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At least 145 records · Page 8

Fine Structure in 3C 120 and 3C 84

Seven epochs of very long baseline radio interferometric observations of the Seyfert galaxies 3C 120 and 3C 84, at 3.8-cm wave length using stations at Westford, Massachusetts, Goldstone, California, Green Bank, West Virginia, and Onsala, Sweden, have been analyzed for source structure. An algorithm for reconstructing the brightness distribution of a spatially confined source from fringe amplitude and so called closure phase data has been developed and successfully applied to artificially generated test data and to data on the above mentioned sources. Over the two year time period of observation, 3C 120 was observed to consist of a double source showing apparent super relativistic expansion and separation velocities. The total flux changes comprising one outburst can be attributed to one of these components. 3C 84 showed much slower changes, evidently involving flux density changes in individual stationary components rather than relative motion.

Hutton, L. K.↗

Time-dependent radio fine structure of the quasar 3C 345

Results are reported for VLBI observations of 3C 345 at a wavelength of 3.8 cm with left circular polarization. Sixteen sets of correlated flux densities and three sets of closure phases were obtained from observations made over a three-year period with six independent interferometers formed from up to four antennas. The mathematical models employed to represent the radio brightness distribution of the source are described, and the data are analyzed on the basis of a two-component model. The results show that the compact source in 3C 345 consists of two components whose separation increased by about 0.3 milliarcsec over the observational period, that the smaller and weaker component is to the east and south of the larger and stronger one, and that the apparent expansion velocity would be about 2.5c for a redshift of 0.6, a Hubble constant of 60 km/sec per Mpc, and a deceleration parameter of unity. It is concluded that these findings are difficult to reconcile with the 'Christmas tree' model for the apparent change of separation with time over any given series of observations.

Wittels, J. J.↗

Observations of far-infrared fine structure lines: o III88.35 micrometer and oI 63.2 micrometer

Observations of the O III 88.35 micrometer line and the O I63.2 micrometer were made with a far infrared spectrometer. The sources M17, NGC 7538, and W51 were mapped in the O III line with 1 arc minute resolution and the emission is found to be quite widespread. In all cases the peak of the emission coincides with the maximum radio continuum. The far infrared continuum was mapped simultaneously and in M17, NGC 7538, and W51 the continuum peak is found to be distinct from the center of ionization. The O III line was also detected in W3, W49, and in a number of positions in the Orion nebula. Upper limits were obtained on NGS 7027, NGC 6572, DR21, G29.9-0.0 and M82. The 63.2 micrometer O I line was detected in M17, M42, and marginally in DR21. A partial map of M42 in this line shows that most of the emission observed arises from the Trapezium and from the bright optical bar to the southeast.

Storey, J. W. V.↗

On the fine structure of cometary plasma tails

It is argued that the small-scale structures observed close to the plasma tail axis of comets may be a consequence of well-known finite-resistivity instabilities in the cometary cross-tail current sheet. While the 'tearing' mode instability, which causes a breakup of the layer along the current flow lines, may be responsible for the observed small condensation or 'knots' near the axis, the 'ripple' mode instability may give rise to the small-scale wavy structures.

Morrison, P. J.↗

The fine structure of lightning return stroke wave forms

The paper describes and analyzes the structure of the radiation fields produced just prior to and during the development of the first and subsequent return strokes in lightning discharges to ground. Stroke data are summarized, and the waveforms of the fields are found to contain several features which are quite alike for different strokes in the same discharge and also for different discharges. The possibility of large currents in the upward connecting discharges, the existence of a large submicrosecond component in all return strokes, and the cause of the small second peak and of the large subsidiary peaks in first strokes are discussed. The similarity of subsequent stroke fields within a flash suggests that the currents and velocities of different subsequent strokes in the same discharge are often very nearly the same.

Weidman, C. D.↗

Rapid changes in the fine structure of a coronal 'bright point' and a small coronal 'active region'

A coronal bright point is resolved into a pattern of emission which, at any given time, consists of 2 or 3 miniature loops (each approximately 2500 km in diameter and approximately 2000 km long). During the half-day lifetime of the 'bright point' individual loops evolved on a time scale approximately 7 min. A small 'active region' seemed to evolve in this way, but the occasional blurring together of several loops made it difficult to follow individual changes.

