A complex diagnostic of solar prominences

We use the polarimetric and intensity measurements of H and HeI D₃ lines in solar prominences to derive the true geometrical thickness for several quiescent prominences. The electron densities, derived from the collisional depolarization in H by Bommier et al. (1994), are used to evaluate the thickness from the emission measure. The emission measure was obtained from the theoretical correlation with the H integrated intensity, according to Gouttebroze, Heinzel, and Vial (1993). Theoretical electron densities obtained by latter authors are also compared with those of Bommier et al. (1994) and we find a very good agreement between them. The prominence geometrical thickness exhibits a relatively large range of values from about 100 km up to a few 10⁴ km. The plasma densities vary by almost two orders of magnitude in the observed structures, but the total column mass in the direction perpendicular to the prominence sheet seems to be fairly constant for the set of prominences studied.     

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).     

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.     

The fine structure of prominences

High spatial resolution spectral observations of five hedgerow prominences were made in H, He i D₃ and Ca ii H and K.The observed relations between the lines were not the same in all prominences. The Ca ii H and K lines were 2–4 times brighter relative to H and D₃ than predicted theoretically. The optical thickness of H was less than for the H and K lines, the H was optically thin in medium faint prominence structures. Faint structures appeared slightly hotter than bright structures.On leave from Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, Oslo 3, Norway.     

Comparison of Prominences in Hα and He II 304 Å

In this letter, we bring attention to prominences which show different morphology in H and Heii 304 Å, as observed simultaneously by BBSO and EIT on board SOHO. Those two lines have been thought to represent similar chromospheric structures although they are formed at significantly different temperatures. We give two examples representing two kinds of anomaly: (1) prominences showing strong H emissions in the lower part and strong Heii emissions in the upper part, and (2) erupting prominences showing extensive Heii emission, but nothing in H. Our results indicate that a part or the whole of a prominence may be too hot to emit H radiation, possibly due to heating or thermal instability. Please note that these are not just two isolated cases, many other prominences show the similar differences in H and Heii 304 Å.     

Axially-symmetric Velocities in the 15 May 2000 Eruptive Prominence

Large Doppler velocities with unique, almost regular elliptical features were observed in the H spectra of the May 15, 2000 eruptive prominence. These features were interpreted in the frame of axially symmetric models of the eruptive prominence. The rotational (7–60 km s^–1), expansion (30–44 km s^–1), axial (3–19 km s^–1), and global (66–160 km s^–1) prominence plasma velocities were derived. The plasma velocity patterns were compared with the observed helical structures of the H prominence. The velocities of selected H blobs in the image plane were determined. The axially symmetric detwisting process of the magnetic flux rope of the eruptive prominence was recognized.     

A Statistical Analysis of Faint Hα Coronal Emission Associated with Prominences

The results from a detailed study of the prominences associated with faint H emission objects in the solar corona are given. The frequency distribution of the prominences by their lifetime, as well as for the prominence groups with and without `disparition brusque' (DB), is presented. The systematic comparison of the time of the prominence DBs and the observation time of the objects with faint H emission, as well as the positions of the faint H emissions and the associated filaments at the limb and on the disk of the Sun, suggests that in the most cases these coronal emissions are probably closely connected with the instability processes operating in the prominence magnetic field configurations and leading to prominence final or temporary DBs.     

Evidence of Magnetic Field Reconnection in the Hα Eruptive Prominence on 18 september 1995

In this paper we present a detailed study of a violent evolution of the 18 September 1995 eruptive prominence observed by the H telescope and the Multichannel Optical Flare Spectrograph in Ondejov. The fast changes of the prominence structure started immediately after a weak radio burst at 3 GHz. This circumstance shows the presence of non-thermal processes. In the later phase of the prominence evolution a comparison of the H filtergrams with the Yohkoh Soft X-ray Telescope pictures was made. For a search of fine structures in the H images, an image processing technique was used. A detailed analysis of observations indicates magnetic field line reconnection, mainly in space below the rising H prominence. These reconnection processes are manifested not only by structural changes of the H prominence and X-ray loops but also by the character of Doppler velocities. Evidence of splitting and rotation was found in the H spectrum formed close to the reconnection space, and the typical velocities of such plasma movement were evaluated. We estimated amplitudes of rotational velocities, giving evidence about the rearrangement of helical structures during the process of the eruptive prominence activation. In the conclusion we discuss some implications of our results.     

