Thermodynamic models and fine structure of prominences

Observed H brightness versus size of emission substructures of quiescent prominences are compared with values predicted from thermodynamical models. The measured size of an emission element of a given brightness is substantially less than the theoretical value.Two possible causes for the discrepancy are suggested: (1) The partial filling of a recording aperture, due to the prominence fine structure, may affect the measurements seriously. Caution is therefore urged against using face values of observed brightness vs ratios in model calculations in cases of partly optically thick lines. (2) Changes of individual fine structure elements on a time scale of a few minutes implies that the prominence plasma may be in a non-stationary radiative state.     

Multiple-Thread Model of a Prominence Observed by Sumer and eit on Soho

A quiescent polar crown prominence was observed at Meudon in Hα and Ca ii lines, and by EIT and SUMER on board SOHO in UV lines from 9 to 10 March 1996. SUMER observed the prominence continuously in a scanning mode between 21:40 UT on 9 March, and 18:13 UT on 10 March, in the nitrogen line N v (λ1238) with a 1 arc sec² resolution. Altogether 190 prominence images (121×108 pixels) were obtained. These are presented in a movie. The prominence is highly dynamic. Large-scale features, such as mixed loop systems and dark cavities are changing on time scales of a few hours. Filamentary structure is evident and is changing within a few frames of the movie. A lifetime of 20–25 min for the fine structure has been found by the autocorrelation method. We have statistically analysed the three moments of the N v line in the prominence: line intensity, Doppler shift and linewidth, in the context of a multiple-thread model. We find that the data are consistent with a model where the prominence is assumed to be an ensemble of small threads. In the brightest parts of the prominence it is possible that there are many unresolved threads (15–20) along the line of sight with diameters smaller than a few hundred kilometers. The filling factor is probably very small and in that case the structures occupy only a fraction of the volume. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005151015043     

Fine structure of motions in a quiescent prominence

The Ca⁺ K line of the quiescent prominence of 15 October 1969 observed at the Abastumani Astrophysical Observatory of the Academy of Sciences of Georgia, U.S.S.R. is studied in detail. The behaviour of various fragments of the prominence with the line-of-sight Doppler velocities is investigated for different places of the prominence along the spectrum height. Due to superposition of the fragments and the radial velocities a complex non-gaussian profile of the spectral line is formed. Decomposition of the non-gaussian profile into suitable gaussian ones is made with a computer yielding objectivity and high accuracy. The rms error for the intensities in the decomposition is about 5%.It is found that the components into which the Ca⁺ K line decomposes at each level, are traced while proceeding from one photometric section to another throughout the spectral height.A hypothesis on the preferably filamentary structure of the prominence is put forward.     

An investigation of macroscopic motions using the Ca+ lines in the prominence of 15 October 1969

The fine structure of the quiescent prominence of 15 October 1969, observed at Abastumani Astrophysical Observatory of the Academy of Sciences of Georgia, U.S.S.R. with the horizontal telescope, is studied.The complex non-gaussian Ca⁺ K line profiles are decomposed into gaussian components and the distribution of the velocity field is plotted.Estimating the direction of the motion by the appearance of the spectral line and by the distribution of the velocity field, we conclude that there is a left-handed screw. Having used the high dispersion spectral data published, we find that prominences showing left-handed screws fall in the northern hemisphere of the Sun and those with right-handed screws in the southern hemisphere.The influence of Coriolis acceleration on quiescent prominences is discussed.     

The small scale velocity field of a quiescent prominence

The small scale velocity field of a large quiescent prominence is studied from simultaneous filtergrams in the red and violet wings of the Ca ii K-line.The average spatial separation of peak velocities is about 3000 km. The duration of individual velocity elements appears to increase with increasing area of the elements. In some positions of the prominence the direction of the velocity persists through the 5 hrs of observations. It is suggested that emitting elements of gas, which flow along the magnetic lines of force, produce peak velocities when they pass through particular locations of the prominence region.There is no clear evidence in the data for oscillations or waves.     

