1 Introduction

The Moon is known to possess a surface bound exosphere (Stern, 1999, and references therein, Grava et al., 2021). The lunar exosphere is known to be produced by a variety of processes such as thermal desorption, photon-stimulated desorption, solar wind ion sputtering as well as meteorite impact vapourization and the loss processes include photoionization, Jeans escape and sticking to the surface (Bhardwaj et al., 2005; Prem et al., 2019; Stern, 1999; Wurz et al., 2022). The variability in the sources are expected to modulate the exosphere composition, and hence the number densities of the constituents. Observations and modeling studies suggest the significant role of solar wind in contributing the neutral exosphere (Hurley et al., 2016) and the ionisation of the species in the exosphere (Poppe et al., 2016). Understanding of these processes and the variability of the exosphere are still evolving. Transient solar events, such as coronal mass ejections (CMEs), can also contribute to the bulk of the exosphere through enhanced sputtering. Killen et al. (2012) have modeled the response of the lunar exosphere to a coronal mass ejection, which suggested an increase in the exospheric neutral number densities by more than an order of magnitude due to the increase in the solar wind ion sputter yield. However, observations of the lunar exosphere during such events were lacking.

The Moon encountered an intense space weather event (NOAA G5 class) on 10 May 2024. Chandra's Atmospheric Composition Explorer-2 (CHACE-2), which is a neutral gas mass spectrometer onboard the Chandrayaan-2 orbiter, was operating during this period. CHACE-2 could make in situ observations of the lunar exosphere during this event. The response of the lunar exosphere as revealed from these first observations are reported here.

2 Instrumentation and Data Sources

CHACE-2, a quadrupole based neutral gas mass spectrometer, is one of the experiments on the Chandrayaan-2 orbiter and aimed at making in situ measurements of the lunar exosphere. The sensor probe consists of an ionizer, quadrupole mass filter, and detector. The neutral atoms and molecules of the medium enter the ionizer section through the aperture and are ionized by the electron impact ionization technique, where the electrons are emitted by a hot filament and accelerated to nearly 70 eV. The ions thus produced are guided into the quadrupole mass analyzer section, where the mass analysis is done by applying suitable radio frequency (RF) and direct current (DC) voltages across the quadrupole rods, and the ions eventually fall on the detector assembly, which consists of a Faraday cup and a channel electron multiplier (CEM). The detector current provides the partial pressure as a function of mass per charge (m/q). In the default mode of operation, the time taken to acquire the mass spectra over the range of 1–100 amu/q is around 38 s. The sensor has a built-in Bayard-Alpert (B/A) gauge as part of the ionizer assembly that independently provides the total pressure. The total pressure data have the same cadence as the mass spectra (one value in 38 s). In this work, we have extensively used the total pressure measurements from the B/A gauge. More details of the instrument can be found in Das et al. (2020), Dhanya et al. (2021). CHACE-2 is a sequel to the CHACE experiment on the Moon Impact Probe (MIP) of the Chandrayaan-1 mission (Das et al., 2016, 2017; Sridharan et al., 2010).

During the observations reported in this work, CHACE-2 was continuously operating for a duration of about 4 hr per Earth day. The duration corresponds to two orbits of Chandrayaan-2. During these orbits, CHACE-2 had observations on the dayside as well as on the nightside of the Moon since the orbital plane of Chandrayaan-2 was at an angle of about 45$${}^{\circ}$$ with respect to the day-night terminator plane. The altitude of Chandrayaan-2 was varying between 60 and 110 km in these orbits and the dayside observations happened around 100–110 km altitude. The methodology to obtain the exospheric number density from the observed partial pressure has been described in Dhanya et al. (2021), while reporting the global distribution of argon-40 in the lunar exosphere. Also, the number densities derived for Argon-40 from CHACE-2 have been normalized with a factor of 500 to match with the LACE/Apollo observations (Dhanya et al., 2021). The same approach has been adopted here to estimate the total number density in the exosphere from the observed total pressure (except that the total number densities are reported at the measured altitudes in this work without converting to surface values).

For the information on the prevailing solar wind plasma parameters and the interplanetary magnetic field (IMF), level-2 data from the Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM) and Magnetometer (MAG) instruments on the Advanced Composition Explorer (ACE) satellite and the data from the 3-D Plasma Analyzer (3DP) from WIND satellite, both located around the first Lagrangian point (L1) of the Sun-Earth system, have been used.

