Author name code: anzer ADS astronomy entries on 2022-09-14 author:"Anzer, Ulrich" ------------------------------------------------------------------------ Title: 3D modelling of magnetic field and plasma structure of entire prominences Authors: Gunár, Stanislav; Anzer, Ulrich; Heinzel, Petr; Mackay, Duncan Bibcode: 2018cosp...42E1315G Altcode: The 3D Whole-Prominence Fine Structure (WPFS) model allows us for the first time to simulate entire prominences/filaments including their numerous fine structures. This model combines a 3D magnetic field configuration of an entire prominence obtained from non-linear force-free field simulations, with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along hundreds of fine structures within the 3D magnetic model. The prominence plasma has realistic density and temperature distributions including the prominence-corona transition region. This allows us to produce synthetic H-alpha images of simulated prominences both in emission on the solar limb and in absorption against the solar disk (viewed as filaments) using a single model.Such 3D WPFS model provides us with consistent information about the prominence magnetic field configuration, prominence fine structure plasma and its radiative output. Moreover, we are able to follow the evolution of modeled prominences caused by changes of the underlying photospheric magnetic flux distribution. Thanks to these capabilities we can study links between the photospheric flux distribution, prominence magnetic field configuration, distribution and composition of the prominence plasma and its observable signatures. These relationships are important for interpretation of the observed imaging and spectral/spectropolarimetric data and for inference of the properties of the prominence magnetic field. Title: Plasma physical parameters of a prominence embedded in the core of a Coronal Mass Ejection Authors: Bemporad, Alessandro; Anzer, Ulrich; Heinzel, Petr; Jejcic, Sonja; Susino, Roberto Bibcode: 2018cosp...42E.267B Altcode: We determine the plasma physical parameters of an erupting prominence embedded in the core of a CME, combininging visible light coronagraphic images from SOHO/LASCO with UV spectra acquired by SOHO/UVCS. Strong UV emissions were detected in the hydrogen Lyman-α and Lyman-β lines and C III line. Visible light and UV intensities have been used to estimate the projected thickness and velocity of the prominence, together with the effective plasma temperature, microturbolent velocity, and column density. These parameters have been used to constrain 1D NLTE modeling of the erupting plasma, taking into account the effects of large flow velocities (Doppler dimming). Roughly one-half of considered points in the prominence body show a non-negligible Lyman-α optical thickness. Comparison between the calculated and the measured intensities of the two Lyman lines was also used to derive the geometrical filling factor. Results show that the erupting prominence plasma is relatively hot, with a low electron density, a wide range of effective thicknesses, a rather narrow range of radial flow velocities, and a microturbulence of about 25 km/s. This analysis provides a basis for future diagnostics of prominences using the METIS coronagraph on board the Solar Orbiter mission. Title: Hot Erupting Prominences in Cores of Cme's Authors: Heinzel, Petr; Bemporad, Alessandro; Anzer, Ulrich; Jejcic, Sonja; Susino, Roberto; Dzifcakova, Elena Bibcode: 2018cosp...42E1421H Altcode: Coronal mass ejections (CME) associated with prominence eruptions exhibit relatively coolmaterial in their cores. Such prominence plasmas were frequently detected in various spectrallines by SOHO/UVCS coronagraph and in the visible light by SOHO/LASCO as well as bySTEREO coronagraphs. UVCS provided excellent spectra of CME-core prominences and anextended catalogue of these data is available. We will present recent results of a hot prominence diagnostics using the hydrogen Lyman lines and the CIII line . The erupting prominence parameters are further constrained by the visible light observations from LASCO-C2. A novel non-LTE modeling based on such observations will be presented and we will highlight the diagnostic potential of the UV and visible light for future space coronagraphs like Metis on board the ESA Solar Orbiter mission. The plasma parameters of such hot prominences are compared with those obtained from numerical MHD simulations of erupting flux ropes surrounded by CMEs. Finally, we will also mention synergies with stellar analogues. Title: Can 3D whole-prominence fine structure models be used for assessment of the prominence plasma mass and distribution prior to the onset of CMEs? Authors: Gunár, Stanislav; Schmieder, Brigitte; Aulanier, Guillaume; Anzer, Ulrich; Heinzel, Petr; Mackay, Duncan; Dudik, Jaroslav Bibcode: 2018cosp...42E1316G Altcode: Two complex 3D models of entire prominences including their numerous fine structures were recently developed. The first 3D Whole-Prominence Fine Structure (WPFS) model was developed by Gunár and Mackay. The second 3D WPFS model was put forward by Gunár, Aulanier, Dudík, Heinzel, and Schmieder. These 3D prominence models combine simulations of the 3D magnetic field configuration of an entire prominence with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along hundreds of fine structures. The assumed prominence plasma has realistic density and temperature distributions including the prominence-corona transition region.These 3D WPFS models allow us to study the distribution and the mass of the prominence plasma contained in prominence magnetic field configurations. These can be crucial during the onset and early evolution of CMEs. Moreover, prominence plasma represents a bulk of the material ejected by CMEs into the interplanetary space. Here, we investigate the potential of using the 3D WPFS models for assessment of the role the prominence plasma plays in the initiation and evolution of CMEs. Title: Quiescent Prominences in the Era of ALMA. II. Kinetic Temperature Diagnostics Authors: Gunár, Stanislav; Heinzel, Petr; Anzer, Ulrich; Mackay, Duncan H. Bibcode: 2018ApJ...853...21G Altcode: We provide the theoretical background for diagnostics of the thermal properties of solar prominences observed by the Atacama Large Millimeter/submillimeter Array (ALMA). To do this, we employ the 3D Whole-Prominence Fine Structure (WPFS) model that produces synthetic ALMA-like observations of a complex simulated prominence. We use synthetic observations derived at two different submillimeter/millimeter (SMM) wavelengths—one at a wavelength at which the simulated prominence is completely optically thin and another at a wavelength at which a significant portion of the simulated prominence is optically thick—as if these were the actual ALMA observations. This allows us to develop a technique for an analysis of the prominence plasma thermal properties from such a pair of simultaneous high-resolution ALMA observations. The 3D WPFS model also provides detailed information about the distribution of the kinetic temperature and the optical thickness along any line of sight. We can thus assess whether the measure of the kinetic temperature derived from observations accurately represents the actual kinetic temperature properties of the observed plasma. We demonstrate here that in a given pixel the optical thickness at the wavelength at which the prominence plasma is optically thick needs to be above unity or even larger to achieve a sufficient accuracy of the derived information about the kinetic temperature of the analyzed plasma. Information about the optical thickness cannot be directly discerned from observations at the SMM wavelengths alone. However, we show that a criterion that can identify those pixels in which the derived kinetic temperature values correspond well to the actual thermal properties in which the observed prominence can be established. Title: Hot prominence detected in the core of a coronal mass ejection. II. Analysis of the C III line detected by SOHO/UVCS Authors: Jejčič, S.; Susino, R.; Heinzel, P.; Dzifčáková, E.; Bemporad, A.; Anzer, U. Bibcode: 2017A&A...607A..80J Altcode: Context. We study the physics of erupting prominences in the core of coronal mass ejections (CMEs) and present a continuation of a previous analysis.
Aims: We determine the kinetic temperature and microturbulent velocity of an erupting prominence embedded in the core of a CME that occurred on August 2, 2000 using the Ultraviolet Coronagraph and Spectrometer observations (UVCS) on board the Solar and Heliospheric Observatory (SOHO) simultaneously in the hydrogen Lα and C III lines. We develop the non-LTE (departures from the local thermodynamic equilibrium - LTE) spectral diagnostics based on Lα and Lβ measured integrated intensities to derive other physical quantities of the hot erupting prominence. Based on this, we synthesize the C III line intensity to compare it with observations.
Methods: Our method is based on non-LTE modeling of eruptive prominences. We used a general non-LTE radiative-transfer code only for optically thin prominence points because optically thick points do not allow the direct determination of the kinetic temperature and microturbulence from the line profiles. The input parameters of the code were the kinetic temperature and microturbulent velocity derived from the Lα and C III line widths, as well as the integrated intensity of the Lα and Lβ lines. The code runs in three loops to compute the radial flow velocity, electron density, and effective thickness as the best fit to the Lα and Lβ integrated intensities within the accuracy defined by the absolute radiometric calibration of UVCS data.
Results: We analyzed 39 observational points along the whole erupting prominence because for these points we found a solution for the kinetic temperature and microturbulent velocity. For these points we ran the non-LTE code to determine best-fit models. All models with τ0(Lα) ≤ 0.3 and τ0(C III) ≤ 0.3 were analyzed further, for which we computed the integrated intensity of the C III line using a two-level atom. The best agreement between computed and observed integrated intensity led to 30 optically thin points along the prominence. The results are presented as histograms of the kinetic temperature, microturbulent velocity, effective thickness, radial flow velocity, electron density, and gas pressure. We also show the relation between the microturbulence and kinetic temperature together with a scatter plot of computed versus observed C III integrated intensities and the ratio of the computed to observed C III integrated intensities versus kinetic temperature.
Conclusions: The erupting prominence embedded in the CME is relatively hot with a low electron density, a wide range of effective thicknesses, a rather narrow range of radial flow velocities, and a microturbulence of about 25 km s-1. This analysis shows a disagreement between observed and synthetic intensities of the C III line, the reason for which most probably is that photoionization is neglected in calculations of the ionization equilibrium. Alternatively, the disagreement might be due to non-equilibrium processes. Title: Quiescent Prominences in the Era of ALMA: Simulated Observations Using the 3D Whole-prominence Fine Structure Model Authors: Gunár, Stanislav; Heinzel, Petr; Mackay, Duncan H.; Anzer, Ulrich Bibcode: 2016ApJ...833..141G Altcode: We use the detailed 3D whole-prominence fine structure model to produce the first simulated high-resolution ALMA observations of a modeled quiescent solar prominence. The maps of synthetic brightness temperature and optical thickness shown in the present paper are produced using a visualization method for synthesis of the submillimeter/millimeter radio continua. We have obtained the simulated observations of both the prominence at the limb and the filament on the disk at wavelengths covering a broad range that encompasses the full potential of ALMA. We demonstrate here extent to which the small-scale and large-scale prominence and filament structures will be visible in the ALMA observations spanning both the optically thin and thick regimes. We analyze the relationship between the brightness and kinetic temperature of the prominence plasma. We also illustrate the opportunities ALMA will provide for studying the thermal structure of the prominence plasma from the cores of the cool prominence fine structure to the prominence-corona transition region. In addition, we show that detailed 3D modeling of entire prominences with their numerous fine structures will be important for the correct interpretation of future ALMA observations of prominences. Title: Hot prominence detected in the core of a coronal mass ejection: Analysis of SOHO/UVCS Lα and SOHO/LASCO visible-light observations Authors: Heinzel, P.; Susino, R.; Jejčič, S.; Bemporad, A.; Anzer, U. Bibcode: 2016A&A...589A.128H Altcode: Context. The paper deals with the physics of erupting prominences in the core of coronal mass ejections (CME).
Aims: We determine the physical parameters of an erupting prominence embedded in the core of a CME using SOHO/UVCS hydrogen Lα and Lβ lines and SOHO/LASCO visible light observations. In particular we analyze the CME event observed on August 2, 2000. We develop the non-LTE (NLTE; I.e. considering departures from the local thermodynamic equilibrium - LTE) spectral diagnostics based on Lα and visible light observations.
Methods: Our method is based on 1D NLTE modeling of eruptive prominences and takes into account the effect of large flow velocities, which reach up to 300 km s-1 for the studied event (the so-called Doppler dimming). The NLTE radiative-transfer method can be used for both optically thin and thick prominence structures. We combine spectroscopic UVCS observations of an erupting prominence in the core of a CME with visible light images from LASCO-C2 in order to derive the geometrical parameters like projected thickness and velocity, together with the effective temperature and column density of electrons. These are then used to constrain our NLTE radiative transfer modeling which provides the kinetic temperature, microturbulent velocity, gas pressure, ionization degree, the line opacities, and the prominence effective thickness (geometrical filling factor).
Results: Analysis was made for 69 observational points (spatial pixels) inside the whole erupting prominence. Roughly one-half of them show a non-negligible Lα optical thickness for flow velocity 300 km s-1 and about one-third for flow velocity 150 km s-1. All pixels with Lατ0 ≤ 0.3 have been considered for further analysis, which is presented in the form of statistical distributions (histograms) of various physical quantities such as the kinetic temperature, gas pressure, and electron density for two representative flow velocities (150 and 300 km s-1) and non-zero microturbulence. For two pixels co-temporal LASCO visible-light data are also available, which further constrains the diagnostics of the electron density and effective thickness. Detailed NLTE modeling is presented for various sets of input parameters.
Conclusions: The studied CME event shows that the erupting prominence expands to large volumes, meaning that it is a low-pressure structure with low electron densities and high temperatures. This analysis provides a basis for future diagnostics using the METIS coronagraph on board the Solar Orbiter mission. Title: Is it Possible to Use the Green Coronal Line Instead of X rays to Cancel an Effect of the Coronal Emissivity Deficit in Estimation of the Prominence Total Mass from Decrease of the EUV-corona Intensities? Authors: Schwartz, P.; Heinzel, P.; Jejčič, S.; Rybák, J.; Kotrč, P.; Fárník, F.; Kupryakov, Yu. A.; Deluca, E. E.; Golub, L.; Jibben, P. R.; Anzer, U.; Tlatov, A. G. .; Guseva, S. A. Bibcode: 2016ASPC..504...89S Altcode: Total masses of six quiescent prominences observed from April through June 2011 were estimated using multi-spectral observations (in EUV, X-rays, Hα, and Ca <small>II</small> H). The method for the total mass estimation is based on the fact that the intensity of the EUV solar corona at wavelengths below 912 Å is reduced at a prominence by the absorption in resonance continua (photoionisation) of hydrogen and possibly by helium and subsequently an amount of absorbed radiation is proportional to the column density of hydrogen and helium plasma. Moreover, the deficit of the coronal emissivity in volume occupied by the cool prominence plasma also contributes to the intensity decrease. The observations in X-rays which are not absorbed by the prominence plasma, allow us to separate these two mechanisms from each other. The X-ray observations of XRT onboard the Hinode satellite made with the Al-mesh focal filter were used because the X-ray coronal radiation formed in plasma of temperatures of the order of 106 K was registered and EUV spectral lines occurring in the 193, 211 and 335 Å channels of the Atmospheric Imaging Assembly of the Solar Dynamics Observatory satellite are also formed at such temperatures. Unfortunately, the Al-mesh filter has a secondary peak of the transmittance at around 171 Å which causes a contribution from the EUV corona to the measured data of up to 11 % in the quiet corona. Thus, absorption in prominence plasma influences XRT X-ray data when using the Al-mesh filter. On the other hand, other X-ray XRT filters are more sensitive to plasma of much higher temperatures (log T of the order of 7), thus observations using these filters cannot be used together with the AIA observations in the method for mass estimations. This problem could be solved using observations in the green coronal line instead of X-rays. Absorption of the green coronal line by a prominence plasma is negligible and this line is formed at temperatures of the order of 106 K. We compare values of the total mass of the prominence observed on 20 October 2012 on the SE limb estimated when using XRT X-ray observations and observations in the green coronal line obtained at Kislovodsk Mountain Astronomical Station of the Pulkovo observatory (Russia). Title: Observed IRIS Profiles of the h and k Doublet of Mg II and Comparison with Profiles from Quiescent Prominence NLTE Models Authors: Vial, Jean-Claude; Pelouze, Gabriel; Heinzel, Petr; Kleint, Lucia; Anzer, Ulrich Bibcode: 2016SoPh..291...67V Altcode: 2015SoPh..tmp..173V With the launch of the Interface Region Imaging Spectrograph (IRIS) mission, it is now possible to obtain high-resolution solar prominence spectra and to begin to distinguish the contributions of the many (apparent or not) threads that structure prominences. We aim at comparing unique observations obtained in the Mg II h and k lines of a polar crown prominence with the radiative outputs from one-dimensional models built with non-local-thermodynamic equilibrium codes (Heinzel et al.Astron. Astrophys.564, A132, 2014). We characterize the profiles obtained through thorough calibration procedures, with attention paid to the absolute values, full-width at half-maximum, and the ratio of k to h intensities. We also show that at the top of some structures, line-of-sight velocities of about 9 kms−1 can be detected. We find a range of static, low-pressure, low-thickness, low-temperature models that could fit k or h observed values, but that cannot satisfy the low observed k/h ratio. We investigate whether these low values might be explained by the inclusion of horizontal flows in small-scale threads. These flows are also necessary in another class of models, where the pressure is kept low but thickness and temperature are increased up to the observed thickness and up to 15 000 K. Title: High-resolution fine-structure synthetic imaging of an entire prominence using 3D whole-prominence fine structure modelling Authors: Gunar, Stanislav; Mackay, Duncan; Heinzel, Petr; Anzer, Ulrich Bibcode: 2015IAUGA..2251323G Altcode: The newly developed 3D whole-prominence fine structure (WPFS) model (Gunár & Mackay 2015) allows us for the first time to simulate entire prominences/filaments including their numerous fine structures. This model combines a 3D magnetic field configuration of an entire prominence obtained from non-linear force-free field simulations, with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along hundreds of fine structures within the 3D magnetic model. The prominence plasma has realistic density and temperature distributions including the prominence-corona transition region.To produce the high-resolution synthetic H-alpha images of the WPFS model we use a novel fast approximate radiative transfer visualization technique (Heinzel et al. 2015). This allows us for the first time to produce images of the prominences in emission on the solar limb and filaments in absorption against the solar disk using a single model. The prominence plasma and magnetic field are described in the WPFS model on scales that allow us to produce synthetic images with resolution matching that of the state-of-the-art observations, or indeed that of the upcoming solar observatories, such as DKIST or Solar-C. Moreover, to complement the prominence/filament synthetic images we have consistent information about the magnetic field and plasma parameters everywhere in the modeled prominences. This allows us to investigate the apparent puzzling nature of the observed prominence and filament fine structures. We can also study the connections between the local configuration of the magnetic field and the observable structure of the finest prominence/filament features. In addition, we are able to investigate the prominence evolution. We can consistently study the influence of the varying photospheric flux distribution on the prominence magnetic field configuration and its effect on the observable prominence plasma. Title: Fast approximate radiative transfer method for visualizing the fine structure of prominences in the hydrogen Hα line Authors: Heinzel, P.; Gunár, S.; Anzer, U. Bibcode: 2015A&A...579A..16H Altcode:
Aims: We present a novel approximate radiative transfer method developed to visualize 3D whole-prominence models with multiple fine structures using the hydrogen Hα spectral line.
