1,046 research outputs found
May 12 1997 Cme Event: I. a Simplified Model of the Pre-Eruptive Magnetic Structure
A simple model of the coronal magnetic field prior to the CME eruption on May
12 1997 is developed. First, the magnetic field is constructed by superimposing
a large-scale background field and a localized bipolar field to model the
active region (AR) in the current-free approximation. Second, this potential
configuration is quasi-statically sheared by photospheric vortex motions
applied to two flux concentrations of the AR. Third, the resulting force-free
field is then evolved by canceling the photospheric magnetic flux with the help
of an appropriate tangential electric field applied to the central part of the
AR.
To understand the structure of the modeled configuration, we use the field
line mapping technique by generalizing it to spherical geometry. It is
demonstrated that the initial potential configuration contains a hyperbolic
flux tube (HFT) which is a union of two intersecting quasi-separatrix layers.
This HFT provides a partition of the closed magnetic flux between the AR and
the global solar magnetic field. The vortex motions applied to the AR interlock
the field lines in the coronal volume to form additionally two new HFTs pinched
into thin current layers. Reconnection in these current layers helps to
redistribute the magnetic flux and current within the AR in the
flux-cancellation phase. In this phase, a magnetic flux rope is formed together
with a bald patch separatrix surface wrapping around the rope. Other important
implications of the identified structural features of the modeled configuration
are also discussed.Comment: 25 pages, 11 figures, to appear in ApJ 200
A novel metric for coronal MHD models
[1] In the interest of quantitatively assessing the capabilities of coronal MHD models, we have developed a metric that compares the structures of the white light corona observed with SOHO LASCO C2 to model predictions. The MAS model is compared to C2 observations from two Carrington rotations during solar cycle 23, CR1913 and CR1984, which were near the minimum and maximum of solar activity, respectively, for three radial heights, 2.5 R⊙, 3.0 R⊙, and 4.5 R⊙. In addition to simulated polarization brightness images, we create a synthetic image based on the field topology along the line of sight in the model. This open-closed brightness is also compared to LASCO C2 after renormalization. In general, the model\u27s magnetic structure is a closer match to observed coronal structures than the model\u27s density structure. This is expected from the simplified energy equations used in current global corona MHD models
A Hot Helium Plasma in the Galactic Center Region
Recent X-ray observations by the space mission Chandra confirmed the
astonishing evidence for a diffuse, hot, thermal plasma at a temperature of 9.
K (8 keV) found by previous surveys to extend over a few hundred parsecs
in the Galactic Centre region. This plasma coexists with the usual components
of the interstellar medium such as cold molecular clouds and a soft (~0.8 keV)
component produced by supernova remnants, and its origin remains uncertain.
First, simple calculations using a mean sound speed for a hydrogen-dominated
plasma have suggested that it should not be gravitationally bound, and thus
requires a huge energy source to heat it in less than the escape time. Second,
an astrophysical mechanism must be found to generate such a high temperature.
No known source has been identified to fulfill both requirements. Here we
address the energetics problem and show that the hot component could actually
be a gravitationally confined helium plasma. We illustrate the new prospects
this opens by discussing the origin of this gas, and by suggesting possible
heating mechanisms.Comment: 9 pages, accepted for publication in APJ
Early characterization of the severity and transmissibility of pandemic influenza using clinical episode data from multiple populations
The potential rapid availability of large-scale clinical episode data during the next influenza pandemic suggests an opportunity for increasing the speed with which novel respiratory pathogens can be characterized. Key intervention decisions will be determined by both the transmissibility of the novel strain (measured by the basic reproductive number R0) and its individual-level severity. The 2009 pandemic illustrated that estimating individual-level severity, as described by the proportion pC of infections that result in clinical cases, can remain uncertain for a prolonged period of time. Here, we use 50 distinct US military populations during 2009 as a retrospective cohort to test the hypothesis that real-time encounter data combined with disease dynamic models can be used to bridge this uncertainty gap. Effectively, we estimated the total number of infections in multiple early-affected communities using the model and divided that number by the known number of clinical cases. Joint estimates of severity and transmissibility clustered within a relatively small region of parameter space, with 40 of the 50 populations bounded by: pC, 0.0133-0.150 and R0, 1.09-2.16. These fits were obtained despite widely varying incidence profiles: some with spring waves, some with fall waves and some with both. To illustrate the benefit of specific pairing of rapidly available data and infectious disease models, we simulated a future moderate pandemic strain with pC approximately Ă—10 that of 2009; the results demonstrating that even before the peak had passed in the first affected population, R0 and pC could be well estimated. This study provides a clear reference in this two-dimensional space against which future novel respiratory pathogens can be rapidly assessed and compared with previous pandemics
