5,311 research outputs found
Does the spacecraft trajectory strongly affect the detection of magnetic clouds?
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections
(ICMEs) where a magnetic flux rope is detected. Is the difference between MCs
and ICMEs without detected flux rope intrinsic or rather due to an
observational bias? As the spacecraft has no relationship with the MC
trajectory, the frequency distribution of MCs versus the spacecraft distance to
the MCs axis is expected to be approximately flat. However, Lepping and Wu
(2010) confirmed that it is a strongly decreasing function of the estimated
impact parameter. Is a flux rope more frequently undetected for larger impact
parameter? In order to answer the questions above, we explore the parameter
space of flux rope models, especially the aspect ratio, boundary shape, and
current distribution. The proposed models are analyzed as MCs by fitting a
circular linear force-free field to the magnetic field computed along simulated
crossings.
We find that the distribution of the twist within the flux rope, the
non-detection due to too low field rotation angle or magnitude are only weakly
affecting the expected frequency distribution of MCs versus impact parameter.
However, the estimated impact parameter is increasingly biased to lower values
as the flux-rope cross section is more elongated orthogonally to the crossing
trajectory. The observed distribution of MCs is a natural consequence of a
flux-rope cross section flattened in average by a factor 2 to 3 depending on
the magnetic twist profile. However, the faster MCs at 1 AU, with V>550 km/s,
present an almost uniform distribution of MCs vs. impact parameter, which is
consistent with round shaped flux ropes, in contrast with the slower ones. We
conclude that either most of the non-MC ICMEs are encountered outside their
flux rope or near the leg region, or they do not contain any
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High-speed multi-dimensional relative navigation for uncooperative space objects
This work proposes a high-speed Light Detection and Ranging (LIDAR) based navigation architecture that is appropriate for uncooperative relative space navigation applications. In contrast to current solutions that exploit 3D LIDAR data, our architecture transforms the odometry problem from the 3D space into multiple 2.5D ones and completes the odometry problem by utilizing a recursive filtering scheme. Trials evaluate several current state-of-the-art 2D keypoint detection and local feature description methods as well as recursive filtering techniques on a number of simulated but credible scenarios that involve a satellite model developed by Thales Alenia Space (France). Most appealing performance is attained by the 2D keypoint detector Good Features to Track (GFFT) combined with the feature descriptor KAZE, that are further combined with either the H∞ or the Kalman recursive filter. Experimental results demonstrate that compared to current algorithms, the GFTT/KAZE combination is highly appealing affording one order of magnitude more accurate odometry and a very low processing burden, which depending on the competitor method, may exceed one order of magnitude faster computation
Cosmic Optical Background: the View from Pioneer 10/11
We present the new constraints on the cosmic optical background (COB)
obtained from an analysis of the Pioneer 10/11 Imaging Photopolarimeter (IPP)
data. After careful examination of data quality, the usable measurements free
from the zodiacal light are integrated into sky maps at the blue (~0.44 um) and
red (~0.64 um) bands. Accurate starlight subtraction is achieved by referring
to all-sky star catalogs and a Galactic stellar population synthesis model down
to 32.0 mag. We find that the residual light is separated into two components:
one component shows a clear correlation with thermal 100 um brightness, while
another betrays a constant level in the lowest 100 um brightness region.
Presence of the second component is significant after all the uncertainties and
possible residual light in the Galaxy are taken into account, thus it most
likely has the extragalactic origin (i.e., the COB). The derived COB brightness
is (1.8 +/- 0.9) x 10^(-9) and (1.2 +/- 0.9) x 10^(-9) erg/s/cm2/sr/A at the
blue and red band, respectively, or 7.9 +/- 4.0 and 7.7 +/- 5.8 nW/m2/sr. Based
on a comparison with the integrated brightness of galaxies, we conclude that
the bulk of the COB is comprised of normal galaxies which have already been
resolved by the current deepest observations. There seems to be little room for
contributions of other populations including "first stars" at these
wavelengths. On the other hand, the first component of the IPP residual light
represents the diffuse Galactic light (DGL) - scattered starlight by the
interstellar dust. We derive the mean DGL-to-100 um brightness ratios of 2.1 x
10^(-3) and 4.6 x 10^(-3) at the two bands, which are roughly consistent with
the previous observations toward denser dust regions. Extended red emission in
the diffuse interstellar medium is also confirmed.Comment: Accepted for publication in the Astrophysical Journal; Typos
correcte
Are There Different Populations of Flux Ropes in the Solar Wind?
