Structural characteristics of eruptive prominences

Nowadays the primordial importance of the magnetic field for coronal plasma physics is well known. However, its determination is only made in cool regions, mainly the photosphere and prominences. The extrapolation to the corona gives some indications of the magnetic structure but is not presently sufficiently reliable. So it is important to consider all the other observable physical effects of the magnetic field.In this puzzle, eruptive prominences may play a key role because the cool plasma is forced to move along field lines, which can then be visualized. In the strongest field regions, flares also give such information, while coronal mass ejections (CME) play such a role at larger scales. The magnetic field, which is at the base of the physical processes, is a common link between these different events.Observed properties of solar prominence eruptions are reviewed, then their relationships with CMEs and flares are discussed, with the help of present models. We emphasize the importance of magnetic measurements in future coordinated observations.Invited paper presented at the IAU Commission 10 Meeting on Dynamics and Structure of Prominences in Buenos Aires.     

Characteristics of the expansion associated with eruptive prominences

Gradients of H and electron scattering intensities derived from instantaneous radial distributions of erupting prominence material observed at several solar radii by Illing and Athay (1986) are often markedly smaller than those inferred by comparing the intensities observed near several radii to average prominence intensities observed near the limb. In this paper, we show that gradients derived by following individual features in their outward progression with time yield values that are consistent with limb observations and that usually exceed the values obtained from instantaneous distributions. We conclude from the diversity of observed gradients that the prominence eruption cannot be described by a self-similar expansion in which the expansion velocity is a function of radius and time only. However, we cannot rule out possible self-similar solutions that allow the expansion velocity to be a function of angular direction.On leave from the Astronomical Observatory of Wrocaw University, Poland.The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.     

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.     

Stability of the lundquist field and the physical nature of eruptive prominences with helical structure

The stability of the Lundquist field is analysed in the light of the energy principle. The results show: (1) form = 0 disturbances, the Lundquist field is stable; (2) form = 1 disturbances, the Lundquist field will turn unstable in certain conditions. Whether the field will remain stable or not depends on parameters such as the helical pinch angle and radius and length of the prominence. Comparison between theory and observations reveals that a kink instability in the Lundquist field may be a critical physical reason for eruptive prominences with helical patterns.     

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.

     

A solar scenario for the associated occurrence of flares, eruptive prominences, coronal mass ejections, coronal holes, and interplanetary shocks

The observation of non-corotating shock fronts in interplanetary space is always associated with the previous occurrence of a coronal mass ejection (CME), which is frequently accompanied by a flare or a prominence eruption. When looking at the solar region of origin of these events, a coronal hole is always found. Here we propose a scenario at the Sun where all these related events can find a place.     

Quiescent prominences

This review surveys recent research on quiescent solar prominences. The main topics considered are magnetic structure, thermal structure, and formation. Sub-arc sec fine-structures undoubtedly play a crucial role in all three topics. Current attempts to model the magnetic and thermal structure are hampered, in part, by the lack of observations with sufficient spatial resolution. The process of formation is quite complicated, but is yielding slowly to detailed numerical simulations. Unfortunately, observations of prominence condensation from the corona (the favored hypothesis) are lacking. Some suggestions for future work are offered.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

A model of ‘disparitions brusques’ (sudden disappearance of eruptive prominences) as an instability driven by mhd waves

A model of ‘disparitions brusques’ (sudden disappearence of eruptive prominences) is discussed based on the Kippenhahn ans Schlüter configuration. It is shown that Kippenhahn and Schlüter's current sheet is very weakly unstable against magnetic reconnecting modes during the lifetime of quiescent prominences. Disturbances in the form of fast magnetosonic waves originating from nearby active regions or the changes of whole magnetic configuration due to newly emerged magnetic flux may trigger a rapid growing instability associated with magnetic field reconnection. This instability gives rise to disruptions of quiescent prominences and also generates high energy particles.     

