Comparison of Dust Impact and Solitary Wave Signatures Detected by Multiple Electric Field Antennas Onboard the MMS Spacecraft

1 Introduction

Electric field instruments are able to detect hypervelocity dust impacts on a spacecraft body as transient pulses in the measured electric field. This fact provides an interesting opportunity to monitor dust in various parts of our solar system. Many spacecraft are equipped with electric field instruments, while only a few spacecraft carry dedicated dust detectors.

Hypervelocity dust impacts on spacecraft materials generate a cloud of free electrons and ions by impact ionization. This impact cloud alters the potential of the spacecraft or the electric field antenna by recollection of impact cloud particles. These potential changes can be detected by the electric field instruments as transient pulses. This method of dust detection is de facto a by-product of electric field measurements typically performed by many spacecraft. The pulses generated by dust impacts were first discovered in the measured electric field by the Voyager spacecraft during a crossing of Saturn's ring plane (Aubier et al., 1983; Gurnett et al., 1983). Nowadays these signals are frequently used for dust impact detection on various missions. The Cassini spacecraft registered dust impacts near Saturn's ring plane (Wang et al., 2006; Ye et al., 2016) and when crossing the orbit of Enceladus (Kurth et al., 2006; Ye et al., 2014). Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft detected dust grains around Mars (Andersson et al., 2015). The Deep Space 1 mission registered dust impacts during the Comet P/Borrelly encounter (Tsurutani et al., 2004). Dust detection from interplanetary space was reported by Meyer-Vernet et al. (2009), Zaslavsky et al. (2012), Zaslavsky (2015), Malaspina et al., (2014, 2015), Wood et al. (2015), Malaspina and Wilson (2015), and Kellogg et al. (2016). Cluster measurements reveal events due to dust impacts in the Earth's magnetosphere (Vaverka et al., 2017a). The method for dust detection based on the electric field power spectra has been introduced by Meyer-Vernet (2001) and used by Meyer-Vernet et al. (2009); Meyer-Vernet et al. (2009); Moncuquet et al. (2009). This method analyzes the shape of the electric field power spectra, which can be influenced by dust impacts in regions rich in dust grains as Saturn's ring plane. Future space missions such as Parker Solar Probe or Solar Orbiter that carry electric field instruments will enter unexplored dust environments in the inner solar system.

An advantage of using field measurements for dust detection is that the entire spacecraft body is used as a detection area, which is much larger than the impact area for specialized dust detectors. A disadvantage is that the detected signal strongly depends on the design of the electric field instrument, its configuration (monopole or dipole), the length of the antenna boom, the instrument electronics, and the ambient environment which makes it worthwhile to study the process further.

In spite of intensive ongoing studies including laboratory experiments (Close et al., 2013; Collette et al., 2014, 2015, 2016; Dietzel et al., 1973; McBride & McDonnell, 1999; Nouzák et al., 2018) and computer simulations (Fletcher et al., 2015; Pantellini et al., 2012), signatures generated by dust impacts in the measured electric field are still not completely understood and explained. The complexity of the signals has been reported and discussed by Kellogg et al. (2016). A frequently neglected problem is the identification of dust impacts in obtained waveforms and their distinction from other signals. Solitary waves are commonly present in space plasma typically as symmetrical bipolar pulses (Pickett et al., 2004, 2005, 2015). The symmetricity of solitary waves is sometimes used for the signal identification (Malaspina et al., 2013). On the other hand, their signatures in the measured electric field can be similar to the pulses generated by dust impacts. The signatures are similar when the electric antenna is not parallel to the propagation direction of the solitary wave (Kojima et al., 1999; Omura et al., 1999). This can lead to misinterpretation resulting in a false identification of dust impact signal especially in regions where solitary waves are commonly present.

In this study, we use data from one of the four Magnetospheric Multiscale mission (MMS) spacecraft equipped with three pairs of electric field antennas (the Spin-plane Double Probe (Lindqvist et al., 2016) and the Axial Double Probe (Ergun et al., 2016)). The fact that this instrument operates simultaneously in the monopole (probe-to-spacecraft potential measurement) and dipole (probe-to-probe potential measurement) configuration allows us to study various signals and to reliably distinguish between changes in the spacecraft potential and structures in ambient plasma or electric field. We show examples of different types of signal, present a technique for reliable dust impact detection, and discuss the risk of possible signal misinterpretation.

