The Energetic Particle Environment of the Lunar Nearside: SEP Influence

The energetic particle environment of the lunar nearside, always facing toward Earth because of tidal lock, is particularly important to manned lunar activities. The reason for this is that the lunar nearside is more convenient than the farside to communicate with Earth. Due to the lack of atmosphere and global magnetosphere, the energetic particles can be of especially great damage to the lunar surface. Galactic cosmic rays (GCR) and solar energetic particles (SEPs) are two primary sources of hazard energetic particles. In comparison with the very high energy GCR, which has a very long time variation period, the SEPs may have a greater impact in practice. Note that the lunar nearside and farside could be much different in receiving energetic particles from the Sun. When the lunar nearside faces the Sun, the Moon is in the Earth's magnetosphere, which can provide protection. When the lunar nearside faces away from to the Sun, the lunar body may provide shielding for most of the time. However, energetic particles with much lower energies than SEPs from the Earth's bow shock would have a stronger influence on the lunar nearside than on the farside.

Since the Moon is mostly in the Earth's magnetosphere during the daytime of the lunar nearside, the blockage of energetic particles by the magnetosphere would be of great interest to lunar exploration using solar power. By magnetically deflecting, the Earth's strong magnetic field can usually shield the inner magnetosphere from energetic particles, including a significant fraction of GCR (Smart & Shea 1994; Størmer 1955; Smart et al. 2000). The magnetospheric shielding at the lunar orbit has also been studied. Some simulation studies found that significant shielding of GeV particles by the magnetosphere is possible, especially on the lunar nearside (Winglee & Harnett 2007) and for storm conditions (Harnett 2010). While, a contrasting simulation study by Huang et al. (2009) suggested that protons of greater than 1 Mev cannot be significantly shielded by the magnetosphere at lunar distance. In situ observations of energetic particle fluxes with energies >14 Mev by LRO (Case et al. 2010) and energies >4 Mev by Chang'E-1 (Wang & Qin 2013) both demonstrated that the magnetosphere can provide almost no shielding at lunar orbit on such energetic particles in quiet time. However, there is no observational study referring to the magnetospheric shielding effectiveness during SEP events.

The Moon is usually treated as an insulator with no global magnetosphere and ionosphere. As a consequence, most of the incident solar wind particles are directly absorbed on the sunlit lunar surface (Colburn et al. 1967; Lyon et al. 1967), which has been confirmed by many lunar wake observations (Halekas et al. 2011; Zhang et al. 2014; Xu et al. 2015a, 2015b). However, the shadowing of SEPs by the Moon is not as obvious as the solar wind low energy particles because the SEP flux can occur from all directions after the beginning of an SEP event, when particles are primarily incident along the direction of the interplanetary magnetic field (Reames et al. 1996). The lunar shadowing of energetic particles has previously been studied (Lin 1968; Van Allen & Ness 1969). Lin (1968) found that the shielding effect is dependent on the gyroradii and the scattering degrees of the SEPs at 1 au. As a result, greater than 50% of energetic electrons can be shielded by the Moon due to their much smaller gyroradii than lunar radius (R_L) and highly field-aligned pitch angle distributions. In contrast, only a very small fraction, which depends on the distance from the Moon to the observing spacecraft, of energetic protons with energies >15 Mev are blocked by the Moon because of their much greater gyroradii and isotropic pitch angle distributions.

It seems that the energetic particles of very high energies (>10 Mev) can hardly be shielded by either the magnetosphere or the Moon. The energetic particle environment of the lunar nearside for such particles may show little difference from that of the farside. However, the shielding of energetic particles of relatively lower energies but with higher fluxes have not been investigated yet. In this paper, we try to study the magnetospheric and the Moon's protections of the lunar nearside from the energetic protons with relatively lower energies during SEP events.

