The compound grating interferometer at the Gauribidanur radio observatory was constructed to detect the existence of structures of small angular scale ( few arc minutes) in the outer solar atmosphere and angular broadening of radio sources at large distances from the Sun. During June 1986 in the course of our observations on the occultation of the radio source Tau A by the solar corona, we noticed a depletion in the outer corona on June 5. Fig. 1 shows the E-W one dimensional brightness distribution of the Sun obtained on this day at 06:30 UT. One can see that the distribution is markedly asymmetric due to the sudden decrease in the observed brightness immediately after the transit of Sun. This kind of distribution is what one would expect if there is a sharp fall in the electron density in the western hemisphere of the Sun. There was a complete restructuring of the corona in 24 hrs such that the asymmetry was not there in our scans obtained the next day. The E-W scans of the Sun obtained with the Nancay radioheliograph at 169 MHz on June 5 and 6 at 12:00 UT also showed a similar behaviour. An inspection of the 50 MHz radio heliogram obtained with the Clark lake radioheliograph (CLRH) on June 5 at 20:00 UT revealed the presence of a coronal hole close to the west limb near the equator (Gopalswamy, personal communication). According to Burkepile & St. Cyr (1993), a CME event was observed with the coronagraph/polarimeter on board the Solar Maximum Mission (SMM) satellite on 1986 June 5 from 01:25 UT to 04:41 UT, close to the west limb at position angle = . (The position angle [PA] is measured counter clockwise from solar north through east). The angular width of the event was , and the estimated speed was km/s. The coronal mass expelled was about gm. The encircled portion in Fig. 2 shows the ejected material. Fig. 3 shows the region of the corona where material depletion was observed in association with this event as compared to the pre-event time. The mass loss corresponding to this depletion was about gm (Stanger, personal communication). The Sun was `quiet' and no non-thermal radio bursts were reported. This particular event was also not accompanied by filament disappearance (Solar Geophysical Data [August 1986; December 1986]). Our observations were calibrated using the radio source 3C134 (Fig. 1). The angular width of the region across which the decrease in the solar brightness took place was estimated from the observed response on the above calibrator source, and the value is . Assuming circular symmetry, we calculated the peak brightness temperature () of the `quiet' Sun and the depletion region, and the values are K and K respectively. The latter is approximately same as that reported for a long lived coronal hole observed with the CLRH at 30.9 MHz by Wang et al. (1987). The estimated of the hole was in the range K.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000