Electrostatic Waves and Electron Heating Observed over Lunar Crustal Magnetic Anomalies
F. Chu, J. S. Halekas, Xin Cao, J. P. McFadden, J. W. Bonnell, K.-H. Glassmeier
mmanuscript submitted to
Geophysical Research Letters
Electrostatic Waves and Electron Heating Observedover Lunar Crustal Magnetic Anomalies
F. Chu , J. S. Halekas , Xin Cao , J. P. McFadden , J. W. Bonnell , andK.-H. Glassmeier Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA Space Sciences Laboratory, University of California, Berkeley, CA, USA Institut fr Geophysik und Extraterrestrische Physik, Technische Universitt Braunschweig, Braunschweig,Germany
Key Points: • Two types of electrostatic instabilities are observed over the lunar crustal mag-netic anomalies during ARTEMIS flyby • Electron two-stream instability and electron cyclotron drift instability may playan important role in driving the electrostatic waves • Electron cyclotron drift instability, along with modified twostream instability, maycause isotropic electron heating
Corresponding author: F. Chu, [email protected] –1– a r X i v : . [ phy s i c s . s p ace - ph ] A ug anuscript submitted to Geophysical Research Letters
Abstract
Above lunar crustal magnetic anomalies, large fractions of solar wind electrons and ionscan be reflected and stream back towards the solar wind flow, leading to a number ofinteresting effects such as electrostatic instabilities and waves. These electrostatic struc-tures can also interact with the background plasma, resulting in electron heating and scat-tering. We study the electrostatic waves and electron heating observed over the lunarmagnetic anomalies by analyzing data from the Acceleration, Reconnection, Turbulence,and Electrodynamics of Moon’s Interaction with the Sun (ARTEMIS) spacecraft. Basedon the analysis of two lunar flyby events in 2011 and 2013, we find that the electron two-stream instability (ETSI) and electron cyclotron drift instability (ECDI) may play animportant role in driving the electrostatic waves. We also find that ECDI, along withthe modified twostream instability (MTSI), may provide the mechanisms responsible forsubstantial isotropic electron heating over the lunar magnetic anomalies.
Plain Language Summary
Without a global magnetic field or a thick atmosphere, the solar wind directly impactsthe surface of the Moon. However, over regions where the lunar crust is strongly mag-netized, the charged particles in the solar wind can be reflected and travel back towardsthe incoming solar wind, generating interesting features like electrostatic waves. Thesewaves can also in turn affect the solar wind by increasing the temperature of its chargedparticles. To understand the mechanisms causing the waves and heating, we analyze datafrom the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon’s In-teraction with the Sun spacecraft. Our results indicate that the lunar environment be-comes unstable because of the reflected charged particles, thereby creating free energiesthat lead to the waves and heating.
In the absence of a global magnetic field and a thick atmosphere, unlike the caseof the Earth, the surface of the Moon directly interacts with the incident solar wind plasma.Traditionally, the Moon has been thought to act as a simple barrier to the solar windflow, causing the absorption of plasma at the upstream surface and formation of a plasmawake in the downstream. However, recent observations from Chandrayaan-1, Kaguya,and ChangE-1 reveal that Moon-solar wind interaction is in fact much more complicated –2–anuscript submitted to
Geophysical Research Letters and dynamic, capable of creating a variety of interesting effects around the Moon. Forexample, the surface of the Moon, immersed in the solar wind plasma, charges to an elec-trostatic potential in order to balance the total incident currents (Whipple, 1981; J. S. Halekaset al., 2002, 2011). Moreover, solar wind sputtering from the lunar surface and ioniza-tion of the tenuous neutral exosphere can produce heavier lunar pickup ions, which canthen be accelerated downstream from the Moon by the motional electric field (Yokotaet al., 2009; J. S. Halekas, Poppe, Delory, et al., 2012; Cao et al., 2020). Some other ex-amples of lunar interaction include backscattering of solar wind ions and photoelectronemission from the lunar surface (Reasoner & Burke, 1972; Goldstein, 1974; Lue et al.,2014; Bhardwaj et al., 2015; Harada et al., 2017).One