344 research outputs found
Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1
Significant proton fluxes were detected in the near wake region of the Moon
by an ion mass spectrometer on board Chandrayaan-1. The energy of these
nightside protons is slightly higher than the energy of the solar wind protons.
The protons are detected close to the lunar equatorial plane at a
solar zenith angle, i.e., ~50 behind the terminator at a height of
100 km. The protons come from just above the local horizon, and move along the
magnetic field in the solar wind reference frame. We compared the observed
proton flux with the predictions from analytical models of an electrostatic
plasma expansion into a vacuum. The observed velocity was higher than the
velocity predicted by analytical models by a factor of 2 to 3. The simple
analytical models cannot explain the observed ion dynamics along the magnetic
field in the vicinity of the Moon.Comment: 28 pages, 7 figure
The interaction between the Moon and the solar wind
We study the interaction between the Moon and the solar wind using a
three-dimensional hybrid plasma solver. The proton fluxes and electromagnetical
fields are presented for typical solar wind conditions with different magnetic
field directions. We find two different wake structures for an interplanetary
magnetic field that is perpendicular to the solar wind flow, and for one that
is parallell to the flow. The wake for intermediate magnetic field directions
will be a mix of these two extreme conditions. Several features are consistent
with a fluid interaction, e.g., the presence of a rarefaction cone, and an
increased magnetic field in the wake. There are however several kinetic
features of the interaction. We find kinks in the magnetic field at the wake
boundary. There are also density and magnetic field variations in the far wake,
maybe from an ion beam instability related to the wake refill. The results are
compared to observations by the WIND spacecraft during a wake crossing. The
model magnetic field and ion velocities are in agreement with the measurements.
The density and the electron temperature in the central wake are not as well
captured by the model, probably from the lack of electron physics in the hybrid
model.Comment: Accepted for publication in Earth, Planets and Spac
X-ray Observation of Mars with Suzaku at Solar Minimun
Mars was observed in X-rays during April 3-5 2008 for 82 ksec with the
Japanese Suzaku observatory. Mars has been known to emit X-rays via the
scattering of solar X-rays and via the charge exchange between neutral atoms in
the exosphere and solar wind ions. Past theoretical studies suggest that the
exospheric neutral density may vary by a factor of up to 10 over the solar
cycle. To investigate a potential change of the exospheric charge exchange
emission, Mars was observed with Suzaku at solar minimum. Significant signals
were not detected at the position of Mars in the energy band of 0.2-5 keV. A 2
sigma upper limit of the O VII line flux in 0.5-0.65 keV was 4.3
ph cm s. Comparing this upper limit to the past Chandra and
XMM-Newton observations conducted near solar maximum, it was found that the
exospheric density at solar minimum does not exceed that near solar maximum by
more than 6-70 times.Comment: 17 pages, 7 figures, accepted for publication in PAS
Dynamics of solar wind protons reflected by the Moon
Solar system bodies that lack a significant atmosphere and significant
internal magnetic fields, such as the Moon and asteroids, have been considered
as passive absorbers of the solar wind. However, ion observations near the Moon
by the SELENE spacecraft show that a fraction of the impacting solar wind
protons are reflected by the surface of the Moon. Using new observations of the
velocity spectrum of these reflected protons by the SARA experiment on-board
the Chandrayaan-1 spacecraft at the Moon, we show by modeling that the
reflection of solar wind protons will affect the global plasma environment.
These global perturbations of the ion fluxes and the magnetic fields will
depend on microscopic properties of the object's reflecting surface. This solar
wind reflection process could explain past ion observations at the Moon, and
the process should occur universally at all atmosphereless non-magnetized
objects.Comment: 12 pages, 8 figure
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
Studying the Lunar-Solar Wind Interaction with the SARA Experiment aboard the Indian Lunar Mission Chandrayaan-1
The first Indian lunar mission Chandrayaan-1 was launched on 22 October 2008.
The Sub-keV Atom Reflecting Analyzer (SARA) instrument onboard Chandrayaan-1
consists of an energetic neutral atom (ENA) imaging mass analyzer called CENA
(Chandrayaan-1 Energetic Neutrals Analyzer), and an ion-mass analyzer called
SWIM (Solar wind Monitor). CENA performed the first ever experiment to study
the solar wind-planetary surface interaction via detection of sputtered neutral
atoms and neutralized backscattered solar wind protons in the energy range
~0.01-3.0 keV. SWIM measures solar wind ions, magnetosheath and magnetotail
ions, as well as ions scattered from lunar surface in the ~0.01-15 keV energy
range. The neutral atom sensor uses conversion of the incoming neutrals to
positive ions, which are then analyzed via surface interaction technique. The
ion mass analyzer is based on similar principle. This paper presents the SARA
instrument and the first results obtained by the SWIM and CENA sensors. SARA
observations suggest that about 20% of the incident solar wind protons are
backscattered as neutral hydrogen and ~1% as protons from the lunar surface.
These findings have important implications for other airless bodies in the
solar system.Comment: 4 pages, 6 figure
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