Methods: This method employs a fast line-of-sight synthesis of the Hα line profiles through the whole 3D prominence volume and realistically reflects the basic properties of the Hα line formation in the cool and low-density prominence medium. The method can be applied both to prominences seen above the limb and filaments seen against the disk.
Results: We provide recipes for the use of this method for visualizing the prominence or filament models that have multiple fine structures. We also perform tests of the method that demonstrate its accuracy under prominence conditions.
Conclusions: We demonstrate that this fast approximate radiative transfer method provides realistic synthetic Hα intensities useful for a reliable visualization of prominences and filaments. Such synthetic high-resolution images of modeled prominences/filaments can be used for a direct comparison with high-resolution observations. Title: Prominence Visibility in Hinode/XRT Images Authors: Schwartz, P.; Jejčič, S.; Heinzel, P.; Anzer, U.; Jibben, P. R. Bibcode: 2015ApJ...807...97S Altcode: 2015arXiv150606078S In this paper we study the soft X-ray (SXR) signatures of one particular prominence. The X-ray observations used here were made by the Hinode/X-Ray Telescope instrument using two different filters. Both of them have a pronounced peak of the response function around 10 Å. One of them has a secondary smaller peak around 170 Å, which leads to a contamination of SXR images. The observed darkening in both of these filters has a very large vertical extension. The position and shape of the darkening correspond nicely with the prominence structure seen in SDO/AIA images. First, we have investigated the possibility that the darkening is caused by X-ray absorption. However, detailed calculations of the optical thickness in this spectral range show clearly that this effect is completely negligible. Therefore, the alternative is the presence of an extended region with a large emissivity deficit, which can be caused by the presence of cool prominence plasmas within an otherwise hot corona. To reproduce the observed darkening, one needs a very large extension along the line of sight of the region amounting to around 105 km. We interpret this region as the prominence spine, which is also consistent with SDO/AIA observations in EUV. Title: On the formation of Mg ii h and k lines in solar prominences Authors: Heinzel, P.; Vial, J. -C.; Anzer, U. Bibcode: 2014A&A...564A.132H Altcode:
Aims: With the recent launch of the IRIS mission, it has become urgent to develop the spectral diagnostics using the Mg ii resonance h and k lines. In this paper, we aim to demonstrate the behavior of these lines under various prominence conditions. Our results serve as a basis for analysis of new IRIS data and for more sophisticated prominence modeling.
Methods: For this exploratory work, we use a canonical 1D prominence-slab model, which is isobaric and may have three different temperature structures: isothermal, PCTR-like (prominence-corona transition region), and consistent with the radiative equilibrium. The slabs are illuminated by a realistic incident solar radiation obtained from the UV observations. A five-level plus continuum Mg ii model atom is used to solve the full NLTE problem of the radiative transfer. We use the numerical code based on the ALI techniques and apply the partial frequency redistribution for both Mg ii resonance lines. We also use the velocity-dependent boundary conditions to study the effect of Doppler dimming in the case of moving prominences. Finally, the relaxation technique is used to compute a grid of models in radiative equilibrium.
Results: We computed the Mg ii h and k line profiles that are emergent from prominence-slab models and show their dependence on kinetic temperature, gas pressure, geometrical extension, and microturbulent velocity. By increasing the line opacity, significant departures from the complete frequency redistribution take place in the line wings. Models with a PCTR temperature structure show that Mg ii becomes ionized to Mg iii in the temperature range between roughly 15 000 and 30 000 K. Doppler dimming is significant for Mg ii resonance lines. At the velocity 300 km s-1, the line intensity decreases to about 20% of the value for static prominences. Finally, we demonstrate the role of Mg ii h and k radiation losses on the prominence energy balance. Their dominant role is at lower pressures, while the losses due to hydrogen and Ca ii dominate at higher pressures. Title: A comparison between observed IRIS profiles of the h & k doublet of Mg II and profiles from quiescent prominence NLTE models Authors: Vial, Jean-Claude; Anzer, Ulrich; Heinzel, Petr; Kleint, Lucia Bibcode: 2014cosp...40E3515V Altcode: With the advent of IRIS, it is now possible to investigate the cool core of prominences through the detailed profiles of the Mg II resonance lines with an unprecedented spatial resolution of 0.33 arcsecond. The slit-jaw camera also allows to record the temporal evolution of the prominence fine structure. We present IRIS observations of quiescent prominence profiles that we analyse in terms of reversal (if any), width, k/h line ratio, prominence/ quiet Sun line ratio. Comparing these parameters with the results of NLTE modelling (see Heinzel et al. 2014), we can derive thermodynamic parameters of the cool prominence plasma, along with the (line-of-sight) velocities and mass flows. Title: Puzzling nature of the fine structure of quiescent prominences and filaments Authors: Gunár, Stanislav; Heinzel, Petr; Anzer, Ulrich; Mackay, Duncan H. Bibcode: 2013JPhCS.440a2035G Altcode: Even after more than 160 years of observations and modelling of solar prominences their true nature contains many open questions. In this work we argue that current 2D prominence fine structure models can help us to understand the puzzling connection between quasi-vertical fine structures often seen in quiescent prominences observed on the solar limb and horizontally aligned dark fibrils representing the fine structures of prominences observed in absorption against the solar disk (filaments). Title: Non-linear force-free magnetic dip models of quiescent prominence fine structures Authors: Gunár, S.; Mackay, D. H.; Anzer, U.; Heinzel, P. Bibcode: 2013A&A...551A...3G Altcode:
Aims: We use 3D non-linear force-free magnetic field modeling of prominence/filament magnetic fields to develop the first 2D models of individual prominence fine structures based on the 3D configuration of the magnetic field of the whole prominence.
Methods: We use an iterative technique to fill the magnetic dips produced by the 3D modeling with realistic prominence plasma in hydrostatic equilibrium and with a temperature structure that contains the prominence-corona transition region. With this well-defined plasma structure the radiative transfer can be treated in detail in 2D and the resulting synthetic emission can be compared with prominence/filament observations.
Results: Newly developed non-linear force-free magnetic dip models are able to produce synthetic hydrogen Lyman spectra in a qualitative agreement with a range of quiescent prominence observations. Moreover, the plasma structure of these models agrees with the gravity induced prominence fine structure models which have already been shown to produce synthetic spectra in good qualitative agreement with several observed prominences.
Conclusions: We describe in detail the iterative technique which can be used to produce realistic plasma models of prominence fine structures located in prominence magnetic field configurations containing dips, obtained using any kind of magnetic field modeling. Title: Velocity Vector, Ionization Degree, and Temperature of Prominence Fine Structures Observed by Hinode/SOT Authors: Schmieder, B.; Mein, P.; Chandra, R.; Molodij, G.; Heinzel, P.; Berlicki, A.; Schwartz, P.; Fárník, F.; Labrosse, N.; Anzer, U.; Watanabe, T. Bibcode: 2012ASPC..454..107S Altcode: Prominences have been successfully observed by Hinode in April 2007 exhibiting a strong dynamics of their fine structures. The dynamics of a prominence is a challenge to understand the formation of cool prominence plasma embedded in the hot corona. Combining simultaneous observations obtained in Hα with Hinode/SOT and the MSDP spectrograph operating at the Meudon solar tower, velocity vectors have been derived. The Doppler-shifts of bright threads are of the same order as the velocities measured perpendicular to the line of sight. This suggests that the vertical structures of the prominence could be a pile up of dips in magnetic field lines viewed in 3D. Using Hα, Hinode/XRT and TRACE data, the hydrogen ionization degree has been determined to be 0.5-0.8, and the optical thickness in Hα between 0.2 and 1.3. The Extreme Ultraviolet Imaging Spectrometer (EIS) on Hinode produced images of the prominence in 11 selected lines with formation temperatures between log(T) = 4.7 and log(T) = 6.1. We comment on the absorption, emissivity blocking and emission involved for interpreting the different structures of the prominence in terms of the temperature and density. Title: Mass of Solar Prominences Estimated from Multi-Wavelength Data Authors: Schwartz, P.; Fárník, F.; Heinzel, P.; Kotrč, P.; Anzer, U. Bibcode: 2012ASPC..454..117S Altcode: The mass of selected prominences was estimated using their multi-wavelength observations: in Hα by the HSFA2 spectrograph of the Ondřejov observatory, in EUV by SoHO/EIT and in the soft X-rays by Hinode XRT. The results are compared with values estimated by other authors. Title: Mass Loading of Quiescent Prominences Authors: Schwartz, P.; Heinzel, 2. P.; Kotrč, P.; Anzer, U.; Kupryakov, Y.; DeLuca, E. Bibcode: 2012ASPC..456...73S Altcode: From April through June 2011 a multi-spectral observing campaign of quiescent prominences was made. Observations were carried out in EUV by SDO/AIA, in soft X-rays by XRT on Hinode and in Hα by horizontal spectrographs of the Ondrřejov observatory. In this work the masses of selected prominences, observed on May 18 and April 19, were determined in order to test the method and our semi-automatic computer code. Title: Radiative equilibrium in solar prominences reconsidered Authors: Heinzel, P.; Anzer, U. Bibcode: 2012A&A...539A..49H Altcode:
Aims: We reconsider the question which kinetic temperatures can lead to prominence configurations that are in radiative equilibrium. We compare these temperatures to those from other calculations.
Methods: For this purpose we solved the full non-LTE radiative-transfer problem for a gas consisting of hydrogen, helium and calcium. We used simple isobaric 1D slabs and began with isothermal models. Then we solved the radiative-relaxation problem and determined the radiative-equilibrium conditions within the whole slab.
Results: By adding the calcium radiative losses, we found that these equilibrium temperatures are considerably lower than those obtained for a pure hydrogen gas. This is because the newly calculated CaII line losses appear to play a significant role in the energy balance, similar to chromospheric conditions. The equilibrium temperatures obtained span the range between 4400-9500 K, depending on the gas pressure and slab thickness. Title: Synthetic differential emission measure curves of prominence fine structures. II. The SoHO/SUMER prominence of 8 June 2004 Authors: Gunár, S.; Parenti, S.; Anzer, U.; Heinzel, P.; Vial, J. -C. Bibcode: 2011A&A...535A.122G Altcode:
Aims: This study is the first attempt to combine the prominence observations in Lyman, UV, and EUV lines with the determination of the prominence differential emission measure derived using two different techniques, one based on the inversion of the observed UV and EUV lines and the other employing 2D non-LTE prominence fine-structure modeling of the Lyman spectra.
Methods: We use a trial-and-error method to derive the 2D multi-thread prominence fine-structure model producing synthetic Lyman spectra in good agreement with the observations. We then employ a numerical method to perform the forward determination of the DEM from 2D multi-thread models and compare the synthetic DEM curves with those derived from observations using inversion techniques.
Results: A set of available observations of the June 8, 2004 prominence allows us to determine the range of input parameters, which contains models producing synthetic Lyman spectra in good agreement with the observations. We select three models, which represent this parametric-space area well and compute the synthetic DEM curves for multi-thread realizations of these models. The synthetic DEM curves of selected models are in good agreement with the DEM curves derived from the observations.
Conclusions: We show that the evaluation of the prominence fine-structure DEM complements the analysis of the prominence hydrogen Lyman spectra and that its combination with the detailed radiative-transfer modeling of prominence fine structures provides a useful tool for investigating the prominence temperature structure from the cool core to the prominence-corona transition region. Title: Synthetic differential emission measure curves of prominence fine structures Authors: Gunár, S.; Heinzel, P.; Anzer, U. Bibcode: 2011A&A...528A..47G Altcode:
Aims: We use 2D single and multi-thread prominence fine-structure models to obtain the synthetic DEM curves. These are then compared with the DEM curves derived from observations.
Methods: We use the temperature and electron density structure resulting from the 2D models and numerically compute the average synthetic DEM curves for different orientations of the threads with respect to the line of sight.
Results: We show that the synthetic DEM curves obtained by 2D modelling are similar to the DEM curves derived from observations of quiescent prominences.
Conclusions: The DEM curves derived from observations, which are most reliable above temperatures of 20 000 K, can be extended towards cool prominence-core temperatures by supplementing them with synthetic DEM values obtained by modelling hydrogen Lyman spectra originating mainly at temperatures below 20 000 K. On the other hand, the observed DEM can constrain the temperature structure of the prominence fine structures above the formation temperatures of the Lyman spectrum. Title: Statistical comparison of the observed and synthetic hydrogen Lyman line profiles in solar prominences Authors: Gunár, S.; Schwartz, P.; Schmieder, B.; Heinzel, P.; Anzer, U. Bibcode: 2010A&A...514A..43G Altcode:
Aims: We analyse a unique set of prominence SOHO/SUMER Lyman spectra by comparing it with synthetic spectra obtained by 2D multi-thread prominence fine-structure models.