Phonon and Elastic Instabilities in MoC and MoN
We present several results related to the instability of MoC and MoN in the
B1 (sodium chloride) structure. These compounds were proposed as potential
superconductors with moderately high transition temperatures. We show that the
elastic instability in B1-structure MoN, demonstrated several years ago,
persists at elevated pressures, thus offering little hope of stabilizing this
material without chemical doping. For MoC, another material for which
stoichiometric fabrication in the B1-structure has not proven possible, we find
that all of the cubic elastic constants are positive, indicating elastic
stability. Instead, we find X-point phonon instabilities in MoC (and in MoN as
well), further illustrating the rich behavior of carbo-nitride materials. We
also present additional electronic structure results for several transition
metal (Zr, Nb and Mo) carbo-nitride systems and discuss systematic trends in
the properties of these materials. Deviations from strict electron counting
dependencies are apparent.Comment: 5 pages and 4 trailing figures. Submitted to PR
Prehospital identification of middle cerebral artery occlusion - a stroke education program and transcranial ultrasound for paramedics
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Metrics for solar wind prediction models: Comparison of empirical, hybrid, and physics-based schemes with 8 years of L1 observations
Space weather effects on technological systems originate with energy carried from the Sun to the terrestrial environment by the solar wind. In this study, we present results of modeling of solar corona-heliosphere processes to predict solar wind conditions at the L1 Lagrangian point upstream of Earth. In particular we calculate performance metrics for (1) empirical, (2) hybrid empirical/physics-based, and (3) full physics-based coupled corona-heliosphere models over an 8-year period (1995–2002). L1 measurements of the radial solar wind speed are the primary basis for validation of the coronal and heliosphere models studied, though other solar wind parameters are also considered. The models are from the Center for Integrated Space-Weather Modeling (CISM) which has developed a coupled model of the whole Sun-to-Earth system, from the solar photosphere to the terrestrial thermosphere. Simple point-by-point analysis techniques, such as mean-square-error and correlation coefficients, indicate that the empirical coronal-heliosphere model currently gives the best forecast of solar wind speed at 1 AU. A more detailed analysis shows that errors in the physics-based models are predominately the result of small timing offsets to solar wind structures and that the large-scale features of the solar wind are actually well modeled. We suggest that additional “tuning” of the coupling between the coronal and heliosphere models could lead to a significant improvement of their accuracy. Furthermore, we note that the physics-based models accurately capture dynamic effects at solar wind stream interaction regions, such as magnetic field compression, flow deflection, and density buildup, which the empirical scheme cannot
Evidence for topological nonequilibrium in magnetic configurations
We use direct numerical simulations to study the evolution, or relaxation, of
magnetic configurations to an equilibrium state. We use the full single-fluid
equations of motion for a magnetized, non-resistive, but viscous fluid; and a
Lagrangian approach is used to obtain exact solutions for the magnetic field.
As a result, the topology of the magnetic field remains unchanged, which makes
it possible to study the case of topological nonequilibrium. We find two cases
for which such nonequilibrium appears, indicating that these configurations may
develop singular current sheets.Comment: 10 pages, 5 figure
Competition between Magnetic and Structural Transition in CrN
CrN is observed to undergo a paramagnetic to antiferromagnetic transition
accompanied by a shear distortion from cubic NaCl-type to orthorhombic
structure. Our first-principle plane wave and ultrasoft pseudopotential
calculations confirm that the distorted antiferromagnetic phase with spin
configuration arranged in double ferromagnetic sheets along [110] is the most
stable. Antiferromagnetic ordering leads to a large depletion of states around
Fermi level, but it does not open a gap. Simultaneous occurence of structural
distortion and antiferromagnetic order is analyzed.Comment: 10 pages, 10 figure
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