Engineering topics

Dulk, G. A.

Publications and source records attributed to Dulk, G. A..

99 records · Page 6

NASA NTRS1978

Coronal magnetic fields

The observational evidence on the strength of the coronal magnetic field above active regions is reviewed. Recent advances in observations and plasma theory are used to determine which data are the more reliable and to revise some earlier estimates of field strength. The results from the different techniques are found to be in general agreement, and the relation where magnetic field strength (in gauss) equals 0.5 times the -1.5 power of distance (in units of solar radius) minus 1 for distances between 1.02 and 10 solar radii is consistent with all the data to within a factor of about 3.

Dulk, G. A.↗

NASA NTRS1978

Radio evidence on the particle distribution functions in the corona following flares

This paper summarizes results of radio studies of the distribution functions (number, energy distribution, and pitch-angle distribution) of the energetic particles that are produced at the time of solar flares. The clues and constraints they impose on the mechanisms of acceleration are considered, bearing in mind that radio evidence implies that there are at least two stages of acceleration in many flares. The distribution functions of the particles resulting from first-phase acceleration, first- and/or second-phase, and second-phase acceleration are discussed.

Dulk, G. A.↗

NASA NTRS1977

Radio and EUV observations of a coronal hole

Current ideas and modeling assumptions concerning the structure of the transition region and corona are summarized. Observations of a specific coronal hole are reported which were made with the EUV spectroheliometer aboard Skylab and with high-resolution radio telescopes in Australia and West Germany. Theoretical EUV line intensities and radio brightness temperatures are calculated, and an attempt is made to analyze the observations on the basis of standard one-dimensional plane-parallel hydrostatic-equilibrium models for the transition region and corona. It is found that any given standard model predicts either higher radio brightnesses or lower EUV line intensities than are observed. Several potential problem areas are examined in order to resolve this conflict. It is concluded that a model involving departures from hydrostatic equilibrium, varying conductive flux, mass outflow, as well as effects of thermal diffusion and nonequilibrium ionization should be investigated in more detail.

Dulk, G. A.↗

NASA NTRS1976

White light and radio studies of the coronal transient of 14-15 September 1973. I - Material motions and magnetic field

Observations of a coronal transient event were obtained in white light by the Skylab coronagraph and at metric wavelengths by the radioheliograph and spectrograph at Culgoora, Australia, and the spectrograph-interferometer at Boulder, Colo. The continuum radio burst was found to originate above the outward-moving white-light loop, a region of compressed material headed by a bow wave. The computed density in the region of radio emission, based on either gyrosynchrotron or harmonic plasma radiation mechanisms, was approximately 10 times the ambient coronal density; this is compatible with the density deduced from the white-light observations. The magnetic-energy density derived from the radio observations was greater than 10 times the thermal energy density, marginally larger than the kinetic energy density in the fastest-moving portion of the transient, and considerably larger in most other regions. The ambient medium, the white-light front, the compression region, the loop, and the slower massive flow of material behind are each examined. It is found that the plasma was magnetically controlled throughout and that magnetic forces provided the principal mechanism for acceleration of the transient material from the sun.

Dulk, G. A.↗

NASA NTRS1974

Observations of coronal disturbances from 1 to 9 solar radii. I - First event of 1973 January 11

H alpha, white-light and radio observations of a coronal disturbance on Jan. 11, 1973, commencing at about 0036 hr UT show that a piston-driven shock wave propagated outward through the corona to heights of at least 9 solar radii. Probably most of the expelled coronal gas originated in a coronal enhancement in the lower corona. An estimate of the kinetic energy and the mass of the expelled gas is obtained which is compatible with observations of piston-driven shock waves near the earth. Shock-wave parameters are evaluated, and a model of the disturbance is outlined.

Stewart, R. T.↗

NASA NTRS1970