Dust impact voltage signatures on Parker Solar Probe: influence of spacecraft floating potential
S. D. Bale, K. Goetz, J. W. Bonnell, A. W. Case, C. H. K. Chen, T. Dudok de Wit, L. C. Gasque, P. R. Harvey, J. C. Kasper, P. J. Kellogg, R. J. MacDowall, M. Maksimovic, D. M. Malaspina, B. F. Page, M. Pulupa, M. L. Stevens, J. R. Szalay, A. Zaslavsky
mmanuscript submitted to
Geophysical Research Letters
Dust impact voltage signatures on Parker Solar Probe:influence of spacecraft floating potential
S. D. Bale , K. Goetz , J. W. Bonnell , A. W. Case , C. H. K. Chen ,T. Dudok de Wit ,L. C. Gasque , P. R. Harvey , J. C. Kasper , P. J. Kellogg , R. J. MacDowall , M.Maksimovic , D. M. Malaspina , B. F. Page , M. Pulupa , M. L. Stevens , J. R.Szalay , A. Zaslavsky Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA Physics Department, University of California, Berkeley, CA 94720-7300, USA School of Physics and Astronomy, University of Minnesota, Minneapolis, 55455, USA Smithsonian Astrophysical Observatory, Cambridge, MA 02138 USA School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK LPC2E, CNRS and University of Orl´eans, Orl´eans, France Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA Solar System Exploration Division, NASA/Goddard Space Flight Center, Greenbelt, MD, 20771 LESIA, Observatoire de Paris, Universit PSL, CNRS, Sorbonne Universit, Universit de Paris, 5 place JulesJanssen, 92195 Meudon, France Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, 80303, USA Department of Astrophysical Sciences, Princeton University, Princeton, NJ, 08544, USA
Key Points: • The Parker Solar Probe (PSP) FIELDS instrument measures millisecond volt-ages impulses associated with dust impacts • The sign of the largest monopole voltage response is a function of the spacecraftfloating potential • These measurements are consistent with models of dynamic charge balancefollowing dust impactsSubmitted : June 2, 2020
Corresponding author: Stuart D. Bale, [email protected] –1– a r X i v : . [ phy s i c s . s p ace - ph ] J un anuscript submitted to Geophysical Research Letters
Abstract
When a fast dust particle hits a spacecraft, it generates a cloud of plasma some ofwhich escapes into space and the momentary charge imbalance perturbs the space-craft voltage with respect to the plasma. Electrons race ahead of ions, however bothrespond to the DC electric field of the spacecraft. If the spacecraft potential is pos-itive with respect to the plasma, it should attract the dust cloud electrons and repelthe ions, and vice versa. Here we use measurements of impulsive voltage signalsfrom dust impacts on the Parker Solar Probe (PSP) spacecraft to show that the peakvoltage amplitude is clearly related to the spacecraft floating potential, consistentwith theoretical models and laboratory measurements. In addition, we examine sometimescales associated with the voltage waveforms and compare to the timescales ofspacecraft charging physics.
Plain Language Summary
When a fast, interplanetary dust particle hits a spacecraft, it generates a shockin the spacecraft material that liberates a hot, ionized plasma. Some of the plasmaions and electrons return immediately to the spacecraft, but some escape into space.The momentary charge imbalance created by the different ion and electron speedsgenerates a transient perturbation to the voltage of the spacecraft. However, theseelectrons and ions can be attracted or repelled depending on the DC electric fieldof the spacecraft itself. In this paper, we show this effect clearly: when the spacecraftfloating voltage is negative with respective to the interplanetary plasma, the electronsfrom the dust impact are repelled and vice-versa.
