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Context: We introduce the computation of global reference frames from Very Long Baseline Interferometry (VLBI) observations at the Vienna International VLBI Service for Geodesy and Astrometry (IVS) Analysis Center (VIE) in detail. We focus on the celestial and terrestrial frames from our two latest solutions VIE2020 and VIE2022b. Aims: The current International Celestial and Terrestrial Reference Frames, ICRF3 and ITRF2020, include VLBI observations until spring 2018 and December 2020, respectively. We provide terrestrial and celestial reference frames including VLBI sessions until June 2022 organized by the IVS. Methods: Vienna terrestrial and celestial reference frames are computed in a common least squares adjustment of geodetic and astrometric VLBI observations with the Vienna VLBI and Satellite Software (VieVS). Results: We provide high-quality celestial and terrestrial reference frames computed from 24-hour IVS observing sessions. The CRF provides positions of 5407 radio sources. In particular, positions of sources with few observations at the time of the ICRF3 calculation could be improved. The frame also includes positions of 870 new radio sources, which are not included in ICRF3. The additional observations beyond the data used for ITRF2020 provide a more reliable estimation of positions and linear velocities of newly established VLBI Global Observing System (VGOS) telescopes., Comment: 11 pages, Submitted to the Astronomy & Astrophysics

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.

From the analysis of Dopplergrams in the K I 7699 A and Na I 5890 A spectral lines observed with the Magneto-Optical filter at Two Heights (MOTH) experiment during the austral summer in 2002-03 we find upward traveling waves in magnetic regions. Our analysis shows that the dispersion relation of these waves strongly depends on whether the wave is detected in the low-beta or high-beta regime. Moreover, the observed dispersion relation does not show the expected decrease of the acoustic cut-off frequency for the field guided slow magnetic wave. Instead, we detected an increase of the travel times below the acoustic cut-off frequency and at the same time a decrease of the travel time above it. To study the formation height of the spectral lines employed by MOTH in greater detail we are currently in the process of employing 3D MHD simulations carried out with CO5BOLD to perform NLTE spectral synthesis., Comment: Proceedings article for NSO Workshop #25 "Chromospheric Structure and Dynamics" (2009), to appear in Memorie della Societa Astronomica Italiana

We propose a method to derive the line-of-sight magnetic flux density from measurements in the chromospheric Ca II IR line at 854.2 nm. The method combines two well-understood techniques, the center-of-gravity and bisector methods, in a single hybrid technique. The technique is tested with magneto-static simulations of a flux tube. We apply the method to observations with the Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn Solar Telescope of the NSO/SP to investigate the morphology of the lower chromosphere, with focus on the chromospheric counterparts to the underlying photospheric magnetic flux elements., Comment: to appear in Memorie della Societa' Astronomica Italiana, 2010, eds. A. Tritschler and K. Reardon

We use joint observations obtained with the Hinode space observatory and the Interferometric Bidimensional Spectrometer (IBIS) installed at the DST of the NSO/SP to investigate the morphology and dynamics of (a) non-magnetic and (b) magnetic regions in the fluctosphere. In inter-network regions with no significant magnetic flux contributions above the detection limit of IBIS, we find intensity structures with similar characteristics as those seen in numerical simulations by Wedemeyer-B\"ohm (2008). The magnetic flux elements in the network are stable and seem to resemble the spatially extended counterparts to the underlying photospheric magnetic elements. We will explain some of the difficulties in deriving the magnetic field vector from observations of the fluctosphere., Comment: to appear in "The Second Hinode Science Meeting: Beyond Discovery - Toward Understanding", ASP Conference Series, Vol. 415, eds. B. Lites, M. Cheung, T. Magara, J. Mariska, and K. Reeves

