478 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
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
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
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
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
Time-dependent hydrogen ionisation in the solar chromosphere. I: Methods and first results
An approximate method for solving the rate equations for the hydrogen
populations was extended and implemented in the three-dimensional radiation
(magneto-)hydrodynamics code CO5BOLD. The method is based on a model atom with
six energy levels and fixed radiative rates. It has been tested extensively in
one-dimensional simulations. The extended method has been used to create a
three-dimensional model that extends from the upper convection zone to the
chromosphere. The ionisation degree of hydrogen in our time-dependent
simulation is comparable to the corresponding equilibrium value up to 500 km
above optical depth unity. Above this height, the non-equilibrium ionisation
degree is fairly constant over time and space, and tends to be at a value set
by hot propagating shock waves. The hydrogen level populations and electron
density are much more constant than the corresponding values for statistical
equilibrium, too. In contrast, the equilibrium ionisation degree varies by more
than 20 orders of magnitude between hot, shocked regions and cool, non-shocked
regions. The simulation shows for the first time in 3D that the chromospheric
hydrogen ionisation degree and electron density cannot be calculated in
equilibrium. Our simulation can provide realistic values of those quantities
for detailed radiative transfer computations.Comment: 8 pages, 7 figure
Small-scale magnetic flux emergence in the quiet Sun
Small bipolar magnetic features are observed to appear in the interior of
individual granules in the quiet Sun, signaling the emergence of tiny magnetic
loops from the solar interior. We study the origin of those features as part of
the magnetoconvection process in the top layers of the convection zone. Two
quiet-Sun magnetoconvection models, calculated with the
radiation-magnetohydrodynamic (MHD) Bifrost code and with domain stretching
from the top layers of the convection zone to the corona, are analyzed. Using
3D visualization as well as a posteriori spectral synthesis of Stokes
parameters, we detect the repeated emergence of small magnetic elements in the
interior of granules, as in the observations. Additionally, we identify the
formation of organized horizontal magnetic sheets covering whole granules. Our
approach is twofold, calculating statistical properties of the system, like
joint probability density functions (JPDFs), and pursuing individual events via
visualization tools. We conclude that the small magnetic loops surfacing within
individual granules in the observations may originate from sites at or near the
downflows in the granular and mesogranular levels, probably in the first 1 or
1.5 Mm below the surface. We also document the creation of granule-covering
magnetic sheet-like structures through the sideways expansion of a small
subphotospheric magnetic concentration picked up, and pulled out of the
interior, by a nascent granule. The sheet-like structures we found in the
models may match the recent observations of Centeno et al. (2017).Comment: 9 pages, 5 figures, Published in The Astrophysical Journal Letter
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
- …