Initial Results from the Active Spacecraft Potential Control Onboard Magnetospheric Multiscale Mission

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Initial Results From the Active Spacecraft Potential Control Onboard Magnetospheric Multiscale Mission

Article in IEEE Transactions on Plasma Science · May 2017 DOI: 10.1109/TPS.2017.2694223

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Rumi Nakamura Maria Andriopoulou Austrian Academy of Sciences Austrian Academy of Sciences

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IEEE TRANSACTIONS ON PLASMA SCIENCE 1 Initial Results From the Active Spacecraft Potential Control Onboard Magnetospheric Multiscale Mission R. Nakamura, K. Torkar, M. Andriopoulou, H. Jeszenszky, C. P. Escoubet, F. Cipriani, P. A. Lindqvist, S. A. Fuselier, C. J. Pollock, B. L. Giles, and Y. Khotyaintsev

Abstract— NASA’s magnetospheric multiscale (MMS) mission was successfully launched in March 2015. The scientific objectives of MMS are to explore and understand fundamental plasma physics processes in the earth’s magnetosphere: magnetic reconnection, particle acceleration, and turbulence. The region of scientific interest of MMS is in a tenuous plasma environment where the positive spacecraft potential may reach an equilibrium as high as several tens of volts. The active spacecraft potential control (ASPOC) instrument neutralizes the spacecraft potential by releasing the positive charge produced by indium ion emitters. While the method has successfully been applied to other spacecraft such as Cluster and Double Star, new developments in the design of the emitters and the electronics are enabling lower spacecraft potentials and higher reliability compared to previous missions. In this paper, we report the initial results from the tests of the ASPOC performance during the commissioning phase and Fig. 1. MMS orbit on September 21, 2015 together with the probable discuss the different effects on the particle and field instruments magnetopause location. The shaded area along the MMS orbit is the region of observed at different plasma environments in the magnetosphere. interest when all instruments are turned ON and operate in burst mode. (Plot from https://Lasp.Colorado.Edu/Mms/Sdc/Public/Historical-Orbit-Plots/). Index Terms— Electric potential, extraterrestrial measurements, ion beam applications, plasma diagnostics, plasma measurements, surface charging. as the magnetopause, the magnetotail lobe, and the plasma sheet. After the launch on March 13, 2015, at 03:44 UT, I. INTRODUCTION the MMS was in the commissioning phase until the end of ASA’s magnetospheric multiscale (MMS) mission [1] is August, and entered the scientific phase on September 1, 2015. Ndesigned to explore the dynamics of the earth’s mag- MMS has a highly eccentric low-inclination orbit with perigee netosphere and its underlying energy transfer processes. Four at 1.2 RE . The apogee was located initially at 12 RE to identically equipped spacecraft are to carry out 3-D measure- optimize the encounter of dayside magnetopause reconnection ments in the key regions in the earth’s magnetosphere such and will then be raised to 25 RE in early 2017 to encounter the magnetotail reconnection region. Fig. 1 shows the MMS orbit Manuscript received July 21, 2016; revised January 1, 2017; accepted on September 21, 2015 from the first science phase. The four March 10, 2017. The work of the Austrian team is supported by Austrian spacecraft are controlled to form a tetrahedron in the region of Science Funds (FWF) under Grant I2016-N20. ASPOC Phase E study is supported by Österreichische Forschungsförderungsgesellschaft under interest, with interspacecraft distances varying from a couple Grant FFG 847969. (Corresponding author: R. Nakamura.) of kilometers to a couple of hundred kilometers depending on R. Nakamura, K. Torkar, M. Andriopoulou, and H. Jeszenszky are with the the target scales to resolve the ion or electron diffusion region. Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria (e-mail: [email protected]; [email protected]; More details about the orbits and phases are given in [2]. [email protected]; [email protected]). The active spacecraft potential control (ASPOC) neutralizes C. P. Escoubet and F. Cipriani are with the ESA-ESTEC, 2201 AZ the spacecraft potential by releasing positive charge produced Noordwijk, The Netherlands (e-mail: [email protected]; [email protected]). by indium ions, thereby controlling the spacecraft potential. P. A. Lindqvist is with the Royal Institute of Technology, 10044 Stockholm, ASPOC enables accurate plasma measurements in sparse Sweden (e-mail: [email protected]). plasma environments also, which is essential to study the S. A. Fuselier is with Southwest Research Institute, San Antonio, TX 78238-5166 USA (e-mail: [email protected]). properties of reconnection. ASPOC was built by a consortium C. J. Pollock is with Denali Scientific, Healy, AK 99743 USA (e-mail: led by the Institut für Weltraumforschung. craig@denaliscientific.org). An ASPOC instrument unit (shown in Fig. 2) contains four B. L. Giles is with NASA, Goddard Space Flight Center, Greenbelt, MD 20771 USA (e-mail: [email protected]). ion emitters, where one emitter per instrument is operating Y. Khotyaintsev is with the Swedish Institute of Space Physics, at a time. Compared to the previous missions, MMS ASPOC 75121 Uppsala, Sweden (e-mail: [email protected]). includes new developments in the design of emitters and the Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. electronics and is equipped with a more capable control soft- Digital Object Identifier 10.1109/TPS.2017.2694223 ware. In particular, unlike the previous missions, for which one 0093-3813 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

