The prominence-corona interface compared with the chromosphere-corona transition region

The intensities of 52 EUV emission lines from each of 9 hedgerow prominences observed at the limb with the Harvard experiment on ATM-Skylab have been compared with intensities from the interior of network cells at the center of the disk, in order to compare the prominence-corona (P-C) interface with the chromosphere-corona (C-C) transition region. The intensity ratio I _cell/I _prominence for each line varies systematically (in all of the prominences observed), with the temperature of formation of the line as T ^–0.6. The density sensitive C iii (formed at T 9 × 10⁴ K) line ratio I ₁₁₇₅/I ₉₇₇ implies an average density 1.3 × 10⁹ electrons cm^–3 in the P-C interface and 4 times this value in the C-C transition of the cells. The total optical thickness at the head of the Lyman continuum is 10 in most of the prominences studied; in two of the prominences, however, we cannot reject the possibility that _o is large. Methods of analysis of these EUV data are developed assuming both a resolved and an unresolved internal prominence structure. Although the systematic differences between the P-C interface and the C-C transition are stressed, the similarities are probably more remarkable and may be a result of fine structure in the C-C transition.Currently on leave from the Institute of Astronomy, Hawaii; at the Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, 80309.     

The fine structure of prominences

The fine structure of nonspot prominences are studied from H filtergrams. The size of the smallest prominence structures increases with height above the chromosphere. Some prominences contain structures close to 1/2 arc second, which is the spatial resolution in the present data. The effective thickness of many nonspot prominences ranges between 4 × 10⁷ cm and 1.5 × 10⁸ cm. An apparent downward directed motion is observed in the majority of the prominences. No preferred direction of the motion is seen in regions composed of comparatively large diffuse structures. Some bright threads are visible for 1 hr and longer. Bright knots have an average observed lifetime of about 8 min. The process of condensation and subsequent destruction of prominence fine structure appears to take place on a very short time scale compared to the life time of the regions where prominences may exist. The observed H brightness of the prominences in the present data may be accounted for as scattered chromospheric radiation.     

Emitting region of sodium lines in solar prominences

We have calculated the emission spectra of hydrogen and sodium atoms in the cool part of prominence models which satisfy simultaneously the constraints of radiative transfer, statistical equilibrium and charge-particle conservations.In the considered range of our model parameters, emission strengths of H and Nai D lines increase with the temperature and the total number density. Low-pressure models raise the ionization rate highly but yield very weak Nai D line intensities, since these model prominences contain small amounts of free electrons and sodium atoms which have a deep relation with the formation of sodium lines. We find that sodium D lines should be emitted in the high pressure region of prominences, and that their intensities are difficult to attain in the cool core of any model prominence with a temperature as low as 4000 K. In order to explain consistently the spectral emissions of H and Nai D lines observed in quiescent prominences, a total number density higher than 4 x 10¹¹ cm^-3 and a temperature over 5000 K are required at least in the cool part of prominences.Contributions from the Kwasan and Hida Observatories, University of Kyoto, No. 282.     

The helium radiation in prominences

It is shown that the emission of quiescent and loop prominences in the helium D₃ line and in the 4686 Å line of He⁺ respectively, occurs at low temperatures, of the order of 7000 K.The ionization of neutral helium is produced by short-wave solar radiation, which is absorbed in the outer layers of filaments composing a prominence. The population of helium triplet levels in prominences is determined by recombinations and subsequent resonance scattering of photospheric radiation. Transitions from triplet to singlet levels caused by electron collisions considerably reduce the line brightness.Emission of ionized helium in the 4686 Å line arises in prominence surface layers as well. In quiescent prominences the emission is very faint and is due to recombination; the second ionization is caused by the far ultraviolet radiation.In flare-like events ionized helium emits due to charge-exchange collisions. The symmetrical resonance charge-exchange of -particles is caused by helium ions in corpuscular streams which are probably generated in photospheric layers. Due to increased radiation losses the temperature of the prominence under the action of the stream is negligibly increased. With a stream density equal to 5 × 10⁸ cm^-3 and velocity 300 km/s the theoretical intensity of the 4686 He⁺ line is some hundreds of microängströms and agrees with observations of Goldberg-Rogozinskaya (1962, 1965) and others.     

