What Determines Solar Flares Producing Interplanetary Type III Radio Bursts?

Energetic electrons accelerated by solar flares often give rise to type III radio bursts at a broad wave band and even interplanetary type III bursts (IT3) if the wavelength extends to a decameter-kilometer. In this Letter, we investigate the probability of the flares that produce IT3, based on the sample of 2272 flares above M-class observed from 1996 to 2016. It is found that only 49.6% of the flares are detected to be accompanied with IT3. The duration, peak flux, and fluence of the flares with and without IT3 both present power-law distributions in the frequency domain, but the corresponding spectral indices for the former (2.06 0.17, 2.04 0.18, and 1.55 0.09) are obviously smaller than that for the latter (2.82 0.22, 2.51 0.19, and 2.40 0.09), showing that the flares with IT3 have longer durations and higher peak fluxes. We further examine the relevance of coronal mass ejections (CMEs) to the two groups of flares. It is found that 58% (655 of 1127) of the flares with IT3 but only 19% (200 of 1078) of the flares without IT3 are associated with CMEs, and that the associated CMEs for the flares with IT3 are inclined to be wider and faster. This indicates that CMEs may also play a role in producing IT3, speculatively facilitating the escape of accelerated electrons from the low corona to the interplanetary space.