Lecturer: Martin Wieser (IRF) Date: 2010-02-04 10:30 Place: Aniara
Energetic neutral atoms from the Moon
Swedish Institute of Space Physics
Findings from two recent papers will be presented and discussed:
M. Wieser, S. Barabash, Y. Futaana, M. HolmstrÃ¶m, A. Bhardwaj, R.
Sridharan, M.B. Dhanya, P. Wurz, A. Schaufelberger, K. Asamura,
Extremely high reflection of solar wind protons as neutral hydrogen
atoms from regolith in space, Planetary and Space Science (2009), doi:
We report on measurements of extremely high reflection rates of solar
wind particles from regolith-covered lunar surfaces. Measurements by the
Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian
Chandrayaan-1 spacecraft in orbit around the Moon show that up to 20% of
the impinging solar wind protons are reflected from the lunar surface
back to space as neutral hydrogen atoms. This finding, generally
applicable to regolith-covered atmosphereless bodies, invalidates the
widely accepted assumption that regolith almost completely absorbs the
impinging solar wind.
Wieser, M., S. Barabash, Y. Futaana, M. Holmstrom, A. Bhardwaj, R.
Sridharan, M.B. Dhanya, A. Schaufelberger, P. Wurz, and K. Asamura
(2010), First observation of a mini-magnetosphere above a lunar magnetic
anomaly using energetic neutral atoms, Geophys. Res. Lett.,
doi:10.1029/2009GL041721, in press.
The Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian
Chandrayaan-1 spacecraft, which is in a 200-km orbit around the Moon,
has produced for the first time an image of a lunar magnetic anomaly in
backscattered hydrogen atoms. The image shows that a partial void of the
solar wind, a mini-magnetosphere, is formed above the strong magnetic
anomaly near the Gerasimovic crater on the lunar farside. The
mini-magnetosphere is 360 km across at the surface and is surrounded by
a 300-km-thick region of enhanced plasma flux that results from the
solar wind flowing around the mini-magnetosphere. The mini-magnetosphere
is visible only in hydrogen atoms with energy exceeding 150 eV. Fluxes
with energies below 100 eV do not show corresponding spatial variations.
While the high-energy atoms result from the backscattering process, the
origin of the low-energy component is puzzling. These observations (1)
reveal a new class of objects, mini-magnetospheres, and (2) demonstrate
a new observational technique to study airless bodies, imaging in
backscattered neutral atoms.