Methods: We employed a novel statistical approach to the analysis of the observed and synthetic Lyman spectra. We compared the statistical distributions of the line properties of the observed and synthetic Lyman spectra using a set of four statistical criteria.
Results: We demonstrate the very good agreement between the observed Lyman spectra and synthetic spectra obtained by modelling.
Conclusions: Our set of statistical criteria is well-suited to analyses of the prominence Lyman spectra because of its sensitivity to a number of different parameters governing the conditions in the prominence fine structures. Title: How do unresolved motions affect the prominence hydrogen Lyman spectrum. Authors: Gunar, Stanislav; Schwartz, Pavol; Schmieder, Brigitte; Heinzel, Petr; Anzer, Ulrich Bibcode: 2010cosp...38.2830G Altcode: 2010cosp.meet.2830G Space-born observations of quiescent prominences in the hydrogen Lyman line series provide a considerable amount of information about their structure and physical properties. Lyman line series thus represents a focus of several detailed studies. However, the substantial asymmetries exhibited by considerable number of observed Lyman line profiles present a strong indication for the existence of hidden fine-structure dynamics. If attributed merely to the Doppler shift effect, these asymmetries would correspond to radial velocities of the order of 100 km/s. Such high velocities, however, were never observed in quiescent prominences. Typical fine-structure velocities are of the order of 10 km/s. To tackle this problem, we have modified our 2D multi-thread prominence models and randomly assigned line-of-sight (LOS) velocities of the order of 10 km/s to each thread. The obtained synthetic Lyman spectrum exhibits substantial asymme-tries of the line profiles comparable with SOHO/SUMER observations. Moreover, our results do indicate that the synthetic Lyman-α profiles may exhibit an opposite asymmetry to that of the higher Lyman lines. This is consistent with observations which often show opposite asym-metries of these lines at the same prominence location. We have demonstrated the agreement of synthetic and observed spectra not only by comparing asymmetries of individual profiles but also by exploiting statistically significant sets of the observed and synthetic data. These results represent an important step in our understanding of hidden fine-structure dynamics of quiescent prominences. Title: Prominence fine structures and corresponding differential emission measures Authors: Gunar, Stanislav; Heinzel, Petr; Anzer, Ulrich Bibcode: 2010cosp...38.2845G Altcode: 2010cosp.meet.2845G We use the temperature and density structure resulting from multi-thread prominence fine-structure models (consisting of individual 2D vertical threads) to compute the differential emission measures (DEM). We derive the DEM at various positions along the length of the foremost thread of the multi-thread model and also for various angles between the line-of-sight and the magnetic field. We compare the calculated DEM values with those obtained by in-versions from the observed intensities of the transition-region UV lines. We show that the unresolved fine-structuring of quiescent prominences along a particular line-of-sight has consid-erable effect on shape of the DEM curve due to fracturing of the prominence-corona transition region into many thin layers corresponding to individual prominence fine structures. Title: Solar quiescent prominences. Filamentary structure and energetics Authors: Heinzel, P.; Anzer, U.; Gunár, S. Bibcode: 2010MmSAI..81..654H Altcode: We present a first attempt to solve the non-LTE radiative-transfer problem within a 2D numerical domain consisting of several randomly distributed vertical threads. This represents a starting point to simulate mutual radiative interaction between such prominence threads. The second part of the paper presents our new results which concern the radiative equilibrium in prominences and in their fine structure. We show that adding the non-hydrogenic radiative losses significantly lowers the central equilibrium temperatures. To match the observed temperatures, an additional heating seems to be unavoidable. Title: Global prominence oscillations Authors: Anzer, U. Bibcode: 2009A&A...497..521A Altcode: Aims: The question of the different oscillation periods for global modes of quiescent prominences is discussed.
Methods: Simple 1D prominence configurations are used to describe the magnetohydrostatic equilibrium and their oscillations for small amplitudes.
Results: Three basic modes of oscillations were found and their periods as a function of the magnetic field configuration and the assumed geometry are given. Title: On Lyman-line asymmetries in quiescent prominences Authors: Gunár, S.; Heinzel, P.; Anzer, U.; Schmieder, B. Bibcode: 2008A&A...490..307G Altcode: Aims: We study the asymmetries of the synthetic hydrogen Lyman lines and the process responsible for their formation.
Methods: To obtain the synthetic Lyman line profiles, we use a multi-thread prominence fine-structure model consisting of identical 2D threads. The 2D thread models are in MHS equilibrium, include an empirical prominence-corona transition region, and solve consistently 2D non-LTE radiative transfer. Each thread of the multi-thread model has a randomly assigned line-of-sight (LOS) velocity.
Results: The synthetic Lyman spectrum obtained by multi-thread modelling exhibits substantial asymmetries of the line profiles, even though the LOS velocities of individual threads are only of the order of 10 km s-1. Moreover, our results indicate that the synthetic Lyman-α profiles may exhibit an opposite asymmetry to that of the higher Lyman lines.
Conclusions: The presence and behaviour of the asymmetrical profiles of the synthetic Lyman lines agree with observed profiles acquired by SUMER.

Appendix A is only available in electronic form at http://www.aanda.org Title: Hinode, TRACE, SOHO, and Ground-based Observations of a Quiescent Prominence Authors: Heinzel, P.; Schmieder, B.; Fárník, F.; Schwartz, P.; Labrosse, N.; Kotrč, P.; Anzer, U.; Molodij, G.; Berlicki, A.; DeLuca, E. E.; Golub, L.; Watanabe, T.; Berger, T. Bibcode: 2008ApJ...686.1383H Altcode: A quiescent prominence was observed by several instruments on 2007 April 25. The temporal evolution was recorded in Hα by the Hinode SOT, in X-rays by the Hinode XRT, and in the 195 Å channel by TRACE. Moreover, ground-based observatories (GBOs) provided calibrated Hα intensities. Simultaneous extreme-UV (EUV) data were also taken by the Hinode EIS and SOHO SUMER and CDS instruments. Here we have selected the SOT Hα image taken at 13:19 UT, which nicely shows the prominence fine structure. We compare this image with cotemporaneous ones taken by the XRT and TRACE and show the intensity variations along several cuts parallel to the solar limb. EIS spectra were obtained about half an hour later. Dark prominence structure clearly seen in the TRACE and EIS 195 Å images is due to the prominence absorption in H I, He I, and He II resonance continua plus the coronal emissivity blocking due to the prominence void (cavity). The void clearly visible in the XRT images is entirely due to X-ray emissivity blocking. We use TRACE, EIS, and XRT data to estimate the amount of absorption and blocking. The Hα integrated intensities independently provide us with an estimate of the Hα opacity, which is related to the opacity of resonance continua as follows from the non-LTE radiative-transfer modeling. However, spatial averaging of the Hα and EUV data have quite different natures, which must be taken into account when evaluating the true opacities. We demonstrate this important effect here for the first time. Finally, based on this multiwavelength analysis, we discuss the determination of the column densities and the ionization degree of hydrogen in the prominence. Title: On Lyman-line Asymmetries in Quiescent Prominences Authors: Gunár, S.; Heinzel, P.; Anzer, U.; Schmieder, B. Bibcode: 2008ESPM...12.3.18G Altcode: We present a new study of the asymmetries of synthetic hydrogen Lyman line profiles and suggest the mechanism of their formation.

In order to obtain the synthetic Lyman line profiles, we use multi-thread prominence fine-structure model consisting of identical vertical 2D threads. The 2D thread models are in magnetohydrostatic (MHS) equilibrium, include an empirical prominence-corona transition region (PCTR) and consistently solve the 2D non-LTE radiative transfer. Individual threads of the multi-thread fine-structure model have a randomly assigned LOS velocities.

The synthetic Lyman spectrum obtained by our multi-thread modelling exhibits substantial asymmetries of the line profiles even thought the LOS velocities of individual threads are only of the order of 10 km s-1. Moreover, our results show that the synthetic Lyman-? profiles may exhibit an opposite asymmetry as compared to higher Lyman lines. These results are in agreement with the observed Lyman line profiles from SOHO/SUMER.

The presence and the behaviour of the asymmetrical profiles of the synthetic Lyman spectrum is in agreement with observed profiles taken by SUMER. Title: Prominence modelling: from observed emission measures to temperature profiles Authors: Anzer, U.; Heinzel, P. Bibcode: 2008A&A...480..537A Altcode: Aims:We outline the construction of prominence - corona transition region models based upon the observations of one particular prominence.
Methods: The differential emission measure curves from observations were approximated by simple analytical functions. On this basis we constructed the temperature curve and calculated the radiative losses, the gains by thermal conduction, and some estimates for the wave heating.
Results: The temperature curve was calculated in the range between 23 000 K and 450 000 K. The resulting transition region can be divided into an inner region where the temperature is low and the radiative losses are very large, a part with a very steep temperature rise from 40 000 K to around 250 000 K over a width of only 500 km, and an extended high temperature region. Both the conductive heating and our estimates for a possible wave cooling/heating peak very sharply in the region with the very large temperature gradient. The consequences for the energy balance are discussed. Title: Limb Prominences Seen in UV, EUV and SXR Authors: Heinzel, P.; Fárník, F.; Anzer, U.; Dammasch, I. Bibcode: 2007ASPC..369..279H Altcode: We use SOHO/SUMER, EIT and Yohkoh/SXT prominence observations to study the absorption and/or volume blocking of UV, EUV and SXR coronal radiation by cool prominence plasma. An example of such a complex behaviour is the limb prominence observed on 5 September 1996. The SUMER spectrograph has detected two coronal lines, MgX at 625 Å, which is absorbed by the hydrogen Lyman continuum and blocked within a volume occupied by cool prominence plasma, and FeXII at 1242 Å, where the prominence appears dark due to blocking (no absorption at this wavelength). A similar behaviour show also the EUV images taken by EIT. We find that a darkening similar to that detected in the FeXII line is quite well visible also in the SXR images obtained by Yohkoh/SXT. To explain this feature, we exclude the absorption by the HeI and HeII resonance continua (hydrogen absorption is quite negligible) and suggest that it is entirely due to the volume blocking. Based on a quantitative analysis of these data, we discuss the physical conditions in cool prominences and their coronal environment. Finally, we propose new high-resolution EUV and SXR observations of prominences by Solar-B, using the EUV imaging spectrometer (EIS) and the X-ray telescope (XRT), respectively. Title: Properties of prominence fine-structure threads derived from SOHO/SUMER hydrogen Lyman lines Authors: Gunár, S.; Heinzel, P.; Schmieder, B.; Schwartz, P.; Anzer, U. Bibcode: 2007A&A...472..929G Altcode: Context: The SOHO/SUMER observations provide us for the first time with the prominence spectra in the Lyman-α line outside the attenuator together with the higher members of the hydrogen Lyman series.
Aims: We derive the prominence fine-structure thread properties by comparing the SOHO/SUMER hydrogen Lyman series observations with the synthetic Lyman lines.
Methods: To obtain the synthetic profiles of the Lyman lines, we used 2D prominence fine-structure thread models with a PCTR and consistently solved the 2D non-LTE multilevel radiative transfer. The trial-and-error method was applied to find the model with the best agreement between the synthetic Lyman line profiles and the observed ones.
Results: The properties of the resulting model with the best match of the synthetic and observed line profiles are central (minimum) temperature T0 = 7000 K, maximum column mass in the centre of the thread M0 = 1.1×10-4 g cm-2, horizontal field strength in the middle of the thread Bx(0) = 6 Gauss and the boundary pressure p0 = 0.015 dyn cm-2.
Conclusions: The Lyman line profiles observed by SOHO/SUMER can be better reproduced by using multi-thread models consisting of a set of the 2D prominence fine-structure threads placed perpendicularly to the line-of-sight, rather than with the single-thread model. Title: Is the magnetic field in quiescent prominences force-free? Authors: Anzer, U.; Heinzel, P. Bibcode: 2007A&A...467.1285A Altcode: Aims:We describe under which conditions the magnetic fields of quiescent prominences are force-free and under which gravity plays the dominant role.
Methods: The existing observational determinations of the magnetic field are summarised and the calculation of the plasma β is outlined. We derive the dependence of β on the prominence weight and the field strength.
Results: We show that in many cases of well-developed quiescent prominences the field can deviate substantially from the force-free situation and gravity fully determines the structure of the magnetic dips. Title: Prominence Parameters from 2D Modeling of Lyman Lines Measured with SUMER Authors: Gunár, S.; Heinzel, P.; Schmieder, B.; Anzer, U. Bibcode: 2007ASPC..368..317G Altcode: We present SOHO/SUMER observations of a solar prominence in the hydrogen Lyman series lines and compare the line profiles with the synthetic ones obtained using our 2D prominence modeling. The observations contain the Ly-β, Ly-γ, and Ly-δ lines on 25 May 2005.

In order to derive the prominence parameters we used our 2D fine structure models of vertical threads in magnetohydrostatic (MHS) equilibrium. By varying the input model parameters (central temperature, boundary pressure, magnetic field, central column mass and turbulent velocity) we obtained a model having the synthetic Lyman line profiles in good agreement with the observed ones. In this way we are able to determine the structure of the magnetic dip and the thermodynamical parameters in the observed prominence. Title: Prominences on the Limb: Diagnostics with UV EUV Lines and the Soft X-Ray Continuum Authors: Anzer, U.; Heinzel, P.; Fárnik, F. Bibcode: 2007SoPh..242...43A Altcode: In this paper we discuss the two mechanisms by which solar prominences on the limb can manifest themselves when observed in coronal UV - EUV lines and in the soft X-ray continuum. These mechanisms are the absorption in the resonance continua of hydrogen and helium on one hand and the reduction of the emissivity in a part of the coronal volume occupied by a prominence on the other one. We briefly describe earlier observations made with SOHO/SUMER, EIT and Yohkoh/SXT. We then discuss how the instruments on the new Japanese satellite Hinode can be used for more detailed studies of prominences. We also propose some combined observations between the Hinode satellite and the SOHO/SUMER instrument. Title: Spectral Diagnostics of the Magnetic Field Orientation in a Prominence Observed with SOHO/SUMER Authors: Schmieder, B.; Gunár, S.; Heinzel, P.; Anzer, U. Bibcode: 2007SoPh..241...53S Altcode: During several campaigns focused on prominences we have obtained coordinated spectral observations from the ground and from space. The SOHO/SUMER spectrometer allows us to observe, among others, the whole Lyman series of hydrogen, while the Hα line was observed by the MSDP spectrograph at the VTT. For the Lyman lines, non-LTE radiative-transfer computations have shown the importance of the optical thickness of the prominence - corona transition region (PCTR) and its relation to the magnetic field orientation for the explanation of the observed line profiles. Moreover, Heinzel, Anzer, and Gunár (2005, Astron. Astrophys.442, 331) developed a 2D magnetostatic model of prominence fine structures that demonstrates how the shapes of Lyman lines vary, depending on the orientation of the magnetic field with respect to the line of sight. To support this result observationally, we focus here on a round-shaped filament observed during three days as it was crossing the limb. The Lyman profiles observed on the limb are different from day to day. We interpret these differences as being due to the change of orientation of the prominence axis (and therefore the magnetic field direction) with respect to the line of sight. The Lyman lines are more reversed if the line of sight is across the prominence axis as compared to the case when it is aligned along its axis. Title: Prominence fine structures in a magnetic equilibrium. III. Lyman continuum in 2D configurations Authors: Gunár, S.; Heinzel, P.; Anzer, U. Bibcode: 2007A&A...463..737G Altcode: Aims:We discuss the behavior of the Lyman continuum profiles studied on the grid of 2D vertical-thread models for prominence fine structures.
Methods: Multilevel non-LTE transfer calculations for a 12-level plus continuum hydrogen model atom are used.