3 Observation

The solar wind parameters during 09–12 May 2024, observed around the Sun-Earth L1 point, are shown in Figure 1. The increase in the solar wind proton density ($${\ge} $$60 $${\text{cm}}^{-3}$$) observed on 10 May 2024 around 16:36 UT (Figure 1a) is associated with a strong coronal mass ejection from the Sun that resulted in an intense geomagnetic storm (NOAA G5 class). This is also marked by an increase in the solar wind speed (Figure 1b), and the solar wind proton flux (Figure 1c) that is calculated from the observed number density and speed. The IMF strength also increases and the components show significant variation such that the z-component ($${\mathrm{B}}_{z}$$) changes polarity (turning southward) (Figure 1d). Also, the alpha-proton ratio in the solar wind has increased (Figure 1e). During this period, the Moon is located on the upstream of Earth's bow shock (Figure 1f) and hence encounters the solar wind plasma stream unobstructed.

A typical variation of the total pressure observed by CHACE-2 around the Moon for two orbits of Chandrayaan-2 (total duration of about 4 hr) on 09 May 2024 along with the ancillary parameters, are shown in Figures 2a–2d. The selenographic latitude and longitude during the observations are shown in Figure 2b, and the solar zenith angle as well as the local solar time in Figure 2c. CHACE-2 was powered on at 14:52:52 UT, near the night-side equator of the Moon, and the filament was switched on around 3 min later (at 14:55:58 UT). From Figure 2a, it can be seen that during the nightside of the Moon (till around 15:12 UT) when the local solar time was 03 hr (panel c), the total pressure observed by CHACE-2 was $${< } 1{0}^{-9}$$ mbar. Subsequently, the spacecraft crossed the north pole (90$${}^{\circ}$$ latitude from panel b) and entered the dayside at around 15:20 UT (local solar time changed close to 15 hr (3 p.m.)), which was marked by an increase in the total pressure to a value of about 3.9 $${\times} 1{0}^{-8}$$ mbar. From 15:30 UT to 16:12 UT, when the spacecraft traverses the latitude range of $$\pm 60{}^{\circ}$$ (Figure 2b) on the dayside, the observed total pressure was nearly steady. Around 16:20 UT, the spacecraft crosses the south pole and enters the nightside, where the total pressure drops to $${< } 1{0}^{-9}$$ mbar. Similar variations in the total pressure were seen in the subsequent orbit (around 16.8 hr UT to 18.8 hr UT), as well. During these observations, Chandrayaan-2 was passing over the longitude of 200$${}^{\circ}$$ on the dayside and around 30$${}^{\circ}$$ on the nightside. The solar wind proton flux was in the range of (1–3)$${\times} 1{0}^{-8}$$ $${\text{cm}}^{-2}$$ $${\mathrm{s}}^{-1}$$.

On the CME arrival day (10 May 2024), the total pressure recorded by CHACE-2 for a duration of nearly 4 hr, starting from around 16:12 UT to 20:03 UT, along with the ancillary parameters are shown in Figures 2e–2h. It can be seen from Figures 2f and 2g, that the selenographic longitude, LST, and SZA were similar to that on 09 May 2024. However, the total pressure on 10 May (Figure 2e) clearly shows an increase around 17 hr UT (near the dayside equator), compared to that on 09 May (Figure 2a). The total pressure observed at 16:59:01 UT is about 9.73 $${\times} 1{0}^{-9}$$ mbar, which increases to 1.26 $${\times} 1{0}^{-8}$$ mbar at 16:59:36 UT, and a value of 1.57 $${\times} 1{0}^{-8}$$ mbar is recorded at 17:00:14 UT. It can be seen from Figure 2h that between 16.5 and 16.6 hr UT (16:30–16:35 UT), the solar wind proton flux increases from about 3 $${\times} 1{0}^{-8}$$ $${\text{cm}}^{-2}$$ $${\mathrm{s}}^{-1}$$ to $${\ge} 3\times 1{0}^{-9}$$ $${\text{cm}}^{-2}$$ $${\mathrm{s}}^{-1}$$, which is almost an order of magnitude. On this day, the average position of the Moon in the GSE coordinate system was around (50.4, 30.3, 4.9)$${\mathrm{R}}_{E}$$ and that of ACE was (223.8, 2.9, 22.4)$${\mathrm{R}}_{E}$$, where $${\mathrm{R}}_{E}$$ is the radius of Earth. Considering these locations and the solar wind speed of 700 km $${\mathrm{s}}^{-1}$$, the enhanced proton flux observed by the ACE (around the L1 point) between 16:30–16:35 UT (Figure 2h) would reach the Moon about 25 min later (at around 17:00 UT). So, the increase in total pressure observed by CHACE-2 at 16:59 UT in the lunar exosphere (at an altitude of 110 km) could be associated with the impact of the CME on the Moon.