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 figures

For many years, there seemed to be significant differences between the continuum intensity distributions derived from observations and simulations of the solar photosphere. In order to settle the discussion on these apparent discrepancies, we present a detailed comparison between simulations and seeing-free observations that takes into account the crucial influence of instrumental image degradation. We use a set of images of quiet Sun granulation taken in the blue, green and red continuum bands of the Broadband Filter Imager of the Solar Optical Telescope (SOT) onboard Hinode. The images are deconvolved with Point Spread Functions (PSF) that account for non-ideal contributions due to instrumental stray-light and imperfections. In addition, synthetic intensity images are degraded with the corresponding PSFs. The results are compared with respect to spatial power spectra, intensity histograms, and the centre-to-limb variation of the intensity contrast. The observational findings are well matched with corresponding synthetic observables from three-dimensional radiation (magneto-)hydrodynamic simulations. We conclude that the intensity contrast of the solar continuum intensity is higher than usually derived from ground-based observations and is well reproduced by modern numerical simulations. Properly accounting for image degradation effects is of crucial importance for comparisons between observations and numerical models. It finally settles the traditionally perceived conflict between observations and simulations., Comment: 4 pages, 2 figures; proceedings article for Joint Discussion 10 at the IAU General Assembly, Rio de Janeiro, Brazil, August 2009

Three-dimensional magnetohydrodynamic simulations of the surface layers of the Sun intrinsically produce a predominantly horizontal magnetic field in the photosphere. This is a robust result in the sense that it arises from simulations with largely different initial and boundary conditions for the magnetic field. While the disk-center synthetic circular and linear polarization signals agree with measurements from Hinode, their center-to-limb variation sensitively depends on the height variation of the horizontal and the vertical field component and they seem to be at variance with the observed behavior., Comment: To appear in "Second Hinode Science Meeting, Beyond Discovery -- Toward Understanding", eds. B. Lites, M. Cheung, K. Reeves, J. Mariska, and T. Magara ASP Conference Series xxx, 2009

The atmosphere of the Sun is characterized by a complex interplay of competing physical processes: convection, radiation, conduction, and magnetic fields. The most obvious imprint of the solar convection and its overshooting in the low atmosphere is the granulation pattern. Beside this dominating scale there is a more or less smooth distribution of spatial scales, both towards smaller and larger scales, making the Sun essentially a multi-scale object. Convection and overshooting give the photosphere its face but also act as drivers for the layers above, namely the chromosphere and corona. The magnetic field configuration effectively couples the atmospheric layers on a multitude of spatial scales, for instance in the form of loops that are anchored in the convection zone and continue through the atmosphere up into the chromosphere and corona. The magnetic field is also an important structuring agent for the small, granulation-size scales, although (hydrodynamic) shock waves also play an important role -- especially in the internetwork atmosphere where mostly weak fields prevail. Based on recent results from observations and numerical simulations, we attempt to present a comprehensive picture of the atmosphere of the quiet Sun as a highly intermittent and dynamic system., Comment: 33 pages, 16 figures; to appear in the Space Science Series of ISSI by Springer and in Space Science Reviews (accepted)

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 accepted

Isotope shifts in dielectronic recombination spectra were studied for Li-like ^{A}Nd^{57+} ions with A=142 and A=150. From the displacement of resonance positions energy shifts \delta E^{142,150}(2s-2p_1/2)= 40.2(3)(6) meV (stat)(sys)) and \delta E^{142,150}(2s-2p_3/2) = 42.3(12)(20) meV of 2s-2p_j transitions were deduced. An evaluation of these values within a full QED treatment yields a change in the mean-square charge radius of ^{142,150}\delta = -1.36(1)(3) fm^2. The approach is conceptually new and combines the advantage of a simple atomic structure with high sensitivity to nuclear size., Comment: 10 pages, 3 figures, accepted for publication in Physical Review Letters

We study the structure of the magnetic elements in network-cell interiors. A quiet Sun area close to the disc centre was observed with the spectro-polarimeter of the Solar Optical Telescope on board the Hinode space mission, which yielded the best spatial resolution ever achieved in polarimetric data of the Fe I 630 nm line pair. For comparison and interpretation, we synthesize a similar data set from a three-dimensional magneto-hydrodynamic simulation. We find several examples of magnetic elements, either roundish (tube) or elongated (sheet), which show a central area of negative Stokes-V area asymmetry framed or surrounded by a peripheral area with larger positive asymmetry. This pattern was predicted some eight years ago on the basis of numerical simulations. Here, we observationally confirm its existence for the first time. We gather convincing evidence that this pattern of Stokes-V area asymmetry is caused by the funnel-shaped boundary of magnetic elements that separates the flux concentration from the weak-field environment. We also conclude that this kind of magnetic element of the internetwork is accompanied by electric current sheets., Comment: 4 pages, 4 figures, to be published in A&A; acceptance date corrected; A&A language editing done

