UBC Theses and Dissertations
Imaging the sun with the Drao synthesis telescope Burke, Ian E.
The Synthesis Telescope at the Dominion Radio Astrophysical Observatory in Penticton, British Columbia, has been modified to make full-disk continuum maps of the Sun at 1420 MHz. The hardware and software systems of the telescope were augmented for this purpose. The standard observing procedure was altered, and image-processing algorithms were written to produce solar images. To reduce the solar signals to an acceptable level for the telescope electronics, 20dB attenuators were introduced in the 1420 MHz receivers. The tracking and fringe-controller software was modified to allow for the motion of the Sun in right ascension. Algorithms were created to remove the effects of declination motion and rotation of the Sun during the observation; these corrections are combined in a user-friendly software package to be run at the data-processing stage. The large field of view (2.6°) and arcminute resolution of the DRAO Synthesis Telescope make it is suitable for producing full-disk solar maps for the purpose of studying the different sources of the slowly-varying (S-) component; the large field allows the entire solar disk to be mapped with high sensitivity. Because of solar variability and rotation, each map had to be made in one day, using one configuration of the antennas. To maximize this sparse u, v coverage, solar "observing" simulations were used to determine the best configurations of the antennas for solar synthesis; these antenna configurations were used in the summer observations of 1992 and 1993. With each 12-hour solar observation, a calibration source, such as 3C48, was observed for half-an-hour. The resulting images, in general, represent both the extended structure (quiet Sun) and the more localized emission (active regions) well. Comparisons between integrated map fluxes, and cotemporal single-antenna flux measurements at 1420 MHz (21 cm-A), indicate that most of the solar brightness distribution was being recovered satisfactorily. The degree to which the solar disk was recovered appears somewhat sensitive to the level and distribution of solar activity. The images have resolutions comparable with other DRAO synthesis maps, and have a dynamic range of about 100:1. A widely-distributed background component, covering large areas of the Sun in anactive latitude belt centered about the equator, is also evident in the solar images; its contribution to the S-component is comparable with that by active regions. Regions of high radio emission in the solar images have a strong spatial correlation with en-hanced magnetic activity in magnetograms; in addition, a linear correlation was found between the total disk magnetic flux and the total radio emission at 21 cm. These results suggest that the majority of the S-component at 21 cm is due to thermal free-free (bremsstrahlung) emission coming from density enhancements in the corona, which are supported by magnetic fields.
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