2 Dust Impacts on the Spacecraft Body and Their Detection

Hypervelocity impacts of dust grains on spacecraft generate free electrons and ions. The amount of released charge strongly depends on the impactor mass and velocity (Collette et al., 2014; Dietzel et al., 1973; McBride & McDonnell, 1999). Released electrons and ions can be subsequently recollected by the charged spacecraft body and/or electric field antenna resulting in a fast reduction of their potential (positively charged spacecraft recollect electrons and repulse positive ions and vice versa). The recollection process and duration of the fast initial potential change are influenced by complex impact cloud dynamics during its expansion and by its interaction with ambient plasma (Meyer-Vernet et al., 2016; Pantellini et al., 2012). The process of the impact cloud recollection and its detection is described in Meyer-Vernet et al. (2014), Vaverka et al. (2017a), and Zaslavsky (2015).

The amplitude of the pulse is determined by the amount of recollected charge, spacecraft capacitance, and potential and the temperature of impact cloud electrons/ions. The mass and velocity of the impactor influence the amount of released charge (Collette et al., 2014; Dietzel et al., 1973; McBride & McDonnell, 1999), while the spacecraft potential and the temperature of electrons/ions affect the amount of the charge recollected by the spacecraft body. For example, only a small amount of electrons can leave a highly positively charged spacecraft surface. In such case the majority of electrons is recollected and the amount of recollected charge is close to the amount of charge released by the impact (Collette et al., 2014). It is possible to use the amplitude of the pulse for a rough estimation of the velocity and mass of impinging grains (Vaverka et al., 2017a; Zaslavsky, 2015). The complicated issue is to distinguish between the impact of small-fast and big-slow grains because the amplitude of the pulse depends on the impactor mass and velocity. The range of expected impact velocities can be used for better estimation of the grain mass. The relaxation of the potential back to the equilibrium value depends on the ambient plasma environment and ultraviolet illumination (photoemission). The relaxation time can significantly vary in different plasma environments due to electron density or ultraviolet illumination because photoemission and electron collection are dominant charging currents (Vaverka et al., 2017b).

The pulse in the spacecraft and/or antenna potential generated by a dust impact can be noted in the measurements of the electric field instruments. This signal strongly depends on the antenna configuration (monopole or dipole). In the monopole configuration the electric field is determined from the potential difference between the one electric antenna and the potential of the spacecraft body (probe-to-spacecraft potential measurement) divided by length of the antenna. This configuration is thus sensitive to changes in the spacecraft potential and can detect dust impacts on the spacecraft body (Meyer-Vernet et al., 2014). On the other hand, the dipole configuration is sensitive only to the potential of the antennas because the electric field is determined from the potential difference between two antennas (probe-to-probe potential measurement). This configuration is sensitive only to a direct dust impact on one of the antennas or to a dust impact on the spacecraft body when fraction of impact cloud particles is recollected by the antenna (Meyer-Vernet et al., 2014).

Signatures of dust impacts are sometimes observed as a pulse with small overshoots. This signal profile can be explained as simultaneous recollection by the spacecraft body and the antenna in the monopole configuration (Zaslavsky, 2015). The overshoot is a result of the fact that the relaxation time for the antenna is longer than that for the spacecraft body (for an unbiased antenna). This explanation is not valid for electric field instruments operating in the dipole configuration where the relaxation time is the same for both antennas. The overshoot detected in dipole configuration cannot be explained by this theory. Aside from that other measure the features can have similar shapes to those generated by the dust impacts.

3 Electrostatic Solitary Waves

Electrostatic solitary waves (ESWs) are frequently observed in space plasma, mainly as isolated symmetrical bipolar pulses (Deng et al., 2006; Kojima et al., 1999; Malaspina et al., 2013; Omura et al., 1999; Pickett et al., 2004, 2005, 2015). For example Pickett et al. (2004) detected thousands of ESWs per hour in the Earth's magnetosphere. These small-scale structures in the electric field are connected to phase space electron/ion holes generated by various plasma instabilities (Graham et al., 2016; Pickett et al., 2004). The electron holes are small (few kilometers) structures in the plasma with reduced electron density. These structures move with speeds up to 3,000 km/s along the magnetic field (Pickett et al., 2004). The electric field measurement through an electron/ion hole results in a bipolar pulse (Omura et al., 1999). The duration of the pulse thus depends on the size of the electron/ion hole and relative spacecraft-hole velocity.