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)

We have measured resonance strengths and energies for dielectronic recombination (DR) of Mg-like Fe XV forming Al-like Fe XIV via N=3 -> N' = 3 core excitations in the electron-ion collision energy range 0-45 eV. All measurements were carried out using the heavy-ion Test Storage Ring at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. We have also carried out new multiconfiguration Breit-Pauli (MCBP) calculations using the AUTOSTRUCTURE code. For electron-ion collision energies < 25 eV we find poor agreement between our experimental and theoretical resonance energies and strengths. From 25 to 42 eV we find good agreement between the two for resonance energies. But in this energy range the theoretical resonance strengths are ~ 31% larger than the experimental results. This is larger than our estimated total experimental uncertainty in this energy range of +/- 26% (at a 90% confidence level). Above 42 eV the difference in the shape between the calculated and measured 3s3p(^1P_1)nl DR series limit we attribute partly to the nl dependence of the detection probabilities of high Rydberg states in the experiment. We have used our measurements, supplemented by our AUTOSTRUCTURE calculations, to produce a Maxwellian-averaged 3 -> 3 DR rate coefficient for Fe XV forming Fe XIV. The resulting rate coefficient is estimated to be accurate to better than +/- 29% (at a 90% confidence level) for k_BT_e > 1 eV. At temperatures of k_BT_e ~ 2.5-15 eV, where Fe XV is predicted to form in photoionized plasmas, significant discrepancies are found between our experimentally-derived rate coefficient and previously published theoretical results. Our new MCBP plasma rate coefficient is 19-28% smaller than our experimental results over this temperature range.

With numerical experiments we explore the feasibility of using high frequency waves for probing the magnetic fields in the photosphere and the chromosphere of the Sun. We track a plane-parallel, monochromatic wave that propagates through a non-stationary, realistic atmosphere, from the convection-zone through the photosphere into the magnetically dominated chromosphere, where it gets refracted and reflected. We compare the wave travel time between two fixed geometrical height levels in the atmosphere (representing the formation height of two spectral lines) with the topography of the surface of equal magnetic and thermal energy density (the magnetic canopy or beta=1 contour) and find good correspondence between the two. We conclude that high frequency waves indeed bear information on the topography of the `magnetic canopy'., Comment: 6 pages, 4 figures, to appear in Astron. Nachrichten, proceedings of the First HELAS Local Helioseismology Workshop "Roadmap for European Local Helioseismology", Nice, 2006

It is widely believed that the heating of the chromosphere in quiet-Sun internetwork regions is provided by dissipation of acoustic waves that are excited by the convective motions close to the top of the convection zone and in the photospheric overshoot layer. This view lately became challenged by observations suggesting that the acoustic energy flux into the chromosphere is too low, by a factor of at least ten. Based on a comparison of TRACE data with synthetic image sequences for a three-dimensional simulation extending from the top layers of the convection zone to the middle chromosphere, we come to the contradicting conclusion that the acoustic flux in the model provides sufficient energy for heating the solar chromosphere of internetwork regions. The role of a weak magnetic field and associated electric current sheets is also discussed., Comment: 10 pages, 3 figures, to appear in the proceedings of the Coimbra Solar Physics Meeting on The Physics of Chromospheric Plasmas, 2006, eds. P. Heinzel, I. Dorotovic, and R. J. Rutten