Plasma wave electric field measurements in space have proven to be powerfuldiagnostic of planetary (Scarf et al., 1983; Gurnett et al., 1987; Meyer-Vernet et al.,2009; Ye et al., 2014) and interplanetary (Gurnett et al., 1997; Zaslavsky et al., 2012;Malaspina et al., 2014; Kellogg et al., 2016) dust processes. A hypervelocity dustimpact onto the spacecraft body or antenna produces a plasma cloud and the mo-mentary charge imbalance generates a rapid perturbation to the spacecraft floatingpotential. Some of the resulting plasma is recollected by the spacecraft and some ofit escapes, depending on the energy of the ions and electrons and the spacecraft-to-plasma electric potential. Monopole (probe-to-spacecraft) or dipole (probe-to-probe)voltage measurements will record millisecond-timescale spikes and/or their spectralcontent. These effects have been explored in several recent papers (Zaslavsky, 2015;Vaverka et al., 2017; Kellogg et al., 2018; Vaverka et al., 2019; Kellogg et al., 2020) andrecently reviewed by Mann et al. (2019).During the first PSP solar encounters, PSP/FIELDS measurements of dust im-pact rates were used to map the radial variation the flux of the interplanetary dust.Page et al. (2020) used dipole voltage signals to infer the dust velocity vector andcomparison with models (Szalay et al., 2020) suggests that this dust population isconsistent with β micrometeoroids on exiting hyperbolic orbits. Malaspina et al.(2020) compared data from Encounters 1-3 to show that the population is variableand probably has its source between 10-30 R S . While the PSP/WISPR instrumentsaw the beginnings of a decrease in F corona intensity (Howard et al., 2019).Here we examine the sign of the monopole antenna measurements from thePSP/FIELDS instrument; these observations show that the spacecraft voltage pertur-bation is influenced by the initial spacecraft potential itself. If the spacecraft is initiallynegatively charged, it will attract more ions and repel more electrons from the plasmacloud producing a large positive perturbation. If initially positively charged, the op- –2–anuscript submitted to Geophysical Research Letters posite occurs and returning electrons produce a large negative polarity spike. We alsoexamine some typical timescales associated with these perturbations and suggest thatthey are associated with the escape process.
The Parker Solar Probe (PSP) mission (Fox et al., 2016) was launched in August2018 into an orbit that will take it deep into the inner heliosphere with a final perihe-lion distance of 9.8 R S from the center of the Sun. This study uses measurements fromthe PSP/FIELDS (Bale et al., 2016) and the PSP/SWEAP (Kasper et al., 2016) instru-ments primarily from PSP Encounter 2, between March 23, 2019 and April 13, 2020 toinvestigate the role of the spacecraft floating (DC) voltage on the voltage signature ofdust impacts onto the spacecraft. Perihelion of Encounter 2 was on April 5, 2019 at ≈ R S ).Our primary measurements are made by the Time Domain Sampler (TDS) sub-system of the PSP/FIELDS instrument (Bale et al., 2016) . The TDS makes rapid sam-ples of waveforms with simultaneous sampling of five analog channels which canbe selected from dipole antennas pairs, monopoles, or a high-frequency search coilmagnetometer (Bale et al., 2016). During PSP Encounters 2 and 3 (used here), theTDS was configured to sample at 1.92 MSa/s and produce 32768-point waveform’events’; therefore each TDS event is 17.067 ms in duration. In addition to the TDSwaveform events, the TDS records a ’TDS Max’ value each 7 seconds during nominalencounter mode. The TDS Max value is the signed extreme of the entire datastreamduring the interval and is dominated by the large voltage signatures of dust impacts.We use TDS measurements of the voltage between the ’V2’ monopole antennaand spacecraft ground V G and we call this measurement δ V = V − V G . The δ emphasizes the fact that the TDS measurement is band-pass filtered (i.e. not DC-coupled); the TDS system has a flat gain and phase response from 1 kHz to 1 MHz(1 ms - 1 µ s) and the waveforms shown here have not been corrected with a transferfunction, which will not change our results on these timescales. The measurementrepresents the voltage perturbation between the probe-spacecraft system. Note thata positive perturbation of the spacecraft ground/potential at a fixed probe potentialis measured as a negative voltage impulse in δ V . The V2 monopole is mounted nearthe plane of the spacecraft heatshield at − cos ( ◦ ) ˆ x − sin ( ◦ ) ˆ y in the spacecraft co-ordinate system. During PSP solar encounter, the spacecraft ˆ x axis points southwardand the ˆ y axis points approximately in the ram direction, therefore the V2 monopoleis on the anti-ram side during the normal solar encounter configuration (Bale et al.,2016). The PSP/FIELDS electric antennas are 2m long, 1/8” diameter thin-walledtubes of C103 Niobium alloy and have a free space capacitance of C A ∼ C B ∼ E ∼ T ph ∼ and a large number of zero-crossings. This algorithmgenerated primarily plasma wave events during Encounter 3. Figure 1 is a histogramof both minimum and maximum amplitude from 511 waveform events on September1, 2019 of Encounter 3 near perihelion. All of these events are plasma waves, as iden-tified by eye, and it can be seen that none exceed ±
25 mV amplitude. We thereforechoose ±
25 mV as our threshold beyond which we consider our measurements to bedust impacts, rather than plasma waves. Page et al. (2020) used a threshold of 50 mV,which produces qualitatively similar results for this analysis, but fewer counts. –3–anuscript submitted to
Geophysical Research Letters
Encounter 3 peak amplitudes -60 -40 -20 0 20 40 60minmax(V -V G ) [mV]1101001000 o f e v en t s Figure 1.