2 IEEE TRANSACTIONS ON PLASMA SCIENCE

Fig. 3. Results from the extended dual-beam test on MMS4 performed Fig. 2. Photograph of an ASPOC instrument unit that consists of two emitter on July 19, 2015. Top: Spacecraft potential from SDP. Bottom: Ion currents modules at the top of an electronics box. from the sum of the two ASPOC units (black), from ASPOC 1 (green), and ASPOC 2 (red). TABLE I ION EMITTER CURRENT LEVEL,MASS EFFICIENCY, AND verification, simultaneous operation of two emitters, test of ESTIMATED LIFETIME OF THE EMITTER WHEN OPERATED AT THE CURRENT LEVEL feedback loop using spacecraft potential information obtained from the spin-plane double probe (SDP) measurements [5], validation of effects on spacecraft potential control when the electron drift instrument (EDI) [6] is emitting electron beams, long-term stability test, and interference test with other instruments. It has already been shown in the early commissioning phase that ASPOC successfully controlled the spacecraft potential to be kept at values below 4 V, fulfilling the science requirement of MMS. As an example of commissioning-phase operations, Fig. 3 shows the results from the extended dual-beam test of ASPOC unit was installed, each MMS spacecraft carries two MMS4 performed on July 19, 2015. The spacecraft potential ASPOC units. The emitted beams are oppositely directed in data transferred onboard ASPOC from SDP, ion currents the spin plane. The beam direction relative to other instruments emitted from ASPOC 1 and ASPOC 2, and the sum of the two will be described in Section V. emitters’ currents are shown in Fig. 3. During the extended The ASPOC-emitted beams are positive indium ions at tests, different current levels and different operation modes energies of order 4 to 12 keV and emitted currents are up are tested. The nominal operation of ASPOC is to emit the to ∼50 μA. The operating time of an emitter is a function ion beams by setting a constant current level for each of of reservoir size, emitted current, and mass efficiency of the the ASPOC units. ASPOC, however, can also be operated in emission process, as summarized in Table I and obtained a way that a target spacecraft potential level is set and the in [3]. At a current of 20 μA per emitter, which is the ASPOC current level is automatically modified by referring originally planned operation level before the launch, the life- to the spacecraft potential level onboard. This mode is called time per emitter is 9112 h. Since the total duration, when the feedback mode and was successfully tested during the time spacecraft pass through the region of interest during the two interval shown in Fig. 3 (between the times indicated by the science phases, is about 8800 h, ASPOC has a significant two vertical dotted lines). This mode also requires master– redundancy in the number of emitters and has a sufficient slave mode of two ASPOCs, for which the slave ASPOC indium reservoir to be able to operate beyond the nominal two- duplicates the beam current of the master. The feedback mode year science phase. Further descriptions of the MMS ASPOC not only allows keeping the spacecraft potential level close instrumentation are given in [4]. In this paper, we report to the target value, but also enables avoiding unnecessary the ASPOC performance in orbit, highlight observations with indium consumptions by emitting very strong ASPOC current. different modes of ASPOC operation from the early phase of During this interval, the target spacecraft potential value was the mission, and discuss the effects of ASPOC on plasma and set to 4 V. At the beginning of this test, the spacecraft potential field observations. was below 2 V and hence the total current level decreased. After a temporary overshoot to ∼6 V, the potential eventually II. EMITTER TESTS DURING THE COMMISSIONING PHASE settled at a correct constant level of 4 V after about 10 min. The ASPOC instruments were switched ON, starting from This procedure was repeated with a different master–slave March 28, during the commissioning phase. The commis- configuration. By varying control loop parameters and the sioning activities consisted of different types of tests: low- average time interval of SDP data, it is possible to tune the voltage checkout, high-voltage checkout and single-emitter response time in order to minimize the overshoot level. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

