6,908 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 Model for the Sources of the Slow Solar Wind
Models for the origin of the slow solar wind must account for two seemingly
contradictory observations: The slow wind has the composition of the closed
field corona, implying that it originates from the continuous opening and
closing of flux at the boundary between open and closed field. On the other
hand, the slow wind also has large angular width, up to ~ 60{\circ}, suggesting
that its source extends far from the open-closed boundary. We propose a model
that can explain both observations. The key idea is that the source of the slow
wind at the Sun is a network of narrow (possibly singular) open-field corridors
that map to a web of separatrices and quasi-separatrix layers in the
heliosphere. We compute analytically the topology of an open-field corridor and
show that it produces a quasi-separatrix layer in the heliosphere that extends
to angles far from the heliospheric current sheet. We then use an MHD code and
MDI/SOHO observations of the photospheric magnetic field to calculate
numerically, with high spatial resolution, the quasi-steady solar wind and
magnetic field for a time period preceding the August 1, 2008 total solar
eclipse. Our numerical results imply that, at least for this time period, a web
of separatrices (which we term an S-web) forms with sufficient density and
extent in the heliosphere to account for the observed properties of the slow
wind. We discuss the implications of our S-web model for the structure and
dynamics of the corona and heliosphere, and propose further tests of the model
Ballistic charge transport in chiral-symmetric few-layer graphene
A transfer matrix approach to study ballistic charge transport in few-layer
graphene with chiral-symmetric stacking configurations is developed. We
demonstrate that the chiral symmetry justifies a non-Abelian gauge
transformation at the spectral degeneracy point (zero energy). This
transformation proves the equivalence of zero-energy transport properties of
the multilayer to those of the system of uncoupled monolayers. Similar
transformation can be applied in order to gauge away an arbitrary magnetic
field, weak strain, and hopping disorder in the bulk of the sample. Finally, we
calculate the full-counting statistics at arbitrary energy for different
stacking configurations. The predicted gate-voltage dependence of conductance
and noise can be measured in clean multilayer samples with generic metallic
leads.Comment: 6 pages, 5 figures; EPL published versio
Towards the electron EDM search: Theoretical study of HfF+
We report first ab initio relativistic correlation calculations of potential
curves for ten low-lying electronic states, effective electric field on the
electron and hyperfine constants for the ^3\Delta_1 state of cation of a heavy
transition metal fluoride, HfF^+, that is suggested to be used as the working
state in experiments to search for the electric dipole moment of the electron.
It is shown that HfF^+ has deeply bound ^1\Sigma^+ ground state, its
dissociation energy is D_e=6.4 eV. The ^3\Delta_1 state is obtained to be the
relatively long-lived first excited state lying about 0.2 eV higher. The
calculated effective electric field E_eff=W_d|\Omega| acting on an electron in
this state is 5.84*10^{24}Hz/(e*cm)Comment: 4 page
The decay of the and resonances in the hidden gauge formalism
Using recent results obtained within the hidden gauge formalism for vector
mesons, in which the and resonances are dynamically
generated resonances from the interaction, we evaluate the
radiative decay of these resonances into . We obtain results for
the width in good agreement with the experimental data for the
state and a width about a factor five smaller for the resonance,
which would agree with preliminary results from the Belle collaboration,
hinting at an order of magnitude smaller width for this resonance than for the
.Comment: 7 pages, 9 figures, proof of gauge invariance adde
Slip-Squashing Factors as a Measure of Three-Dimensional Magnetic Reconnection
A general method for describing magnetic reconnection in arbitrary
three-dimensional magnetic configurations is proposed. The method is based on
the field-line mapping technique previously used only for the analysis of
magnetic structure at a given time. This technique is extended here so as to
analyze the evolution of magnetic structure. Such a generalization is made with
the help of new dimensionless quantities called "slip-squashing factors". Their
large values define the surfaces that border the reconnected or
to-be-reconnected magnetic flux tubes for a given period of time during the
magnetic evolution. The proposed method is universal, since it assumes only
that the time sequence of evolving magnetic field and the tangential boundary
flows are known. The application of the method is illustrated for simple
examples, one of which was considered previously by Hesse and coworkers in the
framework of the general magnetic reconnection theory. The examples help us to
compare these two approaches; they reveal also that, just as for magnetic null
points, hyperbolic and cusp minimum points of a magnetic field may serve as
favorable sites for magnetic reconnection. The new method admits a
straightforward numerical implementation and provides a powerful tool for the
diagnostics of magnetic reconnection in numerical models of solar-flare-like
phenomena in space and laboratory plasmas.Comment: 39 pages, 9 figures, corrected typos, to appear in ApJ, March 200
Quantum Hall criticality and localization in graphene with short-range impurities at the Dirac point
We explore the longitudinal conductivity of graphene at the Dirac point in a
strong magnetic field with two types of short-range scatterers: adatoms that
mix the valleys and "scalar" impurities that do not mix them. A scattering
theory for the Dirac equation is employed to express the conductance of a
graphene sample as a function of impurity coordinates; an averaging over
impurity positions is then performed numerically. The conductivity is
equal to the ballistic value for each disorder realization
provided the number of flux quanta considerably exceeds the number of
impurities. For weaker fields, the conductivity in the presence of scalar
impurities scales to the quantum-Hall critical point with at half filling or to zero away from half filling due to the
onset of Anderson localization. For adatoms, the localization behavior is
obtained also at half filling due to splitting of the critical energy by
intervalley scattering. Our results reveal a complex scaling flow governed by
fixed points of different symmetry classes: remarkably, all key manifestations
of Anderson localization and criticality in two dimensions are observed
numerically in a single setup.Comment: 17 pages, 4 figure
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