of the most interesting and unique Moon-solar wind interactions happens overthe lunar crustal magnetic anomalies. Previous studies have shown that the lunar crustalmagnetic fields can perturb the solar wind flow, causing plasma compressions at the lu-nar limb (Russell & Lichtenstein, 1975). More recent measurements from Kaguya sug-gest that mini-magnetospheres can form over strong magnetic anomaly regions, partiallyshielding the lunar surface from the solar wind (Saito et al., 2010). In addition, local crustalmagnetic fields are found capable of reflecting solar wind ions and electrons from the lu-nar surface (Lue et al., 2011; Saito et al., 2012; J. Halekas et al., 2012; J. S. Halekas, Poppe,Farrell, et al., 2012). Using Chandrayaan-1 data, Lue et al. (2011) reported that on av-erage 10% of the incident solar wind ions reflect over large-scale magnetic anomalies. Thereflection efficiency can reach up to 50% for ions and as much as 100% for electrons aboveregions of strongest crustal fields (J. S. Halekas et al., 2001). The magnetically reflectedions and electrons, along with backscattered particles and photoelectrons, can then formcounter-streaming beams towards the incoming solar wind flow, resulting in a numberof fundamental plasma processes such as electrostatic instabilities and waves. These elec-trostatic structures can also in turn have an impact on the lunar plasma environment,leading to substantial electron heating and scattering.A variety of plasma instabilities and waves of different origin have been previouslyobserved above the lunar crustal magnetic anomalies (Nakagawa, 2016; Harada & Halekas,2016). Tsugawa et al. (2011) reported that monochromatic, lefthand polarized (in thespacecraft frame) whistler waves with frequencies close to 1 Hz were detected by Kaguyanear the Moon. A further statistical analysis suggested that the waves were generatedby the solar wind interaction with lunar magnetic anomalies. In addition, broadband elec- –3–anuscript submitted to
Geophysical Research Letters trostatic mode, resulting from counter-streaming electron beams, is another type of wavescommonly observed in the lunar upstream plasma (Harada et al., 2014).In this paper, we investigate two types of electrostatic instabilities observed overthe lunar crustal magnetic anomalies by Acceleration, Reconnection, Turbulence, andElectrodynamics of Moon’s Interaction with the Sun (ARTEMIS) spacecraft. We reportfor the first time on a class of electrostatic waves propagating perpendicular to the am-bient magnetic field, possibly driven by electron cyclotron drift instabilities. This typeof electrostatic waves is analogous to those observed in the foot region of perpendicu-lar shocks. In the end, we also discuss the mechanisms of electron heating observed alongwith the electrostatic waves over the magnetic anomalies.
NASA’s ARTEMIS spacecraft, consisting of two satellites (P1 and P2) originallyfrom the THEMIS (Time History of Events and Macroscale Interactions During Sub-storms) mission, occupies stable 26-h period elliptical near-equatorial orbits around theMoon (Angelopoulos, 2011). To investigate the plasma environment above the daysidelunar surface, we utilize measurements from four of the instruments: the ElectrostaticAnalyzer (ESA; McFadden et al., 2008), Electric Field Instrument (EFI; Bonnell et al.,2008), Search Coil Magnetometer (SCM; Roux et al., 2008), and Fluxgate Magnetome-ter (FGM; Auster et al., 2008). The ESA measures electron energies in the range of 2eV to 32 keV and ion energies from 1.6 eV to 25 keV (McFadden et al., 2008). The EFIis capable of measuring the three components of the ambient electric fields from ∼ , ,
0] earth radii ( R E ), located in the solar windwell upstream of the Earths bow shock. The data of the flyby obtained from the abovefour instruments are shown in Figure 1. The probe is found to briefly fly over the mag-netic anomaly region between 10:10 UT and 10:12 UT, indicated by an enhancement ofthe fluctuations in the ambient magnetic field in Figure 1b. When the altitude of P2 de-scends below 50 km, two counter-streaming electron beams along the ambient magnetic –4–anuscript submitted to Geophysical Research Letters A l t i t ude [ k m ] B [ n T ] T e [ e V ] E [ m V / m ] E FFT B FFT E l e c t r on PA [ ] ° (a)(b)(c)(d)(e)(f)(g) xyz 10 -10 ( V / m ) / H z xyz n T / H z E f l u x hh:mm (2011 Nov 26) xyz 10 -8 -6 -6 -4 -2 Figure 1.
