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.     

Observed mass motions in limb prominences

Line shifts occurring on Fabry-Perot interferograms of four limb prominences obtained at Boyden Observatory are reported and analyzed in terms of mass motions of the emitting parts of the prominences. The contribution of these motions to the energy balance of prominences is discussed.     

Remark on rotational motions in flares and prominences

From material secured with the McMath Solar Telescope of the Kitt Peak National Observatory rotational (orbital) motion has been found in a prominence ejected from a rotating flare. A period of rotation of 32 min has been derived from the study of a periodic asymmetry of the AL i 3961 absorption line.Presented at the Physics of Prominences-colloquium at Anacapri, September 29, 1971.     

Formation of solar prominences by photospheric shearing motions

A numerical simulation is performed to investigate the prominence formation in a magnetic arcade by photospheric shearing motions. A two-and-a-half-dimensional magnetohydrodynamic (MHD) code is used, in which the gravitational force, radiative cooling, thermal conduction and a simplified form of coronal heating are included. It is found that a footpoint shear induces an expansion of the magnetic arcade and cooling of the plasma in it. Simultaneously the denser material from the lower part of the arcade is pulled up by the expanding field lines. A local enhancement of radiative cooling is thus effected, which leads to the onset of thermal instability and the condensation of coronal plasma. The condensed material grows vertically to form a sheet-like structure making dips on field lines, leading to the formation of the Kippenhahn- Schlüter type prominence. The mass of the prominence is found to be supplied not only by the condensation of the material in the vicinity but also by the siphon-type upflows. The upward growth of the vertical sheet-structure of the prominence is saturated at a certain stage and the newly condensed material is found to slide down from above the prominence along magnetic field lines. This drainage of material leads to the formation of an arc-shaped cavity of low density and low pressure around the prominence. The problem of force and heat balance is addressed and the prominence is found to be not in a static equilibrium but in a dynamic interaction with its environment.     

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.     

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.     

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.     

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.     

Vector magnetic fields in prominences

A preliminary discussion is presented of measurements of the polarization of the He i D₃ multiplet in a quiescent prominence, observed with a wavelength-scanning Stokes polarimeter. For a series of 43 observations in the same prominence, the linear polarization of the major component of D₃ lies primarily in the range 1 to 2% and of the wing component, the range 2 to 5%; the polarization vector angle lies primarily in the range 10–25° for the major component, and 25–35° for the other component. From a more limited data set, the polarization of both components is found to first increase as a function of height in the prominence, and then to decrease; the polarization angles of the major component vary in a random-like way with height, while the wing component shows a systematic change. The amount of polarization and the angle of polarization are governed by the Hanle effect. The collective effect of the group of lines at the peak of D₃ evidently has a different sensitivity to the Hanle effect than does the wing component, thus yielding at least four independent measurements - two polarizations and two angles. With some redundancy, the vector magnetic field can then be established using the detailed theory of the Hanle effect. Since the wing component of D₃ is a simple triplet, an initial estimate of the magnetic field strength and its horizontal orientation, 0, relative to the line of sight, is simply obtained. Examples of such calculations are presented.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.     

Periodic structures in quiescent prominences

The observed structural periodicities in quiescent prominences and filaments are examined in terms of the instability of a plasma supported by a magnetic field against gravity. It is suggested that the spacing of arch-like structures may be identified with the most unstable wavelength of the interface between the prominence and the supporting magnetic field. The results of analysis further suggest that the observed spacing of periodic structures corresponds to the supporting magnetic field which lies at an angle 90° to 60° with respect to the long axis of the prominence.     

Helium radiation diffusion in prominences

The following is shown on the basis of a solution of the integral diffusion equations for radiation in a multilevel helium atom under low-temperature plasma conditions (T _e = 7000 K) in the Vertical slab model: (a) Neutral helium is ionized by coronal radiation mainly in the 100–300 Å spectral region; the degree of helium ionization is maximum at the boundary planes, (b) The photorecombinations to the 2³ S and 2¹ S levels and the photoionizations by the Balmer continuum of the Sun are very nearly balanced and this determines the population of these levels. The 2³ S level is destroyed by electron impacts (this reduces the brightness of the triplet lines), and the 2¹ S level decays via the escape of the quanta of 584 through the 2¹ P level, (c) Emission in the resonance line 584 (2¹ P 1¹ S) occurs due to recombination to 2¹ S with subsequent absorption of quanta of infrared radiation 20581. This is a rare case. (d) The radiation of helium is generated in the vicinity of the boundary planes in the region of penetration of radiation with 200 Å, where the density of matter decreases gradually down to the coronal value. In the subordinate lines, the radiation is conditioned by the quasi-resonance scattering of photospheric radiation, (e) The calculated absolute values of the intensities of the helium and hydrogen lines are in good agreement with the observations (see Figure 6).The helium to hydrogen number ratio is close to 0.05.     

