RADIO SYNTHESIS IMAGING OF ANISOTROPIC ANGULAR BROADENING IN THE SOLAR-WIND

Citation
Kr. Anantharamaiah et Tj. Cornwell, RADIO SYNTHESIS IMAGING OF ANISOTROPIC ANGULAR BROADENING IN THE SOLAR-WIND, Journal of astrophysics and astronomy, 15(4), 1994, pp. 387-414
Citations number
25
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Astronomy & Astrophysics
ISSN journal
0250-6335
Volume
15
Issue
4
Year of publication
1994
Pages
387 - 414
Database
ISI
SICI code
0250-6335(1994)15:4<387:RSIOAA>2.0.ZU;2-G
Abstract
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 win d in the region 2-16 solar radii. Angular broadening is anisotropic wi th axial ratios in the range 2-16. Larger axial ratios are observed pr eferentially at smaller solar distances. Assuming that anisotropy is d ue to scattering blobs elongated along magnetic field lines, the distr ibution of position angles of the elliptically broadened images indica tes that the field lines are non-radial even at the largest heliocentr ic distances observed here. At 5 R., the major axis scattering angle i s similar to 0.7'' at lambda = 6 cm and it varies with heliocentric di stance as R(-1.6). The level of turbulence, characterized by the wave structure function at a scale of 10 km along the major axis, normalize d to lambda = 20 cm, has a value 20 +/- 7 at 5 R and varies with helio centric distance as R(-3). Comparison with earlier results suggest tha t the level of turbulence is higher during solar maximum. Assuming a p ower-law spectrum of electron density fluctuations, the fitted spectra l 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 observ ed structure functions at different solar distances do not show any ev idence for an inner scale; the upper limits are 1 km at 2 R. and 4 km at 13 R.. These upper limits are in conflict with earlier determinatio ns and may suggest a reduced inner scale during solar maximum.