Data from an ARTEMIS P2 lunar flyby over magnetic anomalies on 26 November2011. (a) Altitude of the probe as a function of time. P2 reaches a periselene at an altitude of23.8 km at 10:12 UT. (b) Ambient magnetic field in SSE coordinates. (c) Electron pitch anglespectrum for energies ranging from 2 eV to 32 keV. The differential energy flux has unites ofeV/(eV cm sr s). (d) Electron temperatures parallel ( Z axis) and perpendicular ( X and Y axis)to the magnetic field. (e) Wave burst data in magnetic field-aligned coordinates, Z axis beingparallel to the magnetic field. (f)–(g) FFT wave spectra of electric and magnetic field, respec-tively. Text labels indicate time of day in UT, solar zenith angle (SZA), and spacecraft (X, Y, Z)SSE coordinates in units of lunar radii ( R L ). –5–anuscript submitted to Geophysical Research Letters field can be seen intermittently from the electron pitch angle spectrum in Figure 1c. Sincethe magnetic field is + B x dominated in SSE coordinates, the X axis being the directionpointing from the Moon toward the Sun, the beam with pitch angles around 180 ◦ cantherefore be identified as incoming solar wind electrons. The other beam with ∼ ◦ pitchangles results from the primary electrons reflected from the magnetic anomalies, as wellas photoelectrons emitted from the dayside lunar surface (Whipple, 1981; J. S. Halekas,Poppe, Farrell, et al., 2012).During the time period of the flyby, we observe high frequency electrostatic fluc-tuations ranging from ∼ . E z parallel to the field line), revealing that the electro-static fluctuations are mostly perpendicular to the magnetic field between 10:10 UT and10:11 UT. We also find broadband magnetic fluctuations extending from tens of Hz downto near-DC levels in magnetic field FFT spectrum (Figure 1g). These waves are mostlikely to be whistler mode, as there are really no other electromagnetic modes that canpropagate in this frequency range. Figure 1d shows the electron temperatures parallel( T e,z ) and perpendicular ( T e,x and T e,y ) to the magnetic field, where perpendicular elec-tron heating is observed between 10:10 UT and 10:11 UT. In addition, strong isotropicheating is seen between 10:11 UT and 10:12 UT, accompanied by the intense electrostaticfluctuations.Figure 2 shows an overview of another flyby event (ARTEMIS P1) that occurredon 10 February 2013, when P1 was at average GSE coordinates of [57 , , R E . The sig-natures we see are very similar to the previous event. Electrostatic fluctuations are ob-served in the electric field FFT spectrum (Figure 2f) between 16:28 UT and 16:36 UT,although the field aligned wave burst data (Figure 2e) indicate that the electrostatic fluc-tuations are mainly parallel to the magnetic field this time. Strong isotropic heating isalso observed in the electron temperature profile (Figure 2d) between 16:31 UT and 16:37UT, coinciding with the intense electrostatic fluctuations. In addition, a recent analy-sis by J. S. Halekas et al. (2014) pointed out that this flyby event has many of the as-pects of a collisionless shock, despite the small scale size. –6–anuscript submitted to Geophysical Research Letters A l t i t ude [ k m ] B [ n T ] T e [ e V ] E [ m V / m ] xyz 10 -10 ( V / m ) / H z E FFT B FFT E l e c t r on PA [ ] ° xyz n T / H z E f l u x (a)(b)(c)(d)(e)(f)(g) xyz10 -6 -4 -2 -8 -6 hh:mm (2013 Feb 10) 66.40.4-0.90.1 92.30.0-1.00.0SZAX_SSEY_SSEZ_SSE 79.30.2-1.00.116:2854.00.6-0.80.2 Figure 2.
