2022 ARTEMIS SCIENCE NUGGETS
Multipoint Observation of the Solar Wind Interaction with Strong Lunar Magnetic Anomalies by ARTEMIS Spacecraft and Chang'E-4 Rover
by Lianghai Xie
Zhejiang Provincial State Key Laboratory of Space Weather,
National Space Science Center, Chinese Academy of Sciences, Beijing, China
Introduction
Differing from the Earth, the Moon does not have a global magnetic field, but only some local crustal magnetic fields, known as magnetic anomalies. Some magnetic anomalies were thought to be strong enough to stand off the solar wind and form a local protecting structure with reduced solar wind flux, which is the so-called lunar mini-magnetosphere (LMM). However, all previous observations associated with the LMM only found enhancements in the magnetic field and the plasma density, but without a density cavity. Furthermore, there is no multipoint observation of the solar wind interaction with lunar magnetic anomalies so far, and hence the full structure of the interaction is still unknown. It is suggested that a LMM may be caused when the crustal field is strong and the ion inertial length is small. Here we present a unique multipoint observation of the solar wind interaction with strong lunar magnetic anomalies, in which the Chang’E-4 (CE-4) rover is located on the lunar surface near one of the strongest magnetic anomalies and the two ARTEMIS spacecraft are in orbit. Moreover, the multipoint observation happens under a special solar wind condition with a very small ion inertial length. As a result, this work provides a good chance to check whether a LMM can be completely caused by the solar wind interaction with lunar magnetic anomalies.
Results
During 01:00-08:00 UT on 31 December 2019, ARTEMIS P1 was in the undisturbed solar wind upstream from the lunar wake and observed a very special solar wind, which had a high number density (∼20 cm-3) and a low velocity (∼300 km/s). Meanwhile, ARTEMIS P2 flew across the lunar wake and observed a shock around 02:15 UT before entering the wake, with an altitude of about 800 km. The shock is downstream from a group of strong magnetic anomalies (Imbrium, Serenitatis, and Crisium antipodes, see Figure 1). During the same period of interest, the CE-4 rover was located on the lunar surface near the Imbrium antipode anomaly. The observing geometries of ARTEMIS and CE-4 during this period are shown in Figure 1, where the Selenocentric Solar Ecliptic (SSE) coordinate is used, whose the X axis points from the Moon's center to the Sun, the Z axis is normal to the ecliptic plane, and the Y axis completes the right-handed set of axes.
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| Figure 1. Observing geometries of ARTEMIS and CE-4 during 01:00-08:00 UT on 31 December 2019. The central sphere represents the lunar body with color contours to show the crustal magnetic field on the lunar surface. The white circle on the lunar surface indicates the position of CE-4. The magenta circles indicate the magnetic anomalies at the Imbrium antipode, Serenitatis antipode, and Crisium antipode, respectively. The yellow and gray dots show the trajectories of ARTEMIS P1 and P1, respectively. The solid black dots indicate the time around 02:15 UT when ARMTEMIS P2 encounters the shock. The geometries are shown in the SSE coordinate, in which the solar wind flows in the opposite direction to the X-axis and the Z-axis is normal to the ecliptic plane. |
The shock observed by ARTEMIS P2 is shown in Figure 2, where we can see obvious changes in the plasma properties around 02:15 UT, including an ion deceleration, an electron heating, and some magnetic field fluctuations (Figure 2a-2c). Furthermore, both the number density and the magnetic field are significantly enhanced by factors of 2.1 (Figure 2d) and 2.3 (Figure 2g), respectively. Meanwhile, the average electron temperature increases from 6.9 eV to 8.6 eV (Figure 2e), and there is also a deflection in the solar wind velocity (mainly in the Y direction), apart from the deceleration in the X direction (Figure 2f). The total change in the velocity is ∼46 km/s, which is larger than both the Alfvén velocity of ∼29 km/s and the magnetosonic velocity of ∼42 km/s of the upstream solar wind, implying a discontinuity in the flow. In addition, the magnetic field lines have been rotated across the discontinuity (Figure 2g). All of these features suggest a strong shock observed by ARTEMIS P2. Known from Figure 1, the shock is downstream from a group of strong magnetic anomalies, suggesting that the shock is caused by the solar wind interaction with these magnetic anomalies.
