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

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3. An overview of the observations

We report here on one observation run of the sequence described above, that of 16 July, 1997. On that day the CDS 4x12 arcminute field was directed to the west limb, centred at 1000 arcseconds west of central meridian, and 0 arcseconds north of the east-west centerline. The first 4x12 arcminute raster was started at 10:02 UT (duration 50 minutes) and the last 4x12 arcminute raster was started at 22:32 UT. Thus, we obtained a good EUV scan of the western limb for some 12.5 hours on 16 July.

Over this period the LASCO observers detected one CME event. This event was initially detected at 13:30 UT in the `C2' coronagraph whilst its outer edge was at 2 solar radii above the limb. The central position angle is given as 267 degrees, i.e. 3 degrees south of the western equator. The event was relatively small and slow, with a span of only 16 degrees. The LASCO event catalogue describes the event as a `small bright ejection of material'. Fig. 2 shows the CME, the 13:30 UT image being enhanced by the subtraction of a pre-event frame. Such a subtraction shows enhancements (relative to the pre-event frame) of material in the corona in white and depletions in black. The resulting image does enhance the cosmic ray background hits, which are seen as black and white dots in Fig. 2. It also enhances changes in the intensity of the streamer belt. However, the white enhancement on the western limb, which is the CME, is clearly visible, although it is rather nebulous.

[FIGURE] Fig. 2. The CME of 16 July, 1997 as detected in the LASCO C2 coronagraph. The image is taken at 13:30 UT and an earlier frame is subtracted from the image to enhance the changes to the corona. The solar disc is denoted by the white circle. White areas show regions of mass enhancement, and black shows depletion.

We are interested in the onset of this event and, as a starting point, we require to identify a time-window in which one would reasonably expect the CME onset to occur. We can do this by a consideration of the measured CME ascent speed, and project back in time. However, the measurement of CME speeds and the projection back to an onset time is rather subjective, especially for events which have little clear structure. For this reason, we quote two independent measurements of the CME speed, one due to Chris St Cyr (private communication) at 173 km s-1 and one due to Mark Lyons at 142-162 km s-1.

If we assume a constant speed and a zero altitude onset on the limb, the CME onset would have been in the range 10:50-11:20 UT, using the speeds given above. This would suggest that the CME onset is not only well within the field of view, it is well within the timeframe of the CDS observations (10:02-22:32 UT).

Of course, this onset time is very crude. We have no reason to assume a constant speed under the coronagraph occulting disc and should anticipate some acceleration. This would put the onset earlier. Also, we should not assume that the source material is at zero altitude. If the source is well above the limb, this would suggest a later onset time. In addition, the source may be away from the limb and this would also suggest an earlier onset time. As discussed later, we believe that this event is close to the limb. However, if the source region was 30o from the limb, this would only put the onset some 10-15 minutes earlier.

The desire is simply to define a window which contains the onset and with all of the assumptions and considerations it seems entirely appropriate to take an onset window from 10:30 UT to 11:45 UT, with 11:00 UT being the most likely onset time.

Using the LASCO white-light data for the peak of the event (i.e. when the CME is well within the C2 field), in comparision to a post-event image, and considering the physics of the Thompson scattering process, the change in white-light emission suggests a CME mass of about (5 [FORMULA] 1) x 1010 kg. This calculation assumes that the CME is in the plane of the sky. This is a small CME; a commonly quoted range for CMEs would be 1012 to 1013 kg. If the CME is out of the plane of the sky, the mass estimate would be low. However, as we will see later, the associated `surface' activity is near the limb, so this would appear to be a reasonable assumption. The mass estimate also assumes that the CME depth perpendicular to the plane of the sky is equal to the measured span in the plane of the sky. This is most likely a good approximation for this event. However, we should take the calculated mass as a lower limit.

Before embarking on a detailed description of the EUV data, we briefly examine the state of the Sun in mid-July 1997. The Sun was remarkably quiet. In the lead up to the ejection event, there were only two, very minor sunspot groups reported in the western hemisphere. Active region 8059, located some 25-30 degrees south of the equator crossed central meridian on 5 July and passed the western limb on 12 July. Magnetograms (e.g. Kitt Peak and Mt Wilson reported in Solar Geophysical Data, 1997) show that it was a simple, compact region. Active region 8060, located on the equator, was also compact, with minor sunspots only. This region crossed the equator on 9 July and crossed the western limb on 16 July. The sunspots were minor and, indeed, no sunspots were recorded (Solar Geophysical Data, 1997) from 10 July in the western hemisphere. Active region 8060 was also a simple, compact region in the magnetogram observations. The Stanford magnetogram data indicate one strong magnetic neutral line running north-south through active region 8060 on the limb. There were no regions or significant magnetic features, including filaments/prominences, passing beyond the limb in front of these two active regions. No X-ray flare events were recorded in the GOES 1-8 Å X-ray profiles and no H-alpha flares were reported.

The Yohkoh images (Fig. 3) show region 8059 as an extensive loop system crossing the south-western limb on 12 July. Region 8060 is shown to be extremely compact and crosses the limb on the equator around 15-16 July. The footpoints of the loop system may be a few degrees beyond the limb at the time of the 16 July CME. Fig. 3 stresses the compactness of active regon 8060 and reveals that there are no clear large-scale loop systems extending beyond the compact region. The Yohkoh SXT data show that there are no other major structures on the western hemisphere or passing over the western limb in the days prior to 16 July.

[FIGURE] Fig. 3. Yohkoh SXT images of the west limb at 12:58 UT on 12 July, 13:37 UT on 14 July, 13:09 UT on 15 July and 15:50 UT on 16 July. Active region 8060 can be seen on the equator approaching the western limb whilst active region 8059 is crossing the south-western limb on the 12th.

Given this information, the CME of 16 July 1997 appears to be associated with active region 8060. It lies right over the active region as it crosses the limb. The CME is well north of active region 8059. This would suggest that the CME is from a source region right on the limb. There are no other features in X-rays, magnetogram data or visible light which would suggest any other association, even half a rotation beyond the limb. If we reject the association with active region 8060, and argue that the CME source is behind or in front of the limb, then it is not associated with any known surface feature. This seems to be very unlikely given the known association between CMEs and active regions (e.g. Harrison, 1995).

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Online publication: June 20, 2000