Sheeley, N. R., Jr.↗

Time-dependent radio fine structure of the compact sources NRAO 150 and 4C 39.25

Very long baseline interferometer observations at 7.85 GHz have been used to probe the milliarcsecond structure of the unidentified, very compact radio source NRAO 150 and QSO 4 C 39.25. NRAO 150 exhibited no structural variations from 1972 to the end of 1974. A model with two circular Gaussian components fits the data well. NRAO 150 had a flux density of 7.6 plus or minus 0.5 Jy in the compact component; 4 C 39.25 showed a two-component structure, the components having a separation of (2.02 plus or minus 0.05 arc sec) x 10 to the -3rd power. The upper bound on the speed of transverse separation is 0.0001 arc sec per year or less than 2.7 c. From the spectrum there are also indications of a third, larger component.

Baath, L. B.↗

I-Xe dating of Allende inclusions - Antiquity and fine structure

Two inclusions from the Allende meteorite exhibit multiple I-Xe isochrons corresponding to an age difference of 3.7 + or - 1.5 m.y. for their constituent minerals. Consequently, I-Xe dating can establish, independently of thermodynamic models, a partial sequence of formation for minerals that incorporated radioactive I-1290. Additionally, the results give an estimate of the cooling rate for the solar nebula when validity of thermodynamic models is assumed.

Zaikowski, A.↗

The fine structure of the front side magnetopause during two successive crossings

The magnetopause crossings of November 10, 1977, which are considered to be representative of a class of structures, are investigated, and a procedure is developed to derive approximate surface currents near the magnetopause from ISEE 1 magnetic field data by assuming a constant speed of the magnetopause along its normal for each crossing of the magnetopause layer. Changes in the magnetic field indicate that current sheets exist on each side of the magnetopause layer, which is characterized by an irregular magnetic field and a magnetosheath-like energy distribution. No boundary layer plasma is observed on the magnetospheric field lines inside the magnetopause, and electromagnetic energy is found to be dissipated at the two edges of the magnetopause layer where the current layers are observed.

Formisano, V.↗

Rocket photographs of fine structure and wave patterns in the solar temperature minimum

A new series of high resolution pictures of the sun has been obtained during the second flight of the Transition Region Camera which occurred on September 23, 1980. The qualitative analysis of the results indicates that a substantial portion of the solar surface at the temperature minimum radiates in non-magnetic regions and from features below 1 arcsec in size. Wave patterns are observed on the 160 nm temperature minimum pictures. They are absent on the Lyman alpha pictures. Their physical characteristics are compatible with those of gravitational and acoustic waves generated by exploding granules.

Bonnet, R. M.↗

VLA observations of fine structures in a solar active region at 6 centimeter wavelength

A complex solar active region at 6 cm has been observed with the VLA. Radio images of the region have been constructed and compared with simultaneous optical data from the region. It is found that all radio sources are associated with either sunspots or regions of the corona where strong transverse magnetic fields are likely, that the transverse field sources are brighter than the sunspot sources, and that some of the sunspot sources have a polarized ring structure. The observations are interpreted in terms of thermal gyroresonance emission, with the radiation from the transverse field sources being at the third harmonic of the gyrofrequency and predominantly at the second harmonic from the sunspot sources.

Mcconnell, D.↗

Fine structure and dynamics of the Harang discontinuity - An event study

Two rockets were launched simultaneously from the Andoya Rocket Range in northern Norway into and through a Harang discontinuity region during a small isolated substorm. Each was equipped with an array of instruments for monitoring ionospheric electric fields and auroral particles. Different sized rockets allowed both to traverse a range of 300 km while introducing altitude separations of up to 440 km, and separations between the feet of the magnetic field lines intersecting the payloads of up to 100 km. The data sets, coupled with multi-station ground-based observations, provide information on the structure and dynamics of the Harang discontinuity region. Two boundaries were encountered. The first electric field reversal was observed simultaneously by both payloads in a region of weak precipitation, while the second reversal was associated with an intense auroral band. Since an unambiguous interpretation of these data is not possible, two alternative scenarios are presented. In one the discontinuity becomes realigned during the flight following a decay in the activity. The second involves a triple cell convection system, possibly the result of an eddy in the flow.

Egeland, A.↗