The enhancement in the total pressure is more evident in Figure 3a, where the time series of the total pressure curve observed on each of the days from 07 May to 13 May 2024 are shown together. The duration of each of these observations is 4 hr. The figure clearly shows the higher total pressure observed on 10 May 2024 compared to all other days. The increase has started around the dayside equator, about 45 min from the start of observation, and is seen continuously throughout the 4 hr of operation, except for the deep nightside where the value drops to 2.63 $${\times} 1{0}^{-10}$$ mbar. This effect is not seen on the subsequent day (11 May 2024), where CHACE-2 was operated from 15:27 UT to 19:23 UT. It should be noted that the altitudes of observation during the 4 hr of observation on these days were similar (Figure 3b). The altitude variation for 10 and 11 May was within 1 km, and $${\le} $$5 km with respect to 09 May 2024. The altitude at which the total pressure increase began on 10 May was around 110 km.

The analysis of the mass spectra to understand the mass bins that would have contributed to the observed increase in the total pressure yielded no unambiguous detection of the peaks above the noise. It is to be noted that the total pressure is from an independent measurement using B/A gauge in the ionizer section of CHACE-2, whereas the mass spectra (partial pressure) are recorded after the ions pass through quadrupole mass filter and reach the detector. There could be various losses in this process, which may have affected the signal strength making it too feeble to be detected above the noise level. Hence, in this work, we emphasize the relative enhancement in the total pressure compared to the pre and post CME impact days.

The possibility of any spacecraft operations that could have resulted in the observation of enhanced total pressure (due to degassing) was explored with the mission operations team. It was confirmed that there were no additional or special operations performed on the spacecraft during the observations on 10 May 2024, compared to other days. The observation geometries, such as SZA, LST, latitude, longitude, altitude, roll angle (angle between spacecraft velocity vector and normal to the sensor aperture area), and the position of the Moon, are similar for a few days before and after 10 May. Hence, except for the solar wind, the conditions under which the exospheric observations have happened during the pre CME and post CME days are similar. This is indicative that the observed total pressure enhancement is purely of exospheric origin and could be associated with the impact of the CME on the Moon.

4 Discussion

The total number densities in the exosphere are computed from the observed total pressure values ($${\mathrm{P}}_{t}$$) using the methodology described in Dhanya et al. (2013). Regarding the temperature required for the calculation, as described in Dhanya et al. (2013), the lunar surface temperature as a function of solar zenith angle computed using the functional form provided in Hurley et al. (2015), based on the Diviner observations are used. It is to be noted that the total pressure has contributions from all the species (masses), which could be of lunar or non-lunar origin. The background contributions have to be subtracted from the total pressure before quantifying the number densities on 10 May 2024. Considering the fact that the background total pressure are similar in magnitude on pre-CME and post CME days (09 and 11 May), the total pressure observed on 11 May is used as a reference. Hence, the total pressure observed on 10 May are subtracted from the values observed on 11 May, to get the enhancement in the total pressure associated with the CME event. The total pressure values thus obtained are used to compute the number densities. Further, the derived number densities are corrected for the ram enhancement, which is the effective increase in the observed pressure (number density) due to the spacecraft velocity. The correction factor involves the roll angle, and the thermal velocity of the species. Since the CME impact is expected to enhance the solar wind ion sputtering of the lunar surface that contribute to the lunar exosphere, the species that are likely produced from sputtering are considered for this. Solar wind ion sputtering is expected to be the source of refractory elements, such as Si (28 amu), Ti (48 amu), Al (27 amu), Fe (56 amu), Mg (24 amu), Ca (40 amu), O (16 amu), Na (22 amu), and K (39 amu) in the lunar exosphere (Killen et al., 2012; Vorburger et al., 2014; Wurz et al., 2007). Assuming that these species would have contributed to the enhancement in the observed total pressure, the thermal velocity was computed (sqrt(8 KT/$$\pi $$m)) using an average mass of 30 amu. This approach is indented to provide only an order of magnitude estimate of the number densities.

