321 research outputs found
An Asymmetric Cone Model for Halo Coronal Mass Ejections
Due to projection effects, coronagraphic observations cannot uniquely
determine parameters relevant to the geoeffectiveness of CMEs, such as the true
propagation speed, width, or source location. The Cone Model for Coronal Mass
Ejections (CMEs) has been studied in this respect and it could be used to
obtain these parameters. There are evidences that some CMEs initiate from a
flux-rope topology. It seems that these CMEs should be elongated along the
flux-rope axis and the cross section of the cone base should be rather
elliptical than circular. In the present paper we applied an asymmetric cone
model to get the real space parameters of frontsided halo CMEs (HCMEs) recorded
by SOHO/LASCO coronagraphs in 2002. The cone model parameters are generated
through a fitting procedure to the projected speeds measured at different
position angles on the plane of the sky. We consider models with the apex of
the cone located at the center and surface of the Sun. The results are compared
to the standard symmetric cone model
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
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
Observations of Coronal Mass Ejections with the Coronal Multichannel Polarimeter
The Coronal Multichannel Polarimeter (CoMP) measures not only the
polarization of coronal emission, but also the full radiance profiles of
coronal emission lines. For the first time, CoMP observations provide
high-cadence image sequences of the coronal line intensity, Doppler shift and
line width simultaneously in a large field of view. By studying the Doppler
shift and line width we may explore more of the physical processes of CME
initiation and propagation. Here we identify a list of CMEs observed by CoMP
and present the first results of these observations. Our preliminary analysis
shows that CMEs are usually associated with greatly increased Doppler shift and
enhanced line width. These new observations provide not only valuable
information to constrain CME models and probe various processes during the
initial propagation of CMEs in the low corona, but also offer a possible
cost-effective and low-risk means of space weather monitoring.Comment: 6 figures. Will appear in the special issue of Coronal Magnetism,
Sol. Phy
Non-monotonic variation with salt concentration of the second virial coefficient in protein solutions
The osmotic virial coefficient of globular protein solutions is
calculated as a function of added salt concentration at fixed pH by computer
simulations of the ``primitive model''. The salt and counter-ions as well as a
discrete charge pattern on the protein surface are explicitly incorporated. For
parameters roughly corresponding to lysozyme, we find that first
decreases with added salt concentration up to a threshold concentration, then
increases to a maximum, and then decreases again upon further raising the ionic
strength. Our studies demonstrate that the existence of a discrete charge
pattern on the protein surface profoundly influences the effective interactions
and that non-linear Poisson Boltzmann and Derjaguin-Landau-Verwey-Overbeek
(DLVO) theory fail for large ionic strength. The observed non-monotonicity of
is compared to experiments. Implications for protein crystallization are
discussed.Comment: 43 pages, including 17 figure
Unveiling the structural transitions during activation of a CO2 methanation catalyst Ru0/ZrO2 synthesised from a MOF precursor
Available online 5 May 2020Carbon Capture, Utilisation and Storage (CCUS) technologies are utilised to minimise net CO2 emissions and hence mitigate the impact of anthropogenic emissions on the global climate. One example of CO2 utilisation is the production of carbon-neutral methane fuel via catalytic CO2 reduction with H2 (methanation). Thermal activation of a metal impregnated metal-organic framework (MOF), 1 wt%Ru/UiO-66 in the presence of H2 and CO2 provides in situ synthesis of a highly active methanation catalyst: H2 promotes the formation of Ru0 nanoparticles, and CO2 behaves as a mild oxidant to remove framework carbon and promote ZrO2 crystallisation. The nature of the active MOF-derived Ru0/ZrO2 catalyst was studied by PXRD, TEM, and XAS, and the evolution of the parent 1 wt%Ru/UiO-66 during thermal activation monitored in operando by synchrotron PXRD. The Ru impregnated Zr-based MOF collapses on heating in H2 and CO2 to form an amorphous C and Zr containing phase that subsequently crystallises as tetragonal (t-) ZrO2 nanoparticles. These t-ZrO2 nanoparticles undergo a subsequent phase transition to the more stable monoclinic (m-) ZrO2 polymorph. In situ activation of Ru/UiO-66 generates a highly active catalyst for CO2 methanation by transforming the MOF precursor into a (carbonfree) crystalline t-ZrO2 support that stabilises highly dispersed metallic Ru nanoparticles. This insight may guide the rational design of future MOF-derived catalystsRenata Lippi, Anita M. D, Angelo, Chaoen Li, Shaun C. Howard, Ian C. Madsen, Karen Wilson, Adam F. Lee, Christopher J. Sumby, Christian J. Doonan, Jim Patel, Danielle F. Kenned
