arXiv subjects

Mats Carlsson

Publications and source records attributed to Mats Carlsson.

At least 127 records · Page 7Linked to original sources

arXiv2011

On the minimum temperature of the quiet solar chromosphere

aims: We aim to provide an estimate of the minimum temperature of the quiet solar chromosphere. methods: We perform a 2D radiation-MHD simulation spanning the upper convection zone to the lower corona. The simulation includes non-LTE radiative transfer and {an equation-of-state that includes non-equilibrium ionization of hydrogen and non-equilibrium H_2 molecule formation}. We analyze the reliability of the various assumptions made in our model in order to assess the realism of the simulation. results: Our simulation contains pockets of cool gas with down to 1660 K from 1 Mm up to 3.2 Mm height. It overestimates the radiative heating, and contains non-physical heating below 1660 K. Therefore we conclude that cool pockets in the quiet solar chromosphere might have even lower temperatures than in the simulation, provided that there exist areas in the chromosphere without significant magnetic heating. We suggest off-limb molecular spectroscopy to look for such cool pockets and 3D simulations including a local dynamo and a magnetic carpet to investigate Joule heating in the quiet chromosphere.

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arXiv2011

Solar velocity references from 3D HD photospheric models

The measurement of Doppler velocities in spectroscopic solar observations requires a reference for the local frame of rest. The rotational and radial velocities of the Earth and the rotation of the Sun introduce velocity offsets in the observations. Normally, good references for velocities are missing (e.g. telluric lines), especially in filter-based spectropolarimetric observations. We determine an absolute reference for line-of-sight velocities measured from solar observations for any heliocentric angle, calibrating the convective line shift of spatially-averaged profiles on quiet sun from a 3D hydrodynamical simulation. This method works whenever there is quiet sun in the field-of-view, and it has the advantage of being relatively insensitive to uncertainties in the atomic data. We carry out radiative transfer computations in LTE for selected C I and Fe I lines, whereas the Ca II infrared lines are synthesized in non-LTE. The degree of asymmetry and the line shifts, however, show a clear dependence on the heliocentric angle and the properties of the lines. The profiles at μ= 1 are compared with observed profiles to prove their reliability, and they are tested against errors induced by the LTE calculations, inaccuracies in the atomic data and the 3D simulation. Theoretical quiet-sun profiles of lines commonly used by solar observers are provided to the community. Those can be used as absolute references for line-of-sight velocities. The limb effect is produced by the projection of the 3D atmosphere along the line of sight. We estimate the precision of the disk-center line shifts to be approximately 50 m s^-1, but the off-center profiles remain to be tested against observations.

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arXiv2011

Non-equilibrium calcium ionisation in the solar atmosphere

Our aim is to determine the dominant processes and timescales for the ionisation equilibrium of calcium under solar chromospheric conditions. The study is based on numerical simulations with the RADYN code, which includes hydrodynamics, radiative transfer, and a detailed non-equilibrium treatment of hydrogen, calcium, and helium. The simulations are characterised by upwards propagating shock waves, which cause strong temperature fluctuations and variations of the ionisation degree of calcium. The passage of a hot shock front leads to a strong net ionisation of Ca II, rapidly followed by net recombination. The relaxation timescale of the Ca ionisation state is found to be of the order of a few seconds at the top of the photosphere and 10 to 30 s in the upper chromosphere. Generally, the timescales are significantly reduced in the wakes of hot shock fronts. The timescales can be reliably determined from a simple analysis of the eigenvalues of the transition rate matrix. The timescales are dominated by the radiative recombination from Ca III into the metastable Ca II energy levels of the 4d 2D term. These transitions depend strongly on the density of free electrons and therefore on the (non-equilibrium) ionisation degree of hydrogen, which is the main electron donor. The ionisation/recombination timescales derived here are too long for the assumption of an instantaneous ionisation equilibrium to be valid and, on the other hand, are not long enough to warrant an assumption of a constant ionisation fraction. Fortunately, the ionisation degree of Ca II remains small in the height range, where the cores of the H, K, and the infrared triplet lines are formed. We conclude that the difference due to a detailed treatment of Ca ionisation has only negligible impact on the modelling of spectral lines of Ca II and the plasma properties under the conditions in the quiet solar chromosphere.

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arXiv2010

SICStus Prolog -- the first 25 years

SICStus Prolog has evolved for nearly 25 years. This is an appropriate point in time for revisiting the main language and design decisions, and try to distill some lessons. SICStus Prolog was conceived in a context of multiple, conflicting Prolog dialect camps and a fledgling standardization effort. We reflect on the impact of this effort and role model implementations on our development. After summarizing the development history, we give a guided tour of the system anatomy, exposing some designs that were not published before. We give an overview of our new interactive development environment, and describe a sample of key applications. Finally, we try to identify key good and not so good design decisions.