The ARTEMIS mission (Angelopoulos 2011), which is the extension of the THEMIS mission (Angelopoulos 2008), consists of two identical probes, P1 (THEMIS-B) and P2 (THEMIS-C). Identical instruments are on board the two probes. In this paper, we used magnetic field and plasma measurements from the FluxGate Magnetometer (FGM; Auster et al. 2008) and the ElectroStatic Analyzer (ESA; McFadden et al. 2008). The energetic particle data are from the Solid State Telescope (SST; Angelopoulos 2008). The SST measures ions with energies from 25 keV to 6 Mev and electrons with energies from 25 keV to 1 Mev. The energy range is relatively low in comparison with previous studies as mentioned above. It is important to point out that one should be careful when using the ARTEMIS/SST data. There are several problems associated with the SST data as follows. First, sunlight contamination sometimes occurs in the data. Fortunately, the THEMIS/ARTEMIS team has provided a toolkit to deal with this problem. Usually, the "automatic" method in the calibration toolkit is efficient enough to remove the sunlight contamination. Second, secondary particles (both protons and electrons) with lower energies can be produced when very high energy particles enter into the instrument. Third, the moonlight contamination can occur in the SST data when the ARTEMIS probe's trajectory is close to the lunar dayside. So far, there is no way to remove the moonlight contamination. Finally, cross-contamination between electrons and ions could also happen when the energies of electrons and ions are very high. These problems will be further discussed in our data analysis.

The SEP events used in our analysis are identified by the GOESspacecraft at Geosynchronous orbit by Bob Rutledge of the NOAA Space Weather Prediction Center (https://umbra.nascom.nasa.gov/SEP/). Since the two ARTEMIS probes entered into lunar orbits in 2011 April (Angelopoulos 2011), we choose the SEP events that occured later than 2011 April. To make our analysis more clear, we here list the total 30 events in Table 1. Note that the proton fluxes in this table are integral 5 minute averages for energies >10 Mev, given in Particle Flux Units (pfu): 1 pfu = 1 cm⁻² sr⁻¹ s⁻¹. Measurements from Wind 3DP/SST (Lin et al. 1995) are also shown in the case study since we need to know the characteristics of the SEP events in the solar wind.

Figure 1 shows the Moon's spatial distribution with respect to the Earth's magnetosphere in the Geocentric Solar Ecliptic (GSE) coordinates at the beginnings of the SEP events as shown in the second column of Table 1. Note that the time shift between the Geosynchronous orbit and lunar orbit for each SEP event is less than 30 minutes for a typical 400 km s⁻¹ plasma flow, which is negligible to the period (27.3 days) of the Moon orbiting the Earth. Therefore, it is convenient but acceptable for us to directly use the beginning time in Table 1 to investigate the spatial distribution of the Moon's positions with respect to the geomagnetosphere. The numbers in circles are the same as the event numbers in Table 1. The numbers in red indicate very strong SEP events with proton (>10 Mev) fluxes greater than 1000 pfu. We can see that the Moon is in the solar wind during most of the SEP events. During 8 out of 30 events (26.7%), the Moon is probably protected by the Earth's magnetosphere or sheath. When discussing the magnetic shielding of the magnetosphere, we believe the bow shock should be the boundary because the magnetic field has already been strengthened in the magnetosheath.

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3.1. Magnetospheric Shielding

The ARTEMIS probes observed two strong SEP events (No. 7 and 21 in Table 1) at the lunar orbit in the Earth's magnetosphere. The flux of protons (>10 Mev) was so high in Event 7 that the background signal produced by secondary particles completely contaminated the measurements of the ESA and SST measurements. The fluxes throughout the entire energy channels of ARTEMIS/SST, which turned out to be homogeneously independent of energy, is not suitable for our investigation. Fortunately, the observations of Event 21 are good enough.

Figure 2 shows selected measurements of Event 21: Wind measurements of the whole SEP event from 2013 May 22 to May 26 and ARTEMIS P1 observations during 2013 May 23–24 when the fluxes of the energetic particles are highest. The magnetic field (Figure 2(b)), plasma bulk speed (Figure 2(c)), and ion density (Figure 2(d)) demonstrate that ARTEMIS P1 was in the plasma lobe of the magnetosphere at the observing time. In the solar wind, as shown in Figure 2(a), the fluxes of highest energy portions (4.44 and 6.75 Mev) detected by Wind increased by more than two orders during 2013 May 23. Meanwhile, a strong secondary particle effect was clearly recorded by the energy-independent high flux background in Figure 2(e), which is an indication of energetic ions with very high energies hitting the instrument. The SST measurements are almost filled with secondary particle signals, suggesting that protons with energies throughout the SST observing channels are mostly shielded by the magnetosphere.

