(Solar Phys.). The relationship between Hα absorption features, type III radio bursts and soft X-ray emission has been examined in order to determine the characteristics of the particle acceleration process operating when a Hα-flare may or may not be detectable. The Hα observations were made by Meudon Observatory with a Hα telescope fitted with a 0.75 Å band pass Lyot filter. During a 10 s period, three pictures were obtained – one at the Hα line center, one at Hα + 0.75 Å and one at Hα −0.75 Å. This sequence of three pictures was repeated every one minute. Each picture covered a rectangular area 18 × 24 mm2, the diameter of the complete solar image being 38 mm on this scale. In addition, Meudon Hα films of the whole solar disc were also used. The X-ray observations were made with the University of California (Berkeley) experiment aboard the OGO-5 satellite and the NRL experiment aboard Solrad-9. The wavelength range covered was 0.5–20 Å. The type III radio data was obtained from two sources: The 169 MHz radio-heliograph at Nancay which provided east–west position of the radio burst on the Sun with an accuracy of ~ 1′ and the radio spectra measured by various ground based observatories. The findings are as follows:

Transient Hα activity observed in the absence of reported flares is associated with production of type III radio and soft X-ray emission. Since such optical phenomena are much more frequent than flares themselves, we conclude that instabilities generating fast particles may be produced in the corona in a quasi-continuous way with coincident perturbations in the lower solar atmosphere.

The soft X-ray component is not necessarily the direct product of fast particles, but is probably associated with some type of heating since both the soft X-ray emission and the Hα features exhibit a comparable evolution. The type III bursts, when they are produced, occur near the maximum of this perturbation.

We identify the transient Hα activity (emission or absorption) with the existence of a metastable situation which may or may not lead to the triggering of a flare.