Irrotational motion in quiescent prominences

On the basis of Kippenhahn and Schlüter's magnetohydrostatic model of a quiescent prominence, an attempt has been made to study the effect of irrotational motion existing in the prominences on the magnetic field pattern in it, introducing an irrotational velocity field. It is found that, under such a condition, the magnetic field geometry in the model does not change.     

Oscillatory phenomena in quiescent prominences

A model for horizontal oscillations of prominences is presented. It is found that the model of a freely oscillating prominence surrounded by coronal matter explains satisfactorily the observed periods and damping times, as well as the changes in the prominence shape.On leave from the Astronomical Institute of the Czechoslovak Academy of Sciences, Ondejov.     

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.     

Magnetic fields in quiescent prominences

We discuss the longitudinal component of the magnetic field, B , based on data from about 135 quiescent prominences observed at Climax during the period 1968–1969. The measurements are obtained with the magnetograph which records the Zeeman effect on hydrogen, helium and metal lines. Use of the following lines, H; Hei, D₃, Hei, 4471 Å; Nai, Di and D₂, leads to the same value for the observed magnetic field component in these prominences. For more than half of the prominences their mean field, B , satisfy the inequalities 3 G B 8 G, and the overall mean value for all the prominences is 7.3 G. As a rule, the magnetic field enters the prominence on one side and exits on the other, but in traversing the prominence material, the field tends to run along the long axis of the prominence.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

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.     

Vector magnetic fields in prominences

Observations of linear polarization in two resolved components of HeI D₃ are interpreted using the Hanle effect to determine vector magnetic fields in thirteen prominences. As in all vector magnetic field measurements, there is a two-fold ambiguity in field direction that is symmetric to a 180° rotation about the line-of-sight. The polar angles of the fields show a pronounced preference to be close to 90° from the local solar radius, i.e., the field direction is close to horizontal. Azimuth angles show internal consistency from point to point in a given prominences, but because of the rotational symmetry, the fields may be interpreted, in most cases, as crossing the prominence either in the same sense as the underlying photospheric fields or in the opposite sense. An exceptionally well observed large prominence of approximately planar geometry exhibits no measurable change in the vector magnetic field either with height or with location along the prominence axis. A second well observed large prominence overlying a sharply curved magnetic neutral line, when interpreted assuming that the prominence field has the same sense as the photospheric field, shows a rotation in the azimuth angle of the field relative to the observer by about 150° and relative to the local plane of the prominence by about 65°. In the alternative interpretation in which the prominence field has the opposite sense of the photospheric field, the field still rotates by 150° relative to the observer but remains essentially constant with respect to the plane of the prominence. This prominence erupted shortly after the extended observations. One good quality observation during the course of the eruption gives a vector field fully consistent with the pre-eruption field in the same segment of the prominence.     

Vector magnetic fields in prominences

The Stokes components of He i D₃ emission in two quiescent prominences, using full spectral profile measurements, are analyzed to derive vector magnetic fields. Two independently developed schemes, based on the Hanle effect, are used for interpretation. They involve solutions of the statistical equilibrium equations for the He i D₃ multiplet, including the effect of coherency and full level crossing, which predict the magnetic field dependence of the observed polarization. Derived magnetic field vector solutions for each pair of linear polarization Stokes profiles corresponding to an observational point in the prominence are, intrinsically, not uniquely determined, and a set of possible solutions is usually obtained. However, mutual consistency of these solutions with those independently predicted by the form of the circular polarized component, allow, in almost all cases, rejection of all solutions of a set except one symmetrical pair. Of such a pair, a unique solution can be determined with a high confidence level by reference to independent potential field information. Field vectors are found usually to be close to horizontal and normal to the prominence surface, but extreme exceptions are found. Field values range from 6 G to 60 G. The derived vectorfield configurations and their magnitudes are briefly discussed relative to these prominences and to different quiescent prominence models.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

Velocity fields in quiescent prominences

Three quiescent prominences were observed in the Ca ii K-line and a fourth one also in the H-line at Oslo Solar Observatory, Harestua, and reduced by Rustad (1974) and by Engvold et al. (1980). These data are used to study the distribution of the line-of-sight velocity component, N(u ₀). It is pointed out that in a stationary and isotropic case, N(u ₀) should be a gaussian distribution. For each of the sets of measurements gaussians were therefore fitted by a least square procedure. The range in observed velocities varies considerably between the prominences. For the best observed prominence more than 70% of the kinetic energy is in the supersonic range. In the other cases none or only an insignificant part of the observations exceed the velocity of sound. Considerable deviations from gaussian distributions are apparent for the smallest velocities. This distortion shows up conspicuously in the slope of the energy spectrum, a parameter that may be used as a rough measure of spectral resolution.If it is assumed that we have to do with MHD-turbulence as described by Kraichnan (1965), a characteristic relationship should exist between velocity and eddy size. When supersonic velocities are present, compressibility effects may severely alter this relationship. The possibility of observational confirmation is discussed.If a turbulent velocity field is indeed present, the heat conductivity and other transport coefficients may be significantly altered as compared to the atomic values.     

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.