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arXiv2010

On the solar chromosphere observed at the limb with Hinode

Broad-band images in the Ca II H line, from the BFI instrument on the Hinode spacecraft, show emission from spicules emerging from and visible right down to the observed limb. Surprisingly, little absorption of spicule light is seen along their lengths. We present formal solutions to the transfer equation for given (ad-hoc) source functions, including a stratified chromosphere from which spicules emanate. The model parameters are broadly compatible with earlier studies of spicules. The visibility of Ca II spicules down to the limb in Hinode data seems to require that spicule emission be Doppler shifted relative to the stratified atmosphere, either by supersonic turbulent or organized spicular motion. The non-spicule component of the chromosphere is almost invisible in the broad band BFI data, but we predict that it will be clearly visible in high spectral resolution data. Broad band Ca II H limb images give the false impression that the chromosphere is dominated by spicules. Our analysis serves as a reminder that the absence of a signature can be as significant as its presence.

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arXiv2010

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.

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arXiv2010

Chromospheric heating and structure as determined from high resolution 3D simulations

We have performed 3D radiation MHD simulations extending from the convection zone to the corona covering a box 16 Mm$^3$ at 32 km spatial resolution. The simulations show very fine structure in the chromosphere with acoustic shocks interacting with the magnetic field. Magnetic flux concentrations have a temperature lower than the surroundings in the photosphere but higher in the low chromosphere. The heating is there mostly through ohmic dissipation preferentially at the edges of the flux concentrations. The magnetic field is often wound up around the flux concentrations. When acoustic waves travel up along the field this topology leads to swirling motions seen in chromospheric diagnostic lines such as the calcium infrared triplet.

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arXiv2009

The Temperature Diagnostic Properties of the MG I 457.1 NM Line

We analyze the important formation processes for the Mg I 457.1 nm line. This line is an intercom- bination line and the source function is close to the local thermodynamic equilibrium (LTE) value. The strong coupling to the local temperature and the relatively high population of the lower level (the ground state of Mg I) makes this line an ideal candidate for temperature diagnostics in the lower chromosphere/temperature minimum region. Linking the temperature probed to an absolute phys- ical height is non trivial because of Non-LTE ionization. We analyze the Non-LTE effects and find that photo-ionization from the lower energy levels together with cascading collisional recombination dominate the ionization balance. Taking properly into account the line-blanketing in the UV is essen- tial for obtaining the right photoionization rates. The identification of the main Non-LTE effects in the line allows us to construct a quintessential model atom, ideal for computationally demanding tasks, e.g. full 3D and/or time-dependent radiative transfer. Furthermore we analyze the diagnostic potential to temperature of this line in solar-like atmospheres, by synthesizing the line from a series of parametrized atmospheric models. These models have been constructed with fixed effective temper- ature, but with a variable heat term in the energy equation to obtain a chromospheric temperature rise at different heights. We conclude that the line has a significant potential in the diagnostics of the lower chromosphere temperature structure, especially for cooler atmospheres, such as sunspots.

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arXiv2007

Twisted flux tube emergence from the convection zone to the corona

3D numerical simulations of a horizontal magnetic flux tube emergence with different twist are carried out in a computational domain spanning the upper layers of the convection zone to the lower corona. We use the Oslo Staggered Code to solve the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along the magnetic field lines. The emergence of the magnetic flux tube input at the bottom boundary into a weakly magnetized atmosphere is presented. The photospheric and chromospheric response is described with magnetograms, synthetic images and velocity field distributions. The emergence of a magnetic flux tube into such an atmosphere results in varied atmospheric responses. In the photosphere the granular size increases when the flux tube approaches from below. In the convective overshoot region some 200km above the photosphere adiabatic expansion produces cooling, darker regions with the structure of granulation cells. We also find collapsed granulation in the boundaries of the rising flux tube. Once the flux tube has crossed the photosphere, bright points related with concentrated magnetic field, vorticity, high vertical velocities and heating by compressed material are found at heights up to 500km above the photosphere. At greater heights in the magnetized chromosphere, the rising flux tube produces a cool, magnetized bubble that tends to expel the usual chromospheric oscillations. In addition the rising flux tube dramatically increases the chromospheric scale height, pushing the transition region and corona aside such that the chromosphere extends up to 6Mm above the photosphere. The emergence of magnetic flux tubes through the photosphere to the lower corona is a relatively slow process, taking of order 1 hour.

