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Astron. Astrophys. 337, 887-896 (1998)

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6. Data analysis

To perform the diagnostics of the electrical parameters of the current-carrying magnetic loops we have analyzed flare-associated solar mm-wave bursts observed at the Metsähovi Radio Research Station during 1989-1993. We chose the events with long-term pulsations. The results for sixteen flare bursts measured at 22 GHz (13.5 mm) and 37 GHz (8.2 mm) are given in Table 1. As an example, Figs. 2 and 3 present the temporal behavior of a burst, together with the Pxx - X power spectral density, observed on September 1, 1990. The flare occurred in the NOAA active region 6233 at N13W34, had SN H[FORMULA] importance, and M2.7 X-ray classification. The fine time-structure of the event reveals the main pulsation period P [FORMULA] 1.3 s, which persists throughout the entire burst that lasts for about 30 min. From Table 1 one can see that the pulsation period P varies between 0.7 and 17 s, from event to event, in the bursts under consideration. In the third column of the table radiation fluxes at the burst maximum are given (estimated from spectral slopes if not observed at Metsähovi). Note that as a rule we have analyzed the pulse trains after the burst maximum, e.g., during the period indicated in the second column the mm-wave flux is usually small compared with the burst maximum. The last column in Table 1, with two flare events on 22 June 1989 (Stepanov et al. 1992), and on 7 May 1991 (Urpo et al. 1994), present estimates of the thermal energy of a flare derived from the mm-wave and soft X-ray data.

[FIGURE] Fig. 2. Solar radio burst observed at Metsähovi Radio Research Station on September 1, 1990

[FIGURE] Fig. 3. Part of the Metsähovi burst time profile on September 1, 1990, and its power spectrum


Table 1. Mm-wave bursts with high-Q oscillations and circuit parameters

Changes in the power spectra before, during, and after the bursts were also investigated. The results of the analysis for two homologous bursts on 28 August 1990 are presented in Figs. 4 and 5. One can see that the pulsations have the same period throughout the two events that last for two hours, altogether. This can be considered as an indication in favour of a stable magnetic loop configuration that is not destroyed during the flare.

[FIGURE] Fig. 4. An example of two homologous bursts with high-Q oscillations, observed at Metsähovi on August 28, 1990

[FIGURE] Fig. 5. Parts of the time profiles of the August 28, 1990 bursts, and their power spectra. a  The burst profile at 09:18-09:41 UT, b  power spectrum for the burst at 09:18-09:41 UT, c  the burst profile at 10:20-10:37 UT, d  power spectrum for the burst at 10:20-10:36 UT

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© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998