(a) (b)
Figure 8.1: (a) The dedicated Lunt50mm Hαtelescope being synched with the tracking soft- ware prior to solar observations. (b) The dedicated Hαtelescope (left) observing the Sun on the roof of Building G5at the NWU, Potchefstroom campus and the white light telescope (right) with a solar filter fitted observing the Sun.
8.3 Solar observations
The two telescopes were used in extensive solar observations from2017to the end of2019after which solar observations stopped due to the worldwide coronavirus pandemic. Nonetheless, active regions (ARs), filaments, prominences, an annular solar eclipse, and the Mercury solar transit were observed with the solar telescopes.
8.3.1 Active regions and sunspots
(a) (b)
Figure 8.2: (a) ARs, sunspots and a prominence observed on 18 August2017. (b) The same ARs, sunspots, and prominences observed on20August2017. Details are given in the text.
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Figure 8.3: (a) Two filaments observed on the solar disk on18August2017. (b) A large promi- nence protruding from the south-eastern solar limb on18August2017.
During the decline of solar cycle 24(December 2008to December2019) there were a sudden increase in solar activity, specifically during August and September of2017. A complex active region, designated AR 2671, with several sunspot groups appeared in early-August of 2017.
Figure 8.2 shows the images taken with both the white light and the Hαtelescopes on18August 2017(figure 8.2a) and20August2017(figure 8.2b). The Hαimages taken on18August show the complex sunspot group (top left panel) with the magnetic structures around the sunspots.
The bottom left panel shows a large prominence protruding from the south-eastern solar limb.
The white light image (right panel) shows the AR of the sunspots with the umbrae and the penumbrae clearly visible. Figure 8.2b shows colourised images of the ARs on 20 August.
The bottom left panel shows an arc prominence, with the spine and footpoints visible, while the bottom right panel shows one prominence over the northern solar limb. Comparing the images from the two days, one is able to track the location of the sunspots over the 48 hour timespan. For example, the bottom group of sunspots were three distinct sunspots in figure 8.2a, but two days later, they had merged to form one sunspot with a bow-tie structure.
Also observed on18August2017was one large filament on the solar disk shown in figure 8.3a.
A smaller filament is also visible above and a little to the left of the large filament. The coronal magnetic structures are also seen across the solar disk. To illustrate how large the prominence was on this day, an almost full solar disk image is shown in figure 8.3b with the prominence protruding form the south-eastern solar limb. The length of this prominence is ∼ 0.3 RJ. Both images were taken with the dedicated Hαtelescope. The white light images were taken with a Canon DSLR (digital single-lens reflex) camera, and the Hαimages taken with a CCD (charged-couple device) camera.
126 8.3. SOLAR OBSERVATIONS
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Figure 8.4: (a) The transit of Mercury on11November2019as imaged by the white light tele- scope at the Potchefstroom campus of the NWU. (b) The transit of Mercury as viewed from the SDO spacecraft on the171A channel. Courtesy of NASA/SDO and the AIA, EVE, and HMI˚ science teams.
8.3.2 Mercury transit - 11 November 2019
One of the rarest predictable astronomical phenomena is the transit of Mercury across the solar disk. This event was observed with the white light telescope from the NWU on11November 2019. The next Mercury transit will only take place on13 November2032. Figure 8.4a shows the image of the Mercury transit observed from Potchefstroom, South Africa. The tiny black disk (10arc seconds across and1/150that of the solar diameter) is visible on the yellow solar disk. Due to bad weather across South Africa on that day, the NWU was the only team to observe and image the majority of the transit of Mercury. Figure 8.4b shows the transit of Mercury as viewed from the SDO spacecraft in the171A channel. The transit of Mercury has a˚ rich observational history. Albert Einstein determined that Mercury’s orbit did not conform to Newton’s laws of motion since the Sun’s effect on the local space-time causes the tiny planet to accelerate as it orbits closer to the Sun.
8.3.3 Annular solar eclipse - 26 December 2019
On 26 December 2019, an annular solar eclipse occurred and was observed in the desert of Oman, south of its capitol Muscat, with the two solar telescopes by Mr Ruhann Steyn. The name annular refers to the Latin word for ring, since the solar disk is not completely covered by the Moon’s shadow during an annular eclipse. Rather, a so-called ring of fire (the Sun) is observed around the Moon. Since the Moon is at apogee (its furthest point from the Earth), the lunar disk is smaller than the solar disk as viewed from Earth and therefore the Sun is still visible around the lunar disk. The eclipse lasted for 3 minutes and 40 seconds. Figure 8.5a shows the two solar telescopes in the Omani desert on the morning of26December2019
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Figure 8.5: (a) The Hα(front) and the white light (back) telescopes from the NWU solar tele- scope project in the Omani desert during the annular solar eclipse on26 December2019. (b) The image of the annular solar eclipse at maximum annularity as imaged by the white light telescope.
at sunrise when the eclipse started. Figure 8.5b shows the colourised image observed by the white light telescope. The lunar disk (black) is covering the solar disk (red) with the solar limb still visible. A small prominence is seen on the north-western solar limb. Solar cycle24ended in December2019and thus this eclipse was observed at the trough of solar minimum.