464 research outputs found
Enhanced longitudinal magnetooptic Kerr effect contrast in nanomagnetic structures
We report on enhanced longitudinal magnetooptic Kerr effect signal contrast in thin-film nanomagnetic disks with in-plane magnetization when combined with dielectric layers that provide impedance matching to the structure and the underlying substrate. Kerr signals can increase by a factor of three, while substrate reflectance is almost completely suppressed. This leads to an increase in Kerr ellipticity relative to the background intensity and a subsequent improvement in the measured signal-to-noise ratio. Measurements using a beam focused on opaque 400-nm Ni disks yield contrast improvements of a factor of 8. Arrays of nanodisks demonstrate more complex behavior due to diffraction effects
Progressive transformation of a flux rope to an ICME
The solar wind conditions at one astronomical unit (AU) can be strongly
disturbed by the interplanetary coronal mass ejections (ICMEs). A subset,
called magnetic clouds (MCs), is formed by twisted flux ropes that transport an
important amount of magnetic flux and helicity which is released in CMEs. At 1
AU from the Sun, the magnetic structure of MCs is generally modeled neglecting
their expansion during the spacecraft crossing. However, in some cases, MCs
present a significant expansion. We present here an analysis of the huge and
significantly expanding MC observed by the Wind spacecraft during 9 and 10
November, 2004. After determining an approximated orientation for the flux rope
using the minimum variance method, we precise the orientation of the cloud axis
relating its front and rear magnetic discontinuities using a direct method.
This method takes into account the conservation of the azimuthal magnetic flux
between the in- and out-bound branches, and is valid for a finite impact
parameter (i.e., not necessarily a small distance between the spacecraft
trajectory and the cloud axis). Moreover, using the direct method, we find that
the ICME is formed by a flux rope (MC) followed by an extended coherent
magnetic region. These observations are interpreted considering the existence
of a previous larger flux rope, which partially reconnected with its
environment in the front. These findings imply that the ejected flux rope is
progressively peeled by reconnection and transformed to the observed ICME (with
a remnant flux rope in the front part).Comment: Solar Physics (in press
Genetic diversity among farmer-preferred cassava landraces in Uganda
Understanding of genetic diversity among a breeding population is an
important requirement for crop improvement as it allows for the
selection of diverse parental combinations and formation of heterotic
pools for genetic gain. This study was carried out to determine genetic
diversity within and among 51 farmer-preferred cassava ( Manihot
esculenta ) landraces and 15 elite accessions grown in Uganda. Twenty
six simple sequence repeat (SSR) markers used for genetic diversity
assessment in this study revealed a total of 154 alleles, of which 24%
were unique alleles present only in landraces. The genetic diversity
and observed herozygosity in landraces were slightly higher than in
elite accessions. Elite accessions clustered with some of the landraces
indicating that there were some alleles in common. However, 58.8% of
the landraces fell into 3 different clusters independent of the elite
accessions. Including these landraces with unique alleles in cassava
breeding schemes will increase the chances of producing farmer
preferred adapted elite cultivars. The study also revealed genetic
differentiation among accessions from different regions providing an
opportunity for establishment of heterotic pools within a breeding
programme.La compr\ue9hension de la diversit\ue9 g\ue9n\ue9tique est une
condition importante dans la s\ue9lection de diverses combinaisons
parentales et la formation des groupes h\ue9terotiques
d\u2019int\ue9r\ueat g\ue9n\ue9tique pour
l\u2019am\ue9lioration des cultures. Cette \ue9tude \ue9tait
conduite pour d\ue9terminer la diversit\ue9 g\ue9n\ue9tique
parmi 51 vari\ue9t\ue9s locales de manioc ( Manihot esculenta )
pr\ue9f\ue9r\ue9es des fermiers et 15 accessions \ue9lites
cultiv\ue9es en Ouganda. Vingt six marqueurs simples \ue0 sequence
r\ue9p\ue9t\ue9e (SSR) utilis\ue9s pour l\u2019\ue9valuation
de la diversit\ue9 g\ue9n\ue9tique ont r\ue9v\ue9l\ue9 un
total de 154 all\ue8les, parmi lesquelles 24% \ue9taient des
all\ue8les uniques pr\ue9sentes dans les races locales seulement.
