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

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3. Results

From the study of He II images it is clear that the structures referred to as macrospicules are not just a set of fine long spicules, but they are more like eruptive phenomena with a complex structure, reminding the characteristic morphology of surges. Thus we propose to distinguish them from giant spicules and use the term polar surges .

3.1. Polar surges

Fig. 3 shows the appearance, development and decay of such a polar surge, located a few degrees from the North pole, on December 13. The event began as a small bright bubble which erupted. The polar surge was inclined relative to the vertical and consisted of at least two overlapping mounds. As it decayed, it showed a complex structure of loops, indicating that the material flow was along magnetic field lines. At some locations one can observe bright blobs (small clouds), reminding the structure of a decaying prominence. As the polar surge decayed further, it appears that material was draining from the blob towards the feet.

[FIGURE] Fig. 3. H[FORMULA] center filtergrams showing the appearance, development and decay of a characteristic polar surge observed on December 13. The average sampling time is close to 30 sec. Note the similarity with the impulsive polar event observed with a broad-band filter and a large coronograph by Koutchmy and Loucif (1991); that event was definitely described as a surge-like event and large proper motion and Doppler shifts were reported

Fig. 4 shows sequences of simultaneous H[FORMULA] and He II images of the event at selected times. The polar surge made its appearance first in He II , was relatively faint and showed structures stretching towards the Corona. We cannot see it in H[FORMULA] yet, while diffuse material has spread over a large volume in He II . Ten minutes later, at 18:28:58 UT, we observed an intensity enhancement at the base of the feature in both lines, giving the impression of the beginning of a new, but obviously related event. Subsequently the polar surge expanded upwards in both lines. It faded away first in H[FORMULA] and after about 4 min in He II . Its maximum apparent height reached 45000 km in He II and 15000 km in H[FORMULA]. Its lifetime was about 43 min. in He II and about 30 min. in H[FORMULA].

[FIGURE] Fig. 4. Simultaneous H[FORMULA] and He II 304 Å filtergrams showing the parallel development of the polar surge observed on December 13. Note the difference of the behavior of the event as seen in two lines of quite different temperatures. We assume that the event seen at 18:18:24 in He II is the same as the event seen at 18:28:58, almost 10 min later, in H[FORMULA]

3.2. Giant spicules (macrospicules)

Fig. 5 shows some characteristic cases of giant spicules observed in both lines (marked b,d). Giant spicules are very narrow compared to polar surges (e,c); they consist of just one unresolved (due to the resolution limit) component and their nature is less eruptive. Their maximum apparent height is of the order of 12 - 15 Mm, which is significantly lower than that of polar surges. It is very difficult to follow their development through the time sequence but it seems that their lifetime is of the order of 5-12 minutes.

[FIGURE] Fig. 5. Simultaneous H[FORMULA] and He II 304 Å filtergrams obtained on December 11 1996; time increases from top to bottom and from left to right. The figure shows the time evolution of polar surges (c and e) and giant spicules (b, d and f) observed in both lines. Structure g is a He II spike which is not seen in H[FORMULA], while (a) is a bush of H[FORMULA] spicules without any He II counterpart

3.3. Comparison of He II and H[FORMULA] spikes

From a visual inspection of the images we found that most He II polar surges and giant spicules had corresponding spike-like structures in H[FORMULA] (Figs. 2, 5). In each image we could clearly detect about twenty He II spikes, three quarters of which had corresponding H[FORMULA] structure. We should note that H[FORMULA] spicules are more numerous (their number exceeds considerably what is seen in He II ), making the detailed comparison difficult. However, the fact that the H[FORMULA] evolution parallels that of He II , enhances the possibility that H[FORMULA] structures are counterparts of He II structures. The H[FORMULA] spikes are much shorter and more narrow. In some cases the spikes did not have the same geometry in both lines; this is probably due to the fact that some parts of the spikes emit more strongly in He II , whereas others are better visible in H[FORMULA]. Moreover the geometry of giant spicules sometimes changes in subsequent frames of the same line.

There were some cases where it was difficult to find a clear correspondence between spikes in He II and H[FORMULA] structures. One reason, as we mentioned before, is that they appear first in He II and remain visible for a longer time; this is true especially in the case of polar surges. Another reason is projection effects; polar surges are much more extended in He II , so that we can see structures which have their feet behind the limb. In H[FORMULA] the spikes are much shorter and might be obscured by features in front of them. We should remind the reader that He II spikes are bright even when they are observed on the disk near limb, while H[FORMULA] spicules are dark. We show a characteristic example (c in Fig. 5) of a polar surge in front of the limb.

In some H[FORMULA] images we could observe a diffuse dark feature and only a small bright part overlimb. Finally there might be cases where the spike is very faint in H[FORMULA] which, in combination with poor seeing, makes it difficult to detect. Fig. 5 shows an example (g) of a faint diffuse structure that has no obvious H[FORMULA] counterpart. Further in Fig. 5 we have marked a characteristic case (a) of an H[FORMULA] bush of spicules that did not have He II counterparts, but one could observe a gap in their place. However we should note that the spatial resolution of EIT is not high enough to show really small structures of the solar chromosphere, such as spicules.

3.4. Comparison with previous results

In Table 1 we present the basic properties of polar surges and giant spicules observed in He II and in H[FORMULA]. The comparison of their properties makes obvious that we are dealing with two different classes of structures. The table also shows results from previous investigations. The properties of polar surges are based on a very low sample and are indicative, so that they can be compared with giant spicules.


Table 1. Basic properties of Polar Surges and Giant Spicules.

Bohlin et al. (1975), based on He II observations and from a sample of 25 macrospicules, found that their height ranged from [FORMULA] to [FORMULA] and their lifetime from 8 to more than 45 min. They found that there is an outstanding correlation between maximum height and lifetime: to a first approximation, lifetime in minutes was equal to their height in arc seconds. Our results agree with the ones of Bohlin et al, if we consider as macrospicules both giant spicules and polar surges. Labonte (1979), from a sample of 32 macrospicules observed in H[FORMULA], found an average height of 12000 km and an average lifetime of 11 min. The lifetime and height found by Labonte agree with our results.

Johannesson and Zirin (1997) counted the H[FORMULA] macro-spicules observed in a field of view of [FORMULA] at the poles. They considered as a macrospicule every jet-like feature extending above 7000 km and found up to 20 macrospicules per [FORMULA]. According to Labonte (1979) the rate of appearance of H[FORMULA] macrospicules in the polar quadrants is [FORMULA] events per hour, per [FORMULA]. This means that, observing for one hour over a [FORMULA] field of view we would hardly observe one structure. The counts of Johannesson and Zirin (1997) counted are close (a little more, probably due to better resolution) to the total number of features that we observed, while the rate of appearance computed by Labonte agrees with the number of polar surges we observed.

Moore et al. (1977) in their study that led to the identification of the H[FORMULA] macrospicules with those in He II , stated that it appears that H[FORMULA] macrospicules on BBSO full disk films were a subset (one out of 20) of those in He II . If we assume that what they could observe on H[FORMULA] films were polar surges, then this ratio is in agreement with the ratio of polar surges to the total number of spikes that we observed.

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998