missions CSES-1 and CSES-2. The CSES satellite, 3-axis attitude stabilized, is based on the Chinese CAST2000 platform. It will be placed at a Sun-syncronous circular orbit at an altitude of about 500 km (descending node at 14 : 00 LT; orbit inclination 93∘ ; altitude 507 km). Nine instruments are installed on the satellite that will be launched in the early 2018 with an expected lifetime of 5 years (see website: cses.roma2.infn.it). The CSES-2 mission is designed as a twin version of CSES-1, with some adjustments and differences. CSES-2 is planned to be launched at the same orbit of CSES-1 within 2 years after the begin of the CSES-1 mission, in order to execute coordinated and synchronous measurements, aimed at measuring the background and to exploit the possibility to improve the observation of seismo-associated phenomena with two contemporary satellite missions.
second satellite. The instrument consists of four identical probes located at the tip of four booms deployed from a 3-axes stabilized spacecraft as shown in Fig. 1, and their associated electronics. The attitude information is acquired by star sensor together with gyroscope. There are three star sensors, two of which are guaranteed to be available in case one is illuminated by sunlight. The pointing accuracy of the satellite is better than 0:1∘ (3-Axis, 3σ), the knowledge accuracy is better than 0:03∘ (3-Axis, 3σ) and the stabilization accuracy is better than 0:001∘ =s (3-Axis, 3σ). Even though the design of CSES-2 has not been completed, the booms should be similar to those adopted for CSES-1, that are of the same type of the Kaleva ones installed on the Demeter and FAST missions (Dupuy et al., 2007), (Pankow et al., 2001). In CSES-1 the booms are 4.15 m long, with a minimum distance between probe pairs of 6741.4 mm. The difference of the electric potential between each pair of probes divided by their mutual distance determines the electric field component along the direction defined by the probe locations. The electric field is measured for an extended band of frequencies from quasi-DC up to about 4 MHz, with a sensitivity of the order of 1μV=m in the ULF band. The detector combines low intrinsic noise, high resolution and large bandwidth. The instrument has been conceived on the basis of the most recent technology in device development and digital signal processing. As observed by Demeter mission, perturbations on electric field detectors can be induced by other instruments and ancillary systems. For example, the ICE calibration mode introduced specific lines in the spectrum (625 Hz and 10 kHz). Disturbances up to several tens of mV=m at about 1 Hz due to sweeping voltages Langmuir probes (ISL) were detected in the ICE spectrograms (Lagoutte et al., 2005). At this purpose, the sweeping voltage of the Langmuir probe of CSES-1 has been strongly reduced to avoid significant loss of performance in the electric field detector. In order to minimize the electromagnetic interferences, and in general to reduce the electronic noise, the EFD electronic boards are installed inside a shielding metallic box and the connection between the central unit and the probes are implemented through highly shielded coaxial cables. For what concerns the satellite potential effect, CSES is very similar to Demeter where the spacecraft potential has been observed to be very stable and close (within 0 and 1 V) to the plasma potential, as recorded by ISL instrument. Thus, electrostatic perturbation due to S/C
3. The electrical field detector An electric field detector (EFD), suitable to investigate electromagnetic phenomena in the near Earth environment, such as ionomagnetosphere transition zone, has been developed in the context of the Chinese-Italian CSES collaboration in order to be installed on the 109
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Fig. 4. APU block diagram. The switch matrix permits a complete and independent reconfigurability of the sensor pairs selected for the measurements of the various electric field components. Four chains for LoF channels, three chains for the elaboration of the MeF bands and one for the HiF channel are available. The APU generates also the control voltage of the bias current for the probes through a DAC. Several temperature sensors are mounted in the board. The temperatures of the board as well as those coming from the sensors are digitized from the respective ADCs.
