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.     

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.     

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.     

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.     

The nature of quiescent solar prominences

Quiescent prominences occur as long-lasting cool sheets of matter in the surrounding hot corona at the base of coronal streamers. Seen on the disk they appear as dark filaments dividing regions of opposite magnetic polarity.In this paper a theoretical model is presented, which describes the general appearance of quiescent prominences.It is shown that the neutral sheet between two regions of oppositely directed magnetic fields is thermally unstable. This gives rise to compression and cooling of coronal material to prominence material in a characteristic time of the order of one day for a field strength of 0.5 gauss in the lower corona.It is assumed that due to the finite electrical resistivity of the plasma, filamentary structures are formed by the tearing-mode resistive plasma instability. These structures are thermally insulated from the hot surroundings by the newly formed closed azimuthal magnetic field configuration.It has been shown that for fine structures with a diameter of 300 km the growth rate of the tearing-mode instability is of the same order as the cooling time. The occurrence of fine structures within the prominence is of vital importance for their origin.On leave from the Observatory Sonnenborgh at Utrecht, The Netherlands.     

Measurement of the prominences magnetic field

A new type of magnetograph has been built capable to measure weak magnetic fields in the chromosphere and corona. Measurements of magnetic field in two prominences are demonstrated as examples.     

Stability of prominences exposing helical-like patterns

The internal structure of prominences appearing as twisted tubes was studied. The sample embraced 15 stable and 13 eruptive prominences, exposing patterns which possibly reflect a helical configuration. The equivalent pitch angles () of twisted fine structure features were measured. In some cases the evolution of the internal structure was followed and 49 independent measurements of the parameter were performed in total. The results are presented in the plane relating the parameter and the normalized prominence height. The eruptive prominences occupy the region characterized by > 50° and h > 0.8d, where h and d are the prominence height and the footpoint half-separation, respectively. All prominences characterized by h < 0.6d or by < 35° were stable. Such a result is in good agreement with an order of magnitude treatment of the forces acting in a curved magnetic tube, anchored at both ends in the photosphere.     

Evolution of rising helical prominences in a nonuniform atmosphere

Recent coronagraph observations of rising priminences such as in the 14 April and 5 May, 1980 coronal transient events, as well as other older observations, have shown evidence for helical structure in the prominences. If this is true, then a study of the dynamical evolution of rising helical structures in a nonuniform atmosphere is worthwhile. For this study, three important considerations become apparent: (1) Since the ends of the prominence remain rooted in the photosphere, significant stretching of the configuration will result as it rises, (2) due to the fall-off with height of the external quantities, such as gas and magnetic pressure, the prominence will experience time-varying boundary conditions as it rises, and (3) significant lateral expansion of the prominence is expected as the external conditions weaken with height. The interplay of all these effects togehter result in a quite complex dynamical behavior of the prominence.We have tried to obtain some insight into this general problem through a simple model - that of a helical pinch rising in a low beta atmosphere under the influence of an ambient external magnetic field which declines in strength with radial distance from the solar center. Under the general assumptions of an internal uniform, but time-varying, temperature and neglecting gravitational stratification within the prominence, expressions are derived for associated variations in the prominence structure as it rises. We discuss in some detail, particular quantities which are potentially most accessible to observation such as prominence radius, density, and pitch angle as they vary with height during the eruptive process.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Radiation transfer in prominences with filamentary structure

Observations concerning the structure of sunspots, obtained during the fourth flight of the Soviet Stratospheric Observatory (SSO), are discussed. Objects brighter than the mean photospheric background inside the sunspot penumbra retaining the stable position sometimes vary within time intervals of a few minutes. The brightness change in pores can be explained by their different location at highest levels of the photosphere. The same mechanism can cause the brightness difference of the penumbra filaments. The gradient of the brightness variation inside the pores is determined. The value of this gradient was found to be practically the same for all dark objects. Most penumbral filaments show no magnetic expansion with growing distance from the spot center.     

