1 Introduction

The Lunar Reconnaissance Orbiter (LRO) was launched to establish a lunar return program for later missions to Mars and other destinations. The seven instruments in the science payload of the LRO include the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) (Tooley et al. 2010). The LRO orbiter collects information on the lunar radiation environment in preparation for future human missions to the Moon.

CRaTER (Spence et al. 2010) was designed to identify the global lunar radiation environment and its biological impact. CRaTER is a stacked detector-absorber cosmic-ray telescope system for future human exploration of the solar system. CRaTER examines the effects of galactic cosmic rays (GCRs), solar energetic protons, particularly above 10 MeV, and any secondary radiation at the lunar surface on tissue equivalent plastic (TEP) by measuring the linear energy transfer spectra in lunar and deep space radiation environments.

Mazur et al. (2011) introduced a compact and highly precise micro-dosimeter mounted on the analog electronics board of the CRaTER instrument. The system measures the dose rates below one micro-rad/sec in the lunar radiation environment. Although the micro-dosimeter was not designed for scientific purposes, it effectively monitors even relatively small changes in the GCR intensity. They suggested that the micro-dosimeter can explain the dose rate at energies above 10 MeV.

Using ionization chambers, Forbush (1937) and Hess and Demmelmair (1937) first observed the decreases in the secondary cosmic-ray (CR) count rate, known as the Forbush decrease (FD). Figure 1 shows an FD recorded at the terrestrial South Pole neutron monitor on 16 July 2017. The FD is a sudden decrease in GCR intensity observed on a ground-based neutron monitor. The FD is comprised of initial, main, and recovery phases. Hashim et al. (1972) introduced the term, the main phase, to represent the decreasing part in the GCR intensity time profile. The main phase occurs in less than half a day, whereas the recovery phase can last for several days. FDs are considered to be a blockage of GCR by intense interplanetary magnetic field (IMF) structures of the interplanetary (IP) shock sheath region and/or interplanetary coronal mass ejections (ICMEs) following the IP shocks (Parker 1961; Burlaga et al. 1981, 1982; Klein and Burlaga 1982; Zhang and Burlaga 1988; Lockwood et al. 1991; Badruddin 2002; Oh et al. 2008; Oh and Yi 2012).

Fig. 1
figure 1

Forbush decrease (FD) recorded at the terrestrial South Pole neutron monitor on 16 July 2017

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FDs can be observed by not only ground-based particle detectors but also satellites and interplanetary probes. Recently, Winslow et al. (2018) examined a FD driven by a single ICME on 12 February 2014 at different space positions, such as Mercury, Earth, and Mars, using the data of several space probes. Based on their results, they then suggested that the size of the FD decreases exponentially with increasing heliocentric distance.

Although terrestrial FDs have been studied by experimental observations, the characteristics of an FD in the lunar space environment have not been reported. To develop the study by Winslow et al. (2018), who reported the FD identified by space probes, this study examined the characteristics of FDs in the lunar space environment in association with the ICMEs that drive them. The FDs observed by CRaTER in the lunar space environment were compared with those recorded at the terrestrial South Pole neutron monitor. This paper reports the measurements of FDs in the lunar space environment.

2 Data and methods

The dataset from CRaTER consists of house-keeping data, primary science data, and secondary science data for Levels 0, 1, and 2. The Level-0 data were used only to renovate the other CRaTER data for archival purposes. The detector single count rate parameter of Level-2 secondary science data were provided by the CRaTER flight data of PPI (https://pds-ppi.igpp.ucla.edu/) and CRaTER Legacy Data Products (http://crater-products.sr.unh.edu/). CRaTER is comprised of six silicon detectors (D1–D6) in thin/thick pairs separated by sections of TEP. CRaTER produces a daily file with a one second resolution. The daily files report the number of ‘singles’ (silicon detector) as well as the number of ‘good’, ‘rejected’, and ‘total’ records observed by CRaTER during the monitoring period. Because ‘good’ counts are measured by the qualifying event for all count events, the hourly counter data of ‘good’ counts are used by converting the one second resolution data from June, 2009 to December, 2017. The data processing method was the same as that reported by Sohn et al. (2019). The hourly CR intensity is typically used to produce the FD profile. Because the LRO has an orbital period of 110 min, the hourly counts show oscillations with alternating highs and lows. Figure 2 in the reported by Sohn et al. (2019) compares the time resolution. Although they are averaged, the three-hour smoothed (running averaged) data have 24 intervals for each day. Because they have also been calculated by weighting, the value of each hour can maintain the original trend of the variation. The three-hour smoothed data showed a variation trend in counts because the oscillations from the orbital period are deleted. Therefore, the hourly count and dose rate data were smoothed using the three-hour weighting to determine the onset of an FD.

Fig. 2
figure 2

Comparison of the yearly GCR intensity by CRaTER and yearly sunspot number (2009–2017). The line with circles and line with triangles indicate the GCR intensity and sunspot number, respectively

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The three time profiles were drawn for CR intensity measured by CRaTER, dose rates by a micro-dosimeter within CRaTER, and CR intensity at the terrestrial South Pole neutron monitor. The associated CME lists were then selected from the SECCHI Coronagraphs onboard the STEREO satellite (http://cor1.gsfc.nasa.gov/catalog). The IMF data from the Magnetic Field Experiment (MAG) onboard Advanced Composition Explorer (ACE) were then compared (http://www.srl.caltech.edu/ACE).