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| Figure 2. The shock observed by ARTEMIS. (a)–(c) show the ion energy spectrum, the electron energy spectrum, and the fast Fourier transform of magnetic field fluctuations measured by ARTEMIS P2; (d)–(g) show the number densities and the electron temperatures, the velocity differences, and the magnetic fields, respectively, in which subscripts of 1 and 2 indicate the measurements of P1 and P2, respectively, and the lines in different colors represent the different components in SSE coordinate. The black line in (f) shows the magnitude of velocity change between the upstream P1 measurements and the downstream P2 measurements. The gray and the black lines in (g) show the magnitudes of magnetic field measured by P1 and P2, respectively. |
The ASAN instrument onboard CE-4 rover can measure the backscattered solar wind particles as energetic neutral atoms (ENAs), in an energy range from 10 eV to 10 keV (Wieser et al. 2020). It was found that the ENA fluxes at energies lower than 100 eV could be contaminated by the sputtered ENAs from the lunar surface or the instrument surface (Zhang et al. 2020), which should be excluded when calculating the integrated ENA flux over the energy (JENA). In addition, ASAN only works in the morning and the afternoon of the local lunar day, with a typical period of∼1 hr and a solar zenith angle ranging from 50° to 80°. In particular, ASAN is downstream from the magnetic anomaly for the afternoon measurements but upstream from the magnetic anomaly for the morning measurements. Here we choose three ENA spectra measured during the periods of 05:10–06:06 UT on 2019 December 31, 09:43–10:37 UT on 2020 October 12, and 07:11–08:12 UT on 2020 September 13, respectively, and call them Case 1, Case 2, and Case 3, respectively. Case 1 is downstream from the magnetic anomaly, which happens during the same period of the ARTEMIS observation. Case 2 and Case 3 are upstream from the magnetic anomaly, which are used to compare with Case 1 to show the effect of magnetic anomalies on the solar wind. As shown in Figure 3, in Case 2 and Case 3, the Ecut is almost equal to the Esw and the JENA almost linearly increases with the Jsw,N, which is consistent with the previous conclusions obtained by Zhang et al. (2020) and Xie et al. (2021). Case 1 and Case 3 have similar Esw, and we may expect similar Ecut in these two cases. However, it is found that the ENAs in Case 1 are more gathered at lower energies with a smaller Ecut, suggesting that the solar wind ions have been decelerated by the magnetic anomaly. Case 1 and Case 2 have similar Jsw,N, and the JENA in these two cases may be very close, as we have found that the JENA almost linearly depends on Jsw,N (Xie et al. 2021). Nevertheless, here the JENA for Case 1 is only about 1/3 that of Case 2, which suggests that the solar wind has been partially shielded by the magnetic anomaly. However, it also implies that part of the solar wind ions have penetrated into the magnetic obstacle. As a result, the lunar surface is not fully shielded from the solar wind and there should be no complete LMM.
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| Figure 3. ENA spectra observed by CE-4. The red, green, and blue dots show the measured spectra of Case 1, Case 2, and Case 3, respectively, in which Case 1 is downstream from the magnetic anomaly and correlated with the LMM observed by ARTEMIS, while Case 2 and Case 3 are upstream from the magnetic anomaly with similar solar wind conditions to those of Case 1. The solid lines are the fitting results of the three spectra with exponential functions. The dashed lines indicate the cutoff energies, at which the ENA flux is equal to 5.4% of its initial flux at 105 eV. Texts on the right show the parameters of the three cases, in which Jsw,N is the normal component of the solar wind flux on the lunar surface; JENA is the integrated ENA flux over the energy; Esw is the solar wind energy; and Ecut is the cutoff energy. |
Conclusion
Based on the multipoint observations shown above, we obtain a new physical picture for the solar wind interaction with lunar magnetic anomalies. A lunar magnetic anomaly has many substructures, and the interaction between the solar wind and the magnetic anomaly can be regarded as the sum of the solar wind interactions with all of these substructures. For an individual substructure, the magnetic field is not strong enough to stop the solar wind, but only deflect the solar wind. The deflection is not obvious near the nose of the interaction region and a large number of solar wind particles can penetrate across the magnetic field and impact the lunar surface. Accordingly, there is no shock but only a magnetosonic wake in this region, where the solar wind can be slightly decelerated and compressed. When moving downstream, the solar wind can be further deflected by the following substructures, and the magnetosonic wake is getting more and more significant. Moreover, the magnetosonic wakes caused by different substructures may overlap each other, which jointly form a boundary layer near the lunar surface. Besides, some solar wind ions can be reflected by the magnetic anomaly, which can help decelerate and compress the incoming solar wind. Finally, the solar wind can flow horizontally or even outwardly from the local surface, and a trailing shock appears downstream from the magnetic anomaly. It suggests that a bow shock as well as a complete mini-magnetosphere are not formed here, even if the interaction is expected to be in an obvious fluid manner. Our results imply that a lunar mini-magnetosphere may be never completely formed. As a result, the term of “mini-magnetosphere” might be not appropriate, which needs to be carefully used or redefined. In addition, if the lunar surface is not fully shielded by the magnetic anomaly, all applications associated with the magnetic shielding, such as the formation of a lunar swirl and the protection of a lunar station, need to be reassessed.
References
Wieser, M., Barabash, S., Wang, X.D., et al. (2020). The Advanced Small Analyzer for Neutrals (ASAN) on the Chang’E-4 Rover Yutu-2. Space Sci Rev 216:73. https://doi.org/10.1007/s11214-020-00691-w.Xie, L., Li, L., Zhang, A., Zhang, Y., Cao, J., Wieser, M., et al. (2021). Inside a lunar mini-magnetosphere: First energetic neutral atom measurements on the lunar surface. Geophysical Research Letters, 48, e2021GL093943. https://doi.org/10.1029/2021GL093943.
Xie, L., Li, L., Zhang, A., Wang, H.Z., Shi, Q., et al. (2022). Multipoint Observation of the Solar Wind Interaction with Strong Lunar Magnetic Anomalies by ARTEMIS Spacecraft and Chang'E-4 Rover. The Astrophysical Journal Letters, 937:L5 (5pp). https://doi.org/10.3847/2041-8213/ac903f
Zhang, A., Wieser, M., Wang, C., Barabash, S., Wang, W., et al. (2020). Emission of energetic neutral atoms measured on the lunar surface by Chang’E-4, Planetary and Space Science, 189(15): 104970. doi: https:// doi.org/10.1016/j.pss.2020.104970
Biographical Note
Lianghai Xie is an associate researcher at State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences. He is working on the solar wind interaction with all kinds of bodies, such as the Moon, the Mars, and the Mercury.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu