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Astron. Astrophys. 341, 610-616 (1999)

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4. Summary and conclusions

Based on the present study of high resolution He II and H[FORMULA] limb images, we propose to distinguish between polar surges and giant spicules (macrospicules). We found that most polar surges and giant spicules have corresponding structures in H[FORMULA]. However, there were a few cases in which it was difficult to establish a clear relation between a He II spike and an H[FORMULA] feature. The reason for this is probably projection effects, low resolution and possibly the fact that some spicules are faint in H[FORMULA] due to their very low density. Structures in He II extend higher and spread over a wider area than in H[FORMULA]. Most polar surges we observed became visible in He II well before H[FORMULA] and remained visible longer during the decay phase. Furthermore, there are H[FORMULA] spikes without He II counterparts. We remind the reader that there is considerable difference in the emission temperature of H[FORMULA] and He II . The emission of polar surges and macrospicules in both lines suggests the existence of hotter material around the H[FORMULA] structure.

A number of questions arise from the above analysis, concerning the basic physical processes involved in these two different phenomena. Polar surges are similar to miniature surges. Their structure is complex and the flows complicated, with strong velocity gradients, giving the impression that the phenomenon is triggered by magnetic reconnection. They live longer than giant spicules and it appears that several elementary processes take place in a long lived structure. Moore and Tang (1977) found that some macrospicules coincided with flaring X-ray bright points, which enhances the above speculation.

Our observations suggest that giant spicules are also impulsive phenomena, on a scale smaller than polar surges and much more numerous. They give the impression of jet-like features, similar to ordinary spicules except for their dimensions. Thus they, also, appear to be the result of magnetic reconnection, on a scale smaller than that of polar surges. The fact that giant spicules are observed in He II while ordinary spicules are probably not, may be related with their dimensions, different temperature and density and the fact that giant spicules protrude higher into the hot Corona.

JOP 057 campaign should be repeated, in an effort to achieve better resolution for the ground-based observations and observe for longer time intervals. This will give the opportunity to further investigate the nature and physical processes related with polar surges and giant spicule as well as the relation of He II spikes to the H[FORMULA] ones.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998