electronics block diagram of one of the EFD sensors. The probe consists of three concentric metal shells, an holder and two stubs, St-1 and St-2 in the figure, placed at the opposite sides of the sensor (aligned with the boom axis) to reduce the potential asymmetry caused by the conductive boom connected at the satellite ground. They are bootstrapped at the same potential of the probe in order to prevent disturbances to the sphere potential induced by the presence of the boom which is electrically connected to the satellite ground. As a drawback, however, the stubs may intercept the trajectories of ions and electrons altering the current collection by the probe, producing spurious electric fields in the range of mV=m that should be computed and removed (Diego
body could be neglected if compared to above mentioned effects. In addition, the booms are deployed in proper directions to prevent the probes to fall within the wake of S/C or other booms (Diego et al., 2017a). Although the EFD has been realized to be the candidate for the foreseen second CSES mission, it has been designed to be installed on broad class of satellites devoted to study electromagnetic phenomena of natural and anthropogenic origin. 4. The probe Fig. 2 shows a photograph, some details of mechanical layout and the 110
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Digital Processing & Control Board From/To Host
DSP SPI 31 bit SPI 31 bit SPI 31 bit
100 sps 100 sps
4 CH. LoF Filter Cores
Fig. 5. Digital Processing & Control Board block diagram which describes the functionalities implemented on the Xilinx. The DSP block (Digital Signal Processing) implements the digital filters for the digitized signals generated by the ADCs of the APU board and produces the FFT spectra for the VLF and HF bands. Furthermore, the board generates/controls and manages all the data from/to the DACs and the temperature readings from the APU board. Finally, the board manages all communication to/from a hostPC.
100 sps 100 sps
ULF Waveform - 0 Hz – 16 Hz
SPI 31 bit
21 Ksps 21 Ksps SPI 24 bit SPI 24 bit SPI 24 bit
3 CH. MeF Filter Cores
21 Ksps
ELF Waveform - 13 Hz – 2 KHz 125 Ksps 125 Ksps
3 CH. VLF FFT Cores
125 Ksps
ETH. UDP Interface
VLF Spectrum 1 KHz – 50 KHz Parallel 16 bit
HiF Filter Core
HF FFT Core
16 Msps
HF Spectrum 21 KHz – 4 MHz
Switch SPI/I2C IF
Switch Network State Machine
SPI 15 bit / SPI 12 bit
ADC SPI IF
Temperature reading State Machine
SPI 16 bit
DAC SPI IF
Bias current injecƟon State Machine
prescribed levels of current to the probe. The surface of the probe S and the internal shield C1 (bootstrapped at the sensor potential) form a capacitor between the output and the unity gain amplifier input. By using the Miller theorem, we can easily see that an instability could take place, due to a negative resistance that may rise at high frequency at the input of the amplifier. The low pass filter, reducing the high frequency response of the unity gain amplifier, prevents possible high frequency instability of the circuit. The EFD prototype is an evolution of the Demeter ICE sensor (Berthelier et al., 2006), opportunely updated to improve the measurement accuracy and to extend the bandwidth. The adopted technique for electric field measurements implies a quite efficient probe-plasma coupling, i.e. a low contact impedance (Zc ) with the surrounding plasma, which is, in turn, strongly dependent on the local plasma parameters (density and electron temperature) and possibly on the photoelectron current emitted by the probe surface. The Zc must be small compared to the input impedance of the sensor preamplifier. The value of this impedance is of the order of about 10 GΩ in DC, and decreases as frequency increases. The contact impedance exhibits its minimum close to the plasma potential point (Vpl ). A current injected on the probe, in
Table 2 EFD definition of frequencies bands. W: waveform, S: Spectrum. APU
DPCB
Name
ΔFreq: (Hz)
Name
ΔFreq: (Hz)
LoF
0 16
MeF
13 50k
HiF
21k 4M
ULF ELF VLF HF
0 16 13 2k 1k 50k 21k 4M
Type
Output Data rate
W W S S
100 sps 21 ksps 125 ksps 16 Msps
et al., 2017b). The outer shell, S, with a diameter of 6 cm and coated with conductive material DAG 213 from ACHESON, is the sensing surface. The second shell C1 is a hollow cylinder bootstrapped at the same potential of the probe to minimize the capacitive coupling of the sensor with the ground, thus increasing the frequency response of the readout circuit. The third shell, C2, is a conductive box connected to the ground which shields the electronic board B. The signal coming from the outer sphere is applied to the input of a voltage follower with a very high input impedance. Moreover, a voltage controlled current source injects
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Fig. 6. From upper to lower panel: ULF output voltage noise spectral density; ULF Transfer Function magnitude; ELF output noise voltage spectral density; ELF Transfer Function magnitude. All graphs have been obtained with the two different values of plasma impedance and injected current. The black line is the measured background noise obtained with the probes connected to ground. The small peaks at frequencies: 100 Hz, 500 Hz and 1 kHz are due to the MeF ADC.