Pickett et al. (2004) pointed out a very important fact that many more of ESWs are found to be asymmetric and oddly shaped. Omura et al. (1999) reported that the shape of ESWs depends on the orientation of the electric field antenna with respect to the direction of ESW's propagation (orientation of the magnetic field). It also varies depending on the path on which the spacecraft crosses the hole, center or edge; see Figure 2 in Omura et al. (1999). The solitary wave signatures in the measured electric field can even be monopolar.

A very important fact is that the duration and amplitude of ESWs can be similar to signatures of dust impacts (Pickett et al., 2004). It is important to note that not only electron holes but, for example, also double layers could be responsible for monopolar and tripolar pulses in the electric field (Pickett et al., 2004). The presence of such structures in the space plasma complicates detection of dust impacts by the electric field instruments.

Another interesting fact is that ESWs can be responsible for Broadband Electrostatic Noise (Matsumoto et al., 1994). Broadband Electrostatic Noise could be similar to the power spectra attributed with dust impacts (Meyer-Vernet et al., 2009) and can complicate the dust impact detection from the voltage power spectra.

4 MMS and Cluster Spacecraft

The four NASA MMS spacecraft operating in close formation were launched in 2015 (Burch et al., 2016). These spacecraft are crossing various parts of the Earth's magnetosphere in their highly elliptical orbits providing data from different plasma environments. Each of the spacecraft is equipped with three pairs of electric field antennas, two in the spin plane (120 m tip-to-tip) and one in the axial plane (29 m; Torbert, Russell, et al., 2016). Only spheres (8-cm diameter) located at the end of each spin plane antenna are used as potential sensors. The potential drop between two spheres is used in the dipole configuration (electric field measurement), and the potential drop between one sphere and spacecraft body is used in the monopole configuration. The sensor at the end of the axial antennas is a 1-m-long and 0.64-cm diameter tube. The advantage of this instrument for dust impact detections is that it operates simultaneously in the monopole and dipole configuration. The electric field measurement (dipole configuration) is followed (in 5 μs) by the probe-to-spacecraft potential measurement for three antennas (one from each pair, P1, P3, and P5). The probe-to-spacecraft potentials for the rest of the antennas (P2, P4, and P6) are derived from previous measurements. The antenna configuration for both modes and corresponding color scheme are shown in Figure 1 (monopole left and dipole right). This color scheme is used in Figures 4– 9. The instrument can operate with three different sampling modes: Slow (8 s⁻¹), Fast (32 s⁻¹), and Burst (8,192 s⁻¹). The sampling frequency in the Burst mode is high enough for the dust impact detection. This frequency is too low for derivation of the characteristic rise time of the pulse, which can be used for signal interpretation (Kellogg et al., 2016). On the other hand, dust detection by the MMS spacecraft can profit from the complexity of their antenna systems as we will show later.

The Cluster mission launched in 2000 is very similar to the MMS, also consisting of four spacecraft in highly elliptical orbits around the Earth (Gustafsson et al., 2001). The Wide Band Data instrument processes signals from the electric field antenna providing data with a high sampling frequency (27,400, 54,900, and 219,500 s⁻¹) (Gurnett et al., 1997). We use data from this instrument to illustrate a large variety of pulses presented in the electric field data from the Earth's magnetosphere.