Alfvén waves and turbulence in quiescent prominences

In the dynamical model of quiescent prominences presented in this paper, it is assumed that the ever-changing velocity field and brightness of the fine structure is due to MHD turbulence driven by an Alfvén-wave flux from below. It is shown that these waves become highly non-linear and are dissipated over relatively short scales in prominence matter. For magnetic field strengths lower than those observed in quiescent prominences, no closed arch structure can exist with the physical parameters observed. For higher field strengths the conditions for the creation of turbulence are not fulfilled. The momentum gained by prominence matter in the dissipation process, is shown to be of the right order of magnitude to provide the supporting force against gravity. ‘Edge’ effects find a simple explanation within the framework of this hypothesis. In the upper regions of a prominence one result of the dissipation may be the formation of open magnetic configurations, in keeping with the presence of streamers connected with quiescent prominences. Observational tests are proposed and discussed.     

A self-consistent model of a thermally balanced quiescent prominence in magnetostatic equilibrium in a uniform gravitational field

We present a theoretical model of quiescent prominences in the form of an infinite vertical sheet. Self-consistent solutions are obtained by integrating simultaneously the set of nonlinear equations of magnetostatic equilibrium and thermal balance. The basic features of the models are: (1) The prominence matter is confined to a sheet and supported against gravity by a bowed magnetic field. (2) The thermal flux is channelled along magnetic field lines. (3) The thermal flux is everywhere balanced by Low's (1975b) hypothetical heat sink which is proportional to the local density. (4) A constant component of the magnetic field along the length of the prominence shields the cool plasma from the hot surrounding. We assume that the prominence plasma emits more radiation than it absorbs from the radiation fields of the photosphere, chromosphere and corona, and we interpret the above hypothetical heat sink to represent the amount of radiative loss that must be balanced by a nonradiative energy input. Using a central density and temperature of 10¹¹ particles cm^–3 and 5000 K respectively, a magnetic field strength between 2 to 10 gauss and a thermal conductivity that varies linearly with temperature, we discuss the physical properties implied by the model. The analytic treatment can also be carried out for a class of more complex thermal conductivities. These models provide a useful starting point for investigating the combined requirements of magnetostatic equilibrium and thermal balance in the quiescent prominence.     

A model for quiescent solar prominences with normal polarity

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 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.     

The internal motion of quiescent prominences

A study has been made of fine structure wavelength shift in the K line spectra from quiescent prominences. A persistent small scale motion is found in the prominence main body. In places where we see the characteristic thread like fine structure in the accompanying H filtergrams the average line-of-sight velocity amplitude is about 1 km s^–1. A higher velocity ( 4 km s^–1) is associated with a slightly coarser, mottled prominence fine structure. In the low lying regions, connecting the prominence body and the chromosphere, we do not detect any fine structure line shift (v 1/2 km s^–1).     

A model for quiescent prominences with helical structure

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.On leave from Max-Planck Institut für Physik und Astrophysik, München.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Studies of the prominence-corona transition zone from rocket ultraviolet spectra of the March 1970 eclipse

Slitless VUV spectra of the eclipsed Sun were obtained from a rocket experiment for the first time during the 1970 eclipse. The spatially resolved spectra of a quiescent prominence in the wavelength range 900 Å–2200 Å consist of emission lines from ions formed in the temperature range 3.5 × 10⁴k–3.2 × 10⁵k. The spectral intensities have been interpreted in terms of physical parameters which indicate a transition zone of shell-like layers, the inner the cooler and thinner, the outer the hotter and more extended. The transition zone is about 3 km thick for a model thread of 2000 km in diameter.     

A 2-D model for the support of a polar-crown solar prominence

We present a 2-D potential-field model for the magnetic structure in the environment of a typical quiescent polar-crown prominence. The field is computed using the general method of Titov (1992) in which a curved current sheet, representing the prominence, is supported in equilibrium by upwardly directed Lorentz forces to balance the prominence weight. The mass density of the prominence sheet is computed in this solution using a simple force balance and observed values of the photospheric and prominence magnetic field. This calculation gives a mass density of the correct order of magnitude. The prominence sheet is surrounded by an inverse-polarity field configuration adjacent to a region of vertical, open polar field in agreement with observations.A perturbation analysis provides a method for studying the evolution of the current sheet as the parameters of the system are varied together with an examination of the splitting of an X-type neutral point into a current sheet.Program Systems Institute of the Russian Academy of Sciences, Pereslavl-Zalessky 152140, Russia.     

