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

Abstract

Due to the high absorption of solar wind plasma on the lunar dayside, a large scale wake structure is formed downstream of the Moon. However, recent in-situ observations have revealed the presence of protons in the near-lunar wake (100 km to 200 km from the surface). The solar wind, either directly or after interaction with the lunar surface (including magnetic anomalies), is the source of these protons in the near-wake region. Using the entire data from the SWIM sensor of the SARA experiment onboard Chandrayaan-1, we analysed the velocity distribution of the protons observed in the near-lunar wake. The average velocity distribution functions, computed in the solar wind rest frame, were further separated based on the angle between the upstream solar wind velocity and the IMF. Although the protons enter the wake parallel as well as perpendicular to the IMF, the velocity distribution were not identical for the different IMF orientations, indicating the control of IMF in the proton entry processes. Several proton populations were identified from the velocity distribution and their possible entry mechanism were inferred based on the characteristics of the velocity distribution. These entry mechanisms include (i) diffusion of solar wind protons into the wake along IMF, (ii) the solar wind protons with finite gyro-radii that are aided by the wake boundary electric field, (iii) solar wind protons with gyro-radii larger than lunar radii from the tail of the solar wind velocity distribution, and (iv) scattering of solar wind protons from the dayside lunar surface or from magnetic anomalies. In order to gain more insight into the entry mechanisms associated with different populations, backtracing is carried out for each of these populations. For most of the populations, the source of the protons obtained from backtracing is found to be in agreement with that inferred from the velocity distribution. There are few populations that could not be explained by the known mechanisms and remain unknown.

New suprathermal proton population around the Moon: Observation by SARA on Chandrayaan‐1

“…The velocity distribution of the ions of population “C” in the solar wind rest frame, where the velocities are resolved into components parallel to B IMF ( v ∥ ) and perpendicular to B IMF ( v ⊥ ) (similar to the method discussed in Dhanya et al []) is shown Figure d. For reference, the velocity distribution of the actual solar wind at the location of the Moon (time‐shifted ACE data) is also shown in Figure d (asterisk symbol) in aLSE coordinates.…”

“…The pixel size is 7.6° full width at half maximum in elevation direction and varies from 6.4° to 17.6° full width at half maximum in azimuthal direction, depending on viewing direction. Assisted by the nadir‐pointing spacecraft motion, the FOV covers ∼2 π within half of the orbit (more details about SWIM are provided in Futaana et al [], Bhardwaj et al [], and Dhanya et al [, ]).…”

First Observation of Transport of Solar Wind Protons Scattered From Magnetic Anomalies Into the Near Lunar Wake: Observations by SARA/Chandrayaan‐1

“…For the upstream solar wind plasma parameters, such as solar wind velocity and IMF, level‐2 data of the solar wind electron, proton, and alpha monitor (SWEPAM), and magnetometer (MAG) instruments onboard the Advanced Composition Explorer (ACE) satellite are used. Since ACE makes measurements at the L1 point of Sun‐Earth system, we time shifted the solar wind parameters to the location of the Moon (Dhanya et al, ). The convective electric field (CEF) of the solar wind is derived from the solar wind velocity ( V s w ) and IMF ( B I M F ) as .…”

“…The indirect transport include the entry of solar wind scattered from dayside lunar surface to the near wake region (Nishino, Fujimoto, et al, ; Wang et al, ) and also the solar wind scattered from Earth's bow shock (Nishino et al, ). Further, new population of protons is found in the near lunar wake whose source(s) is(are) yet to be identified (Dhanya et al, , ; Vorburger et al, ).…”

“…It is plausible that the solar wind protons scattered from LMA enter the near wake region and hence a significant source for near wake refilling (Dhanya et al, ; Poppe et al, ). However, there is no direct observational evidence so far for such a process, and also, the energy and flux of such a population in near wake are not known.…”