5 Observations

A reliable identification of events related to dust impacts on the spacecraft body can be a challenging issue in environments where natural waves such as ESWs are commonly present. An example of such a complex event measured by one of the Cluster spacecraft, Cluster 1, on 10 February 2010 at 22:43:31.62 UTC is shown in Figure 2. It is possible to see several different symmetrical and asymmetrical pulses only in 100 ms of data. Such electric field data contain an entire spectrum of pulses from symmetrical bipolar pulses to monopolar ones. Examples of several different pulses detected by Cluster 1 in 2010 are shown in Figure 3. The symmetrical bipolar pulses are typically interpreted as solitary waves and monopolar pulses or pulses with small overshoots are frequently attributed to dust impacts (changes in the spacecraft potential). These figures show that it is not possible to find a dividing line between different kinds of pulses for an automatic dust impact identification. Moreover, such asymmetrical pulses can also be generated by electron holes or double layers (see section 3, for more details; Kojima et al., 1999; Omura et al., 1999). We interpret all events shown in Figure 3 as signatures connected to electron holes (ESWs). It is evident that the complex understanding of the detected signal and reliable distinguishing between structures in the ambient electric field and changes in the spacecraft potential are mandatory for trustworthy dust impacts identification.

Using multiple electric field instruments operating simultaneously in the monopole and dipole configuration can provide complex information for better understanding of the signatures in the measured electric field and allows us to determine the origin of the detected pulses. We used data from one of the four MMS spacecraft (MMS 1) for a time period when the electric field instrument operated in the Burst mode (sampling frequency ∼ 10,000 s⁻¹). Time intervals when the Electron Drift Instrument (Torbert, Vaith, et al., 2016) and the Active Spacecraft Potential Control (Torkar et al., 2016) were active have been skipped to remove events related to these active experiments which can influence the spacecraft potential. We have studied various pulses to understand the detected signal. The events have been preselected automatically by searching for fast changes in the probe-to-spacecraft potential, P and thereafter checked visually. The antenna's configuration for both operating modes (monopole and dipole) and corresponding color scheme are shown in the simple sketch in Figure 1.

5.1 Signatures of Solitary Waves

One of the selected events from 1 February 2016 at 6:38:16.75 UTC is shown in Figure 4. The top panel shows the probe-to-spacecraft potential, P for all antennas (P1–P6). It is possible to see that antennas on one axis (e.g., P1 and P2) register similar signal only with the opposite polarity. The opposite polarity of the signal is the result of the antenna's orientation when the structure in the ambient plasma/electric field are detected (see Figure 1). It is possible to see similar profiles of the signal in the dipole configuration (probe-to-probe measurement), E in Figure 4 (middle panel). E12 represents the electric field measured as the potential difference between probes 1 and 2. These data have shown that observed signals are generated by the structure in the ambient plasma/electric field moving in a direction which is not aligned to any of electric field instrument (antenna) axes. The electron hole moving along one of the instrument axes generates pulse only on this axis (both antennas on this axis in the monopole configuration will detect symmetrical bipolar pulses with the opposite polarity) (Kojima et al., 1999; Omura et al., 1999). It is important to note that the amplitude of the pulses in both configurations (P and E) are similar (e.g., ΔP = 600 mV in probe-to-probe measurements, P corresponds to ΔE∼ 10 mV/m for E12 and E34 and to ΔE∼ 50 mV/m for E56). The last panel of Figure 4 shows the spacecraft potential, Φ_SC derived from the probe-to-spacecraft potential measurements, P. The explanation of this pulse is probably in small temporal shift in the different probe-to-probe potential measurements. The antenna P6 registered the maximum of the pulse sightly before the antenna P5. This fact can provide information about the propagation direction of the detected structure. The pulse in the derived spacecraft potential is not connected to the real spacecraft potential evolution.

Pulses in both monopole and dipole configuration in Figure 4 are similar to the pulses with overshoots which are sometimes attributed to dust impacts. The data from multiple instruments clearly show that this event is not related to the hypervelocity dust impact (change in the spacecraft potential). On the other hand, most of the dust impact detection is based only on a single antenna measurement. Thus, ESWs or other structures in the ambient plasma recorded only by one antenna can be misinterpreted as a pulse generated by a dust impact. The example of ESW from 19 February 2016 at 03:25:01.737 Figure 5 shows another possible problem with the signal interpretation. The two perpendicular antennas operating in the monopole configuration detected very unequal signals during this event (the second bigger peak). It is possible to see that a pulse detected by the antenna P1 is related to ESW. On the other hand, the identification of the pulse detected by the antenna P3 is impossible without data from the other antennas. Such a pulse detected by only one antenna can be misinterpreted as a dust impact. This example represents the spacecraft crossing through the edge of the electron hole see Figure 2 in Omura et al. (1999). The asymmetry in the pulse P3 can be explained by the asymmetrical electron hole.

