356 research outputs found
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
Three-dimensional non-LTE radiative transfer computation of the Ca 8542 infrared line from a radiation-MHD simulation
Interpretation of imagery of the solar chromosphere in the widely used
\CaIIIR infrared line is hampered by its complex, three-dimensional and non-LTE
formation. Forward modelling is required to aid understanding. We use a 3D
non-LTE radiative transfer code to compute synthetic \CaIIIR images from a
radiation-MHD simulation of the solar atmosphere spanning from the convection
zone to the corona. We compare the simulation with observations obtained with
the CRISP filter at the Swedish 1--m Solar Telescope. We find that the
simulation reproduces dark patches in the blue line wing caused by Doppler
shifts, brightenings in the line core caused by upward-propagating shocks and
thin dark elongated structures in the line core that form the interface between
upward and downward gas motion in the chromosphere. The synthetic line core is
narrower than the observed one, indicating that the sun exhibits both more
vigorous large-scale dynamics as well as small scale motions that are not
resolved within the simulation, presumably owing to a lack of spatial
resolution.Comment: accepted as ApJ lette
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&
Numerical Simulations of Shock Wave-Driven Jets
We present the results of numerical simulations of shock wave-driven jets in
the solar atmosphere. The dependence of observable quantities like maximum
velocity and deceleration on parameters such as the period and amplitude of
initial disturbances and the inclination of the magnetic field is investigated.
Our simulations show excellent agreement with observations, and shed new light
on the correlation between velocity and deceleration and on the regional
differences found in observations.Comment: 7 pages, 11 figures, submitted to Ap
The stellar atmosphere simulation code Bifrost
Context: Numerical simulations of stellar convection and photospheres have
been developed to the point where detailed shapes of observed spectral lines
can be explained. Stellar atmospheres are very complex, and very different
physical regimes are present in the convection zone, photosphere, chromosphere,
transition region and corona. To understand the details of the atmosphere it is
necessary to simulate the whole atmosphere since the different layers interact
strongly. These physical regimes are very diverse and it takes a highly
efficient massively parallel numerical code to solve the associated equations.
Aims: The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the convection
zone to the corona.
Methods: The code is subjected to a number of validation tests, among them
the Sod shock tube test, the Orzag-Tang colliding shock test, boundary
condition tests and tests of how the code treats magnetic field advection,
chromospheric radiation, radiative transfer in an isothermal scattering
atmosphere, hydrogen ionization and thermal conduction.
Results: Bifrost completes the tests with good results and shows near linear
efficiency scaling to thousands of computing cores
Multi-Fluid Simulations of Upper Chromospheric Magnetic Reconnection with Helium-Hydrogen mixture
Our understanding of magnetic reconnection (MR) under chromospheric
conditions remains limited. Recent observations have demonstrated the important
role of ion-neutral interactions in the dynamics of the chromosphere.
Furthermore, the comparison between spectral profiles and synthetic
observations of reconnection events suggest that current MHD approaches appear
to be inconsistent with observations. First, collisions and multi-thermal
aspects of the plasma play a role in these regions. Second, hydrogen and helium
ionization effects are relevant to the energy balance of the chromosphere. This
work investigates multi-fluid multi-species (MFMS) effects on MR in conditions
representative of the upper chromosphere using the multi-fluid Ebysus code. We
compare an MFMS approach based on a helium-hydrogen mixture with a two-fluid
MHD model based on hydrogen only. The simulations of MRs are performed in a
Lundquist number regime high enough to develop plasmoids and instabilities. We
study the evolution of the MR and compare the two approaches including the
structure of the current sheet and plasmoids, the decoupling of the particles,
the evolution of the heating mechanisms, and the composition. The presence of
helium species leads to more efficient heating mechanisms than the two-fluid
case. This scenario, which is out of reach of the two-fluid or single-fluid
models, can reach transition region temperatures starting from upper
chromospheric thermodynamic conditions, representative of a quiet Sun scenario.
