106 research outputs found
Carbon monoxide in the solar atmosphere II. Radiative cooling by CO lines
The role of carbon monoxide as a cooling agent for the thermal structure of
the mid-photospheric to low-chromospheric layers of the solar atmosphere in
internetwork regions is investigated. The treatment of radiative cooling via
spectral lines of carbon monoxide (CO) has been added to the radiation
chemo-hydrodynamics code CO5BOLD. [...] The CO opacity indeed causes additional
cooling at the fronts of propagating shock waves in the chromosphere. There,
the time-dependent approach results in a higher CO number density compared to
the equilibrium case and hence in a larger net radiative cooling rate. The
average gas temperature stratification of the model atmosphere, however, is
only reduced by roughly 100 K. Also the temperature fluctuations and the CO
number density are only affected to small extent. A numerical experiment
without dynamics shows that the CO cooling process works in principle and
drives the atmosphere to a cool radiative equilibrium state. At chromospheric
heights, the radiative relaxation of the atmosphere to a cool state takes
several 1000 s. The CO cooling process thus would seem to be too slow compared
to atmospheric dynamics to be responsible for the very cool temperature regions
observed in the solar atmosphere. The hydrodynamical timescales in our solar
atmosphere model are much too short to allow for the radiative relaxation to a
cool state, thus suppressing the potential thermal instability due to carbon
monoxide as a cooling agent. Apparently, the thermal structure and dynamics of
the outer model atmosphere are instead determined primarily by shock waves.Comment: 5 pages, 4 figures. A&A, accepted 06/12/200
Small-scale swirl events in the quiet Sun chromosphere
Recent progress in instrumentation enables solar observations with high
resolution simultaneously in the spatial, temporal, and spectral domains. We
use such high-resolution observations to study small-scale structures and
dynamics in the chromosphere of the quiet Sun. We analyze time series of
spectral scans through the Ca II 854.2nm spectral line obtained with the CRISP
instrument at the Swedish 1-m Solar Telescope. The targets are quiet Sun
regions inside coronal holes close to disc-centre. The line core maps exhibit
relatively few fibrils compared to what is normally observed in quiet Sun
regions outside coronal holes. The time series show a chaotic and dynamic scene
that includes spatially confined "swirl" events. These events feature dark and
bright rotating patches, which can consist of arcs, spiral arms, rings or ring
fragments. The width of the fragments typically appears to be on the order of
only 0.2", which is close to the effective spatial resolution. They exhibit
Doppler shifts of -2 to -4 km/s but sometimes up to -7 km/s, indicating fast
upflows. The diameter of a swirl is usually of the order of 2". At the location
of these swirls, the line wing and wide-band maps show close groups of
photospheric bright points that move with respect to each other. A likely
explanation is that the relative motion of the bright points twists the
associated magnetic field in the chromosphere above. Plasma or propagating
waves may then spiral upwards guided by the magnetic flux structure, thereby
producing the observed intensity signature of Doppler-shifted ring fragments.Comment: 4 pages, 3 figures, A&A Letter, accepted (final version
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
Is the Sun Lighter than the Earth? Isotopic CO in the Photosphere, Viewed through the Lens of 3D Spectrum Synthesis
We consider the formation of solar infrared (2-6 micron) rovibrational bands
of carbon monoxide (CO) in CO5BOLD 3D convection models, with the aim to refine
abundances of the heavy isotopes of carbon (13C) and oxygen (18O,17O), to
compare with direct capture measurements of solar wind light ions by the
Genesis Discovery Mission. We find that previous, mainly 1D, analyses were
systematically biased toward lower isotopic ratios (e.g., R23= 12C/13C),
suggesting an isotopically "heavy" Sun contrary to accepted fractionation
processes thought to have operated in the primitive solar nebula. The new 3D
ratios for 13C and 18O are: R23= 91.4 +/- 1.3 (Rsun= 89.2); and R68= 511 +/- 10
(Rsun= 499), where the uncertainties are 1 sigma and "optimistic." We also