Histogram of minimum and maximum waveform amplitudes on the V R < R S ), comprising 511 waveform events each rep-resented here with a maximum and minimum data value. Encounter 3 uses a burst waveformselection algorithm that selects plasma waves, rather than dust impacts. The measurements hererepresent a mix of ion acoustic, electrostatic whistler, and Langmuir waves. Dotted vertical lines at ±
25 mV are our threshold for dust events during Encounter 2.
Our ’spacecraft potential’ measurement V SC is computed as the negative of theaverage DC-coupled voltage on all four monopole antennas V SC = − ( V + V + V + V )) /4 from the Digital Fields Board (DFB) subsystem (Malaspina et al., 2016).The voltage probes V i are current-biased to hold them near the local plasma poten-tial, therefore this quantity should represent the floating potential of the spacecraft(Pedersen, 1995; Guillemant et al., 2012). However it is important to note that the PSPheatshield on the sunward side of the spacecraft is not directly electrically connectedto the spacecraft body itself; electrical coupling between the heatshield and space-craft body is carried by plasma and body currents only. Therefore this measured V SC may not represent the true floating potential of the entire heatshield-spacecraftsystem. Furthermore, modeling has suggested that for very high photoelectron den-sities, space-charge effects may produce a double layer at the sunward surface of theheatshield and a plasma wake behind that may modify its floating potential (Ergunet al., 2010; Guillemant et al., 2012). Note that we differentiate V SC from V G ; V SC isthe DC-coupled spacecraft potential measured by the DFB, while V G is the spacecraftpotential reference for the TDS band-passed δ V measurement.We also use electron (total plasma) density and electron core temperature mea-surements produced from an analysis of the quasi-thermal noise spectrum (Meyer-Vernet & Perche, 1989) during PSP Encounter 2. This technique was also applied toEncounter 1 data and described in Moncuquet et al. (2020). Ion temperature mea- –4–anuscript submitted to Geophysical Research Letters surements are derived from SWEAP instrument moment calculations of the total iondistribution from the Solar Probe Cup instrument (Kasper et al., 2016; Case et al.,2020).
