Volume 15, Issue 4
December 1994, pages 355-443
pp 355-355 December 1994
pp 357-371 December 1994
The fluctuations of light intensity in the diffraction corona produced by a large number of randomly distributed particles are investigated both theoretically and experimentally. The theoretical considerations show that the view put forward by de Haas is inadequate and that really the fluctuations arise owing to the interference of the waves diffracted byall the particles, which gives rise to a large number of sharp images of the source in monochromatic light. In white light, these spots must spread out, and produce radial streaks. These deductions from theory are all borne out by experiment.
pp 387-414 December 1994
We present Very Large Array observations at wavelengths of 2, 3.5, 6, and 20 cm, of angular broadening of radio sources due to the solar wind in the region 2–16 solar radii. Angular broadening is anisotropic with axial ratios in the range 2–16. Larger axial ratios are observed preferentially at smaller solar distances. Assuming that anisotropy is due to scattering blobs elongated along magnetic field lines, the distribution of position angles of the elliptically broadened images indicates that the field lines are non-radial even at the largest heliocentric distances observed here. At 5R⊙, the major axis scattering angle is ∼ 0.7" atλ= 6 cm and it varies with heliocentric distance asR-1.6. The level of turbulence, characterized by the wave structure function at a scale of 10 km along the major axis, normalized toλ = 20 cm, has a value 20 ± 7 at 5R⊙and varies with heliocentric distance asR-3. Comparison with earlier resu lts suggest that the level of turbulence is higher during solar maximum. Assuming a power-law spectrum of electron density fluctuations, the fitted spectral exponents have values in the range 2.8–3.4 for scale sizes between 2–35 km. The data suggests temporal fluctuations (of up to 10%) in the spectral exponent on a time scale of a few tens of minutes. The observed structure functions at different solar distances do not show any evidence for an inner scale; the upper limits are l k m at 2R⊙ and 4 km at 13R⊙. These upper limits are in conflict with earlier determinations and may suggest a reduced inner scale during solar maximum.
pp 415-443 December 1994
Lack of reliable estimates of distances to most of the local dark clouds has, so far, prevented a quantitative study of their kinematics. Using a statistical approach, we have been able to extract the average spatial distribution as well as the kinematical behaviour of the local dark clouds from their measured radial velocities. For this purpose, we have obtained radial velocities for 115 southern clouds and used the data from the literature for the northern ones. In this paper we present this new data, analyse the combined data and compare our results with those arrived at by earlier studies.
The local clouds are found to be expanding at a speed of ∼ 4 kms-1 which is in general agreement with the estimates from optical and HI studies. However, it is found that the kinematics of the local clouds is not described by the model proposed for the local HI gas where a ring of gas expanding from a point gets sheared by the galactic rotation. Rather, the observed distribution of their radial velocities is best understood in terms of a model in which the local clouds are participating in circular rotation appropriate to their present positions with a small expansion also superimposed. This possibly implies that cloud-cloud collisions are important. The spatial distribution of clouds derived using such a model is in good agreement with the local dust distribution obtained from measurements of reddening and extinction towards nearby stars. In particular, a region of size ∼ 350 pc in diameter enclosing the Sun is found to be devoid of clouds. Intriguingly, most clouds in the longitude range 100‡ to 145‡ appear to have negative radial velocities implying that they are approaching us.
Volume 40 | Issue 5
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