On some transient Hα features associated with metric type III bursts |
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| Authors: | F Axisa M J Martres M Pick I Soru-Escaut |
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| Institution: | 1. Service d'électronique Physique, Centre d'études Nucléaires de Saclay, France
2. Observatoire de Paris, 92-Meudon, Paris, France
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| Abstract: | The question of the chromospheric features type III association is reconsidered by using Hα filtergrams both on and off band. A set of 44 metric type III groups, for which an association can be ascertained with a high degree of confidence leads to the following results: - The type III's have a dual chromospheric association. Sometimes they are related to a flare, sometimes to a perturbation of a different kind. The latter is seen in absorption in the Hα core and ±0.75 Å away. It is interpreted as a rather dense and cool material in motion in the chromosphere or the low corona. A part of which moves downward, the other upward. The type III's are more closely related to the downward motion.
- The type III associated absorbing features take place at the border of an active center and along an H∥ = 0 line. At the present time this appears as the most conspicuous property for marking them off from the great variety of the Hα absorbing features commonly observed on the Sun.
- Most of the type III associated flares are related to an absorbing feature of the same kind, which appears before the flare itself. This indicates that the initial instability which is responsible for the type III emission is basically the same, whether the bursts are flare associated or not.
Our observations give good evidence that an efficient acceleration of 10–100 keV electrons occur also in the absence of flares. Furthermore the chromospheric perturbation involved in this acceleration is, in many cases, clearly associated to the triggering of a flare. A tentative model is proposed. We assume that in relation with the Hα absorbing feature a stream of fast electrons is accelerated which in turn, under suitable conditions, triggers both the flare and the type III's at the same time. |
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