The Moon encountered an extreme space weather event (NOAA G5 class) on 10 May 2024, caused by a series of coronal mass ejections (CMEs). Chandra's Atmospheric Composition Explorer-2 (CHACE-2), a neutral gas mass spectrometer on board Chandrayaan-2 orbiter, made in situ observations of the lunar exosphere during this period. Observations show an increase in total pressure around the arrival time of the CME impact on the Moon. The corresponding total number densities derived from these observations show an enhancement in the total number densities by more than an order of magnitude. The increase in lunar exospheric number densities by a factor > 10, due to the solar wind ion sputter process, is consistent with earlier theoretical modeling. This is the first observational confirmation of the enhancement in lunar exospheric densities during a CME impact.

This work reports the spatial and diurnal variations of the number densities of lunar molecular water (H2O), atomic mass unit (amu) 18 and hydroxyl (OH), amu 17 over low (0 degrees to 30 degrees), middle (31 degrees to 60 degrees) and high (61 degrees to 80 degrees) latitudinal regions of the lunar exosphere during the pre-sunrise, noon, sunset and midnight periods using the mass spectrometric data of CHandra's Atmospheric Composition Explorer-2 (CHACE-2) on board Chandrayaan-2, the second lunar mission developed in India. Both H2O and OH exhibit, particularly in the low latitude regions, a trend of increasing number density after the sunrise and up to noon, followed by a decrease till sunset. An overall higher density of H2O is obtained compared to the previous reports. The findings are justified in terms of the polar orbital height of the instrument and the duration of data procurement. The maximum number density for the low, middle and high latitudes reaches 5225 cm- 3, 5135 cm- 3 and 3747 cm- 3, respectively. The corresponding OH abundances are found to be 5079 cm-3, 5565 cm-3 and 5720 cm- 3. The diurnal variations of H2O and OH and their comparisons, similar to those of the present report may provide suitable means for tracing the lunar water cycle. The CHACE-2 observations imply that the influence of magnetotail passage on volatiles like water is to be further quantified in future missions with other sensors.

CHandra's Atmospheric Composition Explorer-2 (CHACE-2) is a neutral gas mass spectrometer aboard Chandrayaan-2 orbiter. CHACE-2 is a quadrupole based mass spectrometer which detects neutral atoms/molecules in the mass range of 1-300 amu. The data product from CHACE-2 observations provide the partial pressure for different masses that essentially constitute the mass spectra. CHACE-2 scans different masses using suitable voltages such that each mass is contributed by nine mass bins, known as samples. Each spectrum (mass along x -axis and partial pressure along y-axis) is constructed based on these 9 samples, where the fifth sample is expected to be at the center of the peaks. During the actual measurements in space, mass shifts have been observed such that the center of the peaks doesn't coincide with the expected mass bin, but rather shifted to either lower or higher mass bins. Also, the 9 samples that determines the peak shape need not follow the expected pattern. Suitable criteria have been arrived at in order to verify the quality of each spectrum. In view of the large data sets, an algorithm has been developed to determine and calibrate the mass shift, verify the quality of the spectrum based on the criteria and generate suitable flags in the output file. The algorithm is referred to as 'Peak Filter Algorithm'. The output of the algorithm has been validated and the output has been found to be matching with that expected. The details of the algorithm along with the validation results are presented in this paper. The output of the algorithm is significant for the scientific analysis of CHACE-2 data, and also useful for the analysis of data from instruments similar to that of CHACE-2 in future missions.

We report the first observation of Argon-40 (Ar-40) in the mid latitude regions (-60 degrees to +60 degrees) of the lunar exosphere from CHandra's Atmospheric Composition Explorer-2 (CHACE-2) experiment aboard Chandrayaan-2 orbiter. The number density of Ar-40 shows pre-sunrise, sunrise and sunset peaks as well as nightside minima, typical of a condensable gas, which is similar to the features seen at the low latitudes in previous observations. The CHACE-2 observed number densities of Ar-40 and its diurnal variation at low latitudes (-30 degrees to +30 degrees) is consistent with LACE/Apollo observations. CHACE-2 observations show Ar-40 enhancements over certain longitude sectors. In addition to KREEP region, Ar-40 bulges are observed at other longitudes, including the South Pole Aitken (SPA) terrain. The global distribution of Ar-40 shows that the sunrise peak is observed at the same local time over highlands and mare regions. These observations call for a deeper understanding of the surface-exosphere interactions and source distribution. Plain Language Summary The Moon is known to possess a tenuous atmosphere, known as surface bound exosphere. Lunar exosphere exists as a result of a dynamic equilibrium between several sources and sink processes. Noble gases serves as important tracers to understand such processes. Though, Argon-40 (Ar-40) is known to exist in lunar exosphere, the knowledge on its distribution at higher latitudes is lacking. For the first time, CHandra's Atmospheric Composition Explorer-2 (CHACE-2) experiment aboard Chandrayaan-2 orbiter has continuously observed Ar-40 in latitude range of -60 degrees to +60 degrees. It is found that the Ar-40 density variation with local solar time shows the behavior of a condensable gas, which is similar to that observed earlier at low latitudes. The distribution of Ar-40 shows spatial heterogeneity with localized enhancements over KREEP and South Pole Aitken terrain. This suggests that there may be other regions with lower activation energy as the source of Ar-40. The observed global distribution indicates that the interaction of Ar-40 with the surface are similar in low and mid latitude regions. The CHACE-2 observations hint at requirement for improvement in our understanding of the surface-exosphere interactions and source distributions of Ar-40. Key Points First observation of Argon-40 in the mid latitude exosphere of the Moon Observed nightside minimum and sunrise and sunset peaks in Ar-40 abundance is similar to that at low latitudes Enhanced Ar-40 number density is observed at few longitudes, including South Pole Aitken terrain, in addition to KREEP

The CHandra's Atmospheric Composition Explorer-2 (CHACE-2) experiment aboard Chandrayaan-2 orbiter will study in situ, the composition of the lunar neutral exosphere in the mass range 1-300 amu with mass resolution of 0.5 amu. It will address the spatial and temporal variations of the lunar exosphere, and examine water vapour as well as heavier species in it. In this article, results of the major characterization and calibration experiments of CHACE-2 are presented, with an outline of the qualification tests for both the payload and ground segment.