5.2 Signatures of Dust Impacts

A different example of the selected event is shown in Figure 6 (28 March 2016 at 16:31:05.37 UTC). It is possible to see identical pulses in all the probe-to-spacecraft potential measurements (P1–P6) even with the same polarity (top panel). The values for P5 and P6 are very close to each other and a light green line representing P5 data is hidden behind a dark green line of P6. A very interesting fact is that there are no pulses in the electric field measurements (E12–E56; middle panel). Such data can be explained only by a change in the spacecraft potential, Φ_SC. All antennas detect identical pulses in the probe-to-spacecraft potential measurements only when the spacecraft potential is the only variable. Any structure in the ambient plasma/electric field (e.g., ESW) would instead result in a signal with an opposite polarity for antennas on the same axis. No signal detected in the probe-to-probe potential measurements; E supports the hypothesis that the change in the spacecraft potential is responsible for such an event because the electric field measurement is sensitive only to changes in the antenna potential and to the ambient electric field. The pulse in the spacecraft potential, Φ_SC, shown in the bottom panel of Figure 6 corresponds to the real temporal evolution of the spacecraft potential in this case. Such a change of the spacecraft potential can be caused by a hypervelocity dust impact on the spacecraft body and subsequent recollection of impact cloud particles described in section 2. The change in the spacecraft potential ΔΦ_SC = 142 mV corresponds approximately to 14 pC of the recollected charge. Such an amount of charge can be created, for example, by an impact of a micron-sized grain with a velocity of 20 km/s. Derived parameters of impinging grains strongly depend on the surface material in which the impact occurs (Collette et al., 2014). For more details about the derivation of impactor parameters from the amplitude of the pulse in the spacecraft potential see Vaverka et al. (2017a) and Zaslavsky (2015). This example shows that it is possible to reliably distinguish between pulses in the spacecraft potential (dust impacts) and structures in the ambient plasma as ESWs when data from multiple antennas are used.

It is important to note that we have also detected signals in the electric field, E for some of the selected events which we interpreted as changes in the spacecraft potential. An example of such an event is shown in Figure 7 (8 April 2016 at 20:58:13.00 UTC). See that the pulse in the probe-to-spacecraft potential, P5 is different from the pulses (top panel) and that there is one pulse in the probe-to-probe measurement, E56 (middle panel). Such data suggest that not only the spacecraft potential but also the potential of antenna 5, P5 is influenced during this event (the pulse detected by antenna 6, P6 is identical to other pulses). The fact that the amplitude of the pulse in P5 is larger than the other pulses suggest that antenna 5 is influenced by opposite polarity of charge than the spacecraft body. The equilibrium spacecraft potential is slightly above 8 V. It means that the majority of the impact cloud electrons were recollected by the spacecraft body (the temperature of released electrons is few electron volts; Collette et al., 2015, 2016). The potential of antenna 5 could be influenced by expanding positive ions. It is important to note that the surface of the antenna is too small to recollect enough charge since this probe is placed 12.5 m from the spacecraft body. The detected signal could probably be generated by the potential of the expanding ions along antenna 5 after a dust impact on the part of the spacecraft close to this antenna (no signal is detected by probe 6 on the opposite side). Probes 1–4 are placed 60 m from the spacecraft body. This distance is large enough that none of these probes detect any signal. This example shows that it could be possible to use the electric field instruments to study the expansion of the impact cloud after the dust impact.