the best value of the current to be injected for typical plasma conditions at CSES orbit, also by using data from Swarm satellite recorded in April 16, 2014 in an orbit similar to that of CSES. The obtained results are shown in Table 1. Despite at 10 μA the value of the resistance is lower than to that obtained at 5 μA, the latter value is assumed as the optimal one, since for 10 μA the probe potential could exceed the plasma potential (e.g. during exceptional plasma density minima). Anyway, during the in-flight operations, this bias current is adjustable through Telemetry & Telecommand control. Indeed, the contact plasma impedance can be measured during EFD calibration sessions by injecting of AC signals, superimposed to the DC bias current. Then, adjustments in the DC current bias permit to minimize the contact impedance. The AC signal adopted to determine the coupling resistance between probe and plasma consists of a sine current waveform at a frequency of 625 Hz, with an amplitude which can be adjusted from a few tens to a few hundreds of nA. Such a signal is applied to the sensors through the same electronics already used to apply the DC bias current. The AC current
contact with the plasma, modifies the balance among the various currents; in particular, a proper positive current can drive the probe potential close to the local plasma potential. The EFD probe behaves essentially as a floating electrode embedded in plasma. Three main currents determine the floating potential Vf of the conducting probe: the electron and ion collections and the photoelectron emission. The photoelectron emission has been evaluated to be negligible for the EFD probe at CSES orbit. In fact the intensity of the photoelectron current emitted from the EFD probes has been widely discussed in (Diego et al., 2017a), where it was concluded that at the CSES orbit the Iph is of the order of 50 nA. Therefore, if a bias current of 5 μA is applied (see the considerations discussed below), the photoelectron current is about a hundred times lower. Moreover, as the electron current collected by the probes dominates over the ion current, the sensors floats at negative voltages with respect to the local plasma potential. Therefore, the currents injected to the probes, which are the same for all the sensors, must be positive to move the probe voltage toward the plasma potential. A detailed study has been executed in Diego et al. (2017a) to estimate 112
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Fig. 7. From upper to lower panel: VLF output voltage noise spectral density; VLF Transfer Function magnitude; HF output noise voltage spectral density; HF Transfer Function magnitude. All graphs have been carried out with the two different values of plasma impedance and injected current. The black line is the measured background noise obtained with the probes connected to ground. In the HF band small peaks are visible at 2 MHz and 4 MHz as residual interference noise due to the clocks used in the digital electronics.
Table 3 Total output voltage noise VRMS in ULF end ELF bands. Band
ULF
ELF
Table 4 EFD sensitivities.
Current Injected; Plasma Impedance
Noise (μVRMS )
0 μA ; Direct coupling 5 μA ; 25 kΩ//12 pF 10 μA ; 14 kΩ//12 pF 0 μA ; Direct coupling 5 μA ; 25 kΩ//12 pF 10 μA ; 14 kΩ//12 pF
3.3 3.0 3.2 5.6 5.8 5.8
ULF nV= pffiffiffiffiffiffi m⋅ Hz
ELF nV= pffiffiffiffiffiffi m⋅ Hz
VLF nV= pffiffiffiffiffiffi m⋅ Hz
310
60
15
HF nV= pffiffiffiffiffiffi m⋅ Hz
(@ 30 40kHz) (@ 4 MHz)
30 900
5. EFD signal acquisition and data processing system The block diagram of the EFD instrument is shown in Fig. 3. It consists of 4 probes and a complete DAQ system for signal filtering, acquisition and data processing. The four probes are denoted as S0, S1, S2, S3. The data acquisition chain consists of two main units:
signal with amplitude ΔI produces a corresponding variation of potential ΔV in the sensors (according to the probe current-voltage characteristic). The coupling resistance is thus determined from the ratio ΔV=ΔI, that is expected to be of several tens of kΩ. Such a method is similar to that adopted on ICE (Berthelier et al., 2006).
