4. Discussions and conclusions
Using low frequency radio observations we have independently estimated the mass loss associated with a depletion that occurred in the outer solar corona in the aftermath of a CME and found that it agrees well with the value estimated through white light observations of the event. The angular size of the depletion region was . The mass loss associated with the depletion was less compared to the mass of the material ejected during the main CME event by two orders of magnitude. It is to be noted here that the CME event of 1986 June 5 was not associated with any other form of activity on the solar atmosphere like prominence eruption, flare and non-thermal radio bursts. The mass estimates of the dimming events reported earlier (Sterling & Hudson 1997; Gopalswamy & Hanaoka 1998) were accompanied by a prominence eruption and a flare (X-ray + white light) respectively. The latter event was also accompanied by Type II and IV non-thermal radio bursts (Hudson et al. 1998).
According to Zarro et al. (1999), the dimming in the extreme ultraviolet (EUV) pictures taken with the instrument EIT onboard SOHO in the aftermath of the halo CME of 1997 April 7 showed a factor of 4 decrease in average intensity with respect to the ambient. Compared to this, the estimated of the depletion region in the present case was less than the `quiet' corona by a factor of . However the above ratio should be regarded as an upper limit, since it is possible that the actual size of the region might be less than the angular resolution of our instrument. This should give a higher value for the of the depletion region. In this connection we would also like to add that the sizes of the dimming regions seen in the Yohkoh soft X-ray images are only (Sterling & Hudson 1997). This corresponds to a volume of which is lesser than that of the depletion observed in the present case by about an order of magnitude.
Our experience with observations on the `quiet' Sun with the grating interferometer has shown that the angular sizes of the localised emissive regions on the `quiet' Sun are generally at these frequencies. The smaller spatial width of the region over which the temperature decrease has taken place in the present case has important bearing on the theories of scattering of radio radiation by density inhomogeneities in the `quiet' solar corona. It is well known that the smallest source size observed gives an upper limit to the scatter broadened image of a point source. The present observations show that the upper limit at 34.5 MHz is about . This is consistent with our earlier result that there are discrete sources of angular size in the outer solar corona from where the decameter wavelength radiation originates (Ramesh et al. 1999).
Doyle et al. (1999) recently reported that the electron density above a long lived polar coronal hole varies as in the height range . This is two orders of magnitude lesser than the present estimate which varies as . But the latter must be treated with caution since low frequency ( 100 MHz) radio radiation from the solar corona undergoes scattering (by density irregularities of sizes ), and refraction effects.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000