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

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

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

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

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

Inter-network regions of the Sun at millimetre wavelengths

The continuum intensity at wavelengths around 1 mm provides an excellent way to probe the solar chromosphere. Future high-resolution millimetre arrays, such as the Atacama Large Millimeter Array (ALMA), will thus produce valuable input for the ongoing controversy on the thermal structure and the dynamics of this layer. Synthetic brightness temperature maps are calculated on basis of three-dimensional radiation (magneto-)hydrodynamic (MHD) simulations. While the millimetre continuum at 0.3mm originates mainly from the upper photosphere, the longer wavelengths considered here map the low and middle chromosphere. The effective formation height increases generally with wavelength and also from disk-centre towards the solar limb. The average intensity contribution functions are usually rather broad and in some cases they are even double-peaked as there are contributions from hot shock waves and cool post-shock regions in the model chromosphere. Taking into account the deviations from ionisation equilibrium for hydrogen gives a less strong variation of the electron density and with it of the optical depth. The result is a narrower formation height range. The average brightness temperature increases with wavelength and towards the limb. The relative contrast depends on wavelength in the same way as the average intensity but decreases towards the limb. The dependence of the brightness temperature distribution on wavelength and disk-position can be explained with the differences in formation height and the variation of temperature fluctuations with height in the model atmospheres.Comment: 15 pages, 10 figures, accepted for publication in A&A (15.05.07

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