This study examined the FDs with a minimum CR intensity at the main phase that is less than \(-2 \%\) of each GCR background in the lunar space environment. The background was defined as the average of two days before the onset of the FD. When the background was disturbed by an increasing event, we selected the quiet background before CRaTER enhancement (CRE) defined by the onset reported by Sohn et al. (2019). Thus, the CR intensity variations (%) of FDs in the lunar space environment were defined.

Figure 2 shows the yearly GCR intensity measured by CRaTER with the sunspot number from 2009 to 2017 as the long-term analysis of the CRaTER data. The line with circles and that with triangles present the GCR intensity and sunspot number, respectively. The GCR intensity in Fig. 2 indicates the background excluding the increases in CR events, such as CREs in the lunar space environment (Sohn et al. 2019). The GCR intensity in the lunar space environment was also anti-correlated with the sunspot number as an indicator of the solar activity. Figure 2 shows that the GCR intensity is affected by the solar magnetic field varying with the solar cycle because the GCR intensity is detected in space, not on the terrestrial ground. This figure shows the same results by Schwadron et al. (2012, 2014, 2018), which examined the anti-correlation between the sunspot number and dose rate using the CRaTER data.

Thirty-two FDs were identified from June, 2009 to December, 2017, in the lunar space environment. The origins and the characteristics of the FDs in the lunar space environment were examined by comparing the time profiles of CRaTER with those of the terrestrial South Pole neutron monitor and the solar wind parameters of ACE/MAG. The South Pole neutron monitor can detect neutrons produced by protons with all kinds of energy theoretically. Officially, the South Pole neutron monitor has the vertical cutoff rigidity of 0.1 GV (information of the station from NMDB, http://www01.nmdb.eu/station/sopo/). The cutoff rigidity of 0.1 GV is equal to the energy of 5.31 MeV. Therefore, this threshold of the South Pole neutron monitor can be comparable to that of CRaTER. To prove that the FDs observed by CRaTER are real signals, the time profile of the dose rate measured by a micro-dosimeter housed within CRaTER was also checked for any coincidence.

3 Results

Figure 3 gives an example of an FD on 18 February 2011, showing the time profiles of GCR intensity at the terrestrial South Pole neutron monitor (a), lunar GCR intensity observed by CRaTER, and lunar dose rate measured by micro-dosimeter within CRaTER (c). The time profiles of the lunar and terrestrial GCR intensity were similar and showed the typical FD, which has an initial phase, a sharp and rapid decrease in the main phase, and a gradual recovery phase. The time profile of the dose rate was similar to that of the GCR intensity. As mentioned above, the micro-dosimeter can explain the dose rate in energies above 10 MeV. The FD by CRaTER can be verified by the variation of the GCR intensity at the terrestrial South Pole neutron monitor and dose rates by CRaTER for coincidence. The minimum GCR intensity in Fig. 3 was \(-5\%\) on Earth, \(-12 \%\) in the lunar space environment, and the minimum dose rate was \(-20\%\) in the lunar space environment. The lunar FD magnitude was double the terrestrial value.

Fig. 3
figure 3

Forbush decrease (FD) on the 18th of February, 2011: time profiles (a) of the GCR intensity at the terrestrial South Pole neutron monitor, (b) of GCR intensity observed by CRaTER in the lunar space environment, and (c) of the dose rate measured by the micro-dosimeter within CRaTER

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Figure 4 shows the ICME profiles associated with FD in the lunar space environment on the 18th of February, 2011, using the IMF data measured by ACE/MAG. The figure shows the time profile of the GCR intensity observed by CRaTER (a), strength of IMF (|B|) (b), RTN latitude (c), and RTN longitude (d). The vertical lines represent the onset of IP shock (L1) and ICME (L2) that drove the lunar and terrestrial FDs. The strong IMF structure of the IP shock and ICME that reached the Earth and Moon on the 18th of February, 2011, formed a strong magnetic barrier to overtake and surround both. The FDs were then recorded almost simultaneously at the terrestrial South Pole neutron monitor and CRaTER.

Fig. 4
figure 4

ICME profiles associated with the Forbush decrease (FD) in the lunar space environment on the 18th of February, 2011: (a) time profile of the GCR intensity observed by CRaTER, (b) strength of IMF (|B|), (c) RTN latitude, and (d) RTN longitude. The vertical lines indicate the onset of IP shock (L1) and ICME (L2)

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Figure 5 presents the lunar positions of the 32 FDs in lunar orbit according to the in/out region of the Earth’s magnetosphere. Each number indicates the FD number in Table 1. Only six of these 32 FDs occur in the region of the Earth’s magnetosphere in lunar orbit due to the short length of the orbit in the Earth’s magnetosphere, as reported by Sohn et al. (2019). When these 32 FDs are separated into two groups based on being in or out of the Earth’s magnetosphere, they show a statistically insignificant difference. This suggests that the FDs in the lunar space environment may be not influenced by the Earth’s magnetosphere.