Coronal mass-ejections-kinematics of the 19 December 1973 event

An eruptive prominence and coronal transient of 19 December, 1973 comprised one of the best-observed coronal mass ejection events during the skylab period (May, 1973–January, 1974). EUV observations show that the pre-eruptive quiescent prominence was (at 8000 K) not appreciably hotter than other quiescent prominences, but EUV radiation from it and its prominence-corona interface was unusually faint. The prominence material was distributed in helical threads which decreased in pitch angle during the early phases of eruption. No region of the prominence was markedly different from any other just prior to and during the eruption. For the first time, the temperature and density of rising prominence material were determined at great heights in the corona. At 3R , the prominence material was still confined in threads whose temperature and total hydrogen density were 2 × 10⁴ K and 1.5 × 10⁹ cm^–3, respectively. Shortly after this observation ( 7hr after the start of the eruption), the prominence material expanded dramatically. A small portion (1%) of the prominence material was observed draining downward near the solar surface late in the event, and we infer that only a small fraction (10%) of the pre-eruptive prominence mass was expelled from the Sun. The remainder of the prominence apparently lay outside the instruments' fields of view. The bulk of the material expelled did not originate in the prominence. Both coronal and prominence material accelerated outward during the period of observations. A pre-existing streamer was disrupted by the outflowing material.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Simultaneous multifrequency observations of an eruptive prominence at millimeter wavelengths

Radio images and spectra of an eruptive prominence were obtained from simultaneous multifrequency observations at 36 GHz, 89 GHz, and 110 GHz on May 28, 1991 with the 45-m radio telescope at Nobeyama Radio Observatory (NRO), the National Astronomical Observatory, Japan (NAOJ). The radio spectra indicated that the optical depth is rather thick at 36 GHz whereas it is thin at 89 and 110 GHz. The H data, taken at Norikura Solar Observatory, NAOJ, suggest that the eruption of an active region filament was triggered by an H flare. The shape and position of the radio prominence generally coincided with those of H images. The radio emission is explained with an isothermal cool thread model. A lower limit for the electron temperature of the cool threads is estimated to be 6100 K. The range of the surface filling factors of the cool threads is 0.3–1.0 after the H flare, and 0.2–0.5 in the descending phase of the eruptive prominence. The column emission measure and the electron number density are estimated to be of the order of 10²⁸ cm^–5 and 10¹⁰ cm^–3, respectively. The physical parameters of a quiescent prominence are also estimated from the observations. The filling factors of the eruptive prominence are smaller than those of the quiescent prominence, whereas the emission measures and the electron densities are similar. These facts imply that each cool thread of the prominence did not expand after the eruption, while the total volume of the prominence increased.     

Modeling of prominence threads in magnetic fields: Levitation by incompressible MHD waves

The nature of thin, highly inclined threads observed in quiescent prominences has puzzled solar physicists for a long time. When assuming that the threads represent truly inclined magnetic fields, the supporting mechanism of prominence plasma against gravity has remained an open issue. This paper examines the levitation of prominence plasma exerted by weakly damped MHD waves in nearly vertical magnetic flux tubes. It is shown that the wave damping, and resulting `radiation pressure', caused predominantly by ion-neutral collisions in the `cold' prominence plasma, may balance the acceleration of gravity provided the oscillation frequency is 2 rad s^–1 (f0.5 Hz). Such short wave periods may be the result of small-scale magnetic reconnections in the highly fragmentary magnetic field of quiescent prominences. In the proposed model, the wave induced levitation acts predominantly on plasma – neutral gas mixtures.     

