On the coronal source regions of U bursts

Abstract

The projected source positions at 43, 80, and 160 MHz and the sense and degree of circular polarization in the range 24 to 220 MHz, as observed with the Culgoora radioheliograph and spectropolarimeter respectively, are used:(1) To substantiate the hypothesis that metric U bursts originate in high coronal, magnetic loops.(2) To strengthen the hypothesis that U-burst radiation is in the ordinary magneto-ionic mode. The occasional Observation of different senses of circular polarization on either side of the turning point of a U burst suggests that U-burst radiation in these cases reaches its limiting polarization at or near the source. This observation raises the same difficulties as those discussed by Melrose (1973) in connection with the bi-polar nature of type-I storm sources.Solar Physics 57 (1978)415-422. All Rights Reserved

The Polarization of Metre-Wave Solar Emission

“…The inverted-U variety of the Type III burst is believed to be caused by electrons travelling along a closed magnetic loop. This belief is supported by radioheliograph observations showing displaced positions between the forward (ascending) and reverse (descending) arms (Labrum and Stewart 1970;Sheridan et al 1973;Suzuki 1978). Then the polarization of sources in the forward and reverse arms of the U-burst should be similar to that of the bipolar source in the Type I casei.e.…”

“…In low-frequency bursts the degree of polarization of the reverse arm is usually too low to allow a reliable measurement. However, when measurements are made, mainly at or below 80 MHz, the observed polarities of both arms are always opposite (Sheridan et al 1973;Suzuki 1978;Suzuki and Sheridan 1977) (so far only one exception has been seen -in a rather special event at the limb).…”

Relative Positions of Type I and Type III Solar Metre-Wave Bursts

“…2-3 of this paper). The Type I and Type III emissions have the same sense of polarization (Komesaroff 1958;Suzuki 1978). Impressed by this spectral association, Malville (1962) and Boischot et al (1970) suggested that the two types of emission might be excited by the same electron stream.…”

Observation of a Metric Type N Solar Radio Burst

“…These fast drifting bursts are usually recorded at decimetric and metric wavelengths, providing an important diagnostic tool for both accelerated electrons and the background media they travel through (e.g., Dulk & Suzuki 1980;Aschwanden et al 1992;Aurass & Klein 1997;Ning et al 2000;Wang et al 2001;Fernandes et al 2012;Chen et al 2013;Dorovskyy et al 2015). Early studies on type U bursts have shown that the source regions of their two branches are usually spatially separated and associated with different parts of coronal loops or opposite field polarities (e.g., Suzuki 1978;Aurass & Klein 1997), while their polarization shows contradictory results. For instance, at low frequencies (<200 MHz) the two branches present opposite sense of circular polarization (e.g., Sheridan et al 1973;Suzuki 1978), while at high frequencies (e.g., 327 MHz) they show the same sense of polarization indicating a possible polarization reversal (e.g., Benz et al 1977Benz et al , 1979.…”