Figure 2 shows an overview of PSP Encounter 2 measurements from March 27to April 13, 2019. The top panel [a] is the total plasma density n e measured from theplasma frequency peaks in the quasi-thermal noise spectrum (Meyer-Vernet & Perche,1989; Moncuquet et al., 2020). The density reaches a peak value of around n e ∼ − on April 3 at r ∼ R S . Panel [b] is the spacecraft potential V SC as described inSection 2 above, colored for polarity (red >
0, blue < | V | > V > V SC from panel [b]and panel [e] is the number of negative polarity dust events ( V < -25 mV) with bluedots indicating intervals of negative spacecraft potential. The bottom panel [f] showsthe bias current applied to the V2 antenna, indicating an interval of no bias, as well astimes when the fixed bias was disabled during bias current calibration sweeps. Thesweep intervals were deleted from the statistics to keep the sweeps from contaminat-ing the dust impact measurements.The striking feature of Figure 2 is the relationship between the sign of the space-craft potential in panel [b] and the sign of the dust voltage impact signal in panels [d]and [e]; negative polarity spacecraft potential tends to produce negative impulse TDSevents δ V = V − V G , i.e. positive perturbations to V G assuming a fixed V . This isconsistent with the idea that a negatively charged spacecraft V SC < δ V G (therefore a negativeperturbation to δ V = V − V G ), and vice-versa. Note that this behavior is consistentwith plasma cloud ions and electrons having temperatures on the order of ∼ V SC (panel [b]), there appears to be enhanced overall count levels (panel [c]), sug-gesting that the spacecraft floating voltage may influence or modify the overall dustimpact rate by perhaps providing a threshold for the measured charge.This effect can also be seen by examining the waveforms themselves; we usewaveforms from April 3, 2019, the interval between green vertical bars in Figure 2.Figure 3 shows superposed-epoch averaged waveforms, with 1 σ deviations, and or-ganized by intervals of spacecraft potential V SC . The number of waveforms used tocompute the average and variance in each row is given in column 2 of Table 1. Eachindividual waveform is shifted to start at t=0 by finding the initial perturbation above5 ∗ V noise , where V noise is the RMS level of the first 4 ms of the waveform (before thedust signal). This level is typically V noise ≈ T (cid:46) (cid:46) T (cid:46) V noise at times T (cid:38) V SC , with severaltimescales annotated and collected in Table 1. –5–anuscript submitted to Geophysical Research Letters
Table 1.
Parameters associated with TDS epoch-averaged waveforms in Figure 4 and plasmaparameters as described in the text. V SC (cid:104) n e (cid:105) (cid:104) T e (cid:105) (cid:104) T i (cid:105) (cid:104) τ pe (cid:105) (cid:104) τ ce (cid:105) (cid:104) τ SC (cid:105) T [ms] T [ms] T [ms] T [ms] T [ms] [ V ] [ cm − ] [ eV ] [ eV ] [ms] [ms] [ms] 1st z/c 2nd peak 2nd z/c 3rd peak → V noise ( − − )
19 571 25 9 0.005 0.436 0.244 - 0.037 0.310 0.463 3.346 ( − − )
31 515 25 10 0.005 0.448 0.256 0.013 0.076 0.176 0.286 4.208 ( − )
19 352 32 13 0.006 0.403 0.421 0.002 0.066 0.165 0.266 3.532 ( )
57 177 28 11 0.008 0.369 0.797 0.006 0.061 0.192 0.294 2.786 ( )
84 163 30 11 0.009 0.366 0.927 0.006 0.043 0.175 0.288 2.395
The columns in Table 1 list (left-to-right) the interval of V SC in Volts, the num-ber of waveform events in that interval, the average plasma density (cid:104) n e (cid:105) , electroncore temperature (cid:104) T e (cid:105) , ion temperature (cid:104) T i (cid:105) , the electron plasma period (cid:104) τ pe (cid:105) , elec-tron cyclotron period (cid:104) τ ce (cid:105) , the spacecraft RC time (cid:104) τ SC (cid:105) and timescales associatedwith the 1st zero-crossing, 2nd peak, 2nd zero-crossing, 3rd peak, and relaxation to V noise (zero) respectively, as annotated in Figure 4. In addition, all waveforms with V SC > − V show a small (negative) 1st peak at T ≈ T /2, although these valuesstart to approach the granularity of the measurement 1/(1.92 MSa/s) ∼ µ s. Theantenna RC charging time can be estimated as τ A (cid:39) ( C A T ph ) / I bias , which takes val-ues of 0.01 to 0.04 ms during Encounter 2 and is generally less than the spacecraftRC charging time τ SC (cid:39) ( C SC T e ) / ( A SC n e e v th , e ) column 8 in Table 1. We estimate C SC as the free space capacitance of a 1m radius sphere C SC ≈