On 10 May 2024, the total pressure value observed just prior to the enhancement is about $$9.698\times 1{0}^{-9}$$ mbar (at 47.5 min from the start of observation), as indicated in Figure 3c. The first instant where the total pressure showed increase is about 30 s later and the value is $$1.262\times 1{0}^{-8}$$ mbar. The subsequent total pressure values are $$1.584\times 1{0}^{-8}$$ mbar at 48.6 min, $$1.595\times 1{0}^{-8}$$ mbar at 49.25 min, $$1.6\times 1{0}^{-8}$$ mbar at 49.9 min, which is the maximum. These values are subtracted from the values recorded on 11 May 2024 and the total pressure thus obtained are $$3.73\times 1{0}^{-9}$$ mbar at 48 min, $$7.56\times 1{0}^{-9}$$ mbar at 48.6 min, $$7.58\times 1{0}^{-9}$$ mbar at 49.25 min, and $$8.07\times 1{0}^{-9}$$ mbar at 49.9 min. The corresponding number densities estimated are $$7.01\times 1{0}^{4}$$ $${\text{cm}}^{-3}$$, $$1.47\times 1{0}^{5}$$ $${\text{cm}}^{-3}$$, $$1.53\times 1{0}^{5}$$ $${\text{cm}}^{-3}$$, $$1.71\times 1{0}^{5}$$ $${\text{cm}}^{-3}$$. These enhancements are about a factor of 2 in magnitude. Further, the background subtracted total pressure value for the time instant just prior to the enhancement (at 47.5 min) is $$6.33\times 1{0}^{-10}$$ mbar and the corresponding number density is $$1.14\times 1{0}^{4}$$ $${\text{cm}}^{-3}$$. This means that the background is nearly same for both the days. Comparison of the background removed number densities for the time instant just prior to the enhancement ($$1.14\times 1{0}^{4}$$ $${\text{cm}}^{-3}$$) with that of the first enhancement ($$1.47\times 1{0}^{5}$$ $${\text{cm}}^{-3}$$), indicate that the number density increases by at least by an order of magnitude during the observation on 10 May 2024. Even if there could be some uncertainties in the magnitudes (absolute values), the relative change is reliable.

Wurz et al. (2007) have modeled the exosphere densities due to sputtering of the lunar surface for different soil types (highland, KREEP, low-Ti, and high-Ti mare regions), solar wind speeds and proton flux (450 km $${\mathrm{s}}^{-1}$$ and $$2.0\times 1{0}^{8}$$ $${\text{cm}}^{-2}$$ $${\mathrm{s}}^{-1}$$; 530 km $${\mathrm{s}}^{-1}$$ and $$2.65\times 1{0}^{8}$$ $${\text{cm}}^{-2}$$ $${\mathrm{s}}^{-1}$$; 670 km $${\mathrm{s}}^{-1}$$ and $$2.68\times 1{0}^{8}$$ $${\text{cm}}^{-2}$$ $${\mathrm{s}}^{-1}$$). The total number densities obtained from the sum of all calculated elements (O, Na, Mg, Al, S, Si, K, Ca,Ti, Cr, Mn, Fe and Ba) was found to be $${\sim} 1.0\times 1{0}^{7}$$ $${\mathrm{m}}^{-3}$$ (=10 $${\text{cm}}^{-3}$$) at the surface, which was the highest for the solar wind speed of 670 km $${\mathrm{s}}^{-1}$$ and proton flux of $$2.68\times 1{0}^{6}$$ $${\text{cm}}^{-2}$$ $${\mathrm{s}}^{-1}$$. It is to be noted that the flux of sputtered species (and hence the density) is proportional to the flux of the solar wind ions incident on the lunar surface. During the CME event on 10 May 2024, the proton flux has increased by more than an order of magnitude ($${ >} $$$${10}^{9}$$ $${\text{cm}}^{-2}$$ $${\mathrm{s}}^{-1}$$) (Figure 1c), which could have resulted in the enhancement of the sputtered density also by an order of magnitude. Hence, the total number density due to sputtering could have reached above 100 $${\text{cm}}^{-3}$$. Further, as the sputtered atoms are more energetic compared to the thermal release process, resulting in larger scale heights, their densities even at an altitude of 100 km were found to be nearly the same as the surface density values (Figure 4 of Wurz et al. (2007)). Hence, it could be possible to observe such higher exospheric densities during the CME events even at an altitude of 100 km by CHACE-2/Chandrayaan-2. One more important factor to consider is the alpha-proton ratio ($${\mathrm{n}}_{H{e}^{++}}$$/$${\mathrm{n}}_{{H}^{+}}$$) in the solar wind. The $${\text{He}}^{++}$$ ions in the solar wind are known to be a more efficient sputtering agent due to the higher mass and hence the higher momentum imparted during collision with the elements present in the lunar regolith. It is known that 5% alpha particles typically contribute 30% to the total sputter yield (Wurz et al., 2007). The observations from 3DP/WIND have showed an increase in the alpha-proton ratio to $${\ge} $$0.1 after 16:35 UT compared to the value of 0.01–0.04 before (Figure 1e). This increase in the alpha-proton ratio by an order of magnitude would have further enhanced the total number density in the lunar exosphere due to sputtering. These processes could be the source of the increase in the total pressure (hence the density) observed by CHACE-2.