Data from an ARTEMIS P1 lunar flyby over magnetic anomalies on 10 February2013. P1 reaches the lowest altitude of 19.5 km above the lunar surface at 16:34 UT. All panelssame as Figure 1. –7–anuscript submitted to
Geophysical Research Letters
We now consider the generation mechanisms for the electrostatic fluctuations, whichwe suspect may result from a combination of different plasma processes in the complexlunar environment over the magnetic anomalies. The Moon acts as a barrier to the in-coming solar wind flow. Due to the influence of crustal magnetic fields, large fractionsof the solar wind electron and ion populations reflect above the lunar surface and streamtowards the solar wind flow, resulting in varieties of plasma instabilities that could pro-duce the electrostatic fluctuations shown in Section 2. Two possible drivers for the wavesin Figures 1 and 2 are proposed: electron two-stream instability (ETSI) that could causeelectrostatic fluctuations parallel to the ambient magnetic field, and electron cyclotrondrift instability (ECDI), which can generate the electrostatic waves in the perpendicu-lar direction.
Electron two-stream instability driven by counter-streaming electron beams is oneof the most commonly found electrostatic instabilities in space plasmas. For example,ETSI has been reported in the solar wind (Malaspina et al., 2013), Earth’s magnetotail(Matsumoto et al., 1994), and at the bow shock (Bale et al., 1998). The nonlinear evo-lution of ETSI often leads to the formation of time domain structures (Mozer et al., 2015),such as electrostatic solitary waves (Jao & Hau, 2014; Graham et al., 2016), electron phase-space holes (Franz et al., 2005; Hutchinson, 2017; Holmes et al., 2018; Steinvall et al.,2019), and double layers (Andersson et al., 2002; Ergun et al., 2003).We show an example of the time domain structures observed during the ARTEMISP1 lunar flyby in Figure 3a and the corresponding electric field FFT spectrum in Fig-ure 3b. Since the incident and counter-streaming electron beams are mostly adiabatic,the intense electric field spikes that result from the ETSI have significant field-alignedcomponents E z . If we zoom in on the time scale, a series of isolated bipolar electric fieldstructures, known as electron phase-space holes, can be seen in Figure 3c. Similar bipo-lar structures have also been previously observed near the Moon by Kaguya (Hashimotoet al., 2010). Electron phase-space holes are often responsible for heating and scatter-ing background electrons through wave-particle interaction in many space plasmas (Mozeret al., 2016; Vasko et al., 2017). –8–anuscript submitted to Geophysical Research Letters -10 -8 -6 .575 .580 .585-10-505101520 ( V / m ) / H z xyzxyz E [ m V / m ] (a) hh:mm (2013 Feb 10 16:30:01) E [ m V / m ] (c) E FFT (b) hh:mm (2013 Feb 10 16:30:01)-20-100102030 .700 .800 .90010 Figure 3. (a) An example of the time domain structures observed during the ARTEMIS P1lunar flyby on 10 February 2013. (b) Corresponding electric field FFT spectrum showing thebroadband electrostatic fluctuations. (c) Zoom-in on the time scale over the blue-colored regionin (a) to demonstrate electron phase-space holes.–9–anuscript submitted to
Geophysical Research Letters (a) f ( v ) (b) -1000 -800 -600 -400 -200 0 200 400 600 800 1000 V [km/s] -1000-800-600-400-200 V [ k m / s ] -12 -11 -10 -9 -8 para pe r p ReflectedIon Beam Solar WindIon Core
MoonMoon A no m a l y xy xz A no m a l y B S o l a r W I nd S o l a r W I nd B S un S un E pe r p ReflectedIon Beam
Figure 4. (a) An example of a reflected ion beam traversing the solar wind plasma perpen-dicular to the background magnetic field. This sample ion velocity distribution cut, in plasmaframe, was obtained at 10:11:30 UT during the P2 lunar flyby event as shown in Figure 1. (b)Schematic illustration of the magnetic field ( − Y direction) and incoming solar wind ( − X direc-tion) geometry during the flyby in SSE coordinates. Most of the electrostatic instabilities driven by either field-aligned beams or cur-rents can only generate electrostatic fluctuations along the magnetic field. However, elec-tron cyclotron drift instability, which arises from a relative drift between ions and elec-trons across the magnetic field, can lead to electrostatic waves in the electron cyclotronfrequency in the perpendicular direction (Forslund et al., 1970).ECDI results from reactive