The Analog Processing Unit is a signal conditioning unit which provides the first analog filtering of the signals that performs a 113
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VLF and HF. The digital filters were designed to obtain a stopband attenuation of at least 120=150 dB with a multirate architecture to further reduce the ADC data output. The digital decimation filter on the LoF decreases the output data rate to 100 sps, that is a trade-off between data budget constrains of a satellite mission (storage & download) and a good signal reconstruction. The acquired MeF data undergo to a multirate low-pass filtering that reduces the data rate to 125 ksps. The resulting signal is then fed to a pair of different processing flows. The first path provides a multirate filtering with 2 kHz cutoff frequency and a decimation ratio of 6, necessary to obtain the waveform for the ELF band. The second path provides modules for the spectral analysis in the VLF band. The HiF processing unit has a structure similar to the VLF one, with a first stage composed by a cascade of multirate low-pass filters, to reduce the data rate to 16 Msps, followed by a spectral analysis module. The spectral analysis section execute a 2048 points FFT for both the VLF and the HF bands, with a spectral resolutions of 61:04 Hz and 7:81 kHz respectively. Table 2 shows a summary of the various bands implemented by the various filters of APU and DPCB. In addition, the DPCB implements the control and configuration of the APU switch matrix and the setting of the voltages to be applied to the current generators embedded into each Electric Field Probe (EFP). This section also allows the managing of the calibration mode and performs the temperature reading of the EFPs and of critical APU areas. Finally, the DPCB provides the management of all communication between the EFD and a host-PC through a UDP/IP Ethernet interface. Note that, as already introduced at the beginning of this section, this work provides the description of the maximum data acquisition and transfer rate. Nonetheless, a strong data flow reduction will be imposed by the satellite bandwidth downlink.
preliminary sub-division of the signal into frequency bands; generates all the necessary control voltages for the EFD (as the bias current injection of the probe) and, last, performs the Analog/Digital conversion. The Digital Processing & Control Board is a digital unit for data sampling and processing that execute further elaborations of the signals and the Fast Fourier Transform (FFT). Moreover, it implements the control and configuration features of the detector. After the experience of Demeter, and waiting for the results of CSES-1 mission, we have adopted the most flexible data acquisition strategy according to the state-of-art of the updated electronic components. Such strategy is conceived to minimizing intrinsic noise, to optimize resolution, dynamic range and sampling acquisition modes. Considering the maximum expected voltage dynamic (about 2:5V at low frequencies), mainly due to the v x B and to the probe Vfloating as reported in Diego et al. (2017), the ADCs input ranges are consequently chosen as the trade-off among the relevant requirements as: low noise, high S=N ratio, linearity and dynamic ranges. 5.1. The Analog Processing Unit (APU) The block diagram of the APU is shown in Fig. 4. The signals coming from the 4 probes are multiplexed to select couples Si Sj (i 6¼ j ¼ 1; …; 4) needed to evaluate three electric field components. The switch matrix scheme allows the choice of all the possible couples Si Sj , thus ensuring the measurement of at least one or two electric field components in case of failure of one EFD sensor. The signal spectrum is divided via analog filters into three different bands (low, medium and high frequency channels) denoted LoF, MeF and HiF, which are further digitally filtered. The LoF band includes signals up to 16 Hz, the MeF band selects frequencies from 13 Hz to 50 kHz and the HiF band selects frequencies from 21 kHz to 4 MHz. The LoF channel uses a ADC with extremely high-performance and high-accuracy instrumentation having a S=N ratio of 120 dB. The LoF, MeF and HiF ADCs have a voltage input full scale of 2:5 V, 3:0 V and 1:5 V respectively. In each chain the signal is amplified with a gain slightly different from the unity to optimize their effective range as shown in Table 5. In the LoF bands, the acquired signals are the 4 potentials directly measured between each probe and the satellite body which, in turn, is considered as ground reference. These LoF signals are acquired through 4 dedicated ADCs with independent amplification chains. The acquisition in the MeF band is carried out by measuring three potential differences among the selected sensors, while in the HiF band only one potential difference is measured between a selected pair of EFD sensors. All the acquired signals are over-sampled to minimize the intrinsic analog noise, lowering the order of the anti-aliasing filters and reducing the number of the operational amplifiers. The LoF filter is a low-pass third order Butterworth filter implemented with Sallen-Key architecture plus a single pole filter. The signal is sampled by a 31-bit ADC at 1 ksps output data rate. The MeF signal undergoes a pass band filtering, with a second order Butterworth implemented with a Multiple-Feedback (MFB) architecture. The signal is sampled by a 24-bit ADC at 1 Msps output data rate. The HiF signal is elaborated by a band pass network composed by a cascade of a fourth-order Butterworth high-pass and a fourth-order Butterworth low-pass filters with Sallen-Key architectures. The HiF signal is then sampled by a 16-bit ADC operating at 128 Msps output data rate.