Results: Since the Lyman continuum is formed in regions with different temperatures for different orientations between the magnetic field direction and the line-of-sight, our Lyman continuum modeling, together with additional information from Lyman lines, represents a very useful tool for the determination of the thread structure.
Conclusions: .A comparison between our theoretical Lyman continuum models between 800 Å and 911 Å with the observed values shows that such a modeling can give interesting new constraints on the temperature structure in prominence threads. Title: Study of an extended EUV filament using SoHO/SUMER observations of the hydrogen Lyman lines Authors: Schwartz, P.; Heinzel, P.; Schmieder, B.; Anzer, U. Bibcode: 2006A&A...459..651S Altcode: Context: .In previous work we determined the 3D structure of an EUV extension of the filament observed by SoHO/CDS on 15 October 1999. The determination of the 3D structure depends on the optical thickness τ912 at the hydrogen Lyman-continuum edge at 912 Å. The τ912 value was unknown, therefore it was taken as a free parameter for the 3D-structure determinations.
Aims: .The plasma properties (including τ912) of this EUV filament are estimated using non-LTE modelling of profiles of the hydrogen Lyman lines.
Methods: .The lines Lβ, Lδ, L ɛ, L6 and L7 were observed by SoHO/SUMER with the slit positioned across the EUV filament. In our modelling the EUV filament was approximated by a horizontal 1D slab with constant pressure and the temperature decreasing vertically from the edges to interior of the slab. The values of the geometrical thickness of the slab in the EUV extension were taken from its 3D structure determined for the τ912 values estimated iteratively. The synthetic profiles were fitted to observed ones using the χ2 minimization.
Results: .For the Hα-filament part we obtained the expected results: a small geometrical thickness between several thousand and 15 000 km, temperatures around 5000 K in the interior and around 20 000 K in PCTRs, and pressure ~0.1~dyn~cm-2. That leads to an optical thickness in the Hα line center larger than 1. In the EUV extension, temperatures of 10 000-40 000 K in the interior and 35 000-50 000 K in the vertically extended PCTRs and a pressure of ~0.01~dyn~cm-2 were determined. Such large variations of the temperature lead to large variations of τ912.
Conclusions: .The EUV extension is inhomogeneous - areas both optically thick and thin at 912 Å are present close to each other. Parts optically thick at 912 Å are visible also in Hα as faint and tiny dark structures around the very dark and well pronounced Hα filament. The volume blocking mechanism is an important mechanism throughout the whole EUV extension even in its optically thick parts. The absorption dominates only in the Hα-filament part of the EUV filament. Title: Darkening of Coronal Lines by Limb Prominences Observed by SOHO/SUMER Authors: Dammasch, I. E.; Heinzel, P.; Anzer, U. Bibcode: 2006ESASP.617E..66D Altcode: 2006soho...17E..66D No abstract at ADS Title: On the Fine Structure of Solar Filaments Authors: Heinzel, P.; Anzer, U. Bibcode: 2006ApJ...643L..65H Altcode: High-resolution Hα images of solar filaments show a variety of dark fine-structure fibrils, most of them aligned with the magnetic field that is sheared with respect to the filament long axis. Here we demonstrate how such fibrils can be explained in terms of the magnetic field dips produced by realistic mass loading due to plasma condensation along the top of a magnetic prominence arcade. Our interpretation is supported by (2+1)-dimensional radiation-magnetohydrostatic simulations that predict quantitatively the properties of such condensations that are suspended in the dipped magnetic field. Title: A periodic arcade model for extended EUV filaments Authors: Anzer, U.; Heinzel, P. Bibcode: 2006A&A...446..301A Altcode: The extensions of Hα filaments that are observed in EUV spectral lines require the presence of cool material in a large volume surrounding these filaments. This material can only be supported by magnetic fields with dips. Such dips can be generated by a sufficiently strong twisting of magnetic flux tubes. In the present paper we have extended earlier work to more realistic magnetic field configurations that are periodic in the x-direction. We derived approximate equilibria for twisted slender flux tubes and determined the region that contains dips. In addition we investigated the effects on our models of the mass loading due to a heavy prominence. Finally we compared our theoretical models with the EUV - observations. We conclude that these types of models are capable of explaining the basic features of the observations. Title: Prominence Fine Structures in Amagnetic Equilibrium: a Grid Oftwo-Dimensional Models Authors: Gunár, S.; Heinzel, P.; Anzer, U. Bibcode: 2005ESASP.600E..85G Altcode: 2005dysu.confE..85G; 2005ESPM...11...85G No abstract at ADS Title: Radiative effects in supersonic wind accretion onto gravitating objects Authors: Kryukov, I.; Pogorelov, N.; Anzer, U.; Börner, G.; Bisnovatyi-Kogan, G. Bibcode: 2005A&A...441..863K Altcode: We investigate the influence of radiative effects

on supersonic wind accretion onto gravitating objects. The accreting matter is assumed to be optically thin. The physical mechanisms taken into account include cooling due to free-free and free-bound transitions, the Compton heating via X-ray scattering on electrons and the inverse Compton cooling in the regions where the temperature of the matter becomes sufficiently large to be able to transfer part of its internal energy to photons. A wide range of determining parameters was covered, including the values applicable to the Vela X-1 binary system, but our main emphasis is on the study of the effects of radiative processes on the behavior of accretion flows. It is shown that the applicability of polytropic accretion models is very limited and the actual accretion rate can be considerably lower than that provided by the Bondi-Hoyle-Lyttleton formula. The detailed consideration of the realistic radiative effects proved to be of great importance in our understanding of the accretion phenomenon, since they can substantially affect it both qualitatively and quantitatively. Title: Prominence fine structures in a magnetic equilibrium. II. A grid of two-dimensional models Authors: Heinzel, P.; Anzer, U.; Gunár, S. Bibcode: 2005A&A...442..331H Altcode: We construct a grid of 2D vertical-thread models for prominence fine structures which are in magnetohydrostatic (MHS) equilibrium. Such thread models have been described in a previous paper by Heinzel & Anzer (2001), but here we use a modified 2D transfer code with an adaptive MHS grid. Multilevel non-LTE transfer calculations are now performed for a 12-level plus continuum hydrogen model atom, in order to study the behaviour of the Lyman-series lines observed by SOHO/SUMER. Our grid consists of 18 models which cover a range of central column masses, magnetic-field intensities and two parameters characterising the 2D temperature structure of the thread. Since different Lyman lines and their parts (line center, peak, wings) are formed at different places within the thread, the Lyman series may serve as a good diagnostic tool for thermodynamic conditions varying from central cool parts to a prominence-corona transition region. We demonstrate this behaviour for various lines, showing their synthetic profiles as seen from two perpendicular directions along and across the magnetic field lines, respectively, and displaying the respective contribution functions. This study confirms our earlier conclusion that the Lyman line profiles are much more reversed when seen across the field lines, compared to those seen along the lines. The latter can be even unreversed. We also show the geometrical cross-section (shape) of all 18 models. Their thread-like shape with a considerable aspect ratio resembles recent high-resolution Hα images. Finally, we discuss the relation of our thread models to the vertical threads studied by Fontenla et al. (1996, ApJ, 466, 496). Title: On the Nature of Dark Extreme Ultraviolet Structures Seen by SOHO/EIT and TRACE Authors: Anzer, U.; Heinzel, P. Bibcode: 2005ApJ...622..714A Altcode: Spectral lines in the extreme ultraviolet (EUV) observed on the Sun can originate either from the hot corona or the cooler chromosphere to corona transition region. In the present paper we concentrate on coronal lines and in particular on iron lines at 171, 195, and 284 Å, which have been frequently observed by the Solar and Heliospheric Observatory (SOHO)/Extreme Ultraviolet Imaging Telescope (EIT) and the Transition Region and Coronal Explorer (TRACE). The intensity of these lines emitted by the corona can be reduced by the presence of a cool gas, e.g., by prominence-like material, located in the corona. There are two mechanisms that can lead to a reduction of the line intensity: absorption and volume blocking. That part of the coronal radiation that comes from behind the cool gas can be partially absorbed. The main absorbers for the iron lines studied here are neutral hydrogen and neutral plus singly ionized helium. In this paper we calculate the optical thickness of their resonance continua at the wavelength positions of iron lines and compare it with that in the Hα line center. The comparison of the derived values shows that they are quite similar. The second effect is due to the fact that cool structures in the corona do not emit any radiation in the EUV lines, which gives rise to an additional reduction of the observed intensity as compared with the surrounding corona-we have termed this ``volume blocking.'' We further describe in this paper how these two effects can be used in a new way of performing prominence diagnostics. Our results are also applicable to other coronal lines detected by, e.g., the SOHO CDS or SUMER instruments. Title: Determination of the 3D structure of an EUV-filament observed by SoHO/CDS, SoHO/SUMER and VTT/MSDP Authors: Schwartz, P.; Heinzel, P.; Anzer, U.; Schmieder, B. Bibcode: 2004A&A...421..323S Altcode: Heinzel et al. (\cite{hein1}) found that filaments seem to be much more extended in EUV spectral lines than in Hα. In this paper we reconstruct the 3D structure of the extensions of the EUV-filament observed on 15 October 1999 by SoHO/CDS and SoHO/SUMER. Small Hα structures were observed by VTT/MSDP. We computed the bottom and top heights of the EUV-filament extensions for several values of the hydrogen Lyman-continuum optical thickness τ912 using the spectroscopic model of Heinzel et al. (\cite{hein2}). For this we used CDS observations of the two coronal lines Mg X 624.94 Å and Si XII 520.60 Å. Further, we develop a new method for estimating τ912 using the O V line measured by CDS and the O VI measured by SUMER onboard SoHO. The latter line is used as a proxy to deduce the intensity variations of O V underneath the filament. This method also takes into account the presence of prominence-corona transition regions.

Resulting heights of the EUV-filament extensions are around 15 000 km for the bottom and around 40 000 km for the top boundaries. However, there are significant variations around these representative values and we show that in about half of observed pixels the Lyman-continuum absorption dominates in the coronal-line depression, while in the other half it is the volume blocking effect. The latter situation corresponds to a large (larger than 30 000 km) geometrical thickness of the EUV-filament extension. From the heights of the boundaries of the EUV-filament the geometrical thickness can be computed and used to estimate the total mass loading which was found to be comparable to that of typical CMEs. Title: Multiwavelength determination of the density and total mass of the EUV filament observed by SoHO/CDS, SoHO/SUMER and MSDP/VTT Authors: Schwartz, P.; Schmieder, B.; Heinzel, P.; Anzer, U. Bibcode: 2004IAUS..223..219S Altcode: 2005IAUS..223..219S It was found that filaments observed in EUV could be much more extended than in Halpha. These extended dark structures visible in EUV are named EUV filaments. Their parts seen only in EUV (not observable in Halpha because of low opacity at the Halpha wavelength) are called EUV-filament extensions (or simply EUV extensions). For the EUV filament observed by SoHO on 15 October 1999 as northern polar crown filament, only a few small dark structures were seen in Halpha. This suggests that the mass of the EUV extension is larger than, or at least comparable with, the mass of the parts of the filament observed in Halpha. In our previous work we determined the 3D structure of the EUV extension of this EUV filament. In this paper we present the determinations of mass and average density of this EUV extension. For better density estimates we interpret the hydrogen Lyman lines observed by SUMER using non-LTE radiative transfer code. From the best fitting of Lyman lines we obtained a reasonable model of the EUV extension with low gas pressure, temperatures from 2times10^4 K to 10^5 K and with extended prominence-corona transition regions. Title: A Spectroscopic Model of euv Filaments Authors: Heinzel, P.; Anzer, U.; Schmieder, B. Bibcode: 2003SoPh..216..159H Altcode: We propose a new spectroscopic model for extended dark structures around Hα filaments observed in EUV lines. As in previous papers, we call these structures EUV filaments. Our model uses at least three observed EUV lines (located shortward the hydrogen Lyman-continuum limit) to compute iteratively the altitudes at which the EUV filament extensions are located. A transition-region line (O v in the present case) can be used to derive the opacity of the Lyman continuum and the other two coronal lines (e.g., Mg x and Si xii) then give two heights: the bottom and top of the EUV filament. The method takes into account self-consistently the absorption of EUV-line radiation by the Lyman continuum, as well as the volume-blocking effect potentially important for coronal lines. As an example we compute the heights of the EUV filament at one particular position, using CDS data for the 5 May 2000 filament. At this position, the EUV filament extension lies between altitudes 28 700 and 39 000 km, so that the geometrical thickness of the structure is 10300 km (we discuss also the sensitivity of our models to variations of the line intensities). These heights are consistent with the concept of twisted magnetic flux tubes, but there could be also some influence on our results due to additional low lying cool structures from parasitic polarities. Title: Determination of the 3D-topology of an EUV-filament observed by SoHO/CDS SoHO/SUMER and VTT/MSDP Authors: Schwartz, P.; Heinzel, P.; Schmieder, B.; Anzer, U. Bibcode: 2003ESASP.535..495S Altcode: 2003iscs.symp..495S We have constructed a spatial model of an extended EUV-filament observed on 15 October 1999 (polar crown N 38 deg) by SoHO/CDS. The narrow Hα counterparts were observed by VTT/MSDP. As found in 2001, EUV-filaments are much more extended than those parts visible in Hα. We have computed the bottom and top heights of this EUV-filament for the whole CDS raster for different values of the Lyman-continuum optical thickness using CDS measurements of two coronal lines (Mg X 624.94 Å and Si XII 520.60 Å) and the model of Heinzel et al., 2003. For the scale height of the MgX line we used the value from Fludra et al. 1999, assuming that this line is not solar cycle and activity dependent. We created a new method for τ912 determination from O VI and O V line intensities. Because of the inhomogeneity in the O V 629.37 Å line intensity distribution in the vicinitiy of the EUV-filament it was not possible to estimate this intensity under the filament. So we used observations made by SoHO/SUMER of the O VI 1031.91 Å line not absorbed by EUV-filament cold plasma. The knowledge of the heights of the bottom and top boundaries of an EUV-filament gives us the EUV-filament geometrical thickness. From this the mass of the EUV-filament was computed. Title: EUV-filaments and their mass loading Authors: Heinzel, P.; Anzer, U.; Schmieder, B.; Schwartz, P. Bibcode: 2003ESASP.535..447H Altcode: 2003iscs.symp..447H It was found recently (Heinzel et al., 2001; Schmieder et al., 2003) that solar filaments observed in EUV lines by SOHO/CDS are much more extended than their Hα counterparts. This was explained by a large difference between the hydrogen Lyman-continuum and Hα opacities. Two different MHD models were suggested to explain the EUV-filament extensions: the model based on parasitic polarities (Aulanier & Schmieder, 2002) and the model with twisted flux tubes (Anzer & Heinzel, 2003). The latter model can explain our recent findings that at least some parts of the EUV-filament extensions are located relatively high in the corona. These heights can be computed using a new spectroscopic model of EUV-filaments. The mass which is loaded into the EUV-filament extensions is then estimated on the basis of non-LTE transfer calculation. The total filament mass is larger than that derived for the Hα filament itself and this may have consequences for the structure and mass loading of CMEs whenever they form from such filaments - this may answer the question how the extended CME structures can form from rather narrow Hα filaments. We summarize the basic properties of EUV-filaments, present their spectroscopic analysis and give some estimates for mass loading. We then discuss possible relations between EUV-filaments and CMEs, in particular the problems of their masses. Title: On the nature of extended EUV filaments Authors: Anzer, U.; Heinzel, P. Bibcode: 2003A&A...404.1139A Altcode: This paper describes the properties of extended EUV filaments and the theoretical modelling of them. We summarise the general aspects of the depression of EUV-line emission and give an interpretation of recent filament observations in transition-region and coronal lines. The EUV filament was found to be located relatively high in the corona (at least 20 000 km above the solar surface) and this requires an MHD scenario alternative to the parasitic-polarity model of Aulanier & Schmieder (\cite{Aulanier02}). Here we present a new idea for the support of cool gas in the magnetic arcade of a prominence which is capable of explaining both wide and vertically extended EUV filaments. Our mechanism is based upon the twisting of individual flux tubes, similar to the one which was suggested by Priest et al. (\cite{Priest89}). Finally, the consequences of this new model are discussed. Title: The influence of radiative effects on the accretion onto stellar magnetospheres Authors: Kryukov, I. A.; Pogorelov, N. V.; Anzer, U.; Bisnovatyi-Kogan, G. S.; Börner, G. Bibcode: 2003A&A...402...13K Altcode: The influence of radiative effects on the accretion onto stellar magnetospheres is investigated by performing global 2- and 2.5-dimensional simulations on the basis of high-resolution numerical schemes with the application of irregular grids adapted to the shape of the magnetopause. The latter is represented by an impermeable, contracted dipole magnetic field surface with polar holes. Accreting matter is assumed to be optically thin. The physical mechanisms which are taken into account include cooling due to free-free and free-bound transitions, the Compton heating via X-ray scattering on electrons, and the inverse Compton cooling in the regions where the temperature of the matter becomes sufficiently large to be able to transfer part of its internal energy to photons. Depending on the determining parameters, both steady-state solutions with a system of discontinuities and unsteady flows with expanding shock waves can be obtained. It is shown that efficient cooling of the matter can substantially facilitate the penetration of the matter through the polar holes. The detailed consideration of the realistic radiative effects proved to be of great importance in our understanding of the accretion phenomenon, since they can substantially affect it both qualitatively and quantitatively. Title: 2D Radiative Transfer in Magnetically Confined Structures Authors: Heinzel, P.; Anzer, U. Bibcode: 2003ASPC..288..441H Altcode: 2003sam..conf..441H Magnetically confined structures in the solar atmosphere exhibit a large complexity in their shapes and physical conditions. As an example, we show the case of so-called magnetic dips in prominences which are in magnetohydrostatic equilibria. For such models we solve 2D non-LTE multilevel problem for hydrogen with PRD in Lyman resonance lines. The iterative technique used is based on the MALI approach with simple diagonal ALO and SC formal solver.