coupling between electron cyclotron Bernstein modesand ion beam modes (Muschietti & Lemb`ege, 2013). The linear dispersion relation ofthe ECDI can be found in Janhunen et al. (2018). This type of instability has been ob-served in many laboratory plasmas (Ripin & Stenzel, 1973; Stenzel & Ripin, 1973) andoccasionally in space (Wu & Fredricks, 1972; Wilson et al., 2010). ECDI plays an im-portant role in particle acceleration and heating in the foot of supercritical quasi-perpendicularshocks, where a fraction of incoming ions are reflected at the steep front (Matsukiyo &Scholer, 2006). Similar conditions favoring the occurrence of ECDI can be found in the –10–anuscript submitted to
Geophysical Research Letters lunar plasma environment tens of kilometers above the magnetic anomalies. The elec-trons in these regions are strongly magnetized; however, the ions are considered to beunmagnetized due to their very large gyroradii in comparison to the scale of the lunarcrustal magnetic fields. The ion beam reflected from the magnetic anomalies thereforecan stream in any direction, in particular, across the magnetic field, triggering the ECDI.ECDI provides a mechanism capable of driving the perpendicular electrostatic fluc-tuations during the P2 flyby as shown in Figure 1e. Reflected ion beams traversing thesolar wind plasma perpendicular to the background magnetic field are observed from ionvelocity distributions, a good example of which is presented in Figure 4a. Two ion beamscan be correspondingly identified: a dense solar wind ion core close to the origin and thereflected ion beam streaming at ∼
200 km/s across the magnetic field. Figure 4b illus-trates the geometry of the magnetic field ( − Y direction) and incoming solar wind ( − X direction) during the flyby in SSE coordinates. Once the solar wind ions are reflectedfrom the magnetic anomaly, they are accelerated by the motional electric field and streamtowards the − Z direction. The perpendicular configuration of the magnetic field and re-flected ion beam therefore favors the occurrence of ECDI, resulting in the time-domainstructures with perpendicular electric field in Figure 1e. As discussed earlier, ECDI can result in electrostatic waves propagating perpen-dicular to the magnetic field. However, so far we have not considered the effect of theelectron parallel motion in the dispersion relation of the ECDI. If we allow a finite wavevector along the magnetic field in the dispersion relation, then a new type of instabil-ity naturally arises in the solution: modified two-stream instability (MTSI) (Janhunenet al., 2018). Due to the parallel component of the wave fields, previous studies have shownthat the MTSI can cause strong parallel heating of electrons (McBride et al., 1972; Mat-sukiyo & Scholer, 2006). In section 4, we will show that ECDI, along with MTSI, mayprovide a mechanism responsible for substantial isotropic electron heating as observedover lunar magnetic anomalies. –11–anuscript submitted to
Geophysical Research Letters
When waves are traveling through a plasma, the fluctuating wave fields can inter-act with the charged particles of the plasma, resulting in many interesting nonlinear ef-fects. As shown in section 2, one of the important features we notice in the two flyby eventsis the significant electron heating observed over lunar crustal magnetic anomalies (Fig-ures 1d and 2d). Furthermore, the electric and magnetic field FFT wave spectra (Fig-ures 1f–1g and 2f–2g) reveal that the electron heating is always accompanied by signif-icant electrostatic and/or electromagnetic wave power, suggesting that the wave-particleinteraction may play an important role in heating the electrons.To demonstrate the mechanisms causing the electron heating above the magneticanomaly regions, here we only focus on the ARTEMIS P2 lunar flyby as shown in Fig-ure 1. We plot the perpendicular electron temperature and the electromagnetic wave en-ergy density together as a function of time in Figure 5a. We present a similar figure show-ing the parallel electron temperature and the electrostatic wave energy density as a func-tion of time in Figure 5b, where the perpendicular temperature is also shown for com-parison. We notice that there are two peaks (A and B) in the perpendicular electron tem-perature and one peak (C) in the parallel temperature. Further investigation of the cor-relation between the wave