423 research outputs found
Examining Periodic Solar Wind Density Structures Observed in the SECCHI Heliospheric Imagers
We present an analysis of small-scale, periodic, solar-wind density
enhancements (length-scales as small as \approx 1000 Mm) observed in images
from the Heliospheric Imager (HI) aboard STEREO A. We discuss their possible
relationship to periodic fluctuations of the proton density that have been
identified at 1 AU using in-situ plasma measurements. Specifically, Viall,
Kepko, and Spence (2008) examined 11 years of in-situ solar-wind density
measurements at 1 AU and demonstrated that not only turbulent structures, but
also non-turbulent periodic density structures exist in the solar wind with
scale sizes of hundreds to one thousand Mm. In a subsequent paper, Viall,
Spence, and Kasper (2009) analyzed the {\alpha} to proton solar-wind abundance
ratio measured during one such event of periodic density structures,
demonstrating that the plasma behavior was highly suggestive that either
temporally or spatially varying coronal source plasma created those density
structures. Large periodic density structures observed at 1 AU, which were
generated in the corona, can be observable in coronal and heliospheric
white-light images if they possess sufficiently high density contrast. Indeed,
we identify such periodic density structures as they enter the HI field of view
and follow them as they advect with the solar wind through the images. The
smaller periodic density structures that we identify in the images are
comparable in size to the larger structures analyzed in-situ at 1 AU, yielding
further evidence that periodic density enhancements are a consequence of
coronal activity as the solar wind is formed.Comment: 15 pages, 12 figures. The final publication is available at
http://www.springerlink.co
Accuracy and Limitations of Fitting and Stereoscopic Methods to Determine the Direction of Coronal Mass Ejections from Heliospheric Imagers Observations
Using data from the Heliospheric Imagers (HIs) onboard STEREO, it is possible
to derive the direction of propagation of coronal mass ejections (CMEs) in
addition to their speed with a variety of methods. For CMEs observed by both
STEREO spacecraft, it is possible to derive their direction using simultaneous
observations from the twin spacecraft and also, using observations from only
one spacecraft with fitting methods. This makes it possible to test and compare
different analyses techniques. In this article, we propose a new fitting method
based on observations from one spacecraft, which we compare to the commonly
used fitting method of Sheeley et al. (1999). We also compare the results from
these two fitting methods with those from two stereoscopic methods, focusing on
12 CMEs observed simultaneously by the two STEREO spacecraft in 2008 and 2009.
We find evidence that the fitting method of Sheeley et al. (1999) can result in
significant errors in the determination of the CME direction when the CME
propagates outside of 60deg \pm 20 deg from the Sun-spacecraft line. We expect
our new fitting method to be better adapted to the analysis of halo or limb
CMEs with respect to the observing spacecraft. We also find some evidence that
direct triangulation in the HI fields-of-view should only be applied to CMEs
propagating approximatively towards Earth (\pm 20deg from the Sun-Earth line).
Last, we address one of the possible sources of errors of fitting methods: the
assumption of radial propagation. Using stereoscopic methods, we find that at
least seven of the 12 studied CMEs had an heliospheric deflection of less than
20deg as they propagated in the HI fields-of-view, which, we believe, validates
this approximation.Comment: 17 pages, 6 figures, 2 tables, accepted to Solar Physic
Speeds and arrival times of solar transients approximated by self-similar expanding circular fronts
The NASA STEREO mission opened up the possibility to forecast the arrival
times, speeds and directions of solar transients from outside the Sun-Earth
line. In particular, we are interested in predicting potentially geo-effective
Interplanetary Coronal Mass Ejections (ICMEs) from observations of density
structures at large observation angles from the Sun (with the STEREO
Heliospheric Imager instrument). We contribute to this endeavor by deriving
analytical formulas concerning a geometric correction for the ICME speed and
arrival time for the technique introduced by Davies et al. (2012, ApJ, in
press) called Self-Similar Expansion Fitting (SSEF). This model assumes that a
circle propagates outward, along a plane specified by a position angle (e.g.