To compute the hydrogen ionization balance, the preconditioned MALI equations are linearized with respect to atomic level populations and electron density and solved iteratively using the Newton-Raphson scheme. Two additional problems are addressed: (i) an adequate iteration method for cases when the column-mass scale is used in one of the two dimensions but varies along the other dimension (which has a geometrical scaling); and (ii) a possibility of using AMR (Adaptive Mesh Refinement) algorithms to account for steep 2D gradients of selected variables (temperature, density, etc.). Title: Radiative Effects in the Modelling of Accretion onto Stellar Magnetospheres Authors: Pogorelov, Nikolai; Kryukov, Igor; Anzer, Ulrich; Bisnovatyi-Kogan, Guennadii; Börner, Gerhard Bibcode: 2003acfp.conf..473P Altcode: Accretion onto neutron stars and black holes is the main energy supply in galactic X-ray sources. The angular momentum captured by the X-ray star from the optical companion's wind with velocity V_{infty} and the binary period P is proportional to V^{-4}_{infty}P^{-1} [4]. If V_{infty} is high or the binary components are widely separated, the angular momentum is not sufficient for the accretion disk formation at distances of the Alfvén radius where the magnetic pressure of the star is balanced by the dynamic pressure of infalling matter, and the accretion at large distances from the star becomes nearly spherically symmetric. This occurs, for example, in some X-ray sources with a massive companion star where long-periodic pulsars are observed [1], [3]. Title: A prominence with transition region: horizontal two-dimensional filament model Authors: Gorshkov, A. B.; Anzer, U.; Heinzel, P. Bibcode: 2002ESASP.506..405G Altcode: 2002ESPM...10..405G; 2002svco.conf..405G SOHO/SUMER observations of solar filaments in Lyman lines of hydrogen have revealed, in general, a two-peak form of the emission line profiles with peak to center ratio less than an order of magnitude. This fact strongly contradicts the results of numerical radiative transfer simulations, where profiles with strong dips and large (1.5-2 orders of magnitude) peak to center ratio are usually obtained due to the absence of incident radiation from above the filament and, as a consequence, very low atomic populations on the upper energy levels at the top of the filament. Recently it was shown (Schmieder et al. 1998) that this discrepancy can be removed by introducing a prominence-corona transition region (PCTR) to the radiative transfer models. This paper continues a set of publications (Anzer & Heinzel, 1999 and Heinzel & Anzer, 2001) devoted to the modelling of prominences as structures in MHS equilibrium. Here the prominence is represented by a horizontal 2D slab supported by a magnetic field. In our model we use a constant gas pressure and assume that the kinetic temperature profile and the PCTR extension are different in the vertical and horizontal directions due to magnetic field. In the PCTR the temperature sharply increases outwards from 8000K to 50000K on the scale of 100-600 km. To solve the radiative transfer problem we apply a 2D code based on the MALI iteration scheme and the modified long characteristics method. Title: Magnetic dips and the physics of quiescent prominences Authors: Anzer, Ulrich Bibcode: 2002ESASP.506..389A Altcode: 2002svco.conf..389A; 2002ESPM...10..389A This paper discusses the recent developments of prominence modelling. The basic aspect of our discussion is the presence of magnetic dips in all the models. We first show that "initial" dips are needed for prominence formation and stability. We then give some typical examples for different field configurations with dips. Next we discuss those dips which are required for prominence support. We present both 1D (slab) and 2D (thread) models. In addition the problem with the theoretical modelling of extended EUV filaments is described. Then we explain the difficulties with the observational confirmation of dipped structures. We also comment on a model which suggests that equilibria can be obtained for configurations without dips and finally we summarise the most relevant aspects. Title: Spectral diagnostics of the magnetic field orientation in a round-shaped filament Authors: Schmieder, B.; Mein, N.; Heinzel, P.; Anzer, U. Bibcode: 2002ESASP.506..469S Altcode: 2002ESPM...10..469S; 2002svco.conf..469S During several campaigns focused on prominences we have obtained coordinated spectral observations from the ground and from space. The SOHO/SUMER spectrometer allows us to observe, among others, the whole Lyman series of hydrogen, while the Hα line was observed by the MSDP spectrograph at the VTT. For Lyman lines, the non-LTE radiative-transfer computations have shown the importance of the prominence-corona transition region (PCTR) and its relation to the magnetic field orientation for the explanation of the observed line profiles (Schmieder et al. 1998, Heinzel et al. 2001). Moreover, Heinzel and Anzer (2001) developed new 2D models which demonstrate how the shapes of Lyman lines vary depending on the orientation of the magnetic field with respect to the line of sight. To confirm this result observationally, we focus here on a round-shaped filament observed during three days as it was crossing the limb. The Lyman profiles observed on the limb are different from day to day and we interpret these differences by the change of orientation of the prominence axis (and therefore the magnetic field direction) with respect to the line-of-sight. Title: Prominence fine structures in a magnetic equilibrium: Two-dimensional models with multilevel radiative transfer Authors: Heinzel, P.; Anzer, U. Bibcode: 2001A&A...375.1082H Altcode: In this paper we construct theoretical models for vertical prominence threads which are in magnetohydrostatic (MHS) equilibrium. These models are fully two-dimensional (2D) and take the form of vertically infinite threads hanging in a horizontal magnetic field. A typical example of such a 2D magnetic-dip structure is shown for the case when the central cool parts are surrounded by the prominence-corona transition region (PCTR). We display 2D variations of the pressure, density and temperature. While the pressure variations follow from the MHS equilibrium, the kinetic temperature was specified empirically. As a next step, we have solved the 2D multilevel non-LTE transfer problem in such thread-like structures, in order to predict the spatial variations of the emergent hydrogen spectrum. It is demonstrated that the hydrogen Lyman lines (treated with partial redistribution) show significant spatial variations of the intensity and that an important difference exists between the line profiles emergent along and across the magnetic field lines. We also discuss how these intensity variations compare to recent SOHO/SUMER prominence observations, namely we show the effects of line-profile averaging over the fine-structure threads which are below the instrumental resolution. Finally we make some suggestions for future modelling and observations. Title: Hydrodynamic modeling of accretion onto stellar magnetospheres Authors: Kryukov, I. A.; Pogorelov, N. V.; Bisnovatyi-Kogan, G. S.; Anzer, U.; Börner, G. Bibcode: 2000A&A...364..901K Altcode: A numerical investigation is performed of the accretion of slowly rotating matter onto a stellar magnetosphere. The shape of the magnetosphere is represented by an impermeable, contracted dipole magnetic field surface with polar holes. Depending on the governing parameters, both steady-state solutions with a system of discontinuities and unsteady flows with expanding shock waves can be obtained. Certain solutions exhibit extended outflow regions. The simulation is performed using a high-resolution finite volume numerical scheme and structured irregular grids adapted to the shape of the accreting body. Title: Energy considerations for solar prominences with mass inflow Authors: Anzer, U.; Heinzel, P. Bibcode: 2000A&A...358L..75A Altcode: In this Letter we study the inflow of enthalpy and ionisation energy into solar prominences. We use 1D stationary slab models for the prominence to calculate this inflow. We compare the resulting energy gain with the integrated radiative losses obtained for such slab models. We find that for reasonable inflow velocities many of our models can be in energy equilibrium; only the very massive prominences will either require some additional heating or they have to cool down to low central temperatures. We also discuss the possibility or heating the prominence by vertical downflows. Title: The energy balance in solar prominences Authors: Anzer, U.; Heinzel, P. Bibcode: 1999A&A...349..974A Altcode: In this paper we study the energy balance in quiescent solar prominences. For this investigation we use a simple 1D slab model in magneto-hydrostatic equilibrium. We divide the studied region into two parts: The inner region consisting of the prominence itself and the cool part of the transition region (up to 30000 K). In this region the plasma is optically thick in several atomic transitions and therefore we have to solve the non-LTE radiative transfer problem in this part. This allows us to derive self-consistently the gas density, the ionization degree of hydrogen and the hydrogenic radiation losses. The energy transport by thermal conduction can be neglected in this region. In the outer part of the transition region, the thermal conduction becomes very important but the treatment of radiation is simplified by the fact that the plasma is optically thin. We find that energy balance in prominences can only be obtained for special forms of the heating function. This requirement is very stringent for the inner parts, while in the outer region thermal conduction can transport large amounts of the heat energy and therefore a wider class of heating functions will be allowed. Title: Magnetic Dips in Prominences Authors: Heinzel, P.; Anzer, U. Bibcode: 1999SoPh..184..103H Altcode: Magnetic dips are generally assumed to be basic equilibrium configurations in quiescent solar prominences. Here we discuss two types of the magnetic dips which were considered in the literature: (1) dips resulting from a force-free magnetic equilibrium in the corona, and (2) magnetic dips which are formed in situations where the Lorentz force balances the weight of the prominence plasma. An important parameter which decides between these two cases is the plasma β. For β⋘1, the effect of the prominence material on the equilibrium structure is quite negligible and the case (1) holds. If, however, β is larger, say between 0.1 and 1 or even higher, magnetic dips of the second kind are formed and they can be characterized by the angle ψ1 between the vertical and the direction of the field lines at the surface of the prominence structure. A simple and illustratory formula is derived to relate this angle to the plasma β at the prominence center, namely βc≃cot2ψ1. βc=1 thus corresponds to ψ1=45°. Finally, we discuss the range of values of both βc and ψ1 as deduced from various observations and conclude that the dips of the second kind are important for the prominence equilibria. We also suggest a new method for determination of the field-line inclination. Title: Numerical Computation of Two-Dimensional Wind Accretion of Isothermal Gas Authors: Shima, E.; Matsuda, T.; Anzer, U.; Börner, G.; Boffin, H. M. J. Bibcode: 1999ASSL..240..225S Altcode: 1999numa.conf..225S No abstract at ADS Title: Numerical computation of two dimensional wind accretion of isothermal gas Authors: Shima, Eiji; Matsuda, Takuya; Anzer, Ulrich; Boerner, Gerhard; Boffin, Henri M. J. Bibcode: 1998A&A...337..311S Altcode: 1998astro.ph..5343S A new numerical algorithm for calculating isothermal wind accretion flows has been developed and is applied here to the analysis of the hydrodynamics of two-dimensional plane symmetric accretion flows in wind-fed sources. Polar coordinates are used to ensure fine resolution near the object. It is found that a thin accretion column is formed which shows wave-like oscillations. Small accretion disks are formed temporarily around the object. Mass accretion rate and angular momentum accretion rate exhibit quasi-periodic oscillations. The amplitudes of the oscillations depend on the size of the inner boundary, the number of grid points and the method of calculation. For a smaller size of the inner boundary, finer grids and more accurate numerical schemes, the amplitudes of the oscillation become larger. Title: Prominence Parameters Derived from Magnetic-Field Measurements and NLTE Diagnostics Authors: Anzer, U.; Heinzel, P. Bibcode: 1998SoPh..179...75A Altcode: In this paper we present a detailed analysis of a number of quiescent prominences for which the components of the magnetic field as well as the electron density and emission measure were previously obtained from quasi-simultaneous measurements in hydrogen Hα and helium D3 lines. From magnetic equilibrium models of the Kippenhahn-Schlüter type one can calculate the gas pressure, density, column mass and geometrical width. The same set of physical parameters can also be derived from a NLTE hydrogen-line analysis. We have studied the mutual correlations between these two sets of parameters. Very large differences, reaching an order of magnitude, were found between these two sets, both for individual prominences and for the mean values over all prominences used in this investigation. Finally, we discuss some implications of our results. Title: Modelling of Non-Uniform Prominence Slabs Authors: Heinzel, P.; Anzer, U. Bibcode: 1998ASPC..150..213H Altcode: 1998IAUCo.167..213H; 1998npsp.conf..213H No abstract at ADS Title: VELA X-1: how to produce asymmetric eclipses. Authors: Feldmeier, A.; Anzer, U.; Boerner, G.; Nagase, F. Bibcode: 1996A&A...311..793F Altcode: Light curves of the X-ray pulsar Vela X-1 obtained with the ASCA satellite show a strong asymmetry in the hard energy band during the eclipse of the X-ray source: a steep drop at ingress is followed by a gradual decline, whereas only a steep increase is observed at egress. X-ray scattering off the dense accretion wake trailing the neutron star cannot explain the gradual decline because of the long persistence of the latter ({DELTA}φ=0.11). Instead we propose that scattering in an extended photoionization wake is responsible. This wake is caused by the switch-off of the radiative force that drives the B supergiant wind during the passage through the highly ionized Stroemgren region surrounding the X-ray source. The stalled gas then trails the neutron star, which moves relative to the B star surface (no corotation). A model for the Vela X-1 system which assumes that the B star does not rotate gives too large a phase extent for the dense wake and can be ruled out. Including the B star rotation in an approximate way, the relative azimuthal motion of the neutron star is slower and the wake covers a smaller phase interval. Finally, we assume that the Stroemgren sphere does not reach too deep into the wind accelerating region. The wind can then reach a certain fraction of the terminal velocity before the radiative force is switched off. This elongates the photoionization wake further and both its phase extent and its scattering efficiency match the observations. Title: Modeling Accretion onto a Closed Pulsar Magnetosphere Authors: Morris, J. P.; Boerner, G.; Anzer, U. Bibcode: 1995AAS...187.3306M Altcode: 1995BAAS...27Q1330M A method for modeling accretion onto a closed magnetosphere employing the numerical method of SPH is presented. This method allows the dynamic interaction of the magnetosphere and surrounding field-free plasma to be simulated. Results obtained for some steady configurations are compared with previous work for closed magnetospheres. Future application of the technique, with a view to modeling spin up and spin down of pulsars is discussed. Other possible applications and enhancements of the method are outlined. Title: Parameters of Quiescent Prominences Derived from Magnetic Field Measurements Authors: Anzer, Ulrich Bibcode: 1995SoPh..161...49A Altcode: Based upon the new magnetic field vector measurements of Bommieret al. (1994) we construct prominence models which are in magnetohydrostatic equilibrium. We compare these new models with earlier ones and find that they are on average more massive and also considerably narrower. Title: Random variations of the pulse period of X-ray binaries with wind accretion. Authors: Anzer, U.; Boerner, G. Bibcode: 1995A&A...299...62A Altcode: In this paper we first summarise earlier results about the randomness of pulse period variations in wind fed X-ray pulsars. Then we present two types of models for the random spin-up and spin-down of rotating neutron stars which accrete mass from a stellar wind. We show that the irregularities in accretion flows which were discovered in numerical models have too low amplitudes in order to explain the observations. Therefore we discuss an alternative model which is