3. Radio observations as tracers of coronal electron acceleration
3.1. Spectrographic observations
During the 1996 July 9 event enhanced levels of radio emission were observed from 15 GHz in the low corona to some tens of kHz at 1AU. Different aspects of the radio emission during the first minutes of the event were discussed by Mann et al. (1997), Dryer et al. (1998), Karlický (1998), Pick et al. (1998) and Klassen et al. (1999). Fig. 6 shows whole Sun observations between the event onset and 10:30 UT, covering the time interval of the identified electron injections and of most of the p-component proton injection. The times of the impulsive injections of mildly relativistic electrons and the evolution of the p-component injection of protons in the 12-20 MeV energy channel are plotted at the top and bottom of the figure.
The GOES soft X-ray time profile (0.1-0.8 nm; Fig. 6b) shows a rapid rise from a perturbed pre-event background to maximum followed by a decay in three steps with successively longer decay times: 9:11:30 UT (peak time) - 9:17 UT, 9:17 - 9:40 UT, 9:40 UT until about 11:30 UT. The narrow major peak suggests that the event belongs to the class of impulsive soft X-ray flares. However, the complex behavior during the decay of the event probably reveals different episodes of energy release on increasing temporal and spatial scales.
We are going to describe separately the three spectral ranges of radio emission in the following.
3.1.1. Microwaves (8 GHz, 3 GHz)
Microwave radiation (Fig. 6c) is emitted by mildly relativistic electrons (typically 100 keV to a few hundreds of keV) interacting in the low corona (below above the photosphere). The time profiles in Fig. 6c show a strong impulsive peak similar to the hard X-rays and -rays (cf. Fig. 1). The impulsive microwave peak has a spectral maximum near 9 GHz (SGD 1996). It is followed by a series of weaker enhancements, which are less pronounced at 8 GHz than at 3 GHz and extend well into the meter wave range.
3.1.2. Decimetric to decametric emission (2000-40 MHz)
The emissions in this range (Fig. 6d,e) are produced by suprathermal electrons (poorly known energy range, presumably a few tens to a few hundreds of keV) at coronal heights between roughly a few km and (middle corona; the typical source height increases with decreasing frequency). The plotted spectra are obtained after the elimination of frequency channels disturbed by terrestrial transmitters and subtraction of a pre-event spectrum. The initial bright feature between 600 and 100 MHz (9:11-9:13 UT) is a type II burst, due to electrons accelerated at a shock wave. A faint decametric type III group appears at its low frequency end (90-40 MHz, 9:11:30-9:12:30). A series of short broadband enhancements, the counterparts of the late microwave enhancements (Fig. 6c), are seen between 9:13:20 and 9:39 UT. Initially, they appear in the decimetric waves (2000-300 MHz), but the later emissions extend across the entire decimeter-to-decameter wave range. They are associated with a diffuse continuum emission, which is best visible in the spectrogram below 200 MHz, but which also extends to higher frequencies. The low frequency limit of the continuum emission drifts gradually to lower frequencies. The continuum fades a few tens of minutes after the last broadband enhancement. It is no longer seen on the spectrographic records after 11 UT.
The short broadband enhancements display little spectral fine structure. Detailed inspection of the brightening near 9:25 UT in the data of the Tremsdorf Spectrograph and the Nançay Radio Heliograph shows a smoothly evolving emission with only few superposed bursts, the most prominent one being a type III burst between 500 and 600 MHz near 9:25:10 UT. A few faint decametric type III bursts are observed before and afterwards. In the decimeter range the peak brightness temperatures measured with the NRH during the enhancement near 9:25 UT rise from K at 237 MHz to K at 432 MHz, which is consistent with the increasing flux density from low to high frequencies displayed by the dynamic spectrum in this range (Fig. 6e). The spectral shape and the brightness temperatures are consistent with optically thick gyrosynchrotron radiation at decimetric waves, whereas the 3 and 8 GHz fluxes point to optically thin gyrosynchrotron emission. Hence, we suggest that the broadband brightenings are emitted by mildly relativistic electrons in the corona. These enhancements occur during the second part of the soft X-ray decay, which has a longer time constant than the preceding decrease. The last broadband enhancement at about 9:35 UT occurs near the instant when the decaying soft X-ray profile flattens for the second time. It has no counterpart at 8 GHz, suggesting that the electrons radiate in weaker magnetic fields than precedingly.
3.1.3. Hectometer emission (14 MHz)
Radio emission in the range 14-1 MHz (Fig. 6f) is typically emitted between 1 and 20 above the photosphere. The most prominent features are two groups of type III bursts, at 9:11 and 9:24 UT, generated by electron beams traveling from the corona into interplanetary space (energies of a few keV to a few tens of keV). Both type III groups have several components, as seen near their high frequency end and both groups are observed below 1 MHz down to the plasma frequency at the satellite level. The first group, starting at 9:10:57 UT, which is well defined in the entire frequency range below 14 MHz, starts well after the impulsive microwave burst at 9:09:05 UT and therefore appears not to be produced by electrons accelerated in the low corona. Rather, this type III group occurs simultaneously with the decimetric-metric type II burst associated with the shock wave in the middle corona, and with the decametric type III bursts at 9:12 UT. However, this first type III group does have a very weak secondary component at 9:14 UT that becomes visible at frequencies below 5 MHz and which occurs 1 minute after the end time of the metric type II. This weak component also occurs 1 minute after the first weak microwave enhancement and could be related to it.
The second hectometric type III burst has only faint, but nonetheless significant, traces between 14 and 2 MHz. It occurs at about the same time as the broadband cm-m-wave enhancement around 9:25. The frequency drift seems to be faster than during the first type III burst, but it is hard to determine accurately due to the intermittent spectrum at high frequencies and the superposition of both type III bursts at and below 1 MHz.
Between 9:30 and 10 UT a highly intermittent and weak signature of a type II burst is seen between 14 and 6 MHz.
3.2. Source configuration at decimeter and meter waves
Imaging observations were carried out at five wavelengths in the decimetric and metric range by the Nançay Radio Heliograph (henceforth NRH; Kerdraon & Delouis 1997). In the following the evolution of the radio sources is briefly presented in order to demonstrate that magnetic restructuring and electron acceleration continue in and around the active region during several hours after flare onset.
Fig. 7 shows snapshot maps at several frequencies. A noise storm existed above the active region before flare onset (first row). It reveals long lasting electron acceleration which is unrelated to the flare under discussion. The subsequent type II and continuum emission during the first hour of the event have been discussed in detail by Pick et al. (1998). The second row of Fig. 7 (9:14:30 UT) displays sources at 410 and 237 MHz during one of the broadband enhancements.
After 9:40 UT the source configuration evolves in a similar way at all frequencies observed by the NRH. The emission fades progressively at successively lower frequencies, but structural changes continue during several hours at the lowest observed frequencies, suggesting that the main sites of electron acceleration rise gradually from the low to the middle corona:
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000