6. Test measurements on the EFD prototype in Faraday Cage The intrinsic noise, the dynamic range and the transfer function of the EFD instrument has been characterized end to end (probes, APU and DPCB) installing a pair of probes in a Faraday Cage to obtain an efficient shielding from the external electromagnetic environment. Due to the very high input impedance of the probes, a special care has been taken in the design of the harness in order to reduce the external noise. The intrinsic noise, in the various bands, is assumed to be the sensitivity of the instrument, since it represents the bottleneck of the system being in general larger than the LSB (Less Significant Bit) of the ADC: 2VðnFSbit Þ ADC
where VFS is the value of the full-scale sine wave and nbit is the bit numbers of the ADC.
6.1. Test set-up and methods The EFD tests have been performed as a function of the bias current applied to the probes. The bias current is applied directly at the input of the sensors through the built-in constant current source present in each probe. Tests have been carried out by taking into account the estimated average plasma conditions described in previous section (Table 1); thus the two probes have been connected to the measurement setup through a parallel Rpl ==Cpl element which emulates the probe-plasma Zc coupling impedance. The used values have been 25 kΩ==12 pF with the optimal bias current of 5 μA and 14 kΩ==12 pF with the bias current of 10 μA, as previously defined. The Rpl ==Cpl element at each probe input is connected in two different configurations: (i) to perform noise measurements the Rpl ==Cpl are connected to ground; (ii) to perform measurement the dynamic range and transfer function tests the two Rpl ==Cpl elements are connected one at ground and the other to an external signal generator. In the ELF, VLF and HF bands, each signal at the output of the measurement chain is obtained directly as potential difference between the EFD probe pairs, while for the ULF band, an off-line manipulation of data is required
5.2. The Digital Processing & Control Board (DPCB) The Digital Processing & Control Board shown in Fig. 5 is based on a FPGA Xilinx Virtex 6. The LoF, MeF and HiF signals are processed inside a digital signal processing (DSP) block to obtain the four bands ULF, ELF, 114
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from the background noise, thus a local approach (narrow bandwidth) for the VLF and HF dynamic range estimation has been adopted. The frequency bandwidth depends on the frequency resolution δf and on the type of windowing used in the FFT, therefore the correspondent noise is rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn Vi 2 pffiffiffiffi VNOISERMS ¼ ⋅δf where n is the number of points in the i¼1 Hz i 2 i is the power noise spectral narrowband frequency window and pVffiffiffiffi Hz
to obtain the potential difference. The output noise measured at the end of the digital processing chain has to be normalized at the input in order to define the sensitivities and dynamic range. The Transfer Function is obtained by applying a white noise input signal (0-44 MHz band, 1 VRMS ). 6.2. The output voltage noise and the transfer functions
i
The noise and Transfer Function measurements in ULF/ELF and VLF/HF bands are presented in Figs. 6 and 7 respectively. The resulting voltage noise spectral densities in all bands (except HF), starting from 0:2 Hz are essentially independent from both bias current and plasma conditions.