the ecliptic), with constant angular half width (lambda). This is an extension
to earlier, more simple models: Fixed-Phi-Fitting (lambda = 0 degree) and
Harmonic Mean Fitting (lambda = 90 degree). This approach has the advantage
that it is possible to assess clearly, in contrast to previous models, if a
particular location in the heliosphere, such as a planet or spacecraft, might
be expected to be hit by the ICME front. Our correction formulas are especially
significant for glancing hits, where small differences in the direction greatly
influence the expected speeds (up to 100-200 km/s) and arrival times (up to two
days later than the apex). For very wide ICMEs (2 lambda > 120 degree), the
geometric correction becomes very similar to the one derived by M\"ostl et al.
(2011, ApJ, 741, id. 34) for the Harmonic Mean model. These analytic
expressions can also be used for empirical or analytical models to predict the
1 AU arrival time of an ICME by correcting for effects of hits by the flank
rather than the apex, if the width and direction of the ICME in a plane are
known and a circular geometry of the ICME front is assumed.Comment: 15 pages, 5 figures, accepted for publication in "Solar Physics
STEREO and Wind observations of a fast ICME flank triggering a prolonged geomagnetic storm on 5-7 April 2010
On 5 April 2010 an interplanetary (IP) shock was detected by the Wind
spacecraft ahead of Earth, followed by a fast (average speed 650 km/s) IP
coronal mass ejection (ICME). During the subsequent moderate geomagnetic storm
(minimum Dst = -72 nT, maximum Kp=8-), communication with the Galaxy 15
satellite was lost. We link images from STEREO/SECCHI to the near-Earth in situ
observations and show that the ICME did not decelerate much between Sun and
Earth. The ICME flank was responsible for a long storm growth phase. This type
of glancing collision was for the first time directly observed with the STEREO
Heliospheric Imagers. The magnetic cloud (MC) inside the ICME cannot be modeled
with approaches assuming an invariant direction. These observations confirm the
hypotheses that parts of ICMEs classified as (1) long-duration MCs or (2)
magnetic-cloud-like (MCL) structures can be a consequence of a spacecraft
trajectory through the ICME flank.Comment: Geophysical Research Letters (accepted); 3 Figure
Effect of Solar Wind Drag on the Determination of the Properties of Coronal Mass Ejections from Heliospheric Images
The Fixed-\Phi (F\Phi) and Harmonic Mean (HM) fitting methods are two methods
to determine the average direction and velocity of coronal mass ejections
(CMEs) from time-elongation tracks produced by Heliospheric Imagers (HIs), such
as the HIs onboard the STEREO spacecraft. Both methods assume a constant
velocity in their descriptions of the time-elongation profiles of CMEs, which
are used to fit the observed time-elongation data. Here, we analyze the effect
of aerodynamic drag on CMEs propagating through interplanetary space, and how
this drag affects the result of the F\Phi and HM fitting methods. A simple drag
model is used to analytically construct time-elongation profiles which are then
fitted with the two methods. It is found that higher angles and velocities give
rise to greater error in both methods, reaching errors in the direction of
propagation of up to 15 deg and 30 deg for the F\Phi and HM fitting methods,
respectively. This is due to the physical accelerations of the CMEs being
interpreted as geometrical accelerations by the fitting methods. Because of the
geometrical definition of the HM fitting method, it is affected by the
acceleration more greatly than the F\Phi fitting method. Overall, we find that
both techniques overestimate the initial (and final) velocity and direction for
fast CMEs propagating beyond 90 deg from the Sun-spacecraft line, meaning that
arrival times at 1 AU would be predicted early (by up to 12 hours). We also
find that the direction and arrival time of a wide and decelerating CME can be
better reproduced by the F\Phi due to the cancellation of two errors:
neglecting the CME width and neglecting the CME deceleration. Overall, the
inaccuracies of the two fitting methods are expected to play an important role
in the prediction of CME hit and arrival times as we head towards solar maximum
and the STEREO spacecraft further move behind the Sun.Comment: Solar Physics, Online First, 17 page