Complete determination of the magnetic field vector and of the electron density in 14 prominences from linear polarizaton measurements in the HeI D3 and Hα lines

The present paper is devoted to the interpretation of linear polarization data obtained in 14 quiescent prominences with the Pic-du-Midi coronagraph-polarimeter by J. L. Leroy, in the two lines Hei D₃ andH quasi-simultaneously. The linear polarization of the lines is due to scattering of the anisotropic photospheric radiation, modified by the Hanle effect due to the local magnetic field. The interpretation of the polarization data in the two lines is able to provide the 3 components of the magnetic field vector, and one extra parameter, namely the electron density, because the linear polarization of H is also sensitive to the depolarizing effect of collisions with the electrons and protons of the medium. Moreover, by using two lines with different optical thicknesses, namely Hei D₃, which is optically thin, and H, which is optically thick ( = 1), it is possible to solve the fundamental ambiguity, each line providing two field vector solutions that are symmetrical in direction with respect to the line of sight in the case of the optically thin line, and which have a different symmetry in the case of the optically thick line.It is then possible to determine without ambiguity the polarity of the prominence magnetic field with respect to that of the photospheric field: 12 prominences are found to be Inverse polarity prominences, whereas 2 prominences are found to be Normal polarity prominences. It must be noticed that in 12 of the 14 cases, the line-of-sight component of the magnetic field vector has a Normal polarity (to the extent that the notion of polarity of a vector component is meaningful; no polarity can be derived in the 2 remaining cases); this may explain the controversy between the results obtained with methods based on the Hanle effect with results obtained through the Zeeman effect. A dip of the magnetic field lines across the prominence has been assumed, to which the optically thick H line is sensitive, and the optically thin Hei D₃ line is insensitive.For the Inverse prominences, the average field strength is 7.5±1.2 G, the average angle,, between the field vector and the prominence long axis is 36° ± 15°, the average angle, , between the outgoing field lines and the solar surface at the prominence boundary is 29° ± 20°, and the average electron density is 2.1 × 10¹⁰ ± 0.7 × 10¹⁰ cm^–3. For the Normal prominences, the average field strength is 13.2±2.0 G, the average angle,, between the field vector and the prominence long axis is 53° ± 15°, the average angle, , between the outgoing field lines and the solar surface at the prominence boundary is 0° ± 20° (horizontal field), and the average electron density is 8.7 × 10⁹ ± 3.0 × 10⁹ cm^–3.     

On the intensity ratio of emission lines of Nai D1 to D2 in prominences

Spectroscopic observations of the Nai D emission lines of prominences were made with the Domeless Solar Telescope in Hida Observatory. When active prominences are bright in the D₂ emission line, the intensity ratio of D₁ to D₂ is found to deviate significantly from the theoretical ratio of the optically-thin case. On the other hand, the intensity ratio is close to the theoretical ratio for the most part of quiescent prominences. Furthermore, the full widths at half maximum intensity of the D₂ emission line for active prominences become wider than those of the D₁ line, as the intensity of the D₂ line gets higher. These observed features clearly show that the emitting region of the Nai D lines is optically thick in some types of prominences. Non-LTE calculations were made by taking the ionization degree of hydrogen atoms and the thickness of the prominences and the electron temperature as free parameters. It is shown that the electron temperature of the emitting region of the Nai D lines should be as low as 4000 K for an explanation of the large optical thickness of the Nai D lines for active prominences. Brief discussions are included about the possible existence of low temperatures in active prominences.     