On the structure and evolution of a polar crown prominence/filament system
Polar crown prominences are made of chromospheric plasma partially circling
the Suns poles between 60 and 70 degree latitude. We aim to diagnose the 3D
dynamics of a polar crown prominence using high cadence EUV images from the
Solar Dynamics Observatory (SDO)/AIA at 304 and 171A and the Ahead spacecraft
of the Solar Terrestrial Relations Observatory (STEREO-A)/EUVI at 195A. Using
time series across specific structures we compare flows across the disk in 195A
with the prominence dynamics seen on the limb. The densest prominence material
forms vertical columns which are separated by many tens of Mm and connected by
dynamic bridges of plasma that are clearly visible in 304/171A two-color
images. We also observe intermittent but repetitious flows with velocity 15
km/s in the prominence that appear to be associated with EUV bright points on
the solar disk. The boundary between the prominence and the overlying cavity
appears as a sharp edge. We discuss the structure of the coronal cavity seen
both above and around the prominence. SDO/HMI and GONG magnetograms are used to
infer the underlying magnetic topology. The evolution and structure of the
prominence with respect to the magnetic field seems to agree with the filament
linkage model.Comment: 24 pages, 14 figures, Accepted for publication in Solar Physics
Journal, Movies can be found at http://www2.mps.mpg.de/data/outgoing/panesar
Quantifying Between-Cohort and Between-Sex Genetic Heterogeneity in Major Depressive Disorder
Major depressive disorder (MDD) is clinically heterogeneous with prevalence rates twice as high in women as in men. There are many possible sources of heterogeneity in MDD most of which are not measured in a sufficiently comparable way across study samples. Here, we assess genetic heterogeneity based on two fundamental measures, between-cohort and between-sex heterogeneity. First, we used genome-wide association study (GWAS) summary statistics to investigate between-cohort genetic heterogeneity using the 29 research cohorts of the Psychiatric Genomics Consortium (PGC; N cases = 16,823, N controls = 25,632) and found that some of the cohort heterogeneity can be attributed to ascertainment differences (such as recruitment of cases from hospital vs community sources). Second, we evaluated between-sex genetic heterogeneity using GWAS summary statistics from the PGC, Kaiser Permanente GERA, UK Biobank and the Danish iPSYCH studies but did not find convincing evidence for genetic differences between the sexes. We conclude that there is no evidence that the heterogeneity between MDD data sets and between sexes reflects genetic heterogeneity. Larger sample sizes with detailed phenotypic records and genomic data remain the key to overcome heterogeneity inherent in assessment of MDD
Dynamic ensemble prediction of cognitive performance in spaceflight
During spaceflight, astronauts face a unique set of stressors, including microgravity, isolation, and confinement, as well as environmental and operational hazards. These factors can negatively impact sleep, alertness, and neurobehavioral performance, all of which are critical to mission success. In this paper, we predict neurobehavioral performance over the course of a 6-month mission aboard the International Space Station (ISS), using ISS environmental data as well as self-reported and cognitive data collected longitudinally from 24 astronauts. Neurobehavioral performance was repeatedly assessed via a 3-min Psychomotor Vigilance Test (PVT-B) that is highly sensitive to the effects of sleep deprivation. To relate PVT-B performance to time-varying and discordantly-measured environmental, operational, and psychological covariates, we propose an ensemble prediction model comprising of linear mixed effects, random forest, and functional concurrent models. An extensive cross-validation procedure reveals that this ensemble outperforms any one of its components alone. We also identify the most important predictors of PVT-B performance, which include an individual's previous PVT-B performance, reported fatigue and stress, and temperature and radiation dose. This method is broadly applicable to settings where the main goal is accurate, individualized prediction of human behavior involving a mixture of person-level traits and irregularly measured time series
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