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

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1. Introduction

During the Skylab mission, giant spikelike structures were observed in He II 304 Å spectroheliograms (Bohlin et al. 1975). The correlation analysis of the He II images with simultaneous H[FORMULA] images did not reveal a significant correlation between He II structures and H[FORMULA] spicules (Moe et al. 1975). They were thus considered as new solar features and were called macrospicules . Moore et al. (1977) used the term "H[FORMULA] macrospicules" to refer to small, surge-like, quiet region H[FORMULA] limb eruptions, with dimensions significantly larger than those of the usual H[FORMULA] spicules. They found that the shapes of H[FORMULA] macrospicules are mound like (as wide as tall) and that they are similar to the EUV macrospicules in shape, size, motion and duration. From the examination of simultaneous He II and H[FORMULA] observations (6 events) they concluded that an H[FORMULA] macrospicule is the H[FORMULA] component of an EUV macrospicule. Before that, the larger H[FORMULA] macrospicules observed in the polar regions were called "polar limb surges" (Godoli and Mazzuconi 1967).

There is a number of difficulties in comparing H[FORMULA] and He II 304 Å features. Macrospicules can be very faint in H[FORMULA], due to their low density. The spatial and time resolution of He II images are usually significantly lower than that of H[FORMULA] images; it is thus very difficult to observe or resolve small scale, faint features in He II . On the other hand, H[FORMULA] images suffer from variable seeing and are stretched due to atmospheric distortion, which makes difficult the comparison of images over an extended field of view.

The large velocities associated with macrospicules can easily produce Doppler shifts in excess of 0.5 Å at H[FORMULA]. Thus some spicules, in particular those with large inclinations, may not be well observable at the center of H[FORMULA] during their ascending or descending phase. Moreover, the motion of spicules transverse to the line of sight modifies their intensity due to the Doppler brightening effect. As the velocities during the vertical extension phase of the structures are larger than 30 km s-1, the H[FORMULA] intensity is enhanced during this phase due to Doppler brightening (Hyder and Lites 1970). The Doppler brightening mechanism predicts intensity maxima during the ascending and descending phases, when the velocity transverse to the line of sight is large. According to Labonte (1978) the brightening during the descending phase is not observed due to the change in the state of the material in the macrospicule. He suggested that the macrospicule undergoes a monotonic expansion with a corresponding reduction in density.

Finally, the physical conditions of the material (temperature, density, velocity) play an important role and as a result some parts of macrospicules do emit strongly in He II whereas other parts may be better visible in H[FORMULA]. According to Georgakilas et al. (1998) it appears that limb surges become visible in He II well before in H[FORMULA] and remain visible for a longer period during the decay phase.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998