110 pF. Note that weuse the thermal electron temperature to estimate τ SC , rather than the photoelectrontemperature (Zaslavsky, 2015) since the spacecraft body is not illuminated; using T ph would result in smaller values of τ SC by a factor of ∼ τ e ∼ R SC / v e and τ i ∼ R SC / v i can be estimated (Shen, 2020) using impact cloud electron and iontemperatures v e and v i (Collette et al., 2016), where R SC ∼
1m is the spacecraft scalesize. If we assign the values of our first peak T /2 with an electron timescale R SC / v e and our second peak T with an ion timescale R SC / v i we find escape speeds of v e ≈ ∼ v i ≈ ∼ T /2 for thewaveform with V SC < − We show that the sign of the voltage waveforms and onboard voltage extremaare influenced by the DC spacecraft floating potential in a way that is consistent withmodels of dust plasma cloud dynamics. Timescales associated with the waveformsare broadly consistent with expectations for electron and ion escape energies, andthose in turn are at appropriate energies to be influenced by the O ( ± –6–anuscript submitted to Geophysical Research Letters potential measurement. As noted above, the measured V SC may not represent thetrue floating potential of entire spacecraft-heatshield system; as our models of dustplasma cloud escape become more sophisticated, these waveforms may add insightto the actual floating potential of the PSP spacecraft-heatshield system.More detailed, and statistical, analysis of the PSP/FIELDS dust data is likelyto add insight to the physics of dust plasma cloud dynamics, and well as the globaldistribution of interplanetary dust. Acknowledgments
The FIELDS experiment on the Parker Solar Probe spacecraft was designed and de-veloped under NASA contract NNN06AA01C. The PSP/FIELDS team acknowledgesthe extraordinary contributions of the Parker Solar Probe mission operations andspacecraft engineering teams at the Johns Hopkins University Applied Physics Labo-ratory. SDB acknowledges the support of the Leverhulme Trust Visiting Professor-ship program. Data access and processing was done using the SPEDAS IDL en-vironment (Angelopoulos et al., 2019). PSP/FIELDS data is publicly available at http://fields.ssl.berkeley.edu/data/ . –7–anuscript submitted to Geophysical Research Letters
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Bias off[a][b][c][d][e][f ]
Figure 2.
An overview of PSP Encounter 2 data, with perihelion at 35.7 R S on April 5, 2019. Panel[a] is the total plasma density computed from the QTN spectrum. Panel [b] is the spacecraft po-tential proxy V SC , colored for polarity (red >
0, blue < | V | , showing an increase towards perihelion. Panel [d] is thenumber of dust hits in 30 minute intervals with positive polarity, where red dots are intervals of V SC >
0. Panel [e] is the number of dust hits in 30 minute intervals with negative polarity, whereblue dots are intervals of V SC ≤
0. Panel [f] is the value of the applied bias current in µ A . Spikeyevents are twice-daily bias sweeps, and an interval of no bias is indicated. Green vertical bars showthe interval of April 3, 2019 with a large change of density and spacecraft potential/polarity. Thisfigure shows clearly that intervals of negative (positive) V SC correspond to clear enhancements indust impulses with positive (negative) dominated TDS events; note that TDS measures V - V G , sothat rapid voltage changes in V G appear opposite polarity.–11–anuscript submitted to Geophysical Research Letters -5000500 V - V G [ m V ] -5000500 V - V G [ m V ] -5000500 V - V G [ m V ] -5000500 V - V G [ m V ] -1 0 1 2-5000500 V - V G [ m V ] -2.5V < V SC < -1.5V-1.5V < V SC < -0.5V-0.5V < V SC < 0.5V0.5V < V SC < 1.5V1.5V < V SC < 2.5Vtime [ms] Figure 3.
Superposed epoch plots of V2 monopole TDS waveform events, organized by space-craft floating voltage V SC . The black curve is the average waveform and error bars are one-sigmavariations. The instrument saturates at ≈ ± V SC < -1.5V shows that the typical waveform hasits peak value with V - V SC <
0. –12–anuscript submitted to
Geophysical Research Letters -0.2 0.0 0.2 0.4 0.6 0.8time [ms]-600-400-2000200400600 V - V G [ m V ] -2.5V < V SC < -1.5V-1.5V < V SC < -0.5V-0.5V < V SC < 0.5V0.5V < V SC < 1.5V1.5V < V SC < 2.5VT1 T4T2T2 T3T4 T5 Figure 4.
Epoch-averaged waveforms binned and colored by spacecraft potential. These are theaverage waveforms from Figure 3. Note that the waveform with V SC < -1.5V is opposite polar-ity to the others at both the primary peak at T and the secondary peak at T4