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Due to the high absorption of solar wind plasma on the lunar dayside, a largescale wake structure is formed downstream of the Moon. However, recent in-situobservations have revealed the presence of protons in the near-lunar wake (100km to 200 km from the surface). The solar wind, either directly or afterinteraction with the lunar surface (including magnetic anomalies), is thesource of these protons in the near-wake region. Using the entire data from theSWIM sensor of the SARA experiment onboard Chandrayaan-1, we analysed thevelocity distribution of the protons observed in the near-lunar wake. Theaverage velocity distribution functions, computed in the solar wind rest frame,were further separated based on the angle between the upstream solar windvelocity and the IMF. Several proton populations were identified from thevelocity distribution and their possible entry mechanism were inferred based onthe characteristics of the velocity distribution. These entry mechanismsinclude (i) diffusion of solar wind protons into the wake along IMF, (ii) thesolar wind protons with finite gyro-radii that are aided by the wake boundaryelectric field, (iii) solar wind protons with gyro-radii larger than lunarradii from the tail of the solar wind velocity distribution, and (iv)scattering of solar wind protons from the dayside lunar surface or frommagnetic anomalies. In order to gain more insight into the entry mechanismsassociated with different populations, backtracing is carried out for each ofthese populations. For most of the populations, the source of the protonsobtained from backtracing is found to be in agreement with that inferred fromthe velocity distribution. There are few populations that could not beexplained by the known mechanisms and remain unknown.

Characteristics of proton velocity distribution functions in the near-lunar wake from Chandrayaan-1/SWIM observations