9,025 research outputs found
Electronic structure of an electron on the gyroid surface, a helical labyrinth
Previously reported formulation for electrons on curved periodic surfaces is
used to analyze the band structure of an electron bound on the gyroid surface
(the only triply-periodic minimal surface that has screw axes). We find that an
effect of the helical structure appears as the bands multiply sticking together
on the Brillouin zone boundaries. We elaborate how the band sticking is lifted
when the helical and inversion symmetries of the structure are degraded. We
find from this that the symmetries give rise to prominent peaks in the density
of states.Comment: RevTeX, 4 pages, 6 figure
Vortices, shocks, and heating in the solar photosphere: effect of a magnetic field
Aims: We study the differences between non-magnetic and magnetic regions in
the flow and thermal structure of the upper solar photosphere. Methods:
Radiative MHD simulations representing a quiet region and a plage region,
respectively, which extend into the layers around the temperature minimum, are
analyzed. Results: The flow structure in the upper photospheric layers of the
two simulations is considerably different: the non-magnetic simulation is
dominated by a pattern of moving shock fronts while the magnetic simulation
shows vertically extended vortices associated with magnetic flux
concentrations. Both kinds of structures induce substantial local heating. The
resulting average temperature profiles are characterized by a steep rise above
the temperature minimum due to shock heating in the non-magnetic case and by a
flat photospheric temperature gradient mainly caused by Ohmic dissipation in
the magnetic run. Conclusions: Shocks in the quiet Sun and vortices in the
strongly magnetized regions represent the dominant flow structures in the
layers around the temperature minimum. They are closely connected with
dissipation processes providing localized heating.Comment: Accepted for publicaton in A&
The Role of Partial Ionization Effects in the Chromosphere
The energy for the coronal heating must be provided from the convection zone.
The amount and the method by which this energy is transferred into the corona
depends on the properties of the lower atmosphere and the corona itself. We
review: 1) how the energy could be built in the lower solar atmosphere; 2) how
this energy is transferred through the solar atmosphere; and 3) how the energy
is finally dissipated in the chromosphere and/or corona. Any mechanism of
energy transport has to deal with the various physical processes in the lower
atmosphere. We will focus on a physical process that seems to be highly
important in the chromosphere and not deeply studied until recently: the
ion-neutral interaction effects (INIE) in the chromosphere. We review the
relevance and the role of the partial ionization in the chromosphere and show
that this process actually impacts considerably the outer solar atmosphere. We
include analysis of our 2.5D radiative MHD simulations with the Bifrost code
(Gudiksen et al. 2011) including the partial ionization effects on the
chromosphere and corona and thermal conduction along magnetic field lines. The
photosphere, chromosphere and transition region are partially ionized and the
interaction between ionized particles and neutral particles has important
consequences on the magneto-thermodynamics of these layers. The INIE are
treated using generalized Ohm's law, i.e., we consider the Hall term and the
ambipolar diffusion in the induction equation. The interaction between the
different species affects the modeled atmosphere as follows: 1) the ambipolar
diffusion dissipates magnetic energy and increases the minimum temperature in
the chromosphere; 2) the upper chromosphere may get heated and expanded over a
greater range of heights. These processes reveal appreciable differences
between the modeled atmospheres of simulations with and without INIE.Comment: 25 pages, 3 figures, accepted to be published in Royal Societ
Morphology and Dynamics of the Low Solar Chromosphere
The Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn
Solar Telescope of the NSO/SP is used to investigate the morphology and
dynamics of the lower chromosphere and the virtually non-magnetic fluctosphere
below. The study addresses in particular the structure of magnetic elements
that extend into these layers. We choose different quiet Sun regions in and
outside coronal holes. In inter-network regions with no significant magnetic
flux contributions above the detection limit of IBIS, we find intensity
structures with the characteristics of a shock wave pattern. The magnetic flux
elements in the network are long lived and seem to resemble the spatially
extended counterparts to the underlying photospheric magnetic elements. We
suggest a modification to common methods to derive the line-of-sight magnetic
field strength and explain some of the difficulties in deriving the magnetic
field vector from observations of the fluctosphere.Comment: accepted by ApJ, 16 pages, 8 figure
Second-harmonic generation in vortex-induced waveguides
We study the second-harmonic generation and localization of light in a
reconfigurable waveguide induced by an optical vortex soliton in a defocusing
Kerr medium. We show that the vortex-induced waveguide greatly improves
conversion efficiency from the fundamental to the second harmonic field.Comment: 3 pages, 4 figures, submitted to Optics Letter
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