Configuration and gradual dynamics of prominence-related X-ray coronal cavities

We have analyzed X-ray images of the solar corona obtained by the S-054 telescope on Skylab, together with H filtergrams from the Catania Astrophysical Observatory and EUV and magnetic data, to study the morphology and the evolution of the coronal structures associated with prominences (coronal cavities).X-ray cavities are associated with prominences and are enclosed by series of loops of hot plasma typically higher than 5 × 10⁹ cm. Helmet streamers can be observed only at very large heights (> 1 solar radius). The cavities show a higher luminosity when prominences have temporarily disappeared. The density in one of these X-ray cavities ( 3 × 10⁸cm^–3) is insufficient to allow formation of dense ( 10¹¹ cm^–3) prominences by local condensation from the corona.Prominences associated with young (up to three solar rotations) and old (greater than eight) magnetic neutral lines are significantly less stable than those associated with middle-aged neutral lines. Downward bending of the top of the inner magnetic loop, necessary in some models of prominences, is not detected in these X-ray observations. The relevance of these results to models of prominence formation is discussed.Presently at Osservatorio Astrofisico di Arcetri, Firenze, Italy.     

On peculiarities of the H and K Ca ii lines of quiescent prominences

A study of the metallic lines in bright quiescent prominences indicates that the optical thickness in the K line of Ca ii may reach values as high as 10³. This is about 10 times larger than the optical thickness in the H line and may explain some peculiarities of the H and K lines in solar prominences.     

Two-dimensional photometric analysis of emission lines in quiescent prominences

Emission lines from quiescent prominences were observed simultaneously through narrow-band interference filters, thus integrating the total line intensities without the use of a spectrograph. Simultaneous exposures (50 ms) on three electronically connected CCD cameras at the 70 cm VTT on Tenerife assured almost identical influence of the Earth's atmosphere and a spatial resolution of 1 arc sec. The resulting spatially high-resolution two-dimensional images in H, H, and Ca⁺8542, calibrated in units of the disk-center intensities, allow a two-dimensional mapping of emission ratios yielding relevant physical parameters. The emission relation between H and H, which depends on the total optical thickness, confirms earlier photometric results from spectra, however, with a large sample of data points from six prominences. It demonstrates the saturation effects towards brighter prominences or prominence locations. The relation between Ca⁺8542 and H, which depends on the gas pressure, is found to vary between different prominences but is nearly constant within one prominence. Its mean spatial variation of 30% within one prominence may be interpreted in terms of a magnetic field with variations of 5%. The brightness distribution in most prominences is not smooth but indicates preferred values, which are interpreted as superpositions of several fine structures.     

A study of the He λ 10830 line emission from quiescent prominences

Observations of the He 10830 line emission in a number of quiescent prominences are presented. The line shapes are analyzed to obtain values for the Doppler widths and optical thicknesses, leading to a determination of the 2³ P – 2³ S excitation temperature of 4020 K. The results are compared with those of previous observational and theoretical research.     

Coronal environment of quiescent prominences

With thespectro-coronagraph and themultichannel subtractive double pass spectrograph (MSDP) at the Pic du Midi Observatory two quiescent prominences were observed simultaneously. From the spectro-coronagraph observations 2D maps of Hei 10830 , Fexiii 10798 and 10747 line intensities were obtained. In addition, we obtained 2D maps of the ratioR of the two iron lines. This ratio is used as a diagnostic for determining the density of the hot coronal plasma surrounding prominences. We found that the electron density is higher at the location of the prominences than in the corona, whereas small regions (40) of lower electron density are unevenly distributed around the prominences indicating that the surrounding corona is highly inhomogeneous. The density of the cavity is reduced by a factor 1.5 compared to the density of the prominence environment (5 × 10⁸ cm^–3). We discuss the existence of cavities around these prominences according to the orientation of their axes relative to the line of sight and according to the velocity field inside the prominences. Constraints on models for prominence formation are derived.     

Modern observations of solar prominences

We review observational studies of solar prominences with some reference to theoretical understandings. We lay emphasis on the following findings: (1) An important discovery was made by Leroy, Bommier, and Sahal-Bréchot concerning the direction of the magnetic field inside some high-altitude, high-latitude prominences, where the field vector points in the opposite direction from the one which would be expected from the potential field calculated from the observed photospheric magnetic field. (2) Landman suggests the possibility of a high total density of 10^–11 g cm ^–3 for the main body of quiescent prominences, 50 times higher than the value hitherto believed. (3) Flow patterns, nearly parallel to the magnetic neutral lines, were detected in the 10⁵ K plasma near and in prominences. (4) Coronal loop structures were found overlying prominences as viewed from X-ray photographs. We propose also an evolutionary scheme by taking the magnetic field topologies into account.The fundamental question why a prominence is present remains basically unanswered.     