Physical conditions in eruptive prominences at several solar radii

SMM data from the Corograph/Polarimeter experiment giving intensities of H and continuum emission in eight erupting prominences are analyzed to obtain the physical conditions in the regions of H emission. Since the H intensity depends upon three unknowns whereas only two independent observations are available, it is necessary to assume one additional condition in order to obtain unique solutions. Solutions are chosen that give the maximum expansion of the prominence volume as reflected by minimum values of the electron density. These solutions correspond closely with those giving the best agreement between the gas pressure in the prominence and the ambient coronal pressure. Electron densities are found to be of the order of 10⁸ cm^-3 at temperatures near 2 × 10⁴ K.The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.     

The 1 October 2001 Eruptive Prominence: Observed and Modeled Structures

Using TRACE 171 Å image observations and H spectra and images observed at the Ondejov Observatory, the October 1, 2001, eruptive prominence is studied. The evolution of this prominence is described and velocities of specific parts of the prominence are determined. It was found that, after the rising phase of the cold loop-like prominence, its upper part expanded and below this expanding part, around one of its legs a `ring' structure, visible in the TRACE images, was formed. Then, at the same place, a tearing of the prominence leg was recognized. Simultaneous spectral observations of this structure reveal a very broad H line, which indicates strong turbulent motion at these positions. These processes were accompanied by an expanding H envelope. Due to the similarity of the observed `ring' and tearing structures with those modeled by Lau and Finn (1996), the prominence leg tearing is interpreted as a reconnection process between two parallel magnetic ropes having parallel electric currents, but anti-parallel axial magnetic fields.     

Spatial relationship between λ5303 and Hα components of a loop prominence system

There are remarkable similarities in the structure of loop prominences when observed in H and coronal lines, although the lines arise from extremely different excitation conditions. This leads to the consideration of a multi-component model, where different emission lines come from different elements of the structure. The late phases of a large west limb loop prominence system followed by a surge were recorded at Haleakala Observatory on March 6, 1970. Simultaneous filter-grams in H and 5303 were obtained, together with spectra at three heights in the prominence over the range 3850–5950 Å. The positions of bright knots of emission as seen in the green line are compared with associated H knots. With these relative positions determined, the filtergrams are superimposed to demonstrate the two dimensional spatial relationship between H and 5303 structures. The results support a model of cool loops within a closely associated hot loop system.     

Evolution of helically twisted prominence structures of March 11, 1979

Helical structures are generally associated with many eruptive solar prominences. Thus, study of their evolution in the solar atmosphere assumes importance. We present a study of a flare-associated erupting prominence of March 11, 1979, with conspicuous helically twisted structure, observed in H line center. We have attempted to understand the role played by twisted force-free magnetic fields in this event. In the analysis, we have assumed that the helical structures visible in H outline the field lines in which prominence tubes are embedded. Untwisting of observed prominence tubes and later, formation of open prominence structures provide evidence of restructuring of the magnetic field configuration over the active region during the course of prominence eruption. Temporal evolution of the force-free parameter is obtained for two main prominence tubes observed to be intertwined in a rope-like structure. Axial electric currents associated with the prominence tubes are estimated to be of the order of 10¹¹ A which decreased with time. Correspondingly, it is estimated that the rate of energy release was 10²⁸ erg s^–1 during the prominence eruption.     

Dynamics of Helically Twisted Prominence of January 22, 1979

We have studied the dynamics of a macroscopically twisted helical prominence observed in H line on January 22, 1979 from Udaipur Solar Observatory. The analysis carried out is similar to that of March 11, 1979 event (Srivastava et al., 1991) wherein we had studied the role of twisted force-free magnetic fields in the prominence system. In the present study, it is found that of the two helically braided prominence tubes, one was dynamically more active. We have examined the temporal evolution of force-free parameter , and the axial currents associated with the prominence system that decreased with time. We find that the magnitude of the electric currents and also the rate of energy release during the untwisting of the prominence was of comparatively higher order 10³⁰ ergs s^-1 than that of March 11, 1979 event, in agreement with the physical dimensions of the two prominences.     