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However, we can still figure out that protons of about 4 Mev have a strong enhancement flux with respect to the background signal throughout May 23, including when P2 was on top of the lunar nearside. This can also be seen from the line plots in Figure 2(f): despite of a background flux of secondary particles of $\sim 3\times {10}^{3}\,\mathrm{eV}/({\mathrm{cm}}^{2}\,{\rm{s}}\,\mathrm{sr}\,\mathrm{eV})$, the flux of 4 Mev particles is much higher than this background flux on May 23. The gray box shown in Figure 2(g) indicates the lunar shadow crossing. There is no observing difference between fluxes of 4 Mev protons at dayside and nightside, suggesting that the entering SEPs of 4 Mev have the same influence on lunar nearside and farside in the magnetosphere. However, this result cannot be seen during May 24 though the fluxes of 4.44 and 6.75 Mev protons observed by Wind were still much higher than quiet time. The much enhanced flux of 4 Mev ions only seen on May 23 can also be found from measurements of ARTEMIS P2 (not shown). Since the flux of very high energy electrons on May 24 is a little higher than that on May 23 (not shown), it is unlikely that the enhancement of 4 Mev ion flux results from the cross-contamination of electrons. Therefore, during this SEP event, we believe that energetic protons with energies of about 4 Mev could sometimes effectively get access to the lunar orbit in the Earth's magnetosphere. If so, this result does not agree with the suggestion that significant shielding of GeV particles by the magnetosphere is possible, especially on the lunar nearside (Winglee & Harnett 2007) and for storm conditions (Harnett 2010). However, energetic protons of lower energies up to a few 100 KeV from 18:00 to 21:00 on 2013 May 24. These protons were closely associated with the enhancements of low energy particles. They must be from the magnetosphere and could not be SEP penetrating protons.

In comparison, Figure 3 presents the overview of SEP Event 22, which is a relatively weak SEP event with a proton flux of only 14 pfu. Wind observations (Figure 3(a)) show that this event lasted about 14 hr from 18:00 UT June 23 to 08:00 UT 2013 June 24. The magnetic field, plasma bulk speed and ion density shown in Figures 3(b)–(d) also demonstrated that P2 was in the plasma lobe of the magnetosphere. About 2 hr later at 20:00 UT, P2 detected the SEP influence at the lunar orbit (Figure 3(e)). Background signals of secondary particles, though not as strong as in Event 21, were also recorded by P1. However, no measurements of particles with energy of ∼4 Mev has been seen. This result suggests that the magnetosphere has an effective shielding of ∼4 Mev protons in this event. Energetic particles with energies up to a few KeV were also detected by P1 even in quiet time without SEP as shown in Figure 3(e), which further demonstrates that these energetic particles were from the magnetosphere instead of directly from the SEP. They were also closely associated with the enhancements of low energy protons.

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3.2. The Moon's Shielding

The ARTEMIS with two identical probes has a great advantage to study the Moon's shielding of SEPs. We surveyed all of those SEP events in the solar wind expecting to find an SEP event in which one of the ARTEMIS probes is in the lunar shadow while the other is in the solar wind. Most events have one or more problems (such as both P1 and P2 being in the solar wind and the measurements of one or two probes not being valid or strongly affected by the moonlight) that need to be excluded. Fortunately, Event 3 is a good case for our purpose. Figure 4 shows the Wind observations of the whole SEP event from 2011 September 23 to about September 29. The fluxes of energetic protons were highest from 12:00 UT September 25 to 00:00 on September 27.