Ionized helium in prominences and in the chromosphere

Quiescent prominences It is found that Heii 4686 is emitted in the same cold region of 10000 K as hydrogen, metal and neutral helium emission lines. This conclusion is based on the finding that the observed width of 4686 is the same as the calculated width of 4686. The calculated width is derived from the observed widths of hydrogen and metallic lines. The large intensity of Heii 4686 in 10000 K can be explained by the ionization of Heii due to the UV radiation below 228 Å that comes from the corona and the transition region.Loop prominences The very broad width (30 to 50 km s^–1) of 4686 for two post-flare loop prominences shows that the Heii line is emitted in hot regions different from regions of hydrogen and metal emission. From the widths of the Balmer lines and many metallic lines the kinetic temperature for one loop is found to be 16000 K in one part and 7600 K in another part. The electron densities are 10^12.0 cm^–3 and less than 10^11.0 cm^–3 respectively.Chromosphere The intensity of 4686 in the chromosphere can be interpreted in terms of a temperature of 10000 K with the ionization due to UV radiation. But, since observations of the width of 4686 are not available, a definitive conclusion for the chromosphere cannot be reached.     

A model for X-ray emission from loop prominences

A study is made of X-ray line emission observed during the developing stages of a set of post-flare loop prominences. The time behaviour of the line emission can be described by a model consisting of two flux tubes containing plasma heated impulsively at the flash phase; the plasma cools by radiation and by conduction to the chromosphere. These ideas are extended to the possible formation of H prominences from low-lying hot loops.     

Identification of X-ray lines in the spectrum of the arcsec-scale precessing jets of SS 433

The extended X-ray emission observed at arcsec scales along the propagation trajectory of the precessing relativistic jets of the Galactic microquasar SS 433 features a broad emission line, with the position of the centroid being significantly different for the approaching and receding jets (≈7.3 and ≈6.4 keV, respectively). These observed line positions are at odds with the predictions of the kinematic model for any of the plausible bright spectral lines in this band, raising the question of their identification. Here we address this issue by taking into account time delays of the emission coming from the receding regions of the jets relative to that from the approaching ones, which cause a substantial phase shift and distortion of the predicted line positions for the extended (~10¹⁷ cm) emission compared to the X-ray and optical lines observed from the central source (emitted at distances ~10¹¹ and ~10¹⁵ cm, respectively). We demonstrate that the observed line positions are fully consistent with the Fe XXVI Lyα (E ₀ = 6.96 keV) line emerging from a region of size ~6 × 10¹⁶ cm along the jet. This supports the idea that intensive reheating of the jets up to temperatures >10 keV takes place at these distances, probably as a result of partial deceleration of the jets due to interaction with the surrounding medium, which might cause collisions between discrete dense blobs inside the jets.     

Structure and stability of prominences with helical structure

Observations of internal structure and development of four helical prominences are presented. We assume that the helically twisted fine structure threads are outlining magnetic field lines and we found that it is possible to describe the magnetic fields by the uniform twist configuration, with the twists ranging between 2 and 7. The estimated lower limits for the magnetic fields were about 20 G which give lower limits for the currents flowing along the prominences in the range between 2 × 10¹⁰ A and 2 × 10¹¹ A and current densities at the axis of the prominences about 10^-4 A m^-2. The upper limit of electron drift velocity could be estimated as 1 m s^-1, which is far below the critical velocities for the onset of plasma microinstabilities.The stability of the studied prominences is discussed and the criteria for the onset of eruptive instability are established for a prominence modelled as a twisted and elliptically curved magnetic flux tube which is anchored in the photosphere and affected by its mirror-current. The eruption starts when the prominence attains a critical height which must be larger than half of the footpoint separation and depends on the values of twist, radius, and footpoint distance of the magnetic flux tube. The observed examples of eruptive prominences agree very well with the predictions. Possible applications to the two-ribbon flare process are outlined.Properties of stable cylindrical prominences in equilibrium are analyzed and a criterion for the distinction between the Kuperus-Raadu and Kippenhahn-Schlüter types of prominences is proposed. According to established criteria, two of the studied prominences were of the Kuperus-Raadu type, while the other two were of the Kippenhahn-Schlüter type.     