Filter observations of prominences in the D3 and Hα lines

D₃ and H pictures of prominences were obtained with a 21-in. Lyot coronograph and a Fabry-Perot etalon used as a narrow band filter. The monochromatic images of quiescent, quasiquiescent and loop-prominences were studied. The comparison of the isophotes of quiescent and quasi-quiescent prominences in D₃ with those in H shows the similarity of the prominence structure at both wavelength, although there is a strong tendency for an increase in the intensity ratio D₃/H in the upper region of prominences. It seems that it is due to lower temperature in the upper regions of prominences. Probably, the relaxation processes establishing ionization equilibrium play some role. Measurements of the knot intensities of the loop-prominence show strong variations of the intensity ratio D₃/H (more than one order of magnitude).     

Theoretical model of flares and prominences

A theoretical model of flare which explains observed quantities in H, EUV, soft X-ray and flare-associated solar wind is presented. It is assumed that large mass observed in the soft X-ray flare and the solar wind comes from the chromosphere by the process like evaporation while flare is in progress. From mass and pressure balance in the chromosphere and the corona, the high temperature in the soft X-ray flare is shown to be attained by the larger mass loss to the solar wind compared with the mass remained in the corona, in accord with observations. The total energy of 10³² erg, the electron density of 10^13.5 cm^–3 in H flare, the temperature of the X-ray flare of 10^7.3K and the time to attain maximum H brightness (600 s) are derived consistent with observations. It is shown that the top height of the H flare is located about 1000 km lower than that of the active chromosphere because of evaporation. So-called limb flares are assigned to either post-flare loops, surges or rising prominences.The observed small thickness of the H flare is interpreted by free streaming and/or heat conduction. Applications are suggested to explain the maximum temperature of a coronal condensation and the formation of quiescent prominences.     

Hα Doppler brightening and Lyman-α Doppler dimming in moving Hα prominences

We consider the effect that coherent motion has on the observed brightness of moving clouds above the photosphere. We find that steady state clouds (constant N _e and T _e ) that are moving perpendicular to the line of sight will appear brighter in H for speeds between 8 and 100 km/sec and dimmer for speeds greater than 135 km/sec. The brightening and dimming are due to apparent Doppler shifts of the respective H absorption and the Lyman- emission profiles seen by the absorption profile of the moving cloud.We apply this analysis, along with optical depth and geometrical considerations, to the observed brightness variations of the 1 March 1969 limb eruptive prominence. We find that all of the observed brightening and dimming can be explained by the motions, and that no significant change in the prominence N _e or T _e was necessary during the observed H event. This conclusion is significant in interpreting an X-ray burst that began as the prominence velocity increased abruptly at the time of maximum H intensity. The thermal X-ray peak occurred 150 sec later when the prominence had become faint again. There was no associated flare that was visible in H. We discuss the relative brightness of H and D ₃ in a specific moving prominence knot.We note that the observed range of limb speeds (30–150 km/sec) may be due to the combined H Doppler brightening and Lyman- dimming effects. We also discuss generally the H brightness of disk surges (bright and dark) and flares, and sprays and puffs that occur at or near the limb.Now at the Dept. of Physics and Astrophysics, University of Colorado, and High Altitude Observatory (NCAR) Boulder, Colo., U.S.A.     

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.     

Emissions ‘froides’ dans la couronne solaire

Prominences have been photographed through a coronagraph and an H Lyot filter with long exposure times. Faint H emissions are often detected down to the threshold 2 × 10^-6 times of the Sun's brightness; they show definite structures but their relations to the low-level ordinary prominences are not very clear. Estimates are given for the density and thickness of such cool regions.     

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.     

Observational study of the ‘post’-flare loops on June 4, 1991

A limb, two-ribbon H flare on June 4, 1991, associated with a white-light flare and followed by an emission spray and post-flare loops, is studied. A region of rapidly enhanced brightness at the bottom of the H ribbon above the white-light flare is revealed. The energy released by the white-light flare at _eff = 4100 is estimated to be about 1.5 × 10²⁸ erg s^–1.