Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt.........9s&link_type=abstract
Ph.D. Thesis Iowa Univ., Iowa City, IA.
Astronomy and Astrophysics
Astronomy
2
Magnetohydrodynamic Waves, Radio Astronomy, Solar Corona, Solar Magnetic Field, Solar Wind, Extragalactic Radio Sources, Faraday Effect, Time Series Analysis, Very Large Array (Vla), Very Long Base Interferometry
Scientific paper
The plasma characteristics of the solar wind and the solar corona were studied with radio remote sensing techniques using the Very Large Array and the U.S. Very Long Baseline Interferometer network. Ten extragalactic radio sources were observed with VLBI in September 1989 at 4.99 GHz when several sources were close to the Sun. Angular broadening was detected in five sources with elongations less than 61 solar radii. The broadening sizes were consistent with those estimated from the electron density power spectrum model of Coles and Harmon, which consists of a Kolmogorov spectrum at large scale sizes comparable to the VLBI baselines, but with an enhancement in power at small spatial scales near the ion inertial length. No measurable anisotropy in the scattering was detected. The phase structure function was also obtained from the same set of observations, and it also was shown to be consistent with the spectrum of Coles and Harmon. Faraday rotation through the solar corona was measured with the VLA using the polarization position angles of extragalactic radio sources in March 1990 at 1464 and 1636 MHz. The source closest to the Sun showed an excess rotation measure, which is consistent with a coronal magnetic field of -12.5 +/- 2.3 mG at an elongation of 9 solar radii. A time series consisting of hourly measurement of the rotation measure were filtered for the background plasma contribution to extract the component which could be attributed to coronal Alfven waves. The data were corrected for second order polarimeter errors to removed the instrumental errors as well as ionospheric Faraday rotation errors. From the corrected data, an upper limit to the amplitude of coronal Alfven waves was estimated to be less than or equal to approximately 4.2 mG at a solar distance of 10.5 solar radii. This gives an upper limit to the Alfven wave energy flux of less than or equal to approximately 3.4 x 104 ergs/cm2/s at the coronal base, which is lower than what is believed to be the necessary amount for heating and accelerating the solar wind. Our estimate of delta B/B is roughly 0.5-0.7, indicating that Alfven waves at this solar distance are highly nonlinear.
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