Spectral analysis of four quiescent prominences observed at the Peruvian eclipse

Two-dimensional distributions of kinetic temperature, density and turbulent velocity are obtained for four quiescent prominences observed at the Peruvian eclipse of 12 November, 1966.

1. The kinetic temperature derived from line widths is around 6000–7000 K in the central part of prominences and rises to 12000K in both edges and possibly in the top of prominences.
2. The turbulent velocity shows a similar tendency, being 7–9 km/sec in the central part and ≈ 20 km/sec in the outer part. The turbulent velocity also increases slowly towards higher heights in the prominence.
3. The electron density derived both from the Stark effect and the intensity ratio of the continuous spectra turns out to be about 10^10.2–10^10.6 cm^?3 in the central portion of two prominences.
4. From the width and the intensity, neutral helium lines are shown to originate in the same region as hydrogen and metallic lines where the kinetic temperature goes down to 6000 K. This indicates that neutral helium is emitted after the ionization due to UV radiation from the corona and the transition region.

     

The prominence radiation theory

The present work is a review of papers related to the theory of prominence radiation. Special attention is paid to stationary equations and the theory of radiation diffusion in the lines and continua of hydrogen, helium and metals.We conclude that prominences are low-temperature formations T _e 7000 K, of low density 10¹² particles per cm³, n _e 10¹¹ cm^–3, effective thickness 10⁹ cm, and that the chemical composition of prominences and that of the Sun's atmosphere are the same. The prominence radiation in the lines of hydrogen, helium and metals is due mainly to quasiresonance scattering of the photospheric radiation.     

Measurements of the magnetic field in solar prominences with a spectrally scanning magnetograph

We describe observations with a new magnetograph capable of recording the whole profile of emission lines in prominences. Two recordings are used simultaneously to study the Zeeman effect in circularly polarized light. The spectral scan is produced by the action of piezo ceramics of a Perot-Fabry inter ferometer combined with a narrow band interference filter.The instrument is calibrated using 100% circularly polarized light and an emission line produced in Laboratory conditions in a simulated longitudinal magnetic field. The magnetograph was attached to the large coronagraph (Ø 53 cm) of Kislovodsk to give a series of measurements of the H line of several quiescent and active prominences. The observed values of the longitudinal component of the magnetic field are between: -25 G + 13 G with a noise level at ±2 G for a corresponding resolution of 8 arc sec.Effects produced by the instrumental polarization are discussed.S.A.S. Institut d'Astrophysique du CNRS, 98bis, Bd Arago, F 75014 Paris.     

Some calculations bearing on the heating and cooling of quiescent prominences

We discuss the evolution of pulses of heat both along and perpendicular to magnetic fields threading quiescent prominences. We show that while heating of prominence material can take place on a time scale of the order 10³ s (of the same order as the observed winking of H light from prominences and also of the same order as the dynamical Alfvén time scale across a prominence sheet) individual flux tubes are effectively thermally insulated from neighboring tubes, since the transverse (to the ambient supporting magnetic field) heat conduction time scale is of order 10⁴ yr. The exact solution to the one-dimensional parallel heat conduction problem is shown to differ significantly from the approximate solution reported by Ioshpa (1965). We also suggest that uneven heating of a quiescent prominence by the surrounding solar corona may be a contributory mechanism for surges and/or the observed winking phenomenon - both of which are recorded in many quiescent prominences. The signature of such a temperature pulse would be a sharp (10³ s) brightening of continuum radiation with a correlated decrease in the free-bound emission, followed by a slow (10⁴ s) recovery of both to their pre-heat pulse levels.