It is now well known that transient events in the solar atmosphere like flares, prominence eruptions, CMEs, etc. lead to expulsion of material from the corona. The possible manifestations of the resultant voids are coronal depletions (Hansen et al. 1974; Rust & Hildner 1976), transient coronal holes (Rust 1983; Watanabe et al. 1992; Manoharan et al. 1996; Sterling & Hudson 1997; Hudson et al. 1998) and coronal dimming (Hudson et al. 1996; Gopalswamy & Hanaoka 1998; Zarro et al. 1999). These regions are shortlived and their lifetime varies from less than a day to more than 3 days (Sterling & Hudson 1997). The transient coronal hole reported by Kozuka et al. (1995) had a lifetime of only 17 hrs. Observations of these regions are of interest since transient increases in the solar wind speed in the aftermath of some of the CME events are considered to be closely linked to them (Rust 1983). In this respect, low frequency radio observations play an important role since they provide information on the density and temperature structure in the outer solar corona. The observations reported were carried out at 34.5 MHz with the E-W arm of the large Decameter Wave Radio Telescope (GBDRT) and four small groups of antennas at the Gauribidanur radio observatory (Lat: N and Long: E) near Bangalore in India (Sastry 1995). The entire set-up was operated as a one dimensional grating interferometer along the E-W direction with a total baseline length of 5.6 kms. By correlating the output of each one of the small groups of antennas with the E-W arm of the GBDRT, a fan beam of angular resolution in hour angle was synthesized. This is the highest resolution with which observations were ever made on the Sun at this low frequency so far. In this paper, we report the observations of a depletion in the outer solar corona with this instrument in the aftermath of the CME event of 1986 June 5.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000