density of the i-th frequency point. Along with these considerations, an additional constraint is represented by the dynamic ranges quoted in the manufacturer data-sheet for the specific ADCs used: 120 dB for the LoF ADC, 110 dB for the MeF ðELF and VLFÞ ADC and 78 dB for the HiF ADC. Concerning the VLF, the good flatness of the Transfer Function implies that the dynamic range is extended in the whole band. For the HF, the dynamic range is 78 dB from lower frequencies up to about 200 kHz where the Transfer Function exhibits few dB of variation, while it is 58 dB at 4 MHz. The results are summarized in the Table 5.
μV Vnoise < 2:1 pffiffiffiffiffiffi ðULFÞ Hz nV Vnoise < 400 pffiffiffiffiffiffi ðELFÞ Hz nV Vnoise < 100 pffiffiffiffiffiffi ðVLFÞ Hz
7. Test measurements of the EFD prototype in plasma chamber The INAF-IAPS plasma chamber is a facility capable to reproduce at
The total noise in the time-domain for the ULF and ELF bands has been determined as standard deviation from the correspondent waveform data. The results are shown in Table 3. The cut-off frequencies are clearly visible around 16 Hz; 2 kHz and 50 kHz for ULF; ELF and VLF respectively. The HF Transfer Function, differently from all other bands, exhibits a not negligible dependence on the plasma impedance. Such a high frequency dependence has its origin mainly in the sensor input circuit, due to the unavoidable presence of parasitic capacitances at the input node of the voltage follower with respect to ground. At the high frequencies the resulting gain of the chain is lower than unity, and this has to be considered for the overall instrument calibration. Taking into account the Transfer Function, the HF intrinsic noise (i.e. voltage noise spectral density) measured at the two band limits (normalized to the EFD input), is:
Table 5 Dynamic Range in all bands. Band
VFSRMS (VRMS )
VNOISERMS (μVRMS )
DR (dB)
ULF ELF VLF
3.1 1.9 1.9
3 5.8 ADC limited ADC limited 1100
120 110 110 78 58
HF
20 200 kHz up to 4 MHz
0.88
nV Vnoise < 200 pffiffiffiffiffiffi @ð30 40 kHzÞ Hz
μV Vnoise < 6:2 pffiffiffiffiffiffi considering all the band up to 4 MHz Hz 6.3. Electric field sensitivities and resolution Starting from all the voltage noise spectral densities and VRMS values determined above and considering a minimum nominal distance of 6:8 m between probe pairs (see section 3), we can compute the sensitivities of the EFD instrument in terms of electric field. The results are summarized in Table 4. For the DC-ULF band, the resolution in terms of electric field is 0:5 μVmRMS . 6.4. Dynamic range evaluation The dynamic range (DR) is defined as the ratio of the root-meansquare (RMS) of a full-scale sine wave (VFSRMS ) to the noise (VNOISERMS ) which is assumed as minimum discernible signal. DR ¼ 20⋅log10
VFSRMS VNOISERMS
Fig. 8. Upper panel shows the floating potential measured by the EFD for three different bias current levels (0, 0:5 μA, and 5 μA). Measurements are obtained for three electron temperatures: 2800 K (blue markers), 3700 K (green markers), 4400 K (red markers). Lower panel shows the correspondent plasma/probe coupling impedance Zc . The Zc has been computed by applying a sinusoidal signal at 625 Hz. Plasma potential in the chamber is þ2.7 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
(1)
According to Eq. (1), the dynamic ranges for ULF and ELF bands are directly determined using the couple of values: VFSRMS , relevant to ADCs characteristics, and VNOISERMS , previously determined for the two bands (see Table 3). A different criterion has been applied for the VLF and HF spectra. A signal is discerned at a specific frequency when it emerges 115
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Fig. 9. Upper panel left side shows the rotary stage used to change the probe attitude w.r.t. the plasma beam direction. Upper panel right side shows the attitude of the sensors set to perform plasma tests in the INAF plasma chamber. The lower panel shows the I-V curves obtained for probe set perpendicular (black line), parallel (green line), and 45∘ diagonally oriented w.r.t the plasma beam. Measurements have been performed at B ¼ 0 G. Plasma potential in the chamber is þ2.7 V. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
7.1. Bias current effects on Vf and Zc
ground the ionospheric environment. The plasma source produces an Argon plasma with parameters very close to the values encountered in the daytime ionosphere at F layer altitudes (e.g. plasma density in the range 1011 1012 m3 , electron temperature in the range of 2000 4000 K). The plasma generated by the source is accelerated into the chamber at a velocity that can be tuned to simulate the relative motion between an object orbiting in space and the ionosphere ( 7:5 km=s). The EFD has been fully tested in the INAF-IAPS Plasma Chamber which, in particular, allowed the measurement of the EFD floating potential in a real ionospheric plasma environment. The tests were performed under various bias currents and for different probe attitudes with respect to the ion beam (assumed parallel to the satellite velocity vector). In the following subsection we summarize the main results relevant to the EFD performance: floating potential Vf and dependence of plasma/probe coupling impedance Zc on plasma parameters, probe attitude and bias current level.