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 06/12/2006

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 figures

Recent three-dimensional radiation hydrodynamic simulations by Wedemeyer et al. (2004) suggest that the solar chromosphere is highly structured in space and time on scales of only 1000 km and 20-25 sec, resp.. The resulting pattern consists of a network of hot gas and enclosed cool regions which are due to the propagation and interaction of shock fronts. In contrast to many other diagnostics, the radio continuum at millimeter wavelengths is formed in LTE, and provides a rather direct measure of the thermal structure. It thus facilitates the comparison between numerical model and observation. While the involved time and length scales are not accessible with todays equipment for that wavelength range, the next generation of instruments, such as the Atacama Large Millimeter Array (ALMA), will provide a big step towards the required resolution. Here we present results of radiative transfer calculations at mm and sub-mm wavelengths with emphasis on spatial and temporal resolution which are crucial for the ongoing discussion about the chromospheric temperature structure., Comment: 4 pages, 3 figures (1 colour)

The radiation hydrodynamic code CO5BOLD has been supplemented with the time-dependent treatment of chemical reaction networks. Advection of particle densities due to the hydrodynamic flow field is also included. The radiative transfer is treated frequency-independently, i.e. grey, so far. The upgraded code has been applied to two-dimensional simulations of carbon monoxide (CO) in the non-magnetic solar photosphere and low chromosphere. For this purpose a reaction network has been constructed, taking into account the reactions which are most important for the formation and dissociation of CO under the physical conditions of the solar atmosphere. The network has been strongly reduced to 27 reactions, involving the chemical species H, H2, C, O, CO, CH, OH, and a representative metal. The resulting CO number density is highest in the cool regions of the reversed granulation pattern at mid-photospheric heights and decreases strongly above. There, the CO abundance stays close to a value of 8.3 on the usual logarithmic abundance scale with [H]=12 but is reduced in hot shock waves which are a ubiquitous phenomenon of the model atmosphere. For comparison, the corresponding equilibrium densities have been calculated, based on the reaction network but also under assumption of instantaneous chemical equilibrium by applying the Rybicki & Hummer (RH) code by Uitenbroek (2001). Owing to the short chemical timescales, the assumption holds for a large fraction of the atmosphere, in particular the photosphere. In contrast, the CO number density deviates strongly from the corresponding equilibrium value in the vicinity of chromospheric shock waves. Simulations with altered reaction network clearly show that the formation channel via hydroxide (OH) is the most important one under the conditions of the solar atmosphere., Comment: 15 pages, 6 figures, final version will contain online material

The dielectronic recombination rate coefficients of N V and Ne VIII ions have been measured at a heavy-ion storage ring. The investigated energy ranges covered all dielectronic recombination resonances attached to 2s -> 2p (Delta n=0) core excitations. The rate coefficients in a plasma are derived and parameterized by using a convenient fit formula. The experimentally derived rate coefficients are compared with theoretical data by Colgan et al. (2004, A&A, 417, 1183) and Nahar & Pradhan (1997, ApJ, 111, 339) as well as with the recommended rate coefficients by Mazzotta et al. (1998, A&A, 133, 403). The data of Colgan et al. and Nahar & Pradhan reproduce the experiment very well over the temperature ranges where N V and Ne VIII are expected to exist in photoionized as well as in collisionally ionized plasmas. In contrast, the recommendation of Mazzotta et al. agrees with the experimental rate coefficient only in the temperature range of collisional ionization. At lower temperatures it deviates from the measured rate coefficient by orders of magnitude. In addition the influence of external electric fields with field strengths up to 1300 V/cm on the dielectronic recombination rate coefficient has been investigated., Comment: 9 pages, 9 figures, to be published in Astronomy & Astrophysics

The basic approach for measuring electron-ion recombination rate coefficients in merged-beams electron-ion collision experiments at heavy-ion storage rings is outlined. As an example experimental results for the low temperature recombination of C IV ions are compared with the recommended theoretical rate coefficient by Mazzotta et al. The latter deviates by factors of up to 5 from the experimental one., Comment: 5 pages, 3 figures, Workshop on Stellar Coronae in the Chandra and XMM-Newton era, June 2001, Noordwijk, The Netherlands, for a recent theoretical treatment of C IV DR see Pradhan and al., Phys. Rev. Lett. 87, 183201 (2001)