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Astron. Astrophys. 358, 1097-1108 (2000)

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4. The EUV observations

Fig. 1 shows the He I data for this period. At the 20,000 K temperatures identified by this emission line we can see the supergranulation on the solar disc but the striking feature is an activated prominence on the limb, which extends north of active region 8060. Throughout the sequence the prominence shows a significant uplifting and much activity, but it does not show any clear sign of eruption. Furthermore, the position angle spread of the activated He I prominence is 284 to 297 degrees, clearly north of the equator, and north of the spread of the CME which was between 259-275 degrees. The CME does not overlie the activated prominence, but the prominence and CME locations are such that the northern footpoint of the CME and the southern footpoint of the prominence are adjacent. There is no signature in the He I data which appears to relate to the CME in the region of the CME span.

One question to explore briefly is to establish that the apparent uplift of the prominence is not simply due to the rotation of the Sun, that is, is it real `activation' or apparent activation? In the last CDS 4x12 arcminute image the prominence appears to reach altitudes of approximately 1.3 arcminutes (58,500 km) above the limb. This is almost twice the apparent altitude in the first frame, at 0.7 arcminutes or 31,500 km. Let us consider an order of magnitude calculation. If we have a feature on the solar limb which is 58,500 km high, some 12 hours earlier (the duration of the CDS observation run) its apparent height would be reduced by only approximately 10%. Certainly the apparent altitude would not be reduced by a factor of almost 50%. Thus, we conclude that the uplifting is a real feature of the activity and not an apparent effect due to solar rotation. The prominence is almost certainly associated with the north-south running neutral line crossing through active region 8060 on the limb. There are no other candidate neutral lines either side of the limb.

The activity detected in the prominence in He I is also clearly evident in the O V 629 Å data, as shown in Fig. 4. In this image the colour table is set such that the solar disc is over exposed, therefore revealing the fine-scale, ever-changing structure in the corona. This 250,000 K emission is certainly associated with the activated prominence and most likely is a signature of the intermediate temperatures of a cool prominence-hot corona transition region. Again, there is no sign of any eruption and the activity is restricted to position angles north of the CME span.

[FIGURE] Fig. 4. The same display and sequence as Fig. 1 but for the 250,000 K O V 629 Å line.

Moving to coronal temperatures, Fig. 5 shows the same sequence, this time, as observed in the million K Mg IX 368 Å emission line. The striking feature of this sequence is that (a) the coronal plasma seems to be `unaware' of the activated prominence - there is no obvious activity at a million K, and (b) there is no apparent million K signature of the CME. The million K corona seems pretty inactive.

[FIGURE] Fig. 5. The same display and sequence as Fig. 1 but for the 1,000,000 K Mg IX 368 Å line.

However, there are subtle changes in the million K plasmas, which can be revealed through the differencing of images. Fig. 6 shows the same display as Fig. 5, for the Mg IX data, but the initial frame is subtracted from all of the subsequent frames. This reveals a gradual darkening of the corona above the equator, due to a depletion in the emission measure at the million K level. Such a depletion was not evident at lower temperatures. The depleted region lies between position angles 256 to 284 degrees and, thus, is well placed under the CME span of 259 to 275 degrees - especially when one considers that the error on the CME extremes must be of order 5 degrees. The nature of this dimming is discussed in the next section.

[FIGURE] Fig. 6. The same display and sequence as Fig. 5, for the 1,000,000 K Mg IX 368 Å line, but with the first frame subtracted from all subsequent frames. The box in the bottom-left frame denotes an area used for later analysis of the dimming.

The hottest temperature line available to us is the Fe XVI line at 360 Å and the sequence for this line is shown in Fig. 7. This line is not normally significant outside active regions. In this case, we see a bright 2 million K blob on the limb. This blob is clearly associated with active region 8060 and it lies at the intersection of the northern edge of the Mg IX depleted region and the southern edge of the activated prominence seen in He I and O V. In the last few frames (from 21:42 UT) a second, smaller bright blob becomes apparent, further south and just onto the disc. This lies towards the southern side of the Mg IX depleted region. Thus, these two `hot spots' lie at the extremes of the depletion region and the CME and this is suggestive of an association with the CME span, or footpoints.

[FIGURE] Fig. 7. The same display and sequence as Fig. 1 for the 2,000,000 K Fe XVI 360 Å line.

One scenario to explore is the possibility of associated flare activity beyond the limb. We note that the base of the active region is, at the most, just a few degrees beyond the limb. Thus, any flare event would be seen, even if partly occulted, in X-ray data from GOES or Yohkoh, and in the EUV using CDS. The CDS Mg IX and Fe XVI data confirm that there is no such flare. There is a weak increase in intensity from about 11:30 UT in the GOES X-ray intensity profiles and this is most likely associated with the Fe XVI `hotspots' described above.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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