Astron. Astrophys. 342, 279-284 (1999)

3. Results

3.1. Coronal hole: C IV 1548 Å

The sequence in Fig. 5 lasts 200 s and covers a solar area of arc sec². In the first time frame we see a broadening in the C IV line centered at 904 arc sec north of disk center. By the second time frame (06:31:40 UT) we see a blue-shifted component. For the next 40 s, the line is mostly blue-shifted although there is a weak red-shifted feature. At 06:32:41 UT we see another injection of energy resulting in blue and red-shifted plasma, shown more clearly in Fig. 6. By this stage the center of the feature has drifted southward by three to four arc sec. The latter four raster positions show mostly a blue-shifted plasma. The size of the explosive event in the north-south direction had a maximum extent of 6 arc sec. The time frames in Fig. 5 are separated by 1 arc sec (moving eastward), thus the feature is visible over an area of arc sec². The maximum velocity reached in the blue wing was 150 km s^-1 and 100 km s^-1 in the red wing.

Fig. 5. A time series for an explosive event observed in C IV 1548 Å in a northern coronal hole on 14 July 1996. Doppler velocities are shown in a scale from -195 km s-1 (blue-shift) to 203 km s-1 (red-shift).

Fig. 6. The C IV 1548 Å line profile averaged over the whole explosive event as a function of time.

An estimation of the characteristic sound speed, c, in a region is given by the relation km s^-1. This implies that for C IV 1548 Å line ( K) a value for c around 50 km s^-1 can be estimated. Therefore, we are observing a global presence of supersonic upflows and downflows all along the sequence shown in Fig. 5. We have to consider as well that the Doppler shifts calculated from observations are only a minimum value dependent on line-of-sight conditions for the event.

The line-of-sight conditions for this dataset are particularly extreme being centered as it is in the north pole of the solar disk. This condition makes it especially difficult to observe flows in the radial direction. Further considerations on how that might affect our observations is discussed in the next section.

3.2. Active region: O VI 1032 Å

The first explosive sequence in Fig. 7 lasts for over 4 min. In the first time frame (07:25:37 UT) we see a brightening to the blue at approximately -180 arc sec in the N-S direction, this slowly fades until 07:26:38 UT where we see a broadened blue-shifted line profile with a maximum velocity of approximately 120 km s^-1. The mass motion quickly increases in the red wing to velocities of 100 - 150 km s^-1 by 07:26:53 UT, remaining at these supersonic velocities for 45 s. Similar velocities are seen in the blue wing. The event extends 8 arc sec at maximum and 2 arc sec at minimum in a given raster position. Some apparent displacements along the slit of the highest velocities in the mass flows are observed. In the frames between 07:27:54 UT and 07:28:39 UT the velocities fall below the sound speed (95 km s^-1) in the red wing, disappearing at 07:28:39 UT, while they stay very close to the sound speed value for the blue wing over a region of 2 - 4 arc sec along the slit. At 07:28:55 UT there is another injection of energy, with plasma again moving both red-ward and blue-ward at a velocity up to 160 km s^-1. By 07:29:40 UT, the major component is blue-shifted at close to 240-260 km s^-1. Fifteen seconds later this has almost decayed. The later time frames also showed apparent motions, now southward by 3 - 4 arc sec. This sequence is shown more clearly in Fig. 8a.

Fig. 7a-c. A time series for an explosive event observed in O VI 1032 Å in a active region on 10 July 1996. Doppler velocities are shown in a scale from -244 km s-1 (blue-shift) to 385 km s-1 (red-shift).

Fig. 8a-c. O VI 1032 Å: For each time frame in the raster series we show here an averaged profile over the whole explosive event with the vertical line showing the zero velocity reference for a The first two events in Fig. 7, between 07:28:39 UT and 07:29:40 UT (629-643 arc sec E-W), b  One of the twin events between 07:30:26 UT and 07:31:11 UT and c  The other twin event between -210 and -207 arc sec, in the N-S direction.

At 07:30:26 UT two explosive events takes place simultaneous at approximately -202 and -209 arc sec in the N-S direction, with the maximum velocity being approximately 200 km s^-1 in the blue and 180 km s^-1 in the red direction (see Fig. 8b,c). By 07:31:11 UT, both events are gone. These latter events however occur close to a region which showed persistent broadening and evidence for small scale explosive events over an interval of 4 min prior to 07:30:26 UT.

This active region shows a very complex structure which makes it difficult to give an explanation for the supersonic flows found, but it seems probable that what we observe is a burst of explosive events all along the rastered area.

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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