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arXiv2007

On connecting the dynamics of the chromosphere and transition region with Hinode SOT and EIS

We use coordinated Hinode SOT/EIS observations that include high-resolution magnetograms, chromospheric and TR imaging and TR/coronal spectra in a first test to study how the dynamics of the TR are driven by the highly dynamic photospheric magnetic fields and the ubiquitous chromospheric waves. Initial analysis shows that these connections are quite subtle and require a combination of techniques including magnetic field extrapolations, frequency-filtered time-series and comparisons with synthetic chromospheric and TR images from advanced 3D numerical simulations. As a first result, we find signatures of magnetic flux emergence as well as 3 and 5 mHz wave power above regions of enhanced photospheric magnetic field in both chromospheric, transition region and coronal emission.

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arXiv2007

A Tale Of Two Spicules: The Impact of Spicules on the Magnetic Chromosphere

We use high-resolution observations of the Sun in Ca II H 3968 A from the Solar Optical Telescope on Hinode to show that there are at least two types of spicules that dominate the structure of the magnetic solar chromosphere. Both types are tied to the relentless magnetoconvective driving in the photosphere, but have very different dynamic properties. ``Type-I'' spicules are driven by shock waves that form when global oscillations and convective flows leak into the upper atmosphere along magnetic field lines on 3-7 minute timescales. ``Type-II'' spicules are much more dynamic: they form rapidly (in ~10s), are very thin (<200km wide), have lifetimes of 10-150s (at any one height) and seem to be rapidly heated to (at least) transition region temperatures, sending material through the chromosphere at speeds of order 50-150 km/s. The properties of Type II spicules suggest a formation process that is a consequence of magnetic reconnection, typically in the vicinity of magnetic flux concentrations in plage and network. Both types of spicules are observed to carry Alfven waves with significant amplitudes of order 20 km/s.

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arXiv2007

Spectroscopic measurements of dynamic fibrils in the Ca {\small{II}} 8662 Å line

We present high spatial resolution spectroscopic measurements of dynamic fibrils (DFs) in the Ca {\small{II}} 8662 Å line. These data show clear Doppler shifts in the identified DFs, which demonstrates that at least a subset of DFs are actual mass motions in the chromosphere. A statistical analysis of 26 DFs reveals a strong and statistically significant correlation between the maximal velocity and the deceleration. The range of the velocities and the decelerations are substantially lower, about a factor two, in our spectroscopic observations compared to the earlier results based on proper motion in narrow band images. There are fundamental differences in the different observational methods; when DFs are observed spectroscopically the measured Doppler shifts are a result of the atmospheric velocity, weighted with the response function to velocity over an extended height. When the proper motion of DFs is observed in narrow band images, the movement of the top of the DF is observed. This point is sharply defined because of the high contrast between the DF and the surroundings. The observational differences between the two methods are examined by several numerical experiments using both numerical simulations and a time series of narrow band H$α$ images. With basis in the simulations we conclude that the lower maximal velocity is explained by the low formation height of the Ca IR line. We conclude that the present observations support the earlier result that DFs are driven by magneto-acoustic shocks exited by convective flows and p-modes.

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arXiv2007

Can High Frequency Acoustic Waves Heat the Quiet Sun Chromosphere?

We use Hinode/SOT Ca II H-line and blue continuum broadband observations to study the presence and power of high frequency acoustic waves at high spatial resolution. We find that there is no dominant power at small spatial scales; the integrated power using the full resolution of Hinode (0.05'' pixels, 0.16'' resolution) is larger than the power in the data degraded to 0.5'' pixels (TRACE pixel size) by only a factor of 1.2. At 20 mHz the ratio is 1.6. Combining this result with the estimates of the acoustic flux based on TRACE data of Fossum & Carlsson (2006), we conclude that the total energy flux in acoustic waves of frequency 5-40 mHz entering the internetwork chromosphere of the quiet Sun is less than 800 W m$^{-2}$, inadequate to balance the radiative losses in a static chromosphere by a factor of five.

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arXiv2007

3d Numerical Models of the Chromosphere, Transition Region, and Corona

A major goal in solar physics has during the last five decades been to find how energy flux generated in the solar convection zone is transported and dissipated in the outer solar layers. Progress in this field has been slow and painstaking. However, advances in computer hardware and numerical methods, vastly increased observational capabilities and growing physical insight seem finally to be leading towards understanding. Here we present exploratory numerical MHD models that span the entire solar atmosphere from the upper convection zone to the lower corona. These models include non-grey, non-LTE radiative transport in the photosphere and chromosphere, optically thin radiative losses as well as magnetic field-aligned heat conduction in the transition region and corona.