Killen et al. (2012) have modeled the effects of an interplanetary coronal mass ejection (ICME) for the species such as Na, K, Mg, and Ca in the lunar exosphere, for different solar wind types and alpha-proton ratios. The solar wind types included fast wind (speed of 450 km $${\mathrm{s}}^{-1}$$, density of 5 $${\text{cm}}^{-3}$$, and alpha proton ratio of 0.02), slow wind (speed of 450 km $${\mathrm{s}}^{-1}$$ with an alpha proton ratio of 0.04), CME shock plasma (speed of 600 km $${\mathrm{s}}^{-1}$$ with an alpha proton ratio of 0.001), magnetic bubble or early CME period (speed of 650 km $${\mathrm{s}}^{-1}$$ with an alpha proton ratio of 0.1), and driver gas or late CME period (speed of 500 km $${\mathrm{s}}^{-1}$$ with an alpha proton ratio of 0.3). The species Na, K were chosen as example for volatile elements and also detected in the lunar exosphere from ground based observations, whereas Ca and Mg were example for refractory species that are expected at the Moon (observed in Mercury's exosphere). During CHACE-2 observations, the proton density as well as velocity were close to CME driver gas, and the alpha-proton ratio were close to that of magnetic bubble. So, solar plasma wind type could be between that of a magnetic bubble and CME driver gas. The simulations have shown that due to the CME, there is an enhancement in the total sputter yield such that the number densities in the lunar exosphere of sputtered species can increase by a factor $${ >} $$10, depending on the species. This agrees with the CHACE-2 observation of increase in total number density by more than an order of magnitude. Also, a rapid decrease is seen in the number densities when the CME has passed. This may explain the CHACE-2 observation of higher total pressure for both the orbits on 10 May 2024 when the solar wind flux was still high and the absence of any such effects on 11 May 2024 (post CME period), where the values are similar to that on 09 May 2024 (pre CME orbit).

Further, CHACE-2 observations have shown an increase in the total pressure (number density) even beyond the terminator on 10 May, except on the deep nightside (Figure 3). Studies have shown that the solar wind protons can enter the near lunar wake (crossing the terminator) by a variety of processes, which includes direct entry (Dhanya et al., 2013; Futaana et al., 2010), as well as after interaction with the dayside surface or lunar magnetic anomalies (Dhanya et al., 2016, 2018; Nishino, Fujimoto, et al., 2009; Nishino, Maezawa, et al., 2009). These protons are also found to generate the energetic neutral atoms (ENAs) from the lunar nightside surface due to the interaction with the lunar nightside surface (Vorburger et al., 2016). So, the signals observed by CHACE-2 could be associated with the entry of the solar wind to the nightside of the Moon resulting in the sputtering of the nightside surface. Observations have shown less proton population in the deep or central wake region (Dhanya et al., 2016). CHACE-2 observations also show almost no enhancement in the total pressure close to the midnight during both the orbits on 10 May 2024. Also, the modeling results Killen et al. (2012) have shown a global enhancement in the exospheric number densities during a CME event, for different species.

Thus, the observations from CHACE-2/Chandrayaan-2 have provided the first experimental evidence for the enhancement of lunar exospheric densities during a CME. Since such intense CME events are not observed very often and also may not be supported by concurrent observations around the Moon, these results provide vital input to advance the modeling efforts for the lunar exospheric densities during such strong events. The results are significant not only for the Moon, but also for similar objects with surface bound exospheres in our solar system, such as Mercury and asteroids as well as similar objects in the exoplanetary systems.

5 Conclusion

CHACE-2/Chandrayaan-2 has observed an increase in the total pressure in the lunar exosphere on 10 May 2024. The timings of these observations were found to match the expected arrival time of a CME at the Moon. The number densities derived from CHACE-2 observations show an enhancement in the total number density by more than an order of magnitude. Solar wind ion sputtering is known as a potential process that contributes to the lunar exosphere, and the sputtered flux is found to depend on the incident solar wind flux and also on the alpha proton ratio in the solar wind. During the CME event on 10 May 2024, the solar wind speed nearly doubled, and the proton flux as well as the alpha proton ratio has increased by more than an order of magnitude. Previous modeling studies on the effect of ICME on the lunar exosphere suggest an increase in the number density of selected species in the lunar exosphere by a factor $${ >} $$10, in agreement with the CHACE-2 observations. This is the first experimental evidence for the enhancement of lunar exospheric densities during a CME. The results are significant for the Moon as well as similar objects having surface bound exospheres in our solar system, such as Mercury and asteroids as well as similar objects in the exo-planetray systems.