based upon the existence of a Kepler disc around the neutron star. We argue that such a model can give an explanation of the observations. Title: Book Review: Fragmented energy release in sun and stars, the interface between MHD and plasma physics / Kluwer, 1994 Authors: Anzer, U. Bibcode: 1995SoPh..158R.399V Altcode: No abstract at ADS Title: Book reviews Authors: Valníček, B.; Anzer, U. Bibcode: 1995SoPh..158..399V Altcode: 1995SoPh..158..399A No abstract at ADS Title: Bondi-Hoyle accretion simulations including velocity gradients. Authors: Ruffert, M.; Anzer, U. Bibcode: 1995A&A...295..108R Altcode: We investigate the hydrodynamics of three-dimensional classical Bondi-Hoyle accretion, especially the accretion of angular momentum from a non-homogeneous medium and discuss some consequences for models of wind-fed X-ray sources. A medium taken to be an ideal gas (γ=5/3) moves at Mach 3 past a totally absorbing sphere with a radius of 0.1 accretion radii. The velocity of this medium is given a gradient of 3% (per accretion radius distance) perpendicular to the bulk flow. The hydrodynamics is modeled by the "Piecewise Parabolic Method" and the resolution around the accretor is increased using 7 multiply nested grids. The flow in the shock cone is not steady, but does not exhibit large scale "flip-flop"-flow. The average specific angular momentum accreted is 0.097 (in units of accretion radii and sound speed). Although the magnitude is large compared to cases without velocity gradient, it still is not large enough to sustain the random walk model of wind-fed X-ray sources. Title: Numerical studies of wind accretion using SPH I. 2D simulations. Authors: Boffin, Henri M. J.; Anzer, Ulrich Bibcode: 1994A&A...284.1026B Altcode: Two-dimensional numerical simulations of accretion by a gravitating object from a homogeneous wind are performed using SPH. The effects of several parameters changes as well as the algorithm modifications are studied. The flow is in most cases weakly unstable with a very small, quasi-periodic oscillation of the shock cone. Comparison with previous works is also done. Title: Remarks on Two-Dimensional Magnetic Arcade Models of Coronal Structures Authors: Anzer, U. Bibcode: 1994scs..conf..309A Altcode: 1994IAUCo.144..309A A critical evaluation of some recent models for coronal structures using two-dimensional magnetic arcades is given. Prominences with inverse polarity as well as the evolution of sheared arcades are discussed. Title: Tests of Wind Accretion Models Authors: Anzer, U.; Börner, G.; Nagase, F. Bibcode: 1994nhxr.conf..611A Altcode: No abstract at ADS Title: A simple analysis of period noise in binary X-ray pulsars. Authors: de Kool, Martijn; Anzer, Ulrich Bibcode: 1993MNRAS.262..726D Altcode: A simple method is presented for determining whether the noise in the pulse period history of X-ray pulsars can be described by a random walk process. This is accomplished by calculating the mean difference in angular velocity between different observations as a function of the time interval between these observations. The method is applied to 10 X-ray pulsars for which more than 10 period determinations are available. For Vela X-1, earlier results that the pulse period behavior is very well fitted by a random walk in period is confirmed. In GX 301-2 the pulse period behavior is also consistent with a random walk. Analysis of the pulse period history of three X-ray pulsars with a Be companion shows that a random walk is not consistent with the data. Title: Numerical Simulations of Two-dimensional and Three-dimensional Wind Accretion Flows of an Isothermal Gas Authors: Ishii, Takanori; Matsuda, Takuya; Shima, Eiji; Livio, Mario; Anzer, Ulrich; Boerner, Gerhard Bibcode: 1993ApJ...404..706I Altcode: We present the results of 2D and 3D numerical hydrodynamical calculations of accretion flows of an isothermal gas past a gravitating compact object. The calculations were performed both for a homogeneous medium and for a medium containing a transverse density or velocity gradient. We find that 2D isothermal flows exhibit the 'flip-flop' instability (previously seen in adiabatic calculations) both in the homogeneous and the inhomogeneous cases. In the 3D case, while some unsteadiness is observed, the instability is much less violent than in 2D. We calculate (for the first time with a 3D hydrocode) the rate of accretion of mass and angular momentum from an inhomogeneous medium. The mass accretion rate agrees quite well with the prediction of the Bondi-Hoyle theory. However, the rate of accretion of angular momentum is considerably lower than the rate at which angular momentum is deposited into the accretion cylinder. A possible consequence of our results for wind-fed X-ray binaries is discussed. Title: Some Aspects of Magnetic Arcades in the Solar Corona Authors: Anzer, U. Bibcode: 1993ComAp..16..305A Altcode: No abstract at ADS Title: Variations of the pulse periods in X-ray binaries: modelling attempts Authors: Anzer, U. Bibcode: 1993heac.conf...15A Altcode: No abstract at ADS Title: The Fibril Structure of Prominences Authors: Hood, A. W.; Priest, E. R.; Anzer, U. Bibcode: 1992SoPh..138..331H Altcode: In this paper we present several magneto-hydrostatic equilibrium models for prominences with fibril-like fine structure. For all the models ad hoc temperature profiles are used without discussing the energetics. For our models we assume fine structure to occur either across the prominence axis or along it. This approach is intended as a first step towards more realistic models based upon a series of vertical fibril structures. Title: Numerical simulations of two-dimensional and three-dimensional accretion flows Authors: Matsuda, Takuya; Ishii, Takanori; Sekino, Nobuhiro; Sawada, Keisuke; Shima, Eiji; Livio, Mario; Anzer, Ulrich Bibcode: 1992MNRAS.255..183M Altcode: Numerical simulations of 2D and 3D accretion flows past a gravitating compact object from a uniform flow at a large distance upstream are performed by solving the Eulerian equations. The 2D flows exhibit a 'flip-flop instability' if the central accreting body is small. If the central body is enlarged at some instance in the oscillating flow, then the accretion shock shows a rather periodic oscillation similar to the von Karman vortex street. In the case of 3D flows, it is found that the shock cone is much more robust than in 2D, and the flip-flop instability takes a different, probably less violent, form. The causes of the instabilities are discussed. Title: Pulsating X-Ray Sources - Longterm Period Variations Authors: Anzer, U.; Borner, G. Bibcode: 1992ComAp..16...31A Altcode: No abstract at ADS Title: Flip-Flop Instability of Wind-Fed Accretion Flow - 2D and 3D Calculations Authors: Matsuda, T.; Ishii, T.; Livio, M.; Anzer, U.; Börner, G. Bibcode: 1992fxra.conf..177M Altcode: No abstract at ADS Title: On the 'flip-flop' instability of Bondi-Hoyle accretion flows Authors: Livio, Mario; Soker, Noam; Matsuda, Takuya; Anzer, Ulrich Bibcode: 1991MNRAS.253..633L Altcode: A simple physical interpretation is advanced by means of an analysis of the shock cone in the accretion flows past a compact object and with an examination of the accretion-line stability analyses. The stability of the conical shock is examined against small angular deflections with attention given to several simplifying assumptions. A line instability is identified in the Bondi-Hoyle accretion flows that leads to the formation of a large opening-angle shock. When the opening angle becomes large the instability becomes irregular oscillation. The analytical methodology is compared to previous numerical configurations that demonstrate different shock morphologies. The Bondi-Hoyle accretion onto a compact object is concluded to generate a range of nonlinear instabilities in both homogeneous and inhomogeneous cases with a quasiperiodic oscillation in the linear regime. Title: On the stability of wind accretion. Authors: Matsuda, T.; Sekino, N.; Sawada, K.; Shima, E.; Livio, M.; Anzer, U.; Boerner, G. Bibcode: 1991A&A...248..301M Altcode: Results of numerical investigations of accretion flows past a compact object are reported. A strong tendency toward nonsteady flow is found to exist in both 2D and 3D computations. Steady flows are obtained only when the numerical resolution is low or the central absorbing body is large. It is concluded that: (1) accretion from a homogeneous medium onto a compact object is intrinsically nonsteady; (2) the instability manifests itself in 'flip-flop' oscillations of the accretion shock cone, accompanied by short phases of quasi-disk formation, with the sense of rotation reversing periodically; and (3) no density or velocity gradients in the incoming flow are required for the instability to develop. Title: The Fibril Structure of Prominences Authors: Priest, E. R.; Hood, A. W.; Anzer, U. Bibcode: 1991SoPh..132..199P Altcode: We suggest that the fibril structure of prominences may be caused by filamentation during its formation by radiative instability. We also discuss the effects of other types of instability and give a mechanism for the formation of vertical threads. The models indicate that highly inhomogeneous density structures can exist in the presence of smooth profiles for the plasma pressure and magnetic field. In our particular models the plasma pressure of a fibril prominence is higher and the vertical magnetical field is weaker than in a uniform prominence model, while the mass is substantially smaller. Title: Binary X-Ray Sources: Some Aspects of Wind Accretion Authors: Anzer, U.; Börner, G. Bibcode: 1991heac.conf....3A Altcode: No abstract at ADS Title: Magnetic Field Configurations which can Produce Prominences with Inverse Polarity Authors: Anzer, U. Bibcode: 1990SoPh..130..403A Altcode: In this Letter several observational aspects of prominences with inverse magnetic polarity are summarised. It is shown that these features can be explained in a natural way if inverse polarity prominences result from the merging of two adjacent bipolar magnetic regions. Title: Prominence models with line currents - Stabilisation by flux conservation Authors: Anzer, U.; Ballester, J. L. Bibcode: 1990A&A...238..365A Altcode: A model of prominence eruption, based on a filament with circular cross section as different from the filament of a single line current used in earlier models, is presented. For configurations with normal and inverse polarity, the model is considered to be stable with respect to the vertical displacements. This implies that simple configurations cannot model prominence eruptions unless the effect of flux conservation is also included. It is found that if the magnetic flux below a finite size filament is conserved, a run-away instability no longer exists. The results apply to a limited class of 2-D equilibria containing a cylindrical filament and subjected to strictly 2-D perturbation. Models that are more realistic can be unstable. Title: A Model for Quiescent Solar Prominences with Normal Polarity Authors: Hood, A. W.; Anzer, U. Bibcode: 1990SoPh..126..117H Altcode: A class of 2-D models of solar quiescent prominences, with normal polarity, is presented. These represent an extension to the Kippenhahn-Schlüter model for which the prominence configuration matches smoothly onto an external non-potential coronal solution of a constant α field. Using typical prominence values a model is constructed which also matches the coronal conditions. It is found that the magnetic field component along the prominence influences the internal structure of the prominence. A simple extension to the basic models is indicated as a means of taking a lower boundary of the prominence and eliminating parasitic polarities in the photosphere. Title: Structure and stability of prominences Authors: Anzer, U. Bibcode: 1990GMS....58..307A Altcode: The present consideration of the magnetohydrostatic equilibria of quiescent solar prominences proceeds from the conceptual framework of field configurations having normal and inverse magnetic polarities to an evaluation of the principal models for both types of configuration. A novel model based on the twisted flux tube phenomenon is presented with a view to the visualization of the prominence as a sheet within such a flux rope. Questions as to the energy balance of prominences are left out of account. Title: Prominence Models with Line Currents: Stabilization by Flux Conservation Authors: Anzer, U.; Ballester, J. L. Bibcode: 1990PDHO....7..156A Altcode: 1990ESPM....6..156A; 1990dysu.conf..156A No abstract at ADS Title: A normal polarity quiescent prominence model. Authors: Hood, A. W.; Anzer, U. Bibcode: 1990PDHO....7..130H Altcode: 1990dysu.conf..130H A class of simple 2-D non-isothermal models for quiescent prominences with normal polarity are presented. These extended the Kippenhahn-Schlüter and Menzel models by smoothly matching the prominence region onto an external, force-free coronal field. In addition, the field component along the prominences is an integral part of the solution and modifies the internal structure. Title: A Model for Quiescent Solar Prominences with Normal Polarity Authors: Hood, A. W.; Anzer, U. Bibcode: 1990LNP...363..271H Altcode: 1990doqp.coll..271H; 1990IAUCo.117..271H No abstract at ADS Title: A three-dimensional model for solar prominences Authors: Démoulin, P.; Priest, E. R.; Anzer, U. Bibcode: 1990LNP...363..268D Altcode: 1990doqp.coll..268P; 1990LNP...363..268P; 1990IAUCo.117..268P We suggest here a model for the 3D structure of quiescent prominences by a superposition of two fields. A 3D force-free field with constant is assumed to exist in the corona prior to the prominence formation. The prominence itself is represented by a line current which interacts with the coronal field. The three-dimensional field is represented by analytical functions and concentration of the magnetic field at the photospheric level by convection cells is taken into account. When the field created by the photospheric pattern supports the prominence, the prominence feet are found to be located at supergranule centres otherwise; they are located at cell boundaries. Title: A three-dimensional model for solar prominences Authors: Demoulin, P.; Priest, E. R.; Anzer, U. Bibcode: 1989A&A...221..326D Altcode: In an attempt to model the external force field of a prominence, a three-dimensional linear force-free field configuration was studied. The model consists of a fundamental together with a harmonic that is periodic along the prominence. The variation of the prominence height along the prominence is calculated and it is suggested that feet occur where the prominence sags down to low heights. Title: A Twisted Flux-Tube Model for Solar Prominences. I. General Properties Authors: Priest, E. R.; Hood, A. W.; Anzer, U. Bibcode: 1989ApJ...344.1010P Altcode: It is proposed that a solar prominence consists of cool plasma supported in a large-scale curved and twisted magnetic flux tube. As long as the flux tube is untwisted, its curvature is concave toward the solar surface, and so it cannot support dense plasma against gravity. However, when it is twisted sufficiently, individual field lines may acquire a convex curvature near their summits and so provide support. Cool plasma then naturally tends to accumulate in such field line dips either by injection from below or by thermal condensation. As the tube is twisted up further or reconnection takes place below the prominence, one finds a transition from normal to inverse polarity. When the flux tube becomes too long or is twisted too much, it loses stability and its true magnetic geometry as an erupting prominence is revealed more clearly. Title: Inhomogeneous wind accretion - Comparison between 3D and 2D computations Authors: Sawada, K.; Matsuda, T.; Anzer, U.; Boerner, G.; Livio, M. Bibcode: 1989A&A...221..263S Altcode: Full three-dimensional computations of accretion flows which have velocity gradients are presented and compared with two-dimensional computations. It is found that the rate of accretion of the angular momentum in three-dimensional flows is about 5 times smaller than in the two-dimensional cases. The effect that this finding has on the modeling of wind-fed binary X-ray sources is discussed. It is concluded that only three-dimensional calculations can give a reliable answer to the question of angular momentum transfer to the neutron star. Title: A Model for Quiescent Solar Prominences with Normal Polarity Authors: Hood, A. W.; Anzer, U. Bibcode: 1989HvaOB..13..281H Altcode: No abstract at ADS Title: A Three-Dimensional Model for Solar Prominences Authors: Demoulin, P.; Priest, E. R.; Anzer, U. Bibcode: 1989HvaOB..13..253D Altcode: No abstract at ADS Title: Structure and equilibrium of prominences Authors: Anzer, U. Bibcode: 1989ASSL..150..143A Altcode: 1988dsqs.work..143A; 1989dsqs.work..143A The existing models for magnetohydrostatic equilibrium of quiescent prominences are presented and critically discussed. Also considered is the question of how the energy