NAKAMURA et al.: INITIAL RESULTS FROM ASPOC ONBOARD MMS MISSION 3

Fig. 4. Data from the MMS3 spacecraft on August 1, 2015. Electron energy spectra from FPI, spacecraft potential from SDP, ASPOC ion currents (from the sum of the two ASPOC units in black, from ASPOC 1 in green, and from ASPOC 2 in red), magnetic ﬁeld Bx (blue), By (green), Bz (red) in GSE coordinates from FGM, and plasma density from FPI instruments.

ASPOC started routine operation in a pseudonominal configuration at the end of July 2015 after the completion of all the planned tests and the selection of the optimal emitter pairs for the operational phase. The nominal operation mode during the Fig. 5. Proton energy spectra from HPCA, spacecraft potential from SDP, science phase is the constant current mode where a constant ion currents from ASPOC, proton density from HPCA from MMS1 (first μ to fourth panels) and from MMS4 (from fifth to eighth panels) obtained on beam current level of 10 A is set for both emitters. ASPOC September 21, 2015. has always been operating in this nominal mode during the following science phases. All the ASPOC level 2 science data lobe, where the plasma density is lower (bottom panel) and products are publicly available from the Science Data Center the energy of the electrons is lower (top panel). (https://lasp.colorado.edu/mms/sdc/public/). A positively charged spacecraft also affects the measurements of low-energy ions. Namely, when the level of the III. EFFECTS OF ASPOC ON PLASMA MEASUREMENTS spacecraft potential exceeds the energy of the ions, they cannot In sparse plasma regions, the spacecraft is positively be measured due to the potential barrier. Fig. 5 shows an exam- charged, exceeding several tens of volts, and the low-energy ple of such a case observed by the hot plasma composition electron observations are contaminated by photoelectrons orig- analyzer (HPCA) [9] in the dayside magnetosphere (Fig. 1). inating from the spacecraft surface. Here the top four panels show measurements from MMS1, Fig. 4 shows an example of such observations, during where ASPOC emitters were turned ON. The bottom four which ASPOC was turned OFF for two short intervals in panels are from MMS4, where the emitters were turned OFF. the near-earth premidnight magnetotail. The electron mea- The proton energy spectra (top and fifth panels) show that surements (top panel) are from the fast plasma instru- low-energy protons were not observable at MMS4 below the ment (FPI) [7]. Intense photoelectrons can be seen as red level of the spacecraft potential, which was up to about 13 V areas below energies of 10 eV in Fig. 4, coinciding with (sixth panel). Accordingly, the densities derived by the two the time interval when the spacecraft potential (second panel) spacecraft show differences (fourth and eighth panels), demon- exceeds 10 V. This photoelectron appearance is associated with strating the importance of keeping the spacecraft potential the turn-OFF of the ASPOC emitters, as can be seen in the constantly at low values. Fig. 5 shows that an enhanced ASPOC ion current (third panel). The magnetic field data from cold plasma component (plume) developed in the dayside the flux gate magnetometer (FGM) [8] in the fourth panel show magnetosphere. The effect of such cold plasma on magnetic that MMS was first in the magnetotail lobe region, where the reconnection is one of the important research topics that can magnetic field magnitude exceeds about 30 nT. MMS then be investigated by MMS in a comprehensive way. entered the plasma sheet, where the magnetic field magnitude becomes smaller, the Bz component increases, and a more IV. PLASMA DENSITY DERIVATION USING ASPOC CURRENT AND SPACECRAFT POTENTIAL hotter plasma (top panel) is observed. The first ASPOC OFF interval corresponds to the time interval in the magnetotail For a spacecraft with uncontrolled spacecraft poten- lobe, while the second interval to the plasma sheet. The bottom tial (ASPOC OFF), the balance of the currents flowing into panel shows the density moment from electrons (blue) and and out of the spacecraft can be described as ions (red). The photoelectrons are also included in the electron I + I = 0. (1) moment calculation shown in Fig. 4 so that the photoelectron photo e effects can be seen as differences between the red curve and Here, Iphoto is the photoelectron current and Ie is the ambient the blue curve. It can be seen that the effect of turning-OFF electron current, and other contributions such as plasma ion the ASPOC emitters is more prominent in the magnetotail current or secondary emissions are neglected. Since Ie reflects This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