As already introduced in section 4, the EFD payload is a conducting body immersed in plasma and, therefore, moves its potential (floating potential Vf ) in order to have a total collected current equal to zero. An important parameter, which needs to be computed and measured is the contact impedance (Zc ¼ Rpl ==XCpl ) between probe and plasma. At the frequencies used in this test XCpl is very large, therefore in the following the Zc is assumed as Rpl . Measurements of Vf have been performed in Plasma Chamber for three different bias current levels, 0, 0.5 and 5 μA. In addition, the electron temperature has been varied to evaluate the probe response to different plasma conditions encountered by the CSES satellite along its orbit. Vf has been measured for three electron temperature levels: 2800, 3700 and 4400 K, as shown in Fig. 8. The measurements show that the floating potential decreases (i.e. the difference between Vf and Vpl increases) for higher electron temperature because their thermal velocity enhances the electron current collection moving the probe potential more negative. This temperature effect be116
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electron temperature Te (i.e. at constant Vf Vp difference). Note that the Kaufman plasma source, in use at INAF-IAPS plasma chamber, has a separate neutralizer filament for electron emission which can be used to control plasma potential in the chamber. The filament is characterized by a relatively long thermal time constant, therefore there is an intrinsic limitation in following the ion potential at high frequencies (i.e. greater than 0:1 Hz). This test has been performed by applying a sinusoidal signal of 100 mV in the range 1 1000 mHz. Such a signal is directly applied to the neutralizer filament which, in turn, produces a correspondent potential variation of the entire volume of plasma in the chamber. Fig. 10 shows the induced Vf variation by the applied sine signal as well as the attenuation as a function of frequency. The variations are lower than 3 dB for frequencies lower than 100 mHz.