balance can be achieved in these prominences. The main emphasis is on recent developments in these areas of research. Title: Thermal Condensations in Coronal Magnetic Fields Authors: Hood, A.; Anzer, U. Bibcode: 1988SoPh..115...61H Altcode: Conditions under which cool condensations can form in the solar corona are investigated using the powerful phase plane method to analyse the energy and hydrostatic balance equations. The importance of the phase plane approach is that the conclusions deduced are not sensitive to the actual choice of boundary conditions adopted which only determine the actual contour. The importance of heating variations and area divergence are studied as well as the influence of gravity for their effect on the formation of cool condensations. The cool temperature at which optically thin radiation and heating balance is important and the links with other cool solutions are mentioned. Title: Structure and equilibrium of prominences. Authors: Anzer, U. Bibcode: 1988dssp.conf...99A Altcode: The existing models for magnetohydrostatic equilibrium of quiescent prominences are presented and critically discussed. The author also investigates how the energy balance can be achieved in these prominences. The emphasis of this review is on the recent developments in these areas of research. Title: The influence of external magnetic fields on the structure of thin accretion disks. Authors: Anzer, U.; Boerner, G.; Meyer-Hofmeister, E. Bibcode: 1987A&A...188...85A Altcode: The internal structure of thin accretion disks is studied for the case where strong outside magnetic fields interact with these disks. Such configurations are expected to occur in close binary X-ray pulsars such as Her X-1. The authors find that the magnetic pressure exerted on the disk has two main effects: the vertical disk structure shows a density increase towards the interface between disk and magnetosphere; and the thickness of the disk decreases more rapidly as one approaches the inner edge of the disk. The second effect gives a stronger irradiation of the disk surface close to the inner edge and this results in a higher surface temperature. Title: The stability of line tied force-free cylindrical arcades: Is an active region filament a requirement for a two-ribbon flare? Authors: Hood, A.; Anzer, U. Bibcode: 1987SoPh..111..333H Altcode: The MHD stability of force-free, cylindrical arcades is investigated, including the stabilising effect of photospheric line tying. It is found that a wide variety of fields are stable. This suggests that either a departure from a force free equilibrium or suppression of line tying is necessary if a two-ribbon flare is to be triggered. It is postulated that in both circumstances, the existence of an active region filament is an essential preflare requirement for a two-ribbon flare. Title: Disk formation at the magnetosphere of wind-fed pulsars : applicationto VELA X-1. Authors: Boerner, G.; Hayakawa, S.; Nagase, F.; Anzer, U. Bibcode: 1987A&A...182...63B Altcode: The disk formation in a wind-fed, rotating, accreting neutron star is considered in view of the fact that the angular momentum transfer by wind accretion is too small to account for large rates of spin-up and spin-down as observed. Under the assumption that the closed magnetosphere extends to the corotation radius it is shown that a disk will form through the interaction of the spherically accreting matter with the rotating magnetosphere. Long term spin-up and spin-down episodes then become possible due to the difference between the accretion torque transmitted by the disk and the deceleration by the accretion at the magnetosphere surface. Large rates of short-term pulse period change are attributed to the change in the accretion torque transmitted by the disk. Application of this model to the X-ray source Vela X-1 requires a large magnetic moment μ ≡ 1032 Gauss cm3, but once the high value for the magnetic field (1013 Gauss ⪉ B ⪉ 1014 Gauss) is accepted, a variety of observations fit well into the picture. Title: Numerical studies of wind accretion. Authors: Anzer, U.; Boerner, G.; Monaghan, J. J. Bibcode: 1987A&A...176..235A Altcode: Wind accretion from a companion star is assumed to be the power source for wide X-ray binaries. These winds will in general have spatial variations both in density and in the velocity vector. The authors have performed 2-dimensional numerical computations of such inhomogeneous wind flows to obtain estimates on the amount of angular momentum accreted for a given gradient of the velocity vector. They find that the angular momentum flux is about a factor 5 smaller than the previous estimates. This means that there is not enough angular momentum transferred to the neutron star to explain, for example, the long term period changes of Vela X-1. This leads the authors to propose a different mechanism for the spin-up and spin-down of sources of the type of Vela X-1. They outline briefly the physical features of such a model. Title: The influence of external magnetic fields on the structure of thin accretion disks. Authors: Anzer, U.; Börner, G.; Meyer-Hofmeister, E. Bibcode: 1987imfo.work..203A Altcode: There is a generally accepted picture of many binary X-ray sources wherein a rotating magnetized neutron star accretes matter from a surrounding disk of plasma. As the accreting matter approaches the rotating neutron star it is more and more influenced by the stellar magnetic field until eventually its motion is dominated by the field. Finally the plasma will flow along the field lines towards the surface of the neutron star. The equilibrium structure of a thin accretion disk subjected to the pressure of an external magnetic field is described. Title: Stability of Magnetic Arcades Authors: Anzer, U.; Hood, A. W. Bibcode: 1987rfsm.conf..248A Altcode: The authors conclude that sheared force-free arcades cannot become unstable and produce solar flares. They therefore predict that two-ribbon flares must always be associated with extended solar prominences. Title: Structure and Equilibrium of Prominences Authors: Anzer, U. Bibcode: 1987dssp.work...99A Altcode: 1987ASSL..150...99A No abstract at ADS Title: The influence of external magnetic fields on the structure of thin accretion disks. Authors: Anzer, U.; Börner, G.; Meyer-Hofmeister, E. Bibcode: 1987MPARp.282.....A Altcode: No abstract at ADS Title: Modelling of the magnetic field of solar prominences. Authors: Anzer, U. Bibcode: 1987ppcs.work...61A Altcode: Measurements of magnetic fields with closed structures in quiescent prominences are discussed. It is shown that it is not possible to construct simple equilibrium models for quiescent prominences of the Kuperus-Raadu type. An open-field example is considered, and it is concluded that at present there exists no theoretical equililbrium model of the Kuperus-Raadu type which is consistent with observations. Title: Numerical studies of wind accretion. Authors: Anzer, U.; Börner, G.; Monaghan, J. J. Bibcode: 1986MPARp.230.....A Altcode: No abstract at ADS Title: Interaction of the Accretion Disk with the Magnetosphere of a Rotating Neutron Star Authors: Anzer, U.; Borner, G. Bibcode: 1986ppm..conf..101A Altcode: No abstract at ADS Title: The global structure of magnetic fields which support quiescent prominences Authors: Anzer, U. Bibcode: 1985svmf.nasa..101A Altcode: Magnetic fields in quiescent prominences were observed, but only recently has it become possible to measure the full magnetic field vector. The component of the field along the line of sight, B (parallel) can be uniquely determined, whereas for the component perpendicular to the line of sight B (perpendicular) and -B (perpendicular) are indistinguishable. An ambiguity remains in the actual magnetic field vector, in particular with respect to its orientation relative to the prominence axis. A sample of more than 100 prominences were studied. A more detailed analysis of 10 prominences are presented, and then set these prominence fields into relation to the underlying photospheric fields. It is found from statistical analysis of several hundred prominences that in 25% of the cases the field penetrates the prominence directly, whereas in 75% the field orientation in the prominence is reversed. Title: Remarks on the Magnetic Support of Quiescent Prominences Authors: Anzer, U.; Priest, E. Bibcode: 1985SoPh...95..263A Altcode: The development of magnetic field structures which can lead to prominence configurations of the Kuperus-Raadu type is discussed. Starting from streamer type configurations and preserving the total current in the system we find that simple two-dimensional static configurations lead to prominences which in general lie systematically much lower than the heights found from observations. We therefore conclude that either more complex field configurations are needed to explain the recent observations by Leroy et al. (1983) or the initial configurations must be very special. Title: The global structure of magnetic fields which support quiescent prominences. Authors: Anzer, U. Bibcode: 1985NASCP2374..101A Altcode: The author reviews and discusses models of magnetic field configurations in prominences. Title: Theoretical modelling of inhomogeneous fields in the chromosphere. Authors: Anzer, U.; Galloway, D. J. Bibcode: 1985cdm..proc..199A Altcode: Theoretical models of the magnetic field structure above a photosphere with magnetic flux tubes are constructed, on the hypotheses that the field is either current-free or is governed by the force-free equation j = αB, with α constant. The level where the magnetic pressure becomes equal to the ambient gas pressure is calculated, and is shown to depend very little on which of the above hypotheses is adopted. The resulting levels are in agreement with the "canopy heights" given by Giovanelli (1980) and Giovanelli and Jones (1982) for areas around active regions, but are higher than the results for the quiet, unipolar regions in Giovanelli and Jones (1983). The calculations suggest there are large regions of the low to middle quiet chromosphere that require non-magnetic heating mechanisms. Title: Some aspects of the angular momentum transfer between an accretion disk and a rotating, magnetic neutron star Authors: Anzer, U.; Boerner, G. Bibcode: 1984AdSpR...3j.315A Altcode: 1984AdSpR...3..315A No abstract at ADS Title: Remarks on the magnetic support of quiescent prominences. Authors: Anzer, U.; Priest, E. Bibcode: 1984MPARp.157.....A Altcode: No abstract at ADS Title: Some Aspects of the Angular Momentum Transfer Between an Accretion Disk and a Rotating Magnetic Neutron Star Authors: Anzer, U.; Borner, G. Bibcode: 1984heac.conf..315A Altcode: No abstract at ADS Title: Accretion onto rotating, magnetic neutron stars : the inner edge of the disk. Authors: Anzer, U.; Boerner, G. Bibcode: 1983A&A...122...73A Altcode: The surface of an accretion disk in a rotating neutron star's magnetic field is subject to the Kelvin-Helmholtz instability induced by the velocity difference of the rigidly rotating magnetosphere and the Keplerian disk. This instability is analysed in a simplified geometry. It is shown that a large part of the disk surface will become unstable, leading to a turbulent diffusion of the disk plasma into the magnetosphere. The resulting exchange of angular momentum between the neutron star and the accreting matter is described. The present model of a magnetic dipole axis perpendicular to the rotation axis of the neutron star is compared to the model of an aligned rotator. The observational consequences with respect to the rotational state of pulsating X-ray sources are discussed. Title: A model for the magnetic field above supergranules Authors: Anzer, U.; Galloway, D. J. Bibcode: 1983MNRAS.203..637A Altcode: A model is provided for the inhomogeneous magnetic field structure above a stellar photosphere where the field is concentrated in intense tubes at the downflow regions of a convection pattern. By finding where the magnetic energy is strong or weak compared with the local thermal energy of the gas, the relative importance of magnetic and non-magnetic heating mechanisms can be estimated for those regions where the principal radiative losses occur in solar or stellar chromospheres. For the quiet Sun, slightly over half of the chromospheric emission comes from regions which are essentially non-magnetic. For other stars, the calculations yield a simple working rule which suggests that most late-type stars with fields strong enough to be directly observable will have magnetically dominated chromospheres. Title: The magnetic field configuration in solar and stellar chromospheres Authors: Anzer, U.; Galloway, D. J. Bibcode: 1983IAUS..102..339A Altcode: Calculations are presented for the inhomogeneous magnetic field structure above a stellar photosphere which has magnetic flux tubes located at the downdraughts of its supergranulation pattern. Regions can be delineated where the ambient magnetic energy density is large or small compared with the thermal energy density derived from a model atmosphere. This enables the relative importance of magnetic versus non-magnetic heating mechanisms to be assessed. For the quiet Sun, over half the chromospheric emission must be supplied non-magnetically, whilst the network and active regions require a magnetic supply. For other late-type stars, a simple working rule suggests that when the magnetic field is strong enough to be directly observable, the chromosphere will be magnetically dominated. Title: Interaction of accretion disks and rotating magnetic neutron stars. Authors: Anzer, U.; Boerner, G. Bibcode: 1983heac.book..294A Altcode: Consideration is given to some theoretical aspects of the interaction of an accretion disk and a rotating magnetic star. The principle numerical models of the problem are analyzed with respect to parameterizations of the angular momentum exchange between the accretion disk and the magnetic neutron star. On the basis of the analysis, a detailed quantitative model is proposed. In the model, Kelvin-Helmholtz instability leads to turbulent diffusion of disk matter onto the magnetic lines of the neutron star near the region of corotation. The balance and acceleration of the stars are calculated and the angular momentum exchange is obtained. The model calculations are given in a table. Emphasis is given to the need for greater generalization in current numerical models of the problem in order to account for both Kelvin-Helmholtz and Rayleigh-Taylor MHD instabilities. Title: Accretion onto rotating magnetic neutron stars: the inner edge of the disk. Authors: Anzer, U.; Börner, G. Bibcode: 1982ans..conf..155A Altcode: No abstract at ADS Title: Magnetic Reconnection and Coronal Transients Authors: Anzer, U.; Pneuman, G. W. Bibcode: 1982SoPh...79..129A Altcode: Every two-ribbon flare observed during the Skylab period produced an observable coronal transient, provided the flare occurred close enough to the limb. The model presented here treats these two events as a combined process. Transients that occur without flares are believed to involve magnetic fields that are too weak to produce significant chromospheric emission. Adopting the hypothesis that the rising flare loop systems observed during two-ribbon flares are exhibiting magnetic reconnection, a model of a coronal transient is proposed which incorporates this reconnection process as the driving force. When two oppositely directed field lines reconnect a lower loop is created rooted to the solar surface (the flare loop) and an upper disconnected loop is produced which is free to rise. The magnetic flux of these upper loops is proposed as the driver for the transient. The force is produced by the increase in magnetic pressure under the filament and transient. Title: How disk accretion affects a rotating dipole Authors: Anzer, U.; Börner, G.; Zhou, Y. Y. Bibcode: 1982MitAG..55...28A Altcode: No abstract at ADS Title: Kelvin-Helmholtz Instabilities at the Interface Between an Accretion Disk and the Magnetosphere of a Neutron Star Authors: Anzer, U.; Börner, G. Bibcode: 1982ZNatA..37..723A Altcode: 1982ZNaT...37..723A No abstract at ADS Title: Accretion by neutron stars - Accretion disk and rotating magnetic field Authors: Anzer, U.; Boerner, G. Bibcode: 1980A&A....83..133A Altcode: The paper studies the interaction of a thin Keplerian accretion disk with the magnetosphere of a rotating neutron star. The neutron star's axis of rotation is assumed to be perpendicular to and the magnetic dipole axis parallel to the disk plane. In such a configuration the disk can penetrate diamagnetically into the magnetosphere. The velocity difference between the disk material and the magnetosphere leads to a Kelvin-Helmholtz instability of the interface. This instability can grow to large amplitudes only within a narrow ring around the corotation radius. It can give rise to turbulent diffusion of the disk material into the magnetosphere. Part of this material will fall onto the neutron star, the rest will be flung out