4 IEEE TRANSACTIONS ON PLASMA SCIENCE the ambient electron parameters, when a current from random electrons with Maxwellian distribution is assumed, density Ne can be derived [10], [11] as √ 2meπ Ne = Iphoto · √ . (2) Asc|q| kTe(1 +|q|Vsc/kTe)

Here, me, q,andTe are electron mass, charge, and temperature, Asc is the surface area of a spacecraft, Vsc is the spacecraft potential, and k is the Boltzmann constant. For this method to work, therefore, it is essential to deduce a good model of Iphoto. Previous Cluster studies [11], [12] showed that Iphoto can be deduced using also the current balance when ASPOC is emitting current, IASPOC

Iphoto + IASPOC + Ie = 0. (3) Once the photoelectron current curve is deduced based on these current balance relationships, it is possible to derive plasma density estimations from the spacecraft potential vari- Fig. 6. Estimates of the electron density derived from spacecraft potential ations, as described in [10] using (2). These methods are observations during the time interval of 08:00–09:00 UT September 21, 2015. particularly useful during periods when the FPI instrument is The first panel depicts the electron energy spectrogram, and the second panel not operating. In such cases, the photoelectron current curve depicts the electron density from the FPI instrument from MMS3. The third and fourth panels show the plasma density estimates using spacecraft potential is deduced by combining measurements of the ASPOC ion measurements of the spacecraft under active potential control (MMS3) and current and spacecraft potential taken simultaneously from at without potential control (MMS4), respectively. The fifth panel shows mag- least two spacecraft, one with ASPOC and one without. Here netic field observations in GSM coordinates (Bx in blue, By in green, Bz in red, B total in black). it is assumed that those spacecraft are located in the same magnetospheric environment. This photoelectron information The third and fourth panels show plasma density estimates for together with the ASPOC ion current enables reconstruction of a spacecraft with ASPOC ON (MMS3) and one with ASPOC the spacecraft potential to values we would get if ASPOC was OFF (MMS4), respectively, while the second panel shows the OFF. Using the derived curve that describes the photoelectron electron density observations from FPI. Comparison of FPI emission, plasma density can then be estimated using space- observations with our preliminary estimates are in a good craft potential observations from spacecraft with or without agreement, especially for the controlled spacecraft. A study spacecraft control. on more accurate determination of the photoelectron emission Using data from the MMS commissioning phase, in order to improve the current estimates during that period March–August 2015, the average photoelectron emission curve is ongoing. One of such improvements will be to remove was estimated and spacecraft potential was reconstructed dur- the effects of the noise due to the presence of significant ing several time intervals with at least one of the spacecraft electric fields visible in the spacecraft potential observations having ASPOC ON [13]. The derived model curves are of the controlled spacecraft. Methods of such noise removal − Vsc − Vsc V V are currently under investigation. Iphoto = I01 · e 01 + I02 · e 02 (4) V. E FFECTS OF ASPOC ON ELECTRIC where Io1, Vo1, Io2,andVo2 are 116.5 [μA], 1.09 [V], 85.1 [μA], 3.3 [V] for April–May 2015, and 128.6 [μA], FIELD MEASUREMENTS 1.99 [V], 2.2 [μA], and 7.19 [V] for June–July, 2015, Whereas the benefits of spacecraft potential control for respectively. Plasma density was estimated using (2) and by plasma measurements have been demonstrated in previ- assuming an average temperature of 1 keV, which is a typical ous missions and were confirmed in the previous sections, average value in the magnetotail region. The MMS surface the interaction with electric field measurements requires addi- area was considered to be 34 m2. The criteria when such tional attention. Single ion emitter configurations in previous reconstructions are possible were examined. It was concluded missions such as Cluster have led to improvements of electric that, for ASPOC currents up to 30 μA, spacecraft potential field measurements in the presence of wakes by streaming reconstructions are largely successful (at least for periods cold plasma [14]. The configuration on MMS with dual beams when the spacecraft are not traversing very tenuous regions, within the spin plane is new, and initial results are presented i.e., below 0.1 cm−3). More details on the methods and results here. of this study using MMS observations can be found in [13]. The electric field measurements are achieved by four probes This paper is currently being extended to the science phases in the spin plane (SDP) located at the end of 60-m wires of the mission starting from September 1, 2015. Fig. 6 shows and of two axial probes (ADP) [15] mounted on coiled an example of plasma density estimates in the dayside magne- booms and separated by ∼29.2 m effective length. The two tosphere, and specifically for the same time interval depicted orthogonal spin-plane components of the electric field in in Fig. 5, namely, September 21, 2015, 08:00–09:00 UT. rotating spacecraft coordinates are measured by the SDP probe This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

