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Solar Highlights for April 17


Solar Rotation Nos: 2188 to 2190

A slight rise in sunspot activity compared to last month. There was a burst of sunspot activity in early April and in the last half of the month we saw the return of two long-lived sunspot groups. There was some interesting activity in hydrogen-alpha and we had an aurora alert.

Here are the solar highlights of April 2017 together with a selection of images.


1st to 8th April
At the beginning of April, we saw Active Region (AR)2645 lying across the Central Meridian (CM), just north of the solar equator. This group had developed rapidly in the last week of March and this came as a surprise to many. Slightly farther west was AR2644, and quite rare of late it was just south of the solar equator. It was these sunspots that we saw with our telescopes at the SPA Convention, in Cambridge, on the 1st. AR2644 was classed type Eao and AR2645 as Eko that day. There were two M-class (moderately powerful) solar flares in AR2644 on the 2nd bringing about two Coronal Mass Ejections (CMEs). Also on the 2nd, AR2648 had appeared on the east limb. There was a minor geomagnetic storm on the 4th (believed to have been caused by AR2644) which was mostly visible from the Arctic Circle. Both AR2644 and AR2645 decayed as they neared the western limb bringing about a much quieter Sun.

9th to 16th April
It was so quiet in fact that by the 9th we saw a blank solar disc (no sunspots at all). AR2650 appeared over the east limb on the 10th. It was classed Cro that day and was on the northern hemisphere. This small sunspot hardly changed at all as it crossed the solar disc until the 16th onwards when it decayed and disappeared. A coronal hole was seen at the centre of the solar disc on the 16th.

17th to 30th April
With AR2650 gone, this left us with a blank solar disc on the 17th. AR2651 appeared on the east limb on the 18th and was the remnants of a former sunspot group: AR2644. This had gone across the far side of the Sun and had returned. On the 19th, AR2651 was seen to hurl a CME into space by the ESA/NASA Solar and Heliophysics Observatory satellite. It had become obvious by the 21st that AR2651 was in fact now starting to decay. On the 22nd the northern lights were seen after dark across parts of the United States and it was thought that the CME eruption on the 19th was the chief cause. We had another returning sunspot group reappear over the east limb on the 23rd. This was now labelled AR2653, but it was in fact another former sunspot group: AR2645. A second large coronal hole was seen on the solar disc on the 23rd and it is thought to have led to an aurora on the night of the 23rd/24th. This time we were more fortunate and an SPA aurora alert was issued by email at midnight on the 23rd that for those living in Scotland and northern UK that given clear skies they might see an aurora. In terms of sunspots, in the last few days of April only small sunspots were visible. AR2651 and AR2653 were still there as was AR2654 which had emerged from the east limb on the 29th.  

SPA Sunspot Mean Daily Frequency (MDF): 1.66 (was 0.90).
SPA Relative Sunspot Number: 24.01 (was 12.97).

Solar Prominences, Plage, Filaments and Flares

1st to 15th April
In hydrogen-alpha light, with AR2544 and AR2645 crossing the Sun, there was a lot to see in the first days of April. Areas of bright plage, with a few short dark filaments embedded nearby, were seen surrounding the sunspots. As well as this, there many small intricate prominences on the NE and SW solar limbs.

16th to 30th April
By the middle of the April, a small number of filaments and prominences were visible. Of note was a long dark filament on the southern hemisphere and ‘filaprom’ (a filament that then become a prominence) on the east limb on the 17th. The emergence of AR2651 on the east limb brought a lot of plage, filament and prominence activity to that area from the 18th onwards. This activity was added to by the appearance of sunspots AR2652 and AR2653 on the 22nd. A very broad dusky filament was observed on the 23rd between AR2652 and the NE limb. Even with the disappearance AR2652 on the 28th there was still several areas of bright plage and dark filaments around AR2651 and AR2653, particularly the looped prominence on the SE limb, the tall prominence on the NE limb, and another filaprom on the NW limb.

SPA Prominence Mean Daily Frequency (MDF): 4.95 (was 3.60).

Well done to Brian Gordon-States and Alan Heath who both observed 30 days in April and to Jonathan Shanklin, Bob Steele and Michael Fullerton who observed 26, 25 and 24 days respectively. A fantastic effort!

Thank you to everyone who visited the Solar Section stand at the SPA Convention, in Cambridge, on the 1st. I thoroughly enjoyed the day and I hope you did too!

Detailed count records of Active Regions and Relative Sunspot Numbers came from: Brian Gordon-States, Michael Fullerton, Alan Heath, Mick Jenkins, Ian Lee, Jonathan Shanklin and Bob Steele.

Images and drawings were supplied by: Carl Bowron, Alan Clitherow, Mick Jenkins, Ian Lee, Cliff Meredith, Julia Wilkinson and Brian Woosnam.

Geoff Elston


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