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arXiv2007

Observations and Simulations of Fibrils and Mottles

With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. In this paper, we briefly summarize results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in H-alpha) above and about active region plage. Using extremely high-resolution observations obtained at the SST, and advanced numerical 2D radiative MHD simulations, we show that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. In addition, we present some preliminary observations of quiet Sun jets or mottles. We find that the mechanism that produces fibrils in active regions is most likely also at work in quiet Sun regions, although it is modified by the weaker magnetic field and the presence of more mixed-polarity. A comparison with numerical simulations suggests that the weaker magnetic field in quiet Sun allows for significantly stronger (than in active regions) transverse motions that are superposed on the field-aligned, shock-driven motions. This leads to a more dynamic, and much more complex environment than in active region plage. In addition, our observations of the mixed polarity environment in quiet Sun regions suggest that other mechanisms, such as reconnection, may well play a significant role in the formation of some quiet Sun jets.

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arXiv2006

Velocities measured in small scale solar magnetic elements

We have obtained high resolution spectrograms of small scale magnetic structures with the Swedish 1-m Solar Telescope. We present Doppler measurements at $0\farcs{2}$ spatial resolution of bright points, ribbons and flowers and their immediate surroundings, in the C {\small{I}} 5380.3 Å line (formed in the deep photosphere) and the two Fe {\small{I}} lines at 5379.6 Å and 5386.3 Å. The velocity inside the flowers and ribbons are measured to be almost zero, while we observe downflows at the edges. These downflows are increasing with decreasing height. We also analyze realistic magneto-convective simulations to obtain a better understanding of the interpretation of the observed signal. We calculate how the Doppler signal depends on the velocity field in various structures. Both the smearing effect of the non-negligible width of this velocity response function along the line of sight and of the smearing from the telescope and atmospheric point spread function are discussed. These studies lead us to the conclusion that the velocity inside the magnetic elements are really upflow of the order 1--2 km s${}^{-1}$ while the downflows at the edges really are much stronger than observed, of the order 1.5--3.3 km s${}^{-1}$.

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arXiv2006

Radiative Hydrodynamic Models of Optical and Ultraviolet Emission from M Dwarf Flares

We report on radiative hydrodynamic simulations of M dwarf stellar flares and compare the model predictions to observations of several flares. The flares were simulated by calculating the hydrodynamic response of a model M dwarf atmosphere to a beam of non-thermal electrons. Radiative backwarming through numerous soft X-ray, extreme ultraviolet, and ultraviolet transitions are also included. The equations of radiative transfer and statistical equilibrium are treated in non-LTE for many transitions of hydrogen, helium and the Ca II ion allowing the calculation of detailed line profiles and continuum radiation. Two simulations were carried out, with electron beam fluxes corresponding to moderate and strong beam heating. In both cases we find the dynamics can be naturally divided into two phases: an initial gentle phase in which hydrogen and helium radiate away much of the beam energy, and an explosive phase characterized by large hydrodynamic waves. During the initial phase, lower chromospheric material is evaporated into higher regions of the atmosphere causing many lines and continua to brighten dramatically. The He II Lya line is especially enhanced, becoming the brightest line in the flaring spectrum. The hydrogen Balmer lines also become much brighter and show very broad line widths, in agreement with observations. We compare our predicted Balmer decrements to decrements calculated for several flare observations and find the predictions to be in general agreement with the observations. During the explosive phase both condensation and evaporation waves are produced. The moderate flare simulation predicts a peak evaporation wave of 130 km/s and a condensation wave of 30 km/s. The velocity of the condensation wave matches velocities observed in several transition region lines.

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arXiv2005

Non-LTE neutral carbon spectral line formation in late-type stars

We present non-Local Thermodynamic Equilibrium (non-LTE) calculations for neutral carbon spectral line formation, carried out for a grid of model atmospheres covering the range of late-type stars. The results of our detailed calculations suggest that the carbon non-LTE corrections in these stars are higher than usually adopted, remaining substantial even at low metallicity. For the most metal-poor stars in the sample of Akerman et al. (2004), the non-LTE abundance corrections are of the order of -0.35...-0.45 dex (when neglecting H collisions). Applying our results to those observations, the apparent [C/O] upturn seen in their LTE analysis is no longer present, thus revealing no need to invoke contributions from Pop. III stars to the carbon nucleosynthesis.

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