La diversit\ue9 g\ue9n\ue9tique et
l\u2019h\ue9terozygosit\ue9 observ\ue9es dans les races locales
\ue9taient l\ue9g\ue8rement sup\ue9rieures que dans les
accessions \ue9lites. Celles-ci s\u2019\ue9taient regroup\ue9es
avec quelques vari\ue9t\ue9s locales indiquant donc qu\u2019elles
avaient quelques all\ue8les en commun. Par ailleurs, 58.8% des races
locales ont form\ue9 trois diff\ue9rents groupes
ind\ue9pendamment des accessions \ue9lites. Inclure ces races
locales \ue0 all\ue8les uniques dans les syst\ue8mes
d\u2019am\ue9lioration du manioc permettra d\u2019augmenter les
chances de produire des cultivars \ue9lites pr\ue9f\ue9r\ue9s
des fermiers. L\u2019\ue9tude montre aussi que la
diff\ue9renciation g\ue9n\ue9tique trouv\ue9e parmi les
accessions de diff\ue9rentes r\ue9gions est une opportunit\ue9
offerte pour l\u2019\ue9tablissement des groupes h\ue9terotiques
dans le programme d\u2019am\ue9lioration
How Many CMEs Have Flux Ropes? Deciphering the Signatures of Shocks, Flux Ropes, and Prominences in Coronagraph Observations of CMEs
We intend to provide a comprehensive answer to the question on whether all
Coronal Mass Ejections (CMEs) have flux rope structure. To achieve this, we
present a synthesis of the LASCO CME observations over the last sixteen years,
assisted by 3D MHD simulations of the breakout model, EUV and coronagraphic
observations from STEREO and SDO, and statistics from a revised LASCO CME
database. We argue that the bright loop often seen as the CME leading edge is
the result of pileup at the boundary of the erupting flux rope irrespective of
whether a cavity or, more generally, a 3-part CME can be identified. Based on
our previous work on white light shock detection and supported by the MHD
simulations, we identify a new type of morphology, the `two-front' morphology.
It consists of a faint front followed by diffuse emission and the bright
loop-like CME leading edge. We show that the faint front is caused by density
compression at a wave (or possibly shock) front driven by the CME. We also
present high-detailed multi-wavelength EUV observations that clarify the
relative positioning of the prominence at the bottom of a coronal cavity with
clear flux rope structure. Finally, we visually check the full LASCO CME
database for flux rope structures. In the process, we classify the events into
two clear flux rope classes (`3-part', `Loop'), jets and outflows (no clear
structure). We find that at least 40% of the observed CMEs have clear flux rope
structures. We propose a new definition for flux rope CMEs (FR-CMEs) as a
coherent magnetic, twist-carrying coronal structure with angular width of at
least 40 deg and able to reach beyond 10 Rsun which erupts on a time scale of a
few minutes to several hours. We conclude that flux ropes are a common
occurrence in CMEs and pose a challenge for future studies to identify CMEs
that are clearly not FR-CMEs.Comment: 26 pages, 9 figs, to be published in Solar Physics Topical Issue
"Flux Rope Structure of CMEs
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
Flux-rope twist in eruptive flares and CMEs : due to zipper and main-phase reconnection
Funding: UK Science and Technology Facilities CouncilThe nature of three-dimensional reconnection when a twisted flux tube erupts during an eruptive flare or coronal mass ejection is considered. The reconnection has two phases: first of all, 3D âzipper reconnectionâ propagates along the initial coronal arcade, parallel to the polarity inversion line (PIL); then subsequent quasi-2D âmain phase reconnectionâ in the low corona around a flux rope during its eruption produces coronal loops and chromospheric ribbons that propagate away from the PIL in a direction normal to it. One scenario starts with a sheared arcade: the zipper reconnection creates a twisted flux rope of roughly one turn (2Ï radians of twist), and then main phase reconnection builds up the bulk of the erupting flux rope with a relatively uniform twist of a few turns. A second scenario starts with a pre-existing flux rope under the arcade. Here the zipper phase can create a core with many turns that depend on the ratio of the magnetic fluxes in the newly formed flare ribbons and the new flux rope. Main phase reconnection then adds a layer of roughly uniform twist to the twisted central core. Both phases and scenarios are modeled in a simple way that assumes the initial magnetic flux is fragmented along the PIL. The model uses conservation of magnetic helicity and flux, together with equipartition of magnetic helicity, to deduce the twist of the erupting flux rope in terms the geometry of the initial configuration. Interplanetary observations show some flux ropes have a fairly uniform twist, which could be produced when the zipper phase and any pre-existing flux rope possess small or moderate twist (up to one or two turns). Other interplanetary flux ropes have highly twisted cores (up to five turns), which could be produced when there is a pre-existing flux rope and an active zipper phase that creates substantial extra twist.PostprintPublisher PDFPeer reviewe
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in âs = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fbâ1 of protonâproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
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