111 research outputs found
Modeling Repeatedly Flaring Sunspots
Active regions (AR) appearing on the surface of the Sun are classified into
, , , and by the rules of the Mount Wilson
Observatory, California on the basis of their topological complexity. Amongst
these, the -sunspots are known to be super-active and produce the most
X-ray flares. Here, we present results from a simulation of the Sun by
mimicking the upper layers and the corona, but starting at a more primitive
stage than any earlier treatment. We find that this initial state consisting of
only a thin sub-photospheric magnetic sheet breaks into multiple flux-tubes
which evolve into a colliding-merging system of spots of opposite polarity upon
surface emergence, similar to those often seen on the Sun. The simulation goes
on to produce many exotic -sunspot associated phenomena: repeated
flaring in the range of typical solar flare energy release and ejective helical
flux ropes with embedded cool-dense plasma filaments resembling solar coronal
mass ejections.Comment: Minor changes consistent with Phys Rev Lett versio
Numerical Simulations of Coronal Heating through Footpoint Braiding
Advanced 3D radiative MHD simulations now reproduce many properties of the
outer solar atmosphere. When including a domain from the convection zone into
the corona, a hot chromosphere and corona are self-consistently maintained.
Here we study two realistic models, with different simulated area, magnetic
field strength and topology, and numerical resolution. These are compared in
order to characterize the heating in the 3D-MHD simulations which
self-consistently maintains the structure of the atmosphere. We analyze the
heating at both large and small scales and find that heating is episodic and
highly structured in space, but occurs along loop shaped structures, and moves
along with the magnetic field. On large scales we find that the heating per
particle is maximal near the transition region and that widely distributed
opposite-polarity field in the photosphere leads to a greater heating scale
height in the corona. On smaller scales, heating is concentrated in current
sheets, the thicknesses of which are set by the numerical resolution. Some
current sheets fragment in time, this process occurring more readily in the
higher-resolution model leading to spatially highly intermittent heating. The
large scale heating structures are found to fade in less than about five
minutes, while the smaller, local, heating shows time scales of the order of 2
minutes in one model and 1 minutes in the other, higher-resolution, model.Comment: 20 pages, accepted by Ap
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
The Formation of IRIS diagnostics. IV. The Mg II triplet lines as a new diagnostic for lower chromospheric heating
A triplet of subordinate lines of Mg II exists in the region around the h&k
lines. In solar spectra these lines are seen mostly in absorption, but in some
cases can become emission lines. The aim of this work is to study the formation
of this triplet, and investigate any diagnostic value they can bring. Using 3D
radiative magnetohydrodynamic simulations of quiet Sun and flaring flux
emergence, we synthesize spectra and investigate how spectral features respond
to the underlying atmosphere. We find that emission in the lines is rare and is
typically caused by a steep temperature increase in the lower chromosphere
(above 1500 K, with electron densities above 10 m). In both
simulations the lines are sensitive to temperature increases taking place at
column masses >= 5e-4 g cm. Additional information can also be inferred
from the peak-to-wing ratio and shape of the line profiles. Using observations
from NASA's Interface Region Imaging Spectrograph we find both absorption and
emission line profiles with similar shapes to the synthetic spectra, which
suggests that these lines represent a useful diagnostic that complements the
MgII h&k lines.Comment: 8 pages, 7 figures. Accepted for publication in Ap
Emergence of granular-sized magnetic bubbles through the solar atmosphere. II. Non-LTE chromospheric diagnostics and inversions
Magnetic flux emergence into the outer layers of the Sun is a fundamental
mechanism for releasing energy into the chromosphere and the corona. In this
paper, we study the emergence of granular-sized flux concentrations and the
structuring of the corresponding physical parameters and atmospheric
diagnostics in the upper photo- sphere and in the chromosphere. We make use of
a realistic 3D MHD simulation of the outer layers of the Sun to study the
formation of the Ca II 8542 line. We also derive semi-empirical 3D models from
non-LTE inversions of our observations. These models contain depth-dependent
information of the temperature and line-of-sight stratification. Our analysis
explains the peculiar Ca II 8542 profiles observed in the flux-emerging region.
In addition, we derive detailed temperature and velocity maps describing the
ascent of magnetic bubbles from the photosphere to the chromosphere. The
inversions suggest that, in active regions, granular-sized bubbles emerge up to
the lower chromosphere where the existing large-scale field hinders their
ascent. We report hints of heating when the field reaches the chromosphere.Comment: Submitted to ApJ, 10 Figure
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