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During 11:30–12:30 UT on September 26, ARTEMIS P1 crossed the lunar shadow while P2 stayed in the solar wind. Figure 5 shows the trajectories of ARTEMIS P1 and P2 in the Selenocentric Solar Ecliptic (SSE) coordinates. The SSE coordinate system has its X axis along the instantaneous Sun–Moon line, positive toward the Sun. The Z axis is parallel to the upward normal to the Earth's ecliptic plane, and Y completes the right-handed set. The SSE coordinate system is very similar to the GSE system except for the origin. From their trajectories, we can see that P1 was much closer to the lunar surface than P2, totally in the lunar shadow. It is estimated that the average distances from the lunar center to P1 and P2 to be 1.5 and 5.5 R_L, respectively. The pink dashed lines indicate the boundaries of the lunar shadow.

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Figure 6 shows the observations of the time interval when P1 crossed the lunar shadow. Left panels of P1 observations clearly show that it crossed the lunar shadow. A lunar wake structure was detected. In the lunar wake, most of the low-energy particles were absent (Figure 6(g)). Plasma density decreases to nearly zero (Figure 6(e)). These characteristics are the same as those of lunar wake in quiet solar wind or magnetosheath. Meanwhile, P2 was in the background solar wind. The measurements of magnetic field allow us to know better how magnetic field affects the lunar shielding of energetic particles. Although Lin (1968) observed the lunar shielding of energetic electrons, they could not confirm whether these electrons were magnetically shielded without the observations of magnetic field in their data. Figure 6(b) shows that during the lunar shadow crossing of P1, the background magnetic field stayed relatively steady. The three directions of the components in GSE coordinates remained unchanged. The average background magnetic field was [+5,−5,−5] nT in GSE during most of the time that P1 crossed the lunar shadow. The magnetic field lines are shown in Figure 5(a). We can see that P1 first entered into the lunar shadow and then went into the magnetic wake. Figure 6(f) presents the energetic proton observations of both ARTEMIS probes. In this event, secondary ions have also been recorded by the SST instrument. However, in order to show more clearly the lower energy portions of energetic particles, we removed the background secondary particles by only showing the fluxes above 10³ eV/(cm² s sr eV) as indicated by the color bar. Unlike the low-energy particles in Figure 6(g), no obvious difference of energetic particles between P1 and P2 can be seen.

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The omnidirectional fluxes of every energy channel of P1 and P2 are shown in Figure 7. Although the two instruments on board P1 and P2 are identical, the energy channels are slightly different: every energy channel of P2 is higher than that of P1 by 4000 eV. As a result, due to the power law, the flux in every energy channel of energetic particles by P1 should be higher than those by P2 correspondingly if the two probes observed the same SEPs. However, only fluxes of energetic particles with energies above 150 KeV by P1 are higher than those by P2. Fluxes of below 100 KeV energetic particles are lower. This result suggests that the Moon has no additional shielding on SEPs above 150 eV despite the fact that the distance from the Moon to P1 (1.5 R_L) is much shorter than that from Moon to P2 (5.5 R_L) and P1 was in the lunar shadow. The shielding effect of the moon for various energetic particles should depend on the size of their gyroradii (Lin 1968). Protons of 100 eV and 150 eV have radii of 2.6 and 3.2 R_L, respectively. Their trajectories cannot be treated as straight lines, which are complicated. In addition, the measurements have been strongly affected by instrumental secondary particles. Therefore, it is very hard for us to explain why the two critical energies are 100 and 150 KeV. Maybe numerical simulations can give a better understanding.