comes smaller for higher bias current. On the other hand, the values of Zc show a dependence on the electron temperature greater than that induced by the bias current. In fact, the Zc reduction obtained by applying 5 μA is about three times in a plasma with Te ¼ 2800 K with respect to the case with Te ¼ 4400 K. 7.2. Probe attitude effect on Vf In order to check the effect of the stubs on the ion collecting surface of EFD probes, a specific set of tests have been performed by injecting in the probe a bias current in the range 0 30 μA (much larger than that typically foreseen in the CSES satellite operation) for different probe attitude w.r.t. the plasma beam direction. Since the thermal velocity of electron (for Te of about 2000 K) is about 105 m=s, while the satellite moves at 7:5⋅103 m=s, we may consider the electrons isotropic collected, and thus, not dependent on the probe attitude. On the contrary, the ion thermal velocity is about 103 m=s for Ti of 1500K (according to data from IRI), which is lower than that of satellite, therefore these are collected only on the probe surface portion perpendicular to the ram direction (i.e. a disk with surface of π ⋅R2 ). For this reason, the effective ion collecting surface of the probe depends on the presence of stubs (conducting cylinders extending in the boom direction on both inner and outer side of the each probe). The stub dimensions are: radius 1:7 cm, outer stub length 2:8 cm and inner stub length 5:15 cm (see Fig. 9 right panel). For different boom attitudes w.r.t. the flight directions such stubs produce a shadowing effect that reduces the ion collection. In order to verify the resulting Vf variation, a dual axis rotary mechanism has been developed to change the polar and azimuth angles, while maintaining the center of the probe in the same point of the plasma chamber (i.e. at the same distance from plasma source and, consequently, at the same plasma conditions). Fig. 9 (top panel left side) shows a sketch of the rotary stage with the EFD probe installed. Top panel right side shows the probe attitudes set to perform tests in the plasma chamber. The system is remote controlled and designed for obtain an accuracy of 1∘ . Fig. 9 lower panel shows the results of the tests to determine the floating potential variation induced by the EFD sensor orientation w.r.t. the satellite orbital direction. The measurements have been performed at B ¼ 0 G (obtained applied a proper current in the Helmholtz coil system of the IAPS Plasma Chamber) to avoid spurious effects due to the plasma beam bending. Lower panel of Fig. 9 shows three I V curves obtained for three different attitudes. The curve coloured in black refers to the configuration (a) with the boom axis perpendicular to the plasma beam. This represents the case of maximum ion collecting surface being almost null the stub shadow. In fact, the black I V curve exhibits the largest positive values of the probe floating potential Vf , irrespective of the bias current level. The other positions (b and c) have been obtained by setting the boom parallel (green line) and 45∘ diagonally oriented (red line) w.r.t. the plasma beam, respectively. In these two cases we have an ion collecting surface reduction of about 10% w.r.t. of the full disk obtained in the first configuration. The probe moves its potential to balance the total current of about 10% more negative w.r.t. the full disk collecting area, as expected. These measurements confirm early suggestions about a spurious electric field that will be sensed by EFD probes during the flight due to the different probes attitudes w.r.t. the satellite ram direction (i.e. the ion flux direction).
8. Conclusions An electric field detector able to measure electric field in a wide band, from DC up to about 4 MHz, has been developed, realized and fully tested for space missions. Primarily, tests have been performed in a Faraday Cage, in order to verify accurately all the electrical characteristics and specifications. Further tests took place in a Plasma Chamber to evaluate plasma-probe coupling effects in a real ionospheric environment. The instrument consists of four identical spherical sensors with embedded preamplifier and a controlled current source in order to obtain accurate measurements of the three components of the electric field. Compared with its predecessor ICE experiment onboard Demeter, important and substantial improvements have been achieved. The improved performance of EFD have been obtained by adopting electronic components (ADC and amplifiers) with low noise and a higher bit resolution, and with a careful design of the electronic board (reduced cross-talk, shielding, etc.) conceived for reducing the electronic noise of the whole signal processing chain. The introduction of a switch matrix permits a complete reconfigurability of the selected sensor pairs through telecommand, useful also in case of failure of one or more EFD sensors. Expected plasma conditions at CSES have been studied in order to
7.3. Plasma potential variation effect on Vf
Fig. 10. Upper panel shows the induced plasma potential sinusoidal oscillation (black line) with 100 mV amplitude at 1 Hz. The red line represents the corresponding oscillation of the probe floating potential. Bottom panel shows attenuation vs frequency in the range 1 1000 mHz. Though Vf oscillation is largely attenuated, it maintains the sine shape of the input up to 1 Hz. In the upper panel the phase between the two signals is arbitrary being independently acquired vs time. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
As already introduced in sect. 7.1, for a probe immersed in a plasma the difference between Vp and Vf depends on the electron temperature Te . On the other hand, the Te variation time scale is widely larger than that of plasma fluctuation we want to detect. For this reason, we have tested the EFD ability to follow plasma potential fluctuations specifically induced in the plasma chamber during the tests, maintaining constant the 117
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characterize the EFD measurements. In particular, the analysis of ion and electron current collection along the orbit allowed the identification of the best value of the bias current to be injected on the sensor during the flight to reduce the plasma-coupling impedance.
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