NAKAMURA et al.: INITIAL RESULTS FROM ASPOC ONBOARD MMS MISSION 5

Fig. 7. Location of the ASPOC beams and the spin-plane booms. pairs 1, 2 (E12) and 3, 4 (E34). The two ASPOC ion beams point at half-angle between the SDP probes 1 and 3, and the probes 2 and 4. Fig. 7 shows the location of the beams and spin-plane booms including the magnetometer booms (5-m length). Baffles limit the width of the beams to stay off the probes. In spite of the symmetric position of the ion beams with respect to SDP, the initial data reveal some residual effects which require further analysis and eventually further dedicated calibration of the SDP data when ASPOC is active. It is difficult to disentangle any instrumental effects from natural variations of the electric field, but general trends can be seen, as illustrated by the example from MMS2 in Fig. 8. In this period of 45 min, the ion beams have been commanded into Fig. 8. Top: Spacecraft potential varying with ASPOC beam currents five different combinations, as indicated in Fig. 8. The top as indicated. Bottom: Electric field components E12 (red) and E34 (green) panel contains the spacecraft potential. The value of ∼21 V averaged over a spin period normalized to the zero current state, horizontal lines are averages for constant ion beam current. with ASPOC OFF suggests an ambient plasma density below 1 cm−3. The bottom panel shows the electric field components was ∼13.5 V. In an ideal situation both curves should be E12 (red) and E34 (green) averaged over a spin period sine functions and coincide. In reality one can see a constant and normalized to the conditions when the ASPOC beams offset between E12 and E34 and different profiles from a were OFF. Horizontal lines are averages over the time interval sine curve in these raw data. Calibration schemes to eliminate of a constant ion beam current. Asymmetric ASPOC beams these E12 and E34 differences, which were observed also (0 + 20 μA, 10 + 0 μA) result in larger offsets, as expected. by Cluster under presence of ambient electric fields, have When only ASPOC 2 is active (0 + 20 μA, second phase) it been developed in [16] and are applied for MMS level 2 creates positive space charge in the vicinity of probes 2 and 4, science data. The minima near 0° are consistent with the so that both E12 and E34 become negative. On the contrary, majority of photoelectrons being located at the sunlit part ASPOC 1 being active in phase four results in positive offsets. of the spacecraft. There is also a significant deviation from When both ASPOCs are active, a small negative offset appears, a sinusoidal shape, i.e., a dip around 180°, which suggests which differs between phases 1 and 5, and is somehow additional modifications of the photosheath by the spacecraft related to, however not simply proportional to, the beam body and the axial booms. currents. The bottom panel shows the situation one minute before The effect of ASPOC on the spin variation of the electric with 2 × 10 μA ASPOC ion beams. There are additional field measurements is best shown by comparing data imme- deviations from the sinusoidal shape as well as global shifts diately before and after ASPOC is turned OFF.Fig.9shows of the curves toward lower (E12, red) or higher (E34, green) raw electric field components E12 and E34 in the rotating phase angles. This influence of ASPOC on the shape of spacecraft frame over spin phase for one spin period of ∼20 s electric field raw data and other effects of ASPOC in different from MMS1 on May 26, 2016. Zero phase angle refers to plasma environments requires further analysis. PIC simulation probe 1 or 3, respectively, pointing sunward. The top panel studies are ongoing to support the interpretation of these shows the case for ASPOC OFF. The uncontrolled potential measurements (see [17]). This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

6 IEEE TRANSACTIONS ON PLASMA SCIENCE

team, M. Oka, and N. Kitamura for their help in using SPEDAS and K. J. Genestreti for the new HPCA data processing tool.

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