Flux ropes are twisted magnetic structures, which can be detected by in situ
measurements in the solar wind. However, different properties of detected flux
ropes suggest different types of flux-rope population. As such, are there
different populations of flux ropes? The answer is positive, and is the result
of the analysis of four lists of flux ropes, including magnetic clouds (MCs),
observed at 1 AU. The in situ data for the four lists have been fitted with the
same cylindrical force-free field model, which provides an estimation of the
local flux-rope parameters such as its radius and orientation. Since the
flux-rope distributions have a large dynamic range, we go beyond a simple
histogram analysis by developing a partition technique that uniformly
distributes the statistical fluctuations over the radius range. By doing so, we
find that small flux ropes with radius R<0.1 AU have a steep power-law
distribution in contrast to the larger flux ropes (identified as MCs), which
have a Gaussian-like distribution. Next, from four CME catalogs, we estimate
the expected flux-rope frequency per year at 1 AU. We find that the predicted
numbers are similar to the frequencies of MCs observed in situ. However, we
also find that small flux ropes are at least ten times too abundant to
correspond to CMEs, even to narrow ones. Investigating the different possible
scenarios for the origin of those small flux ropes, we conclude that these
twisted structures can be formed by blowout jets in the low corona or in
coronal streamers.Comment: 24 pages, 6 figure
Dust in the Local Interstellar Wind
The gas-to-dust mass ratios found for interstellar dust within the Solar
System, versus values determined astronomically for the cloud around the Solar
System, suggest that large and small interstellar grains have separate
histories, and that large interstellar grains preferentially detected by
spacecraft are not formed exclusively by mass exchange with nearby interstellar
gas. Observations by the Ulysses and Galileo satellites of the mass spectrum
and flux rate of interstellar dust within the heliosphere are combined with
information about the density, composition, and relative flow speed and
direction of interstellar gas in the cloud surrounding the solar system to
derive an in situ value for the gas-to-dust mass ratio, . Hubble observations of the cloud surrounding the solar system
yield a gas-to-dust mass ratio of Rg/d=551+61-251 when B-star reference
abundances are assumed. The exclusion of small dust grains from the heliosheath
and heliosphere regions are modeled, increasing the discrepancy between
interstellar and in situ observations. The shock destruction of interstellar
grains is considered, and comparisons are made with interplanetary and presolar
dust grains.Comment: 87 pages, 9 figures, 6 tables, accepted for publication in
Astrophysical Journal. Uses AASTe
Overcoming the Challenges Associated with Image-based Mapping of Small Bodies in Preparation for the OSIRIS-REx Mission to (101955) Bennu
The OSIRIS-REx Asteroid Sample Return Mission is the third mission in NASA's
New Frontiers Program and is the first U.S. mission to return samples from an
asteroid to Earth. The most important decision ahead of the OSIRIS-REx team is
the selection of a prime sample-site on the surface of asteroid (101955) Bennu.
Mission success hinges on identifying a site that is safe and has regolith that
can readily be ingested by the spacecraft's sampling mechanism. To inform this
mission-critical decision, the surface of Bennu is mapped using the OSIRIS-REx
Camera Suite and the images are used to develop several foundational data
products. Acquiring the necessary inputs to these data products requires
observational strategies that are defined specifically to overcome the
challenges associated with mapping a small irregular body. We present these
strategies in the context of assessing candidate sample-sites at Bennu
according to a framework of decisions regarding the relative safety,
sampleability, and scientific value across the asteroid's surface. To create
data products that aid these assessments, we describe the best practices
developed by the OSIRIS-REx team for image-based mapping of irregular small
bodies. We emphasize the importance of using 3D shape models and the ability to
work in body-fixed rectangular coordinates when dealing with planetary surfaces
that cannot be uniquely addressed by body-fixed latitude and longitude.Comment: 31 pages, 10 figures, 2 table
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