of the system. This can lead to an approximate balance between braking and acceleration of the neutron star. Application of the model to the pulsating X-ray source Her X-1 gives a natural explanation of the observed temporal variation of the pulse period. Title: The preflare state Authors: van Hoven, G.; Barbosa, D. D.; Birn, J.; Cheng, C. -C.; Hansen, R. T.; Jackson, B. V.; Martin, S. F.; McIntosh, P. S.; Nakagawa, Y.; Anzer, U. Bibcode: 1980sfsl.work...17V Altcode: 1980sofl.symp...17V The accumulation, storage and irreversible release of the free energy necessary for a solar flare are discussed on the basis of data obtained from the Apollo Telescope Mount on Skylab and other pertinent sources. Skylab and OSO 7 observations of possible flare precursors and flare evolution are presented, and the evolution of the flare of Sept. 5, 1973, the most completely observed flare of the Skylab program, is described in detail, with account given to magnetic structures and H alpha radiation. Theories of the preflare state are then reviewed, with attention given to the force-free fields and coronal arcades, thermal and magnetic structures and the MHD stability of coronal loops. Title: MHD aspects of coronal transients Authors: Anzer, U. Bibcode: 1980IAUS...91..263A Altcode: No abstract at ADS Title: Instabilities of accretion disk - magnetosphere interfaces. Authors: Anzer, U.; Boerner, G. Bibcode: 1979ESASP.148..259A Altcode: 1979mbl..rept..259A Configurations which consist of a Keplerian accretion disk and a magnetosphere with solid rotation are investigated. Because of the velocity difference between disk and magnetosphere the boundary between the two is unstable to the Kelvin-Helmholtz instability. This instability is strongest within a ring around the radius of corotation. It leads to turbulent diffusion of disk material into the magnetosphere, and thus gives a sufficiently high accretion rate. Title: Interaction of accretion disk and rotating magnetic field of a neutron star. Authors: Anzer, U.; Boerner, G. Bibcode: 1979ESASP.148..263A Altcode: 1979mbl..rept..263A Utilizing the instabilities of the boundary between disk and magnetosphere, a model for the flow of disk material into the neutron star's magnetosphere is constructed. It is shown that within a thin ring around the corotation radius, gas flows over. The gas is partly accelerated towards the neutron star producing the accretion partly outwards. In terms of this model, certain observed properties of pulsating X-ray sources are explained. Title: Mass flow in loop type coronal transients. Authors: Anzer, U.; Poland, A. I. Bibcode: 1979SoPh...61...95A Altcode: The white light coronagraph on Skylab observed many loop type coronal transients. These loops travel through the coronagraph's field of view (2-6R) over a period of a few hours, after which the legs of the loops usually remain visible for a few days. In this paper we investigate the temporal changes in density and mass per unit length measured along the legs of such loops during the several days after the initial eruption. Examination of 8 transients shows that the mass and density in the legs decrease during the few hours after the top of the loop has travelled beyond the coronagraph's field of view. The mass and density then increase slowly, during the next one half to one day, then decrease again over approximately the same period. These changes are generally shown to be too rapid to be explained by solar rotation, indicating that the transient legs have a lifetime of only a few days. Title: Models of Structure and Dynamics of Prominences. Authors: Anzer, U. Bibcode: 1979phsp.coll..322A Altcode: 1979IAUCo..44..322A; 1979phsp.conf..322A The degree and direction of the linear polarization of the helium D3 lines in quiescent prominences is examined. Three types of prominence models including the Kippenhahn-Schlueter, Kuperus-Raadu, and Matville models, each having current flows inside of them that give rise to Lorentz forces, keeping the material from falling into the sun, are described in detail. The main emphasis of measurements of the magnetic force field centered on the Hanle and Zeeman effects as well as the motions and fine structures of prominences. Results showed that it is possible to obtain information on the field surrounding the prominences from polarization measurements in the radio spectrum. Title: Discussion Authors: Anzer, U.; Maltby, P.; Spicer, D. S. Bibcode: 1979phsp.coll..348A Altcode: 1979IAUCo..44..348A No abstract at ADS Title: Discussion Authors: Anzer, U.; Raadu, M. A.; Rompolt, B.; Rust, D. M.; Sheeley, N.; Spicer, D. S. Bibcode: 1979phsp.coll..171A Altcode: 1979IAUCo..44..171A No abstract at ADS Title: Discussion Authors: Acton, L. W.; Anzer, U.; Engvold, O.; Martin, S. F.; Pneuman, G. W.; Rust, D. M.; Tandberg-Hanssen, E.; Zirin, H. Bibcode: 1979phsp.coll..164A Altcode: 1979IAUCo..44..164A No abstract at ADS Title: Discussion Authors: Anzer, U.; Engvold, O.; Spicer, D. S.; van Tend, W.; Zirin, H. Bibcode: 1979phsp.coll..331A Altcode: 1979IAUCo..44..331A No abstract at ADS Title: Discussion Authors: Anzer, U.; Chiuderi-Drago, F.; Kundu, M. R.; Leroy, J. L.; Malville, J.; Rompolt, B.; Sheeley, N.; Stenflo, J.; Tandberg-Hanssen, E.; Öhman, Y. Bibcode: 1979phsp.coll...77A Altcode: 1979IAUCo..44...77A No abstract at ADS Title: Can coronal loop transients be driven magnetically? Authors: Anzer, U. Bibcode: 1978SoPh...57..111A Altcode: In this note we investigate the possibility of magnetic driving of loop transients. The action of local magnetic forces to balance gravity in a coasting loop and to confine the loop has been proposed by Mouschovias and Poland (1977). In this paper we use similar configurations but deal with the global field structure and present models which show both the initial phase of large acceleration and the later phase of almost constant velocity. We use very simple one-dimensional models consisting of a ring current which is subjected to gravitational attraction. The velocity curves calculated for these models are in good agreement with the observations. Therefore we conclude that if such ring currents can be produced fast enough in the solar corona, they are capable of driving the loop transients observed in the ATM white light coronagraph. Title: Accretion disk and rotating magnetic field of a neutron star. Authors: Börner, G.; Anzer, U. Bibcode: 1978sss..meet..B16B Altcode: No abstract at ADS Title: X-ray Stars: Disc Accretion onto a Rotating Magnetized Neutron Star Authors: Anzer, U.; Boerner, G. Bibcode: 1978MitAG..43..274A Altcode: The interaction between a disk and a neutron star's magnetic field is analyzed. It is assumed that the neutron star has a strong dipole field with its axis in the plane of the disk and perpendicular to the rotation axis, and a reason for the disk to behave like a diamagnetic material in the stars's magnetic field is given. The role of the Kelvin-Helmholtz instability in the interaction of disk matter and magnetic field is examined, and some implications for the disk accretion model are discussed. Title: Shock waves in stellar wind accretion and gamma -bursts. Authors: Anzer, U.; Boerner, G.; Meszaros, P. Bibcode: 1976A&A....50..305A Altcode: A model of cosmic gamma-ray bursts is proposed, involving spherical accretion on compact objects from the stellar wind of a companion star. The gamma-bursts arise when occasional flares in the companion drive shocks in the wind, which then produce a short increase in the accretion rate on the compact object. In the interval between flares, the steady accretion from the wind leads to an X-ray source behavior compatible with that of observed galactic X-ray sources. The model is flexible enough to reproduce the large range of observed gamma-ray burst characteristics, including luminosities, timescales and approximate spectra. Specifically, it can explain the recently suggested possibility that Cygnus X-1 is responsible for some of the gamma-ray events, as well as the recently reported orbit-modulated absorption features. Title: Bremsstrahlung and the Spectra of Cosmic Gamma Bursts Authors: Anzer, U.; Boerner, G. Bibcode: 1976MitAG..38..121A Altcode: The possibility had been considered by Anzer and Boerner (1975) that cosmic gamma-bursts are bremsstrahlung which is produced by the interaction of a mildly relativistic beam of electrons with the protons of a neutral plasma. An investigation concerning an occurrence of a thermal bremsstrahlung is conducted, taking into account a comparison between an observed spectrum reported by Metzger et al. (1974) and calculated spectra of thermal bremsstrahlung. Title: Shock waves in stellar wind accretion and gamma -bursts. Bremsstrahlung and the spectra of cosmic gamma bursts. Authors: Anzer, U.; Boerner, G.; Meszaros, P. Bibcode: 1976swsw.book.....A Altcode: No abstract at ADS Title: Why Syrovatskii's mechanism of dynamic dissipation of magnetic fields does not work. Authors: Anzer, U. Bibcode: 1976str..book..375A Altcode: No abstract at ADS Title: Why Syrovatskii's mechanism of dynamic dissipation of magnetic fields does not work Authors: Anzer, U. Bibcode: 1976CoSka...6..375A Altcode: No abstract at ADS Title: A possible mechanism for cosmic gamma ray bursts. Authors: Anzer, U.; Boerner, G. Bibcode: 1975A&A....40..123A Altcode: The recently discovered cosmic gamma-ray bursts can be explained by the bremsstrahlung of a beam of relativistic electrons hitting a region of high proton density. Spectra are computed, and estimates on the particle densities and on the geometrical dimensions are derived from the observations. The question of associated optical emission is also discussed. Title: Why Syrovatskii's Mechanism of Dynamic Dissipation of Magnetic Fields Does Not Work Authors: Anzer, Ulrich Bibcode: 1973SoPh...30..459A Altcode: Syrovatskii's mechanism of `dynamic dissipation of magnetic field' is reinvestigated. In order to have this kind of `dynamic dissipation' at a neutral line the ratio of current density to particle density must exceed a certain critical value. For conditions in the solar atmosphere near sunspots, this value can only be reached by a mechanism which produces a very large compression of the magnetic field as well as an extreme rarefaction of the density. Syrovatskii claims that his mechanism provides both these features. His enormous field compression, however, can only be obtained if one neglects the restoring Lorentz force (e.g. in Syrovatskii's model the compressed field near the neutral line is about one order of magnitude larger than the field of the sunspots which generates it). The second effect, i.e. the large plasma rarefaction around the neutral line, also is not real. This rarefaction is due to the particular flow field of Syrovatskii's model which allows for a free reconnection of the field lines across the neutral line; the magnetic field is treated like a vacuum field, the effects of the field accumulation near the neutral line being neglected. The aim of the present paper is to show how more realistic models modify Syrovatskii's results. Our numerical calculations lead to a maximum current to density ratio which is a factor of 106 smaller than the one obtained by Syrovatskii. Therefore one has to conclude that in the solar atmosphere one cannot produce in the way described by Syrovatskii the configurations which are necessary for `dynamic dissipation'. Title: On a cinematographic observation of the tail of comet Kohoutek 1973f. Authors: Anzer, U.; Jockers, K.; Schmidt, H. U. Bibcode: 1973JASWA..25...14A Altcode: No abstract at ADS Title: Zur kinematographischen Beobachtung des Schweifs des Kometen Kohoutek 1973f. Authors: Anzer, U.; Jockers, K.; Schmidt, H. U. Bibcode: 1973S&W....12..293A Altcode: No abstract at ADS Title: Kritik an Syrovatskiis Mechanismus der dynamischen Dissipation von Magnetfeldern Authors: Anzer, U. Bibcode: 1973MitAG..32..171A Altcode: No abstract at ADS Title: A Method to Calculate Electric Currents in Quiescent Prominences Authors: Anzer, Ulrich Bibcode: 1972SoPh...24..324A Altcode: A 2-dimensional model of the magnetic field associated with quiescent prominences is presented. The coronal field is assumed to be current-free, currents are only allowed in the photosphere and inside the prominence. The prominence is taken to be infinitely thin. For this model a method is given to calculate the field configuration from the observed normal component of the field both in the photosphere and the prominence. The normal field components are inferred from disc observations and Hα limb observations. The sheet currents inside the prominence are calculated and the resulting Lorentz force is compared with the gravitational force. Within the range of uncertainty in the total hydrogen density of quiescent prominences it is possible to give models where the gravity is balanced by the Lorentz force. Title: Energy Balance in Cool Quiescent Prominences Authors: Poland, A.; Anzer, U. Bibcode: 1971SoPh...19..401P Altcode: The energy balance for cool quiescent prominences is examined using a 6000 km, 6000 K isothermal slab model prominence with a density gradient dictated by a modified Kippenhahn-Schlüter model. The model is irradiated from both sides by the coronal, chromospheric, and photospheric radiation fields. The radiative transfer problem is solved in detail for the Lyman continuum and Hα to determine the net radiative energy loss for hydrogen. An estimate of the energy loss for Ca II H and K indicates that this source of energy loss is unimportant when compared with the hydrogen radiation. The radiative energy loss is easily balanced by the conductive energy gain from the corona. Title: Energy balance in quiescent prominences. Authors: Poland, A.; Anzer, U. Bibcode: 1971BAAS....3..353P Altcode: No abstract at ADS Title: On the Orientation of Magnetic Fields in Quiescent Prominences Authors: Anzer, U.; Tandberg-Hanssen, E. Bibcode: 1971IAUS...43..656A Altcode: No abstract at ADS Title: The Orientation of Magnetic Fields in Quiescent Prominences Authors: Tandberg-Hanssen, E.; Anzer, Ulrich Bibcode: 1970SoPh...15..158T Altcode: We have measured the longitudinal component, B∥, of the magnetic field in quiescent prominences and obtained a relationship between B∥ and θ, where θ is the angle between the long axis of the prominence and the north-south direction on the sun. From this relationship we deduce a distribution function for the magnetic field vector in quiescent prominences in terms of the angle α between the field and the long axis of the prominence. The mean angle, α, for our data is small, ∼ - 15°, indicating that the magnetic field traverses quiescent prominences under a small, but finite angle. Title: A Model for Quiescent Prominences with Helical Structure Authors: Anzer, Ulrich; Tandberg-Hanssen, E. Bibcode: 1970SoPh...11...61A Altcode: We present a model for quiescent prominences with helical structure. The model is described by two magnetic fields, one produced by photospheric or subphotospheric currents, the other due to currents along the cylindrical model prominence. Title: Stability Analysis of the Kippenhahn-Schlüter Model of Solar Filaments Authors: Anzer, U. Bibcode: 1969SoPh....8...37A Altcode: In this paper the stability of the Kippenhahn-Schlüter model of solar filaments against arbitrary perturbations is investigated. The problem is treated in the MHD approximation and a modification of the energy principle of Bernstein et al. is used. Two necessary and sufficient stability conditions are found: (a) [Bz] dBx/dz ≧ 0 and (b) Bx d[Bz]/dz ≦ 0. Condition (a), in the limit of small currents, leads to the condition already discussed by Kippenhahn and Schlüter; condition (b) requires that the current density in the stable filament decreases with height. Title: On the Stability of Solar Filaments Authors: Anzer, Ulrich Bibcode: 1969BAAS....1R.271A Altcode: No abstract at ADS Title: Remarks on my Paper about Force-Free Magnetic Fields and Solar Flares Authors: Anzer, Ulrich Bibcode: 1968SoPh....4..101A Altcode: No abstract at ADS Title: The Stability of Force-Free Magnetic Fields with Cylindrical Symmetry in the Context of Solar Flares Authors: Anzer, Ulrich Bibcode: 1968SoPh....3..298A Altcode: The problem of the accumulation and storage of the energy released in solar flares is discussed; it is proposed that convective energy of the photosphere is transformed into magnetic energy of the chromosphere and corona. The consequences of a large ratio of magnetic pressure to gas pressure are investigated. In this case the field must be approximately force-free. The only suitable force-free fields which allow an analytical treatment are those of cylindrical symmetry. The stability of these fields is studied with the energy principle. It is shown that they are always unstable due to kink type instabilities. The shape of the unstable perturbations is described in detail and an upper limit for their amplitude is estimated. The consequences for the proposed mechanism of energy storage are briefly discussed.