Rotation in prominences

We have studied rotation in non-eruptive limb prominences; in most cases dopplergrams could be used to confirm proper motion measurements. In some cases part of the prominence rotates; in the others, the entire body is in rotation. Velocities of 15–75 km s^-1 are found. Of fifty-one prominences studied in 1978, five showed rotation.     

Splintering loop prominences

A new type of sunspot prominences (splintering loop prominences) is described. They have loop structure, but their material seems to originate from the lower layers. A tentative interpretation of the new type is given. In the splintering loop prominences on October 7, 1967 a phenomenon was observed which might be interpreted as the capture of an ejected prominence streamer by transverse magnetic fields of the loops.On leave from the Astronomical Institute of the Czechoslovak Academy of Sciences, Ondejov.     

Kinematics of solar prominences

Two sequences of OSO-4 spectroheliograms in Mg x and Si xii obtained during October–November 1967 and covering the intervals of 83 and 22 hr, respectively, have been analyzed to reveal quasi-periodic oscillations of EUV flux from solar sources with a periodicity of 5–14 hr. The oscillation periods of the emission flux from local sources over sunspots and magnetic field enhancements in plages without spots have been investigated in correlation with characteristics of the respective AR and plages. The greatest periods (> 8 hr) are shown to be peculiar of small sunspots or sunspot groups at the initial or final stage of their development, whereas the smallest periods ( 5–6 hr) are observed in the case of large well-developed groups at the maximum stage of development. In quiet regions on the Sun and plages without spots, the oscillation periods are 6–8 hr. The surface areas in which the oscillations are synchronous and coincide in phase have typical dimensions of 1 in quiet and 1 to 5 in active regions. These areas form a spatial structure similar to the chromospheric network and supergranules. The characteristic lifetime of the structure elements is 1.5–2 days.     

Macroscopic motions in prominences

Macroscopic velocity fields have been studied in a solar prominence. The spectra and monochromatic images were analyzed, and the existence of a contracting motion, possibly due to a pinch effect, is discussed. A helical shape of the prominence is proposed.     

Macroscopic motions in prominences

We have studied three pairs of prominences, two of them clearly showing the effect of interaction by a dynamical plasma bridge. The other one was a superimposed optical pair which also showed, after being spectroscopically studied, the possible effect of interaction. We have calculated the atom density in the prominence and bridges, and applied the Goldberg-Unno method for the determination of Doppler-widths of the calcium H and K lines.     

Origin of quiescent prominences

For stable equilibrium, prominences must be supported with magnetic lines of force leaning upon the photosphere and concave in their tops; however the general structure may be more complicated. If such a field appears in the corona, the heating of the gas near the upper pit should be low, because Alfvén and slow waves do not propagate across magnetic lines and fast mode waves attenuate because of refraction. The gas of the corona, distributed along the magnetic lines tube, cannot keep balance, it should flow down in the pit, condense there and fall down into the chromosphere in some places. The prominence, therefore, originates in the matter of the chromosphere which is situated at the other end of the magnetic lines and flows through the corona under the effect of a siphon-type mechanism. A similar mechanism for chromospheric structures was earlier suggested by Meyer and Schmidt. A stationary stream along the tube has been calculated with allowance for the heat conductivity and radiative cooling of the corona gas. The stream is subsonic and is about 10¹⁵ cm⁻² sec⁻¹ which corresponds to the prominence formation time of the order of a day.     

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.     

Oscillatory processes in prominences

A new type of oscillations, being long-period oscillations of line-of-sight velocities, with periods ranging from 42 to 82 min and amplitudes in excess of 200 m s^-1 has been discovered in prominences. These oscillations may be interpreted as a combination of torsional and longitudinal ones.     

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.     

Motion of ascending prominences

Apparent motions of a number of ascending prominences of the limb-SD type (sudden disappearances) are investigated. The direction, the velocity of ascent and the correlation with flares are studied. The maximum velocity, which seems to deviate systematically from the radial direction, increases with height and shows a clear dependence on the distance to an initiating flare and its importance.A good correlation between ascending prominences and solar radio emission has been found.     

