SEQUENCES OF CORRELATED HARD X-RAY AND TYPE-III BURSTS DURING SOLAR-FLARES

Acceleration and injection of electron beams in solar flares can be traced from radio type III (or type U) bursts and correlated hard X-ray pulses with similar timescales and nonthermal spectra. We perform a systematic survey of such correlated radio and HXR pulses with timescales of less than or similar to 2 s in flares simultaneously observed with the radio spectrometer Ikarus and the Hard X-Ray Burst Spectrometer (HXRBS) on SMM. We applied an epoch-folding technique to enhance correlated time patterns in burst sequences at the two wavelengths. We present the results from the strongest (10) flares with a HXRBS count rate greater than or equal to 3000 counts s(-1), which have a satisfactory signal-to-noise ratio for subsecond pulses. The major findings of this study are as follows: 1. Sequences of correlated HXR and radio bursts with equal burst durations and intervals at the two wavelengths are found in all strong flares. 2. The sequences were found to contain between two and nine correlated pulses in HXR and radio. 3. The epoch-folded HXR pulses (with respect to the peak times of the type III bursts) were found to have a high statistical significance of 6-57 sigma. 4. The duration of these correlated pulses varies from 0.2 to 2.0 s, the pulses have a duty cycle of about 50%, and the pulse duration in HXR and radio is highly correlated. 5. The radio bursts (detected at approximate to 300 MHz) are systematically delayed with respect to the HXR pulses by 0.4 +/- 0.6 s. 6. The best correlation was found for a lower energy cutoff of 40 +/- 22 keV in HXR. 7. The altitude of the particle injection site, inferred from energy-dependent (time-of-flight) delays of HXR-emitting electrons, was found in the range of 18-32 Mm in three flares. These observations strongly suggest that particle acceleration in solar flares occurs in a pulsed mode where electron beams are simultaneously injected in upward and downward directions. Since the sequences of correlated HXR and radio bursts show identical durations and intervals at the two wavelengths, they are believed to reflect most directly the temporal dynamics of the underlying common accelerator. As a consequence, thick-target models should be reconsidered under the aspect of electron injection with pulse durations of 0.2-2.0 s and duty cycles of approximate to 50%.