37,782 research outputs found
The Brazilian report to the 7th LANDSAT Technical Working Group (LTWG) meeting
Described is the current status of the INPE LANDSAT receiving and processing facilities, as well as the experience in the related activities during the period from June 1984 to February 1985
The formation of IRIS diagnostics I. A quintessential model atom of Mg II and general formation properties of the Mg II h&k lines
NASA's Interface Region Imaging Spectrograph (IRIS) space mission will study
how the solar atmosphere is energized. IRIS contains an imaging spectrograph
that covers the Mg II h&k lines as well as a slit-jaw imager centered at Mg II
k. Understanding the observations will require forward modeling of Mg II h&k
line formation from 3D radiation-MHD models. This paper is the first in a
series where we undertake this forward modeling. We discuss the atomic physics
pertinent to h&k line formation, present a quintessential model atom that can
be used in radiative transfer computations and discuss the effect of partial
redistribution (PRD) and 3D radiative transfer on the emergent line profiles.
We conclude that Mg II h&k can be modeled accurately with a 4-level plus
continuum Mg II model atom. Ideally radiative transfer computations should be
done in 3D including PRD effects. In practice this is currently not possible. A
reasonable compromise is to use 1D PRD computations to model the line profile
up to and including the central emission peaks, and use 3D transfer assuming
complete redistribution to model the central depression.Comment: 13 pages, 13 figures, accepted for Ap
The formation of IRIS diagnostics II. The formation of the Mg II h&k lines in the solar atmosphere
NASA's Interface Region Imaging Spectrograph (IRIS) small explorer mission
will study how the solar atmosphere is energized. IRIS contains an imaging
spectrograph that covers the Mg II h&k lines as well as a slit-jaw imager
centered at Mg II k. Understanding the observations requires forward modeling
of Mg II h&k line formation from 3D radiation-MHD models.
We compute the vertically emergent h&k intensity from a snapshot of a dynamic
3D radiation-MHD model of the solar atmosphere, and investigate which
diagnostic information about the atmosphere is contained in the synthetic line
profiles. We find that the Doppler shift of the central line depression
correlates strongly with the vertical velocity at optical depth unity, which is
typically located less than 200 km below the transition region (TR). By
combining the Doppler shifts of the h and the k line we can retrieve the sign
of the velocity gradient just below the TR. The intensity in the central line
depression is anticorrelated with the formation height, especially in subfields
of a few square Mm. This intensity could thus be used to measure the spatial
variation of the height of the transition region. The intensity in the
line-core emission peaks correlates with the temperature at its formation
height, especially for strong emission peaks. The peaks can thus be exploited
as a temperature diagnostic. The wavelength difference between the blue and red
peaks provides a diagnostic of the velocity gradients in the upper
chromosphere. The intensity ratio of the blue and red peaks correlates strongly
with the average velocity in the upper chromosphere. We conclude that the Mg II
h&k lines are excellent probes of the very upper chromosphere just below the
transition region, a height regime that is impossible to probe with other
spectral lines.Comment: 15 pages, 12 figures, accepted for ApJ, astro-ph abstract shortened
to confirm to submission requirement
Vortices in the presence of a nonmagnetic atom impurity in 2D XY ferromagnets
Using a model of nonmagnetic impurity potential, we have examined the
behavior of planar vortex solutions in the classical two-dimensional XY
ferromagnets in the presence of a spin vacancy localized out of the vortex
core. Our results show that a spinless atom impurity gives rise to an effective
potential that repels the vortex structure.Comment: 6 pages, 2 figures, RevTex
Ellerman bombs and UV bursts: transient events in chromospheric current sheets
Ellerman bombs (EBs) and UV bursts are both brightenings related to flux
emergence regions and specifically to magnetic flux of opposite polarity that
meet in the photosphere. These two reconnection-related phenomena, nominally
formed far apart, occasionally occur in the same location and at the same time,
thus challenging our understanding of reconnection and heating of the lower
solar atmosphere. We consider the formation of an active region, including long
fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic
flux sheet, injected ~Mm below the photosphere, is studied as it pierces
the photosphere and interacts with the preexisting ambient field. Specifically,
we aim to study whether EBs and UV bursts are generated as a result of such
flux emergence and examine their physical relationship. The Bifrost radiative
magnetohydrodynamics code was used to model flux emerging into a model
atmosphere that contained a fairly strong ambient field, constraining the
emerging field to a limited volume wherein multiple reconnection events occur
as the field breaks through the photosphere and expands into the outer
atmosphere. Synthetic spectra of the different reconnection events were
computed using the D RH code and the fully 3D MULTI3D code. The formation
of UV bursts and EBs at intensities and with line profiles that are highly
reminiscent of observed spectra are understood to be a result of the
reconnection of emerging flux with itself in a long-lasting current sheet that
extends over several scale heights through the chromosphere. Synthetic
diagnostics suggest that there are no compelling reasons to assume that UV
bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally
formed at opposite ends of a long current sheet that resides in an extended
bubble of cool gas.Comment: 10 pages, 8 figures, accepted by A&
The Coulomb impurity problem in graphene
We address the problem of an unscreened Coulomb charge in graphene, and
calculate the local density of states and displaced charge as a function of
energy and distance from the impurity. This is done non-perturbatively in two
different ways: (1) solving the problem exactly by studying numerically the
tight-binding model on the lattice; (2) using the continuum description in
terms of the 2D Dirac equation. We show that the Dirac equation, when properly
regularized, provides a qualitative and quantitative low energy description of
the problem. The lattice solution shows extra features that cannot be described
by the Dirac equation, namely bound state formation and strong renormalization
of the van Hove singularities.Comment: 3 Figures; minor typo corrections and minor update in Fig. 3
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