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It is important to point out that the shielding of the Moon from energetic protons ≤100 KeV began at 11:30 UT, which is prior to the P1 encounter with the lunar shadow (vertical pink lines). However, it seems that the shielding ended at different times for protons with different energies: earlier than the trailing lunar shadow boundary for 100 KeV protons and later for protons ≤74 KeV. We can see that the shielding effect of the Moon is strongest during the time interval bounded by the two vertical black lines in Figure 7. We believe that this region is the magnetic wake as shown in Figure 5(a). We calculated the power-law indexes at 12:00 UT for number flux measurements (in units of 1/(cm² s sr eV)) by P1 and P2 as well as that by Wind. We found that the power-law index is −2.05 for Wind, −1.95 for P2, and −1.75 for P1. The energetic proton spectrum by Wind is softest while the spectrum by P1, which is most shielded by the Moon, is hardest. Secondary particles emitted by very high energy particles can increase relatively more of the high energy portions of the energy range (37 KeV to 4 Mev in this event) measured by ARTEMIS. Therefore, the spectra by P1 and P2 are both harder than that by Wind. In addition, low energy portions (≤100 KeV) of energetic particles have been strongly shielded by the Moon. As a result, the spectrum by P1 is even harder than that by P2. Unfortunately, because of the ARTEMIS instrumental problem, we cannot do any quantitative analysis.

We studied the shielding of the Earth's magentosphere and the Moon from SEPs using ARTEMIS SST data. Despite of the instrumental problem, we can still qualitatively obtain some new results, which could be of interest to manned lunar exploration.

When the Moon is in the Earth's magneosphere, protons with energies ≤4 MeV can be shielded efficiently. We found that 4 Mev protons got access to the lunar orbit in the magnetosphere in only one 1 of 8 events. Note that this result is obtained under storm conditions. Although it is suggested by Huang et al. (2009) that Earth's magnetosphere does not provide any substantial magnetic shielding from protons of energy greater than 1 Mev at the Moon's orbit, our result strongly suggests that ≤1 Mev protons can be shielded very efficiently for storm conditions and 4 Mev protons can be shielded for most of storm time. Meanwhile, 4 Mev protons could probably get access to the Moon's orbit in the magnetosphere. If so, 4 Mev is probably near the lower limit of the accessible energy range. Furthermore, the entering SEPs of 4 Mev have no different influence on the lunar nearside and farside in the magnetosphere. This is not consistent with the simulations, which suggest that up to GeV protons can be effectively shielded by the magnetosphere especially at the lunar nearside (Winglee & Harnett 2007; Harnett 2010). However, protons with energies up to a few 100 KeV from the magnetosphere can often get access to the Moon's orbit even at quiet time.

When the Moon is in the solar wind, the lunar nearside is back to the Sun for most of the time. The lunar body can provide shielding from lower energy portions of SEPs. The measurements in our case at a distance of about 1.5 R_L away from the lunar center indicate that SEPs with energies above 150 KeV are probably isotropic in pitch angle distribution at the lunar orbit. Then, the lunar body cannot provide any effective protection for such SEPs. Therefore, the lunar nearside shows no difference from the lunar farside in receiving SEPs above 150 KeV. However, probably due to the relatively small gyroradii (with respect to 1 R_L) and highly anisotropic pitch angle distribution, SEPs below 100 KeV can be partially shielded by the Moon. These two critical energies (100 and 150 KeV) may be related to the distance of the observing spacecraft from the Moon. However, unfortunately, no quantitative result of how much fluxes of lower energy portions being shielded can be derived because of the data problems.

Another important source of energetic particles is the Earth's bow shock. These energetic particles can move upwards into the interplanetary space along magnetic field lines, which has been demonstrated by a lot of previous studies (Lin et al. 1974; Gosling et al. 1978). Although these particles have much lower energies than those SEPs, they have higher fluxes and more frequent occurrences. And importantly, energetic particles from the bow shock can only have influences on the lunar nearside. A following statistical study on energetic particles from the Earth's bow shock reaching the Moon's orbit will be carried out.

We acknowledge NASA contract NAS5-02099 for use of data from the THEMIS Mission. Specifically, we thank C. W. Carlson and J. P. McFadden for use of ESA, K. H. Glassmeier, U. Auster, and W. Baumjohann for use of FGM data and D. Larson and R. P. Lin for use of SST data. We also thank Wind teams and NASA CDAWeb for providing the data. This work is supported by the Science and Technology Development Fund of Macao SAR (008/2016/A1 and 039/2013/A2), National Natural Science Foundation of China (NSFC) under grants 41564007 and 41374174.