RETRATO DE NIÑOS [Material gráfico]
Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte, 201
Seeds for effective oligonucleotide design
Background: DNA oligonucleotides are a very useful tool in biology. The best algorithms for designing good DNA oligonucleotides are filtering out unsuitable regions using a seeding approach. Determining the quality of the seeds is crucial for the performance of these algorithms.\ud
Results: We present a sound framework for evaluating the quality of seeds for oligonucleotide design. The F-score is used to measure the accuracy of each seed. A number of natural candidates are tested: contiguous (BLAST-like), spaced, transitions-constrained, and multiple spaced seeds. Multiple spaced seeds are the best, with more seeds providing better accuracy. Single spaced and transition seeds are very close whereas, as expected, contiguous seeds come last. Increased accuracy comes at the price of reduced efficiency. An exception is that single spaced and transitions-constrained seeds are both more accurate and more efficient than contiguous ones.\ud
Conclusions: Our work confirms another application where multiple spaced seeds perform the best. It will be useful in improving the algorithms for oligonucleotide desig
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LINCS Canvas Browser: interactive web app to query, browse and interrogate LINCS L1000 gene expression signatures
For the Library of Integrated Network-based Cellular Signatures (LINCS) project many gene expression signatures using the L1000 technology have been produced. The L1000 technology is a cost-effective method to profile gene expression in large scale. LINCS Canvas Browser (LCB) is an interactive HTML5 web-based software application that facilitates querying, browsing and interrogating many of the currently available LINCS L1000 data. LCB implements two compacted layered canvases, one to visualize clustered L1000 expression data, and the other to display enrichment analysis results using 30 different gene set libraries. Clicking on an experimental condition highlights gene-sets enriched for the differentially expressed genes from the selected experiment. A search interface allows users to input gene lists and query them against over 100 000 conditions to find the top matching experiments. The tool integrates many resources for an unprecedented potential for new discoveries in systems biology and systems pharmacology. The LCB application is available at http://www.maayanlab.net/LINCS/LCB. Customized versions will be made part of the http://lincscloud.org and http://lincs.hms.harvard.edu websites
Heliospheric Observations of STEREO-Directed Coronal Mass Ejections in 2008--2010: Lessons for Future Observations of Earth-Directed CMEs
We present a study of coronal mass ejections (CMEs) which impacted one of the
STEREO spacecraft between January 2008 and early 2010. We focus our study on 20
CMEs which were observed remotely by the Heliospheric Imagers (HIs) onboard the
other STEREO spacecraft up to large heliocentric distances. We compare the
predictions of the Fixed-Phi and Harmonic Mean (HM) fitting methods, which only
differ by the assumed geometry of the CME. It is possible to use these
techniques to determine from remote-sensing observations the CME direction of
propagation, arrival time and final speed which are compared to in situ
measurements. We find evidence that for large viewing angles, the HM fitting
method predicts the CME direction better. However, this may be due to the fact
that only wide CMEs can be successfully observed when the CME propagates more
than 100 deg from the observing spacecraft. Overall eight CMEs, originating
from behind the limb as seen by one of the STEREO spacecraft can be tracked and
their arrival time at the other STEREO spacecraft can be successfully
predicted. This includes CMEs, such as the events on 4 December 2009 and 9
April 2010, which were viewed 130 deg away from their direction of propagation.
Therefore, we predict that some Earth-directed CMEs will be observed by the HIs
until early 2013, when the separation between Earth and one of the STEREO
spacecraft will be similar to the separation of the two STEREO spacecraft in
2009--2010.Comment: 21 pages, accepted to Solar Physic
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