Fig. 5
figure 5

Lunar positions of 32 FDs in lunar orbit according to the in/out region of the Earth’s magnetosphere. Each number indicates the FD number in Table 1

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Table 1 List of 32 Forbush decreases (FDs) in the lunar environment during the period of June, 2009 – December, 2017
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Table 1 lists the 32 FDs in the lunar space environment from June, 2009 to December, 2017. The data consist of three parts for the associated ICME, and information on the lunar (CRaTER) and terrestrial (South Pole neutron monitor) FDs. The leftmost column contains the number of the event, year, onset of ICME, and maximum IMF strength of ICME from the IMF data of ACE/MAG. For lunar FD, the table displays the date of the background, averaged count during the background period at the baseline, onset time and minimum time (the end time of the main phase), minimum GCR intensity at the minimum time, duration of the main phase, and position. The position indicates whether the point is located in or out of the Earth’s magnetosphere in lunar orbit when each FD occurs. All data for terrestrial FD are displayed in the same manner as the lunar FD except for the position.

Figure 6 shows the correlation between the maximum IMF strength of the associated ICME and the minimum GCR intensity of the main phase at the terrestrial South Pole neutron monitor (a) and by CRaTER (b). The line in each panel represents the linear regression. For both the minimum lunar and the terrestrial GCR intensity of the main phase, the FD magnitude shows an increase in the negative value due to an increase in the maximum IMF strength of the associated ICME. The duration of enhanced IMF strength may also determine the FD magnitude. The lunar FD magnitude showed a slightly better correlation with the maximum IMF strength of ICME than the terrestrial value.

Fig. 6
figure 6

Comparison between the maximum IMF strength of the associated ICME and minimum GCR intensity of the main phase: (a) minimum GCR intensity of the main phase at the terrestrial South Pole neutron monitor, and (b) minimum GCR intensity of the main phase by CRaTER

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Figure 7 shows the correlation of the minimum GCR intensity between the lunar (CRaTER) and terrestrial (South Pole neutron monitor) main phases. Their FD magnitude showed a good correlation coefficient, but the lunar FD was double the terrestrial value. As mentioned above, the threshold of CRaTER is comparable to the cutoff rigidity of the South Pole neutron monitor. Although the threshold of CRaTER is slightly larger than that of the South Pole neutron monitor, the lunar FDs have a larger magnitude than the terrestrial FDs. The orbit of the LRO is free from the Earth’s magnetic field except for the lunar position in the magnetotail of the Earth’s magnetosphere. The ICME can form an effective and strong barrier for entering CR particles in the lunar space environment without any interactions because the lunar FDs are detected by CRaTER in free space not on the ground. Table 2 lists the averaged lunar and terrestrial properties of the FD as well as the associated ICME. During the analysis period, ICME associated with FDs has a mean IMF strength of \(\sim 20\) nT, which is strong. The duration of the main phase shows a similar length for both lunar and terrestrial FDs. On the other hand, the minimum GCR intensity of the main phase (the lunar FD magnitude) is double that of the terrestrial value. The difference in magnitude between the terrestrial and lunar FDs was examined statistically using a Student t-Test and Paired t-Test. At a significance level of 0.05, the magnitudes of the terrestrial and lunar FDs showed a significant difference between the two groups and between each FD event. This result is presented in the last row of Table 2 and agrees with the results in Fig. 7.

Fig. 7
figure 7

Comparison of the minimum GCR intensity of lunar (CRaTER) and terrestrial (South Pole neutron monitor) main phases

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Table 2 Averaged properties of the lunar and terrestrial FD, and the associated ICME
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4 Summary

The GCR decreasing events in the lunar space environment were examined using the CR data observed by CRaTER. Thirty-two FDs were found in the lunar space environment during the period of June, 2009 – December, 2017, by a one-to-one correspondence with FDs recorded at the terrestrial South Pole neutron monitor and the associated ICME. The lunar and terrestrial FDs were similar and showed a distinctive decreasing profile. The minimum GCR intensity of the main phase in the lunar space environment showed a correlation with the maximum IMF strength of the associated ICME that was double the terrestrial FD magnitude. ICME can form an effective and strong barrier for the entering CR particles because the lunar FDs are recorded by CRaTER in space not on the ground; hence, the lunar FDs can be strong. That is, the FDs in the lunar space environment are stronger and they are unaffected by the Earth’s magnetosphere. In addition, the background lunar GCR intensity is anti-correlated with the solar activity.

During the analysis period, FDs were less frequent than CREs. Sohn et al. (2019) reported 96 CREs during the same period. The lunar space environment allows CR particles to enter without any obstacle except for the existence of ICME. This can be expressed by the bombardment of energetic particles. Although ICME reaches the lunar space environment, the preceding solar proton event can attenuate the shielding by IMF. Hence, weak FDs can be buried by CRE in the lunar space environment. These study results highlight the need for careful consideration of the space environment when planning future space missions.