5.3 Impact of Millimeter-Sized Object

Impacts of small dust and meteoroid on spacecraft can cause anomalies resulting in the worst case in a loss of spacecraft (Caswell et al., 1995; Goel & Close, 2015; Lai et al., 2002). One of the Wind wired antennas has been cut twice presumably by the dust impacts (Kellogg et al., 2016; Malaspina et al., 2014). A detection of the impact of a millimeter-sized body (micrometeoroid/orbital debris) on the MMS 4 spacecraft was detected in attitude dynamics flight data on 2 February 2016 (Williams et al., 2016). The signatures of this event were detected simultaneously by accelerometers, star cameras, and electric field instruments. The size of the impactor was estimated from the angular and linear momentum applied to the spacecraft by the impact (Williams et al., 2016). This interesting event represents direct evidence that the electric field instruments are able to detect dust impacts on the spacecraft body as a transient pulse in the spacecraft potential. We used this event to study the signal detected by the electric field instrument in both configurations and compare it with the events described above. The temporal evolution of the spacecraft potential, Φ_SC, during this impact is shown in Figure 8. The figure shows that the spacecraft potential dropped from 5.7 to 2.3 V, but it is possible to expect that the reduction of the spacecraft potential was even higher because of a low sampling frequency. The electric field instrument was unfortunately operating with the lowest sampling frequency (8 s⁻¹). This event can be detected with such low sampling frequency only due to slow recovery time (1 s), which is much longer than the spacecraft potential relaxation time derived by using the orbit motion limited theory (Vaverka et al., 2017b). A possible explanation is that the amount of charge released by this extreme impact is large enough to shield the spacecraft body from the ambient plasma. The presence of the shielding charge around the spacecraft body results in a significantly longer relaxation time than expected (Vaverka et al., 2017b) and reported from impacts of micron-sized grains (e.g., Zaslavsky, 2015). Laboratory experiments with micron-sized grains show that the amount of the released charge is proportional to the impactor mass (Collette et al., 2014). It is not possible to extrapolate these results to millimeter-sized grains, but it is possible to expect that 10⁹ times heavier objects release dramatically more charge.

Figure 9 shows the probe-to-spacecraft potential measurements, P, and probe-to-probe measurements, E, for this event. It is possible to see that there are some pulses in all of the electric field measurements, E. The expanding impact cloud influencing the potential of antennas on all spacecraft axes including spin plane probes located 60 m away from the spacecraft body could explain such data. This can be possible only due to a large amount of charge released by this extreme impact. Large amounts of released material and presence of shining objects after this event have been reported also by star cameras (Williams et al., 2016). An unexpected result is an extremely large (1-min) recovery time for probe 6, P6 (top panel). It is very unlikely that released charge can be responsible for a such long recovery time. Another explanation could be that the change in the spacecraft potential can influence spacecraft electronics which could result in artificial pulses in the measured electric field. Very interesting fact is that location of the impact derived from the accelerometer data was close to this antenna (Williams et al., 2016).

6 Discussion and Conclusion

We have studied various pulses detected by multiple electric field antennas simultaneously in the dipole and monopole configurations onboard one of the MMS spacecraft. This study shows that the identification of dust impacts only by a single antenna is a very challenging issue. For example, pulses generated by the electrostatic solitary waves can be similar to pulses triggered by hypervelocity dust impacts, Figures 4 (with overshoots) and 5. This fact is very important for the automatic dust impact identification by the electric field instruments. It is mandatory to keep in mind that not all pulses in the electric field data are triggered by dust impacts.

We have shown that it is possible to distinguish changes in the spacecraft potential from other pulses by using multiple electric field antennas operating in the monopole configuration (compare Figures 4 and 6). The change in the spacecraft potential generates identical pulses on all antennas operating in the monopole configuration, but no signal in the electric field data measured in the dipole configuration. Using multiple antennas can prevent possible signal misinterpretation. These results could help to understand data from other spacecraft as, for example, Cassini, STEREO, Wind, and MAVEN and to improve the precision of impact detection by the electric field instruments.

Figure 7 indicates that the impact cloud is probably able to influence the potential of one of the antennas in some cases. In this case, the probe was located 12.5 m from the spacecraft body. This fact could be used to study the expansion of the impact cloud particles.

The simultaneous detection of dust impacts by the accelerometers, star cameras, and electric field instrument provides direct evidence that the electric field instruments are able to detect dust impacts on the spacecraft body as a transient pulse in the spacecraft potential (Figure 8).

It is important to note that changes in the spacecraft potential can be generated by other mechanisms than the dust impacts. For example, active experiments onboard or electric discharges can generate such pulses. This is the reason why time periods when active experiments (Electron Drift Instrument and Active Spacecraft Potential Control) were on were skipped. Another important remark is that we detected changes in the spacecraft potential where the positive potential increases for a short time (inverse pulse than in Figure 6). We have no explanation for such pulses yet. The existence of these positive pulses and statistical analyses of various pulses detected by the MMS spacecraft will be the main part of our future study.