Multi-Wavelength Eclipse Observations of a Quiescent Prominence

We construct the maps of temperatures, geometrical thicknesses, electron densities and gas pressures in a quiescent prominence. For this we use the RGB signal of the prominence visible-light emission detected during the total solar eclipse of 1 August 2008 in Mongolia and quasi-simultaneous Hα spectra taken at Ond?ejov Observatory. The method of disentangling the electron density and geometrical (effective) thickness was described by Jej?i? and Heinzel (Solar Phys. 254, 89?–?100, 2009) and is used here for the first time to analyse the spatial variations of prominence parameters. For the studied prominence we obtained the following range of parameters: temperature 6000?–?15?000 K, effective thickness 200?–?15000 km, electron density 5×10⁹?–?10¹¹ cm^?3 and gas pressure 0.02?–?0.2 dyn?cm^?2 (assuming a fixed ionisation degree n _p/n _H=0.5). The electron density increases towards the bottom of the prominence, which we explain by an enhanced photoionisation due to the incident solar radiation. To confirm this, we construct a two-dimensional radiative-transfer model with realistic prominence illumination.     

Energy balance in cool quiescent prominences

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 H 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.The only difficulty with our model is that the total hydrogen density must be of the order of 3 × 10¹²/ cm³ to match the n = 2 population density of 5 × 10⁴/cm³ obtained from observation. To support a prominence of this density and a thickness of 6000 km against gravity requires magnetic fields of the order of 20 G which is much higher than the average magnetic field in quiescent prominences deduced from limb observations. Two possible explanations for this discrepancy are given.Currently at the Max-Planck-Institut für Physik und Astrophysik, München, Germany.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Accurate Determination of Electron Densities in Active and Quiescent Prominences: the Mid-Infrared Advantage

We have analyzed all lines in the MIR (8 to 20 micron) spectra of a quiescent and two time-frames of an active prominence. In the quiescent prominence, in addition to those lines found by Zirker (1985), we have identified a higher excitation hydrogen line and two helium recombination lines. Accounting for instrumental broadening, we can further separate out the Doppler and the Stark contributions to the line width. The former yields maximum temperatures of 6200 K, 34000 K and 12000 K and the latter electric field strengths of 7, 17, and 10 V cm^-1 for the above prominences, respectively. We show that these electric fields when divided by 2.2 are equal to the normal electric field in Holtsmark's quasi-static Stark broadening theory. Hence, we obtain electron densities of N₃=2.4(0.3), 9.1(1.2), and 5.5(0.6) in units of 10¹⁰ cm^-3 respectively. Using the same assumptions as made by Zirker, namely, (1) the strongest line (7-6) is optically thin, (2) the population of the lower level (n=6) is determined by direct radiative recombination and photo-ionization, (3) the equality of proton and electron densities, and (4) the thickness of the prominence is at least 10⁸ cm, we derive a new inequality, N_e 1.83 × 10⁸ T^0.75 e^-2195/T. Substituting our maximum temperatures into the right-hand side, we find upper bound N_e values of 9, 43, and 30 in the same units as above. These upper bound values are comfortably higher than our measurement, unlike those of Zirker's derived from the same set of assumptions. We have also observed the helium recombination spectrum which has been postulated by Tandberg-Hanssen as one of three possible ways of equilibrating the triplet/singlet ratio. Surprisingly, it is present in the quiescent as well as in the active prominence. We show that no meaningful values can be found for the turbulent velocities by combining the helium with the hydrogen line widths.     

Numerical models of quiescent normal polarity prominences

We present 2-D numerical models of quiescent solar prominences with normal magnetic polarity. These models represent an extension to the classical Kippenhahn-Schlüter model in that the prominence is treated as having finite width and height and the external coronal field is matched smoothly to the internal prominence field so that there are no current sheets at the prominence sides. Using typical prominence and coronal values we find solutions to the generalised Grad-Shafranov equation which illustrate the necessary magnetic support. We also discuss some extensions to the basic model.     

Piecewise mass flows within a solar prominence observed by the New Vacuum Solar Telescope

The material of solar prominences is often observed in a state of flowing. These mass flows (MF) are important and useful for us to understand the internal structure and dynamics of prominences. In this paper, we present a high resolution H\(\alpha \) observation of MFs within a quiescent solar prominence. From the observation, we find that the plasma primarily has a circular motion and a downward motion separately in the middle section and legs of the prominence, which creates a piecewise mass flow along the observed prominence. Moreover, the observation also shows a clear displacement of MF’s velocity peaks in the middle section of the prominence. All of these provide us with a detailed record of MFs within a solar prominence and show a new approach to detecting the physical properties of prominence.