obtained R67= 2738 +/- 118 (Rsun= 2632), but we caution that the observed
12C17O features are extremely weak. The new solar ratios for the oxygen
isotopes fall between the terrestrial values and those reported by Genesis
(R68= 530, R6= 2798), although including both within 2 sigma error flags, and
go in the direction favoring recent theories for the oxygen isotope composition
of Ca-Al inclusions (CAI) in primitive meteorites. While not a major focus of
this work, we derive an oxygen abundance of 603 +/- 9 ppm (relative to
hydrogen; 8.78 on the logarithmic H= 12 scale). That the Sun likely is lighter
than the Earth, isotopically speaking, removes the necessity to invoke exotic
fractionation processes during the early construction of the inner solar
system
Morphology and Dynamics of the Low Solar Chromosphere
The Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn
Solar Telescope of the NSO/SP is used to investigate the morphology and
dynamics of the lower chromosphere and the virtually non-magnetic fluctosphere
below. The study addresses in particular the structure of magnetic elements
that extend into these layers. We choose different quiet Sun regions in and
outside coronal holes. In inter-network regions with no significant magnetic
flux contributions above the detection limit of IBIS, we find intensity
structures with the characteristics of a shock wave pattern. The magnetic flux
elements in the network are long lived and seem to resemble the spatially
extended counterparts to the underlying photospheric magnetic elements. We
suggest a modification to common methods to derive the line-of-sight magnetic
field strength and explain some of the difficulties in deriving the magnetic
field vector from observations of the fluctosphere.Comment: accepted by ApJ, 16 pages, 8 figure
Observation of a short-lived pattern in the solar chromosphere
In this work we investigate the dynamic behavior of inter-network regions of
the solar chromosphere. We observed the chromosphere of the quiet Sun using a
narrow-band Lyot filter centered at the Ca II K 2v emission peak with a
bandpass of 0.3A. We achieved a spatial resolution of on average 0.7" at a
cadence of 10s. In the inter-network we find a mesh-like pattern that features
bright grains at the vertices. The pattern has a typical spatial scale of 1.95"
and a mean evolution time scale of 53s with a standard deviation of 10s. A
comparison of our results with a recent three-dimensional radiation
hydrodynamical model implies that the observed pattern is of chromospheric
origin. The measured time scales are not compatible with those of reversed
granulation in the photosphere although the appearance is similar. A direct
comparison between network and inter-network structure shows that their typical
time scales differ by at least a factor of two. The existence of a rapidly
evolving small-scale pattern in the inter-network regions supports the picture
of the lower chromosphere as a highly dynamical and intermittent phenomenon.Comment: Letter A&A 4 pages 5 figure
The horizontal internetwork magnetic field: numerical simulations in comparison to observations with Hinode
Observations with the Hinode space observatory led to the discovery of
predominantly horizontal magnetic fields in the photosphere of the quiet
internetwork region. Here we investigate realistic numerical simulations of the
surface layers of the Sun with respect to horizontal magnetic fields and
compute the corresponding polarimetric response in the Fe I 630 nm line pair.
We find a local maximum in the mean strength of the horizontal field component
at a height of around 500 km in the photosphere, where it surpasses the
vertical component by a factor of 2.0 or 5.6, depending on the initial and
boundary conditions. From the synthesized Stokes profiles we derive a mean
horizontal field component that is, respectively, 1.6 and 4.3 times stronger
than the vertical component. This is a consequence of both the intrinsically
stronger flux density of, and the larger area occupied by the horizontal
fields. We find that convective overshooting expels horizontal fields to the
upper photosphere, making the Poynting flux positive in the photosphere, while
this quantity is negative in the convectively unstable layer below it.Comment: 4 pages, 3 figures, minor revisions, esp. concerning top boundary
cond., ApJL accepte
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