Acceleration of electrons in absence of detectable optical flares deduced from type III radio bursts, H? activity and soft X-ray emission

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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 not be detectable. It is found that transient Hα activity observed in the absence of reported flares is associated with production of relatively weak type III radio and soft X-ray emission. Since such optical phenomena are much more frequent than flares themselves, it is concluded 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, which is similar to the precursor in flares, 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 similar evolution, the type III bursts occurring near the maximum of this perturbation. The observations are consistent with a model in which the electron acceleration region is located at an altitude where the ion density is ∼ 109 cm−3 and most of the accelerated electrons(≳ 20 keV) are confined to coronal altitudes where the ion density is ≲ 1010 cm−3.