The different dynamics between helium and hydrogen species could lead to
chemical fractionation and, under certain conditions, enrichment of helium in
the strongest outflows. This could be of significance for recent observations
of helium enrichment in the solar wind in switchbacks and CMEs
Disentangling flows in the solar transition region
The measured average velocities in solar and stellar spectral lines formed at
transition region temperatures have been difficult to interpret. However,
realistic three-dimensional radiation magnetohydrodynamics (3D rMHD) models of
the solar atmosphere are able to reproduce the observed dominant line shifts
and may thus hold the key to resolve these issues. Our new 3D rMHD simulations
aim to shed light on how mass flows between the chromosphere and corona and on
how the coronal mass is maintained. Passive tracer particles, so-called corks,
allow the tracking of parcels of plasma over time and thus the study of changes
in plasma temperature and velocity not only locally, but also in a co-moving
frame. By following the trajectories of the corks, we can investigate mass and
energy flows and understand the composition of the observed velocities. Our
findings show that most of the transition region mass is cooling. The
preponderance of transition region redshifts in the model can be explained by
the higher percentage of downflowing mass in the lower and middle transition
region. The average upflows in the upper transition region can be explained by
a combination of both stronger upflows than downflows and a higher percentage
of upflowing mass. The most common combination at lower and middle transition
region temperatures are corks that are cooling and traveling downward. For
these corks, a strong correlation between the pressure gradient along the
magnetic field line and the velocity along the magnetic field line has been
observed, indicating a formation mechanism that is related to downward
propagating pressure disturbances. Corks at upper transition region
temperatures are subject to a rather slow and highly variable but continuous
heating process.Comment: 13 pages, 10 figures, online movi
ALMA observations of transient heating in a solar active region
We aim to investigate the temperature enhancements and formation heights of
impulsive heating phenomena in solar active-regions such as Ellerman bombs
(EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs)
using interferometric observations in the millimeter (mm) continuum provided by
the Atacama Large Millimeter/submillimeter Array (ALMA). We examined 3 mm
signatures of heating events identified in Solar Dynamics Observatory (SDO)
observations of an active region and compared the results with synthetic
spectra from a 3D radiative magnetohydrodynamic simulation. We estimated the
contribution from the corona to the mm brightness using differential emission
measure analysis. We report the null detection of EBs in the 3 mm continuum at
" spatial resolution, which is evidence that they are sub-canopy
events that do not significantly contribute to heating the upper chromosphere.
In contrast, we find the active region to be populated with multiple compact,
bright, flickering mm bursts -- reminiscent of UVBs. The high brightness
temperatures of up to K in some events have a significant
contribution (up to 7%) from the corona. We also detect FAF-like events
in the 3 mm continuum that show rapid motions of K plasma launched
with high plane-of-sky velocities () from bright
kernels. The mm FAFs are the brightest class of warm canopy fibrils that
connect magnetic regions of opposite polarities. The simulation confirms that
ALMA should be able to detect the mm counterparts of UVBs and small flares and
thus provide a complementary diagnostic for localized heating in the solar
chromosphere.Comment: revised; accepted in Astronomy & Astrophysic
Intermittent reconnection and plasmoids in UV bursts in the low solar atmosphere
Magnetic reconnection is thought to drive a wide variety of dynamic phenomena
in the solar atmosphere. Yet the detailed physical mechanisms driving
reconnection are difficult to discern in the remote sensing observations that
are used to study the solar atmosphere. In this paper we exploit the
high-resolution instruments Interface Region Imaging Spectrograph (IRIS) and
the new CHROMIS Fabry-Perot instrument at the Swedish 1-m Solar Telescope (SST)
to identify the intermittency of magnetic reconnection and its association with
the formation of plasmoids in so-called UV bursts in the low solar atmosphere.
The Si IV 1403A UV burst spectra from the transition region show evidence of
highly broadened line profiles with often non-Gaussian and triangular shapes,
in addition to signatures of bidirectional flows. Such profiles had previously
been linked, in idealized numerical simulations, to magnetic reconnection
driven by the plasmoid instability. Simultaneous CHROMIS images in the
chromospheric Ca II K 3934A line now provide compelling evidence for the
presence of plasmoids, by revealing highly dynamic and rapidly moving
brightenings that are smaller than 0.2 arcsec and that evolve on timescales of
order seconds. Our interpretation of the observations is supported by detailed
comparisons with synthetic observables from advanced numerical simulations of
magnetic reconnection and associated plasmoids in the chromosphere. Our results
highlight how subarcsecond imaging spectroscopy sensitive to a wide range of
temperatures combined with advanced numerical simulations that are realistic
enough to compare with observations can directly reveal the small-scale
physical processes that drive the wide range of phenomena in the solar
atmosphere.Comment: Accepted for publication in Astrophysical Journal Letters. Movies are
available at http://folk.uio.no/rouppe/plasmoids_chromis
Bombs and flares at the surface and lower atmosphere of the Sun
This research was supported by the Research Council of Norway and by the European Research Council under the European Union's Seventh Framework Programme (FP7/2007–2013)/ERC Grant agreement no. 291058.A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Hα line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma "bombs" (UV bursts) with high temperatures of perhaps up to 8 × 104 K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (106 K) in the upper chromosphere. These results provide new insights into the emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere.Publisher PDFPeer reviewe
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