No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
The helical prominence of March 17, 1977
Abstract
The dynamics of a prominence with helical structure is studied. The slow
rising motion accompanied by detwisting indicates that mass loss
probably caused the prominence rising and decrease of the azimuthal
component of the magnetic field. The radial dependence of current
density within the prominence cylinder is discussed.
The kinematics and the development of the internal structure in the
eruptive prominence of August 16, 1988 are described. The prominence
exposed helical structure, and the pitch of the fine structure filaments
was measured. The evolution of the pitch was measured in the legs of the
prominence and at its summit, from the preeruptive phase up to the late
phases of the eruption. The pitch angle was decreasing in the legs as
well as at the summit. However, the observations indicate that the
integral twist remained constant. The prominence was twisted more at the
summit, where it was wider than in the legs. The 'effective' twist at
the prominence summit was approximately 20pi and in the legs it amounted
to about 8pi.
The stability of prominences and the dynamics of an eruption are studied. The prominence is represented by an uniformly twisted, curved, magnetic tube, anchored at both ends in the photosphere. Several stages of the eruption are analyzed, from the pre-eruptive phase and the onset of the instability, up to the late phases of the process. Before the eruption, the prominence evolves through a series of equilibrium states, slowly ascending either due to an increase of the electric current or to mass loss. The eruption starts when the ratio of the current to the total mass attains a critical value after which no neighbouring equilibrium exists. The linearized equation of motion was used to obtain the instability threshold, which is presented in a form enabling comparison with the observations. The height at which the prominence erupts depends on the twist, and is typically comparable with the footpoint half-separation. Low-lying prominences are stable even for large twists. The importance of the external field reconnection below the filament, and the mass loss through the legs in the early phases of the eruption is stressed. The oscillations of stable prominences with periods on the Alfvn time-scale are discussed. The results are compared with the observations.
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 2π and 7π. 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.
The prominence which erupted at the SE limb on August 18, 1980 is one of the best observed disparition brusque events: high-resolution monochromatic ground-based observations in the Hα line were supplemented by the SMM and Solwind satellite coronographic observations; the radio wavelength range was well covered by single-frequency and spectral observations, and the prominence magnetic fields were measured two days before the eruption.
The prominence showed a helical-like internal structure from the pre-eruptive phase, up to the late phases of eruption. The pitch angles of the helical-like threads were measured at several positions 31 along the prominence axis, and the evolution of twist was followed during the eruption. These measurements provide an estimate of the parameters which are directly comparable with theoretical models. The pitch angles of the helical threads decreased during the eruption. A redistribution of twist along the prominence axis could not be detected within the accuracy of measurements, although there are indications that the twist was partly transformed into an external kink-type screw of the prominence axis. The value of the total twist did not change during the eruption within the accuracy of the measurements.
The kinematics of the process was followed, and accompanying events in the radio-range and soft X-rays are listed. Measurements of the magnetic field vector in the prominence are reviewed briefly. The observations were compared with predictions of cylindrical models, considering the forces acting at the prominence summit. Observational implications and constraints are discussed, and the decrease of the axial electric current and the mass loss are inferred.
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 d 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="">
ResearchGate has not been able to resolve any references for this publication.