Flows in quiescent prominences

Earlier studies of quiescent prominences claim that there is a systematic downward directed motion of the small-scale structure. Disk observations, on the other hand, have detected mass motions both upwards and downwards. The earlier high-resolution observations of limb prominences have been re-examined using local cross-correlation techniques for measurements of motion perpendicular to the line of sight. The new measurements reveal flow speeds and directions that are in good agreement with current Doppler measurements on the disk.     

Physics of solar prominences

Summary Conclusion This colloquium on solar prominences - the first ever held - has shown that a major part of activity in prominence research in recent years concentrated on both observation and computation of the magnetic conditions which were found to play a crucial role for the development and the maintainance of prominences. Remarkable progress was made in fine-scale measurements of photospheric magnetic fields around filaments and in internal field measurements in prominences. In addition, important information on the structure of the magnetic fields in the chromosphere adjacent to the filaments may be derived from high resolution photographs of the H fine structure around filaments which have become available recently; unfortunately, an unambiguous determination of the vector field in the chromosphere is not yet possible.It is quite clear, now, that stable filaments extend along neutral lines which divide regions of opposite longitudinal magnetic fields. Different types of neutral lines are possible, depending on the history and relationship of the opposite field regions. There is convincing evidence that the magnetic field in the neighbouring chromosphere may run nearly parallel to the filament axis and that there are two field components in stable prominences: an axial field dominant in the lower parts and a transverse field dominant in the higher parts.Methods for the computation of possible prominence field configurations from measured longitudinal photospheric fields were developed in recent years. In a number of cases (e.g. for loop prominences) the observed configuration could be perfectly represented by a force-free or even a potential field; poor agreement was found between computed and measured field strengths in quiescent prominences. In order to reconcile both of them it is necessary to assume electric currents. Unambiguous solutions will not be found until measurements of the vector field in the photosphere and in the prominences are available.The two-dimensional Kippenhahn-Schlüter model is still considered a useful tool for the study of prominence support and stability. However, a more refined model taking into account both field components and considering also thermal stability conditions is available now. It was proposed that quiescent prominences may form in magnetically neutral sheets in the corona where fields of opposite directions meet.As for the problem of the origin of the dense prominence material there are still two opposite processes under discussion. The injection of material from below, which was mainly applied to loop prominences, has recently been considered also a possible mechanism for the formation of quiescent prominences. On the other hand, the main objections against the condensation mechanism could be removed: it was shown that (1) sufficient material is available in the surrounding corona, and that (2) coronal matter can be condensed to prominence densities and cooled to prominence temperatures in a sufficiently short time.The energy balance in prominences is largely dependent on their fine structure. It seems that a much better radiative loss function for optically thin matter is now available. The problem of the heat conduction can only be treated properly if the field configuration is known. Very little is known on the heating of the corona and the prominence in a complicated field configuration. For the optically thick prominences the energy balance becomes a complicated radiative transfer problem.Still little is known on the first days of prominence development and on the mechanism of first formation which, both, are crucial for the unterstanding of the prominence phenomenon. As a first important step, it was shown in high resolution H photographs that the chromospheric fine structure becomes aligned along the direction of the neutral line already before first filament appearance. More H studies and magnetic field measurements are badly needed.Recent studies have shown that even in stable prominences strong small-scale internal rotational or helical motions exist; they are not yet understood. On the other hand, no generally agreed interpretation of large-scale motions of prominences seems to exist. A first attempt to explain the ascendance of prominences, the Disparitions Brusques, as the result of a kink instability was made recently.New opportunities in prominence research are offered by the study of invisible radiations: X-rays and meterwaves provide important information, not available otherwise, on physical conditions in the coronal surroundings of prominences; EUV observations will provide data on the thin transition layer between the cool prominence and the hot coronal plasma.Mitt. aus dem Fraunhofer Institut No. 111.