power and the electron temperature shows that these peaksare caused by different heating mechanisms.We note that peak A in the perpendicular electron temperature is very well alignedwith the electromagnetic wave power in Figure 5a, suggesting that the heating likely re-sults from the cyclotron resonance with the whistler modes. When the electrons encounterthe whistler waves Doppler-shifted to their cyclotron frequency or higher harmonics, theycan strongly interact with the wave fields, gaining perpendicular energy and causing thewaves to damp (Tsurutani & Lakhina, 1997; Stenzel, 2016). Similar perpendicular heat-ing by electromagnetic waves near the Moon has been reported in J. S. Halekas, Poppe,Farrell, et al. (2012) – though in this case it happens in the magnetotail. In addition tocyclotron damping, we also note that the peak A coincides with the peaks of the elec-trostatic wave power in Figure 5b, suggesting that the perpendicular electrostatic wavesdriven by ECDI are likely to be another source contributing to the perpendicular heat-ing in peak A. This electron heating mechanism resulting from ECDI is also observedin the foot region of perpendicular shocks (Matsukiyo & Scholer, 2006). –12–anuscript submitted to
Geophysical Research Letters (a) (b)
A B C -1000100 E [ m V / m ] xyz10:09 10:10 10:11 10:12 10:13 Time [hh:mm 2011 Nov 26] T e pe r p [ e V ] E l e c t r o m agne t i c W a v e E ne r g y D en s i t y [ J / m ] -14 Time [hh:mm 2011 Nov 26] T e pa r a [ e V ] E l e c t r o s t a t i c W a v e E ne r g y D en s i t y [ J / m ] -16 T e perp B Figure 5. (a) Perpendicular electron temperature and electromagnetic wave energy density(frequencies ranging from near-DC levels to tens of Hz) as a function of time for the P2 lunarflyby event shown in Figure 1. (b) Parallel electron temperature and electrostatic wave energydensity (frequencies ranging from 10 Hz to 8 kHz) as a function of time for the same flyby. Theperpendicular temperature is also shown in the background for comparison. Inset shows the sameelectric field as Figure 1e. The largest two peaks in the perpendicular temperature is denoted byA and B, respectively. The largest peak in the parallel temperature is denoted by C.
As to peak B, since it is only accompanied by the peaks of the electrostatic wavepower in Figure 5b, this heating therefore is likely to be caused by ECDI as well. Lastbut not the least, peak C may seem to be quite puzzling at first. Even though it is alignedwith peak B and accompanied by intense electrostatic wave power, the perpendicular elec-tric fields resulting from ECDI cannot heat the electrons in the parallel direction. How-ever, as discussed in section 3.3, MTSI can be driven unstable in the similar conditionsas ECDI. In fact, ECDI and MTSI can often be excited simultaneously, allowing for sub-stantial electron heating both perpendicular and parallel to the magnetic field (Wu etal., 1984; Muschietti & Lemb`ege, 2013; Janhunen et al., 2018). Since ETSI can also leadto parallel heating, therefore, the isotropic heating seen in peaks B and C (Figure 5b)may be caused by a combination of contributions from ECDI, MTSI, and ETSI.
In conclusion, we have investigated two types of electrostatic instabilities observedover the lunar crustal magnetic anomalies during ARTEMIS flyby. The electrostatic waves –13–anuscript submitted to
Geophysical Research Letters propagating parallel to the ambient magnetic field are attributed to upward electron beamsreflected by the crustal magnetic fields. We have also reported for the first time on ob-servations of another class of electrostatic waves propagating perpendicular to the mag-netic field. A proposed freeenergy source is associated with reflected ion beams stream-ing across the background magnetic field. Finally, our analysis suggests that the perpen-dicular electron heating observed above the magnetic anomalies is mainly caused by cy-clotron damping and ECDI. The isotropic heating, on the other hand, may result fromjoint effects due to ECDI, MTSI, and ETSI.
Acknowledgments
We acknowledge support from Solar System Exploration Research Virtual Institute andthe THEMIS Contract NAS502099. All ARTEMIS data used in this paper are publiclyavailable at NASAs CDAWeb ( https://cdaweb.sci.gsfc.nasa.gov ) and the ARTEMISsite ( http://artemis.ssl.berkeley.edu ). References
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