Based on an analysis of three active filaments in AR 6891, we find that vector magnetic fields at the base of filaments and the filament environment is characterized by the following: (1) The transverse field is parallel along the magnetic neutral line for most of the filaments. The average transverse field beneath the filaments is more than 400 G. (2) The horizontal gradient of the line-of-sight field crossing the neutral line is, more or less, constant along the major fraction of a filament, but very steep at the two ends. The average gradient is 0.06 G/km. (3) For each of the filaments there is a squeezing magnetic structure which represents an intrusion of a satellite bipole into the main flux system. The neutral line for a squeezing magnetic structure has a large curvature, a strong sheared transverse field of more than 1 KG, and a steep field gradient of approximate 0.3 G/km. (4) The transverse field and field gradient are clearly enhanced before the filament eruption.

Some of the most dramatic images of prominences show helical structure. Helical structure, as well as other structural features such as twist, shear, and linking, can be quantified using helicity integrals. This paper reviews how the calculation of helicity may be applied to prominence models. Recent observations indicate that the sign of helicity in an active region depends on which hemisphere the region is in. The source of this asymmetry is an important problem in solar physics. The total helicity of each hemisphere obeys a Poynting-like theorem which describes how helicity is transferred across the photosphere and the equator. Estimating this helicity transfer may help us in understanding the helicity balance of the sun.

There is a type of filament that forms closed contours encircling regions of one polarity of magnetic field, while the surrounding region has the opposite polarity. One distribution of ring-filaments has sizes 2R = 40,000 to 160,000 km with the maximum around 100,000 km; the other distribution (with filaments and filament channels) has bigger sizes, with a maximum around 300,000 km. At low and mid latitudes the radial component Brs (radial for the Sun) in the region outlined by filaments varies from 50 to 100 gauss, while the longitudinal component Bϕ , varies from 10 to 30 gauss. The total current inside the filament is 1010 – 1011 A, the ratio R/a ≈ 5 to 10, the magnetic flux crossing the surface is ≈ 1021 – 1022Mx; the magnetic energy is ≈ 1029 – 1031erg. Polar ring-filaments at latitudes 60° – 80° are related to the polar magnetic field reversal and the quasi-flare processes at the poles during the field reversal. Correlation between Brs and Bϕ , for the polar filament bands cannot be satisfactorily explained. A theoretical model using the conservation laws is used to study the relations between the various fields, currents, etc. of shrinking polar ring-filaments and their evolution. According to this theory ring-filaments should rise higher above the photosphere when shrinking. However, observations show that the height lowers. Presumably the discrepancy is due to the lack of dissipation in the model.

In a two-dimensional quadrupolar magnetic field geometry, it is demonstrated that various magnetohydrostatic equilibrium field configurations containing current sheets can be formed either by changes in thermal properties or by footpoint displacement. The shape of current sheets depends on the mass distribution in flux tubes. If the mass per flux tube is maximum at the center of the low-lying bipolar arcades, a current sheet is formed between these two arcades along the separatrix line as the plasma temperature (or the shearing footpoint displacement) is increased above a critical value. If the mass per flux tube is higher in the outer tubes of the bipolar arcades than in the inner ones, a current sheet with a sharp downward-pointed tip hanging at a distance above the bottom boundary is formed when the temperature is decreased below a critical value. Resistive magnetic reconnection in the latter type current sheet results in a magnetic island wrapped in dipped field lines, which is regarded as a plausible prominence field configuration.

Measurements of plasma electric fields offer, in principle, a direct test for the presence of neutral sheets in prominences. Macroscopic electric field intensities of order 1–10 V cm−1 are an essential element of MHD models of prominences containing neutral sheets. These fields should be detectable with our electrograph techniques using the high Paschen-series lines in the NIR, and the 15–9 and 16–9 transitions of H I around 10.5 μ. We discuss the upper limits of 1 V cm−1 we have achieved so far, and their implication for our ability to distinguish prominence models of the ideal-MHD type (e.g., Kippenhahn-Schluter 1957), from those fundamentally different models (e.g., Kuperus and Tandberg-Hanssen 1967, Martens and Kuin 1989) in which neutral sheets play a central role.

Induced electric fields and nonstationary drift motions are considered. Nonstationary electric drifts may be important in erupting prominences, rising and shrinking coronal loops, and other phenomena in the solar atmosphere. Theoretical limitations are indicated for the concept of dominant plasma motions along magnetic field lines.

We consider large coronal streamers as confined thin plasma sheets overlying the filament and/or the magnetic polarity inversion lines. Confirmation is found in recent synoptic LASCO/SOHO data and simulations. We discuss the 3D axi-symmetric consistent MHD solution which provides the necessary properties of confinement and equilibrium by including gravity forces in the Grad-Shafranov equation.

By studying line-of-sight velocity time variations in prominences, we found that velocity oscillations with a period over 40 min have a reasonably well-marked dependence of the period length with heliolatitude. Simultaneous observations of line-of-sight velocities in the photosphere and chromosphere showed that quasi-hourly oscillation periods at these levels in the solar atmosphere and in prominences have a similar latitudinal behavior.

We present preliminary results on solar filaments observed from the ground and from SOHO in September 1996. Motions in the network and close to the filament “barbs” are shown at different levels of the chromosphere and the transition region.

