Type IV solar radio bursts exhibit a wealth of well-documented fine structures between 100 MHz and 1 GHz. The BAO radiospectrograph aims to carry out high time and frequency observations at higher frequencies where fine structures are still poorly known. In this paper, we have reported an observation by this instrument of a short type IV-DCIM burst lasting 3 minutes, between 3 and 4 GHz.
This event exhibits remarkable fine structures which do not seem to be related to zebra-patterns, but rather to the EEL reported by Chernov et al. (1998). We describe in detail three characteristic phases where the EEL is clearly visible as one, two or three simultaneous emission strips, whose frequencies fluctuate in parallel at the high frequency limit of the event.
EELs are the only example of a narrow, isolated and continuous emission line in solar radio bursts. Such a spectral line is potentially rich in information on the physical parameters inside the radiation source. It may deserve substantial theoretical attention. A major question to be solved in the future is whether an EEL is the signature of a current sheet. Another question is why is this kind of feature so rare; is it because emission lines are usually too wide to show up as such, or is it because the emission mechanism itself requires a specific and narrow range of local parameters? We will also have to eliminate the remote possibility that such fine structures are the result of propagation effects through the inhomogeneous corona. The final question is to judge the possible continuity among all the fine structures on that day, including the 3 EELs in Fig. 2.
The present observation constitutes the second detection of an EEL, and the first one at frequencies higher than 1 GHz. It gives us the incentive to look for more EEL examples, throughout a wide possible frequency range, between approximately 100 MHz and 10 GHz. We need to get a statistical idea of its properties: frequency of occurrence, distribution in frequency, association with emissions at other wavelengths, etc.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001