1,367 research outputs found
On red shifs in the transition region and corona
We present evidence that transition region red-shifts are naturally produced
in episodically heated models where the average volumetric heating scale height
lies between that of the chromospheric pressure scale height of 200 km and the
coronal scale height of 50 Mm. In order to do so we present results from 3d MHD
models spanning the upper convection zone up to the corona, 15 Mm above the
photosphere. Transition region and coronal heating in these models is due both
the stressing of the magnetic field by photospheric and convection `zone
dynamics, but also in some models by the injection of emerging magnetic flux.Comment: 8 pages, 9 figures, NSO Workshop #25 Chromospheric Structure and
Dynamic
Clusters of small eruptive flares produced by magnetic reconnection in the sun
We report on the formation of small solar flares produced by patchy magnetic
reconnection between interacting magnetic loops. A three-dimensional (3D)
magnetohydrodynamic (MHD) numerical experiment was performed, where a uniform
magnetic flux sheet was injected into a fully developed convective layer. The
gradual emergence of the field into the solar atmosphere results in a network
of magnetic loops, which interact dynamically forming current layers at their
interfaces. The formation and ejection of plasmoids out of the current layers
leads to patchy reconnection and the spontaneous formation of several small
(size ? 1-2Mm) flares. We find that these flares are short-lived (30 s - 3 min)
bursts of energy in the range O(10^25 - 10^27) ergs, which is basically the
nanoflare-microflare range. Their persistent formation and co-operative action
and evolution leads to recurrent emission of fast EUV/X-ray jets and
considerable plasma heating in the active corona.Comment: 5 pages, 5 figure
Observational Signatures of Simulated Reconnection Events in the Solar Chromosphere and Transition Region
We present the results of numerical simulations of wave-induced magnetic
reconnection in a model of the solar atmosphere. In the magnetic field geometry
we study in this article, the waves, driven by a monochromatic piston and a
driver taken from Hinode observations, induce periodic reconnection of the
magnetic field, and this reconnection appears to help drive long-period
chromospheric jets. By synthesizing observations for a variety of wavelengths
that are sensitive to a wide range of temperatures, we shed light on the often
confusing relationship between the plethora of jet-like phenomena in the solar
atmosphere, e.g., explosive events, spicules, blinkers, and other phenomena
thought to be caused by reconnection.Comment: 13 pages, 22 figures. Submitted to The Astrophysical Journa
Observing the Roots of Solar Coronal Heating - in the Chromosphere
The Sun's corona is millions of degrees hotter than its 5,000 K photosphere.
This heating enigma is typically addressed by invoking the deposition at
coronal heights of non-thermal energy generated by the interplay between
convection and magnetic field near the photosphere. However, it remains unclear
how and where coronal heating occurs and how the corona is filled with hot
plasma. We show that energy deposition at coronal heights cannot be the only
source of coronal heating, by revealing a significant coronal mass supply
mechanism that is driven from below, in the chromosphere. We quantify the
asymmetry of spectral lines observed with Hinode and SOHO and identify faint
but ubiquitous upflows with velocities that are similar (50-100 km/s) across a
wide range of magnetic field configurations and for temperatures from 100,000
to several million degrees. These upflows are spatio-temporally correlated with
and have similar upward velocities as recently discovered, cool (10,000 K)
chromospheric jets or (type II) spicules. We find these upflows to be pervasive
and universal. Order of magnitude estimates constrained by conservation of mass
and observed emission measures indicate that the mass supplied by these
spicules can play a significant role in supplying the corona with hot plasma.
The properties of these events are incompatible with coronal loop models that
only include nanoflares at coronal heights. Our results suggest that a
significant part of the heating and energizing of the corona occurs at
chromospheric heights, in association with chromospheric jets.Comment: 14 pages, 5 figures, accepted for publication in ApJ letter
Forward modeling of emission in SDO/AIA passbands from dynamic 3D simulations
It is typically assumed that emission in the passbands of the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is
dominated by single or several strong lines from ions that under equilibrium
conditions are formed in a narrow range of temperatures. However, most SDO/AIA
channels also contain contributions from lines of ions that have formation
temperatures that are significantly different from the "dominant" ion(s). We
investigate the importance of these lines by forward modeling the emission in
the SDO/AIA channels with 3D radiative MHD simulations of a model that spans
the upper layer of the convection zone to the low corona. The model is highly
dynamic. In addition, we pump a steadily increasing magnetic flux into the
corona, in order to increase the coronal temperature through the dissipation of
magnetic stresses. As a consequence, the model covers different ranges of
coronal temperatures as time progresses. The model covers coronal temperatures
that are representative of plasma conditions in coronal holes and quiet sun.
The 131, 171, and 304 \AA{} AIA passbands are found to be least influenced by
the so-called "non-dominant" ions, and the emission observed in these channels
comes mostly from plasma at temperatures near the formation temperature of the
dominant ion(s). On the other hand, the other channels are strongly influenced
by the non-dominant ions, and therefore significant emission in these channels
comes from plasma at temperatures that are different from the "canonical"
values. We have also studied the influence of non-dominant ions on the AIA
passbands when different element abundances are assumed (photospheric and
coronal), and when the effects of the electron density on the contribution
function are taken into account.Comment: 48 pages, 14 figures, accepted to be publish in Ap
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
Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere
The ionization of hydrogen in the solar chromosphere and transition region
does not obey LTE or instantaneous statistical equilibrium because the
timescale is long compared with important hydrodynamical timescales, especially
of magneto-acoustic shocks. We implement an algorithm to compute
non-equilibrium hydrogen ionization and its coupling into the MHD equations
within an existing radiation MHD code, and perform a two-dimensional simulation
of the solar atmosphere from the convection zone to the corona. Analysis of the
simulation results and comparison to a companion simulation assuming LTE shows
that: a) Non-equilibrium computation delivers much smaller variations of the
chromospheric hydrogen ionization than for LTE. The ionization is smaller
within shocks but subsequently remains high in the cool intershock phases. As a
result, the chromospheric temperature variations are much larger than for LTE
because in non-equilibrium, hydrogen ionization is a less effective internal
energy buffer. The actual shock temperatures are therefore higher and the
intershock temperatures lower. b) The chromospheric populations of the hydrogen
n = 2 level, which governs the opacity of Halpha, are coupled to the ion
populations. They are set by the high temperature in shocks and subsequently
remain high in the cool intershock phases. c) The temperature structure and the
hydrogen level populations differ much between the chromosphere above
photospheric magnetic elements and above quiet internetwork. d) The hydrogen n
= 2 population and column density are persistently high in dynamic fibrils,
suggesting that these obtain their visibility from being optically thick in
Halpha also at low temperature.Comment: 10 pages, 4 figure
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