We present the results of a high spatial resolution investigation of Doppler oscillations in a solar filament, using the He I 10830Å infrared line. Fourier power spectra of Doppler shifts reveal the presence of periodic signals. Two features, showing oscillations at 2.7 min and 12.5 min, have been studied. The use of the so-called wavelet analysis enables us to estimate the size of both features at 2.7 arc sec and 4.75 arc sec, respectively. Their approximate lifetimes are 10 min and 20 min.

Recent theoretical investigations of prominence oscillations indicate that the prominence-corona transition region (PCTR) may have a great influence on the magnetohydrodynamic modes of the whole system. We here consider an isothermal prominence embedded in an isothermal corona with a thin PCTR providing a smooth temperature transition between prominence and corona. The effect of the PCTR on the oscillations of the prominence body are investigated.

We model vertical prominence dynamics, describing the evolution of the magnetic field in a self-consistent way. Since the photosphere imposes a boundary condition on the field (flux conservation), the Alfvén crossing time τ 0/2 between prominence and photosphere has to be taken into account. Using an electrodynamical description of the prominence we are able to compare two basic prominence models: Normal Polarity (NP) and Inverse Polarity (IP).

The results indicate that for IP prominences, the stability properties are sensitive to ωτ 0 (ω: oscillation frequency of prominence). For ωτ 0 ≳ 1 instability results. Forced oscillations of five minutes are efficiently excited in IP prominences that meet certain criteria only. NP prominences on the other hand, are insensitive to the Alfvén crossing time. Forced oscillations of five minutes are difficult to excite in NP prominences.

Korteweg-de Vries equations for slow body and torsional weakly nonlinear Alfvén waves in twisted magnetic flux tubes are derived. Slow body solitons appear as a narrowing of the tube in a low β plasma and widening of the tube, when β ≫ 1. Alfvén torsional solitons appear as a widening (β > 1) and narrowing (β < 1) of the tube, where there is a local increase of tube twisting. Two scenarios of nonlinear dissipation of strongly nonlinear waves in twisted flux tubes are proposed.

The advanced thin flux tube approximation for force-free thin magnetic flux tubes is used to derive a dispersion relation for linear waves. All wave modes appear to be coupled in a twisted flux tube. In the case of a weakly twisted flux tube, it has been found that torsional Alfvén waves have dispersion and produce pressure and temperature fluctuations. The effect of tube rotation is pointed out. These properties of linear waves have an impact on prominence oscillations.

An arch-shaped prominence was observed with SUMERSOHO spectrometer on March 31–April 1, 1996. The observations were performed at three wavelengths in the Lyman continuum. Ten successive images were obtained at 41 minute time intervals. Based on the computed models of Gouttebroze-Heinzel-Vial we have determined the temperature distribution of the prominence using the intensity ratio of 876 Å and 907 Å. The observed time sequence shows that the prominence disappears by heating, i.e., ionization of hydrogen: a thermal DB. We model the heat input with the linearized MHD equations using a prescribed initial density and a broad-band spectrum of Alfvén waves and find a good qualitative agreement with observations. In the model the prominence is heated by the resonant absorption of Alfvén waves with frequencies that match the resonant condition for a particular flux tube structure that is determined by the magnetic field topology and density.

We describe in this paper some of the findings of the Yohkoh satellite about the coronal structure surrounding dark filaments in the pre-event and initial phases of high latitude arcade formation events. The knowledge of pre-event structure and its change is essential for the proper understanding of the arcade flaring process from the causality point of view. The wide dynamic range and high sensitivity obervations by Yohkoh allow us to look into the faint structures and their changes with the use of a faint-feature-enhancing technique in the image analysis.

We have studied coronal soft X-ray (SXR) arcades around filament channels, as observed with the Yohkoh Soft X-ray Telescope (SXT). We have found about a dozen events (1991–1993). Comparisons with Hα filaments have revealed an association between some SXR structures and filaments. These bright thread-like SXR structures (often) appear to be parallel to and co-spatial with Hα filaments. The SXR structures are occasionally maintained for days, although transient brightening of axial structures is more common.

This paper reviews the diagnostic techniques currently used to establish estimates of the physical properties of prominences. Most often the fine structure of prominences cannot be resolved. Because of this and the complex structures and the varied forms of prominences, it is difficult to establish definitive values for temperature, density, magnetic field, electric field, differential emission measure, mass flows, etc. Nevertheless, there are many useful techniques ranging from spectroscopic analysis to measurements of prominence oscillations. Most of the major techniques are reviewed in this paper, with examples of the results and an extensive bibliography. Special attention is paid to the potential diagnostic value of optically thick lines. Suggestions are made for further progress based on SOHO and other observations.

We briefly summarize the most important aspects of NLTE formation of hydrogen Lyman lines in prominences and filaments. A modified MALI code of Heinzel (1995) is described and its applicability to the interpretation of SOHO/SUMER data is discussed.

The Sacramento Peak Observatory VTT Echelle Spectrograph was used to obtain high resolution spectrograms of the filament at W06° S27° observed on April 9, 1991 in hydrogen Hα, Hβ, and CaII H and λ 8498 lines. The line profiles were measured and digitized. A numerical code based on the MALI approach with partial frequency redistribution (PRD) in resonance lines was applied to obtain theoretical profiles of hydrogen and CaII lines. As a result of fitting the observed and calculated profiles, the physical parameters of the filament plasma were obtained. They include temperature, gas pressure, electron density and turbulent velocity.