Magnetic fields in flares and active prominences

The locations of flares and chromospheric absorption features on May 21 and 23, 1967, are compared with a series of H magnetograms. Each of the four major flares included in the study developed as double emission ribbons lying at positions of steep field gradient on opposite sides of the boundary between regions of opposite magnetic polarity. At certain stages, the flare outlines followed closely the isogauss contours of the longitudinal field. A fluctuating field of 75 gauss was measured directly in the importance two flare of May 21. Modifications in the magnetic structure of the active region followed the flares of May 23.     

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.     

Photoelectric measurements of the green coronal line during the eclipse of November 12, 1966

A line to continuum ratio of the green line equal to 2.1 Å was measured above the West limb at the height p = 1.75 solar radii during the eclipse. The lower limit to the abundance of iron relative to hydrogen obtained from these measurements is 4 × 10^–5.     

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.     

The growth of electric current in the eruptive prominence of June 8, 1974

The observed twisting of bright threads during the eruption of a limb prominence is described in terms of the transfer of energy from the large-scale to the small-scale magnetic field. Modeling the process by means of an inductive circuit, the time constant for the growth of current along the bundle of threads appears to be consistent with the observed rate of twisting.Visiting astronomer on leave from the Department of Astro-Geophysics, University of Colorado, Boulder, Colorado 80309, U.S.A.     

Impulsive brightenings and velocity transients in prominences

Observations of quiescent prominences with the Zeiss Universal Birefringent Filter at Sacramento Peak show short-lived brightenings and velocity transients in H and D₃. The larger events range in area from 25 to 170 square arc sec, have lifetimes of approximately 30 min, velocities of 30 km s^–1, and total energy excesses up to 7 × 10²⁷ ergs. These events do not disrupt the stable structure of the prominence, and are interpreted as either condensation events or low energy flares.Visiting astronomer, Sacramento Peak Observatory, operated by the Association of Universities for Research in Astronomy Inc. under contrast AST-78-17292 with the National Science Foundation.Visiting student, Sacramento Peak Observatory, operated by the Association of Universities for Research in Astronomy Inc. under contract AST-78-17292 with the National Science Foundation.     

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.