The fine structure of prominences

Faint, Doppler shifted, emission features are detected in high resolution spectra of limb prominences. Their average line-of-sight velocity is about 3 × 10⁶ cm s^-1, their average life time is 300 s, and their angular sizes are 10⁸ cm in our spectrograms. The emission line width of the spectral features increases with increasing line shift.Some implications on the stability of prominences by these structures are discussed briefly.Visiting Astronomer, on leave from the Department of Astro-Geophysics, University of Colorado, Boulder, Colorado.     

The fibril structure of prominences

In this paper we present several magneto-hydrostatic equilibrium models for prominences with fibril-like fine structure. For all the models ad hoc temperature profiles are used without discussing the energetics. For our models we assume fine structure to occur either across the prominence axis or along it. This approach is intended as a first step towards more realistic models based upon a series of vertical fibril 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.     

The fibril structure of prominences

We suggest that the fibril structure of prominences may be caused by filamentation during its formation by radiative instability. We also discuss the effects of other types of instability and give a mechanism for the formation of vertical threads. The models indicate that highly inhomogeneous density structures can exist in the presence of smooth profiles for the plasma pressure and magnetic field. In our particular models the plasma pressure of a fibril prominence is higher and the vertical magnetical field is weaker than in a uniform prominence model, while the mass is substantially smaller.     

The nature of velocity fields in quiescent prominences

It is shown that for certain definite conditions of symmetry imposed on the permitting magnetic field geometry for an isothermal case in Kippenhahn and Schlüter's (1957) model of a quiescent prominence, any irrotational velocity field would quickly get converted to rotational.     

The fine structure of prominences

Ions falling in vertically aligned magnetic structures of quiescent prominences may experience a vertical Lorentz force as flux ropes are distorted from the force-free condition. The terminal velocity of such ions may be sub-Alfvénic and may correspond to the 5–15 km s^–1 velocity of down falling material observed in many quiescent prominences. The higher velocities of down falling material found in active prominences and coronal rain may occur because of higher terminal velocities occurring in stronger magnetic fields.Visiting Astronomer, on leave from the Department of Astro-Geophysics, 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.     

Magnetodynamic stability of a fluid cylinder under the lundquist force-free magnetic field

The magnetodynamic (in)stability of a conducting fluid cylinder subject to the capillarity and electromagnetic forces has been developed. The cylinder is pervaded by a uniform magnetic field but embedded in the Lundquist force-free varying field that allows for flowing a current surrounding the fluid. A general eigenvalue relation is derived based on a study of the equilibrium and perturbed states. The stability criterion is discussed analytically in general terms. The surface tension is destabilizing for small axisymmetric mode and stable for all others. The principle of the exchange of stability is allowed for the present problem due to the non-uniform behaviour of the force-free field. Each of the axial and transverse force-free fields separately exerts a stabilizing influence in the most dangerous mode but the combined contribution of them is strongly destabilizing. Whether the model is acted upon the electromagnetic force (with the Lundquist field) the stability restrictions or/and the capillarity force are identified.Several reported works can be recovered as limiting cases with appropriate simplifications.     

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.     

Stability analysis of two-dimensional models of quiescent prominences

Using the MHD energy principle of Bernstein et al. (1958) we develop a formalism in order to analyze the stability properties of two-dimensional magnetostatic plasma equilibria. We apply this to four models of quiescent prominences, namely those of Menzel (1951), Dungey (1953), Kippenhahn and Schlüter (1957), and finally Lerche and Low (1980). For the observed parameter range, all models are stable and they explain reasonably well the reported flare-initiated oscillations in quiescent prominences. We also investigate other parameters regions, which may be relevant in some stellar atmospheres. It is found that, with the exception of the Kippenhahn and Schlüter model, all models become unstable. The instabilities that occur show simultaneously several features of well-known MHD-instabilities. However, an unequivocal assignment of the instabilities to specific instability prototypes is not possible. Our formalism allows one to investigate not only more realistic prominence equilibria, but also arbitrary one- and two-dimensional static ideal MHD-equilibria.     

Magnetic field configurations which can produce prominences with inverse polarity

In this Letter several observational aspects of prominences with inverse magnetic polarity are summarised. It is shown that these features can be explained in a natural way if inverse polarity prominences result from the merging of two adjacent bipolar magnetic regions.