4 research outputs found
Characteristics of proton velocity distribution functions in the near-lunar wake from Chandrayaan-1/SWIM observations
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. 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.Comment: 8 figures, paper accepted in Icarus (2016),
http://dx.doi.org/10.1016/j.icarus.2016.01.03
First Observation of Transport of Solar Wind Protons Scattered From Magnetic Anomalies Into the Near Lunar Wake: Observations by SARA/Chandrayaan-1
We report the first observational evidence for the transport of the solar wind protons scattered from the lunar magnetic anomaly (LMA) into the near wake region from SWIM/SubâkeV Atom Reflecting Analyzer (SARA) aboard Chandrayaanâ1. These protons with high angular spread are observed in the near wake region for specific orientations of interplanetary magnetic field. The typical energy range is 600â1,000 eV, which is either smaller or comparable to that of solar wind. Using our backtracing model, the source region of these protons is found to be the large LMA at South PoleâAitken basin on the dayside, suggesting that these are solar wind protons forward scattered from LMA at the South PoleâAitken. The flux of these protons is ~5 Ă 10â»âŽ of the solar wind proton flux, which is comparable to the proton population in near wake due to other known processes. Such protons can significantly affect the electromagnetic environment in near wake region