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Solar Highlights for April 15

Solar Rotation Nos: 2162 to 2163

A slight increase in the average sunspot count (MDF) but we still saw a much lower level of activity compared to this time last year. We saw more sunspots in mid-to-late April (17th to 24th to be precise) but then had two spotless days at the end of April (29th and 30th). There was more activity to be seen for those with hydrogen-alpha telescopes at their disposal.

Here are the highlights for April 2015 together with a small selection of images (shown above in date order) from the many that were sent to the Solar Section.


We had a quiet start to April. On the 1st, Active Region (AR) 2305 was nearing the west (W) limb and as it did so it showed the Wilson Effect quite clearly. After a couple of days it went over the W limb leaving just AR2317 visible now nearing the W limb.

It wasn’t until the 5th that activity picked up with the sudden emergence overnight of sunspot group AR2320 on the eastern hemisphere. It developed pretty slowly into a trail of small spots over the next few days so that by the 8th, as it crossed the Central Meridian (CM) together with a smaller separate group AR2318 just to the north, it had become quite long and with an intricate structure. It was though that AR2320 might produce solar flares but as it headed towards the W limb it was clear this was not going to happen.

By the 12th our attention was drawn towards the E limb again as an active group, AR2321, came into view. We had a clearer view of the group by the following day as it travelled further away from the limb. Preceding it was separate sunspot group AR2322 and it was apparent that two more sunspots were following behind AR2321. None of these were large enough to be seen with the protected naked eye but AR2321 displayed three main spots each one about the same size as the Earth.

By mid-month we had the much smaller sunspots of AR2320 and AR2323 now lying very near the W limb but over towards the E limb AR2321, AR2324 and AR2325 were clustered together and putting on a good show. By the 18th the trio of sunspot groups (AR2321, AR2324 and AR2325) were nearing the CM and another group AR2327 had also appeared over the E limb. It was from about this time up to the 24th that we saw the highest number of sunspots at any one time as all this activity was carried from east to west by the rotation of the Sun. What we did not see among these sunspots were any solar flares until Alan Heath reported a long-lasting solar flare at the preceding limb on the 21st (see below). It seems there were several M-class solar flares connected with AR2322 as it went over the W limb.

By the 25th we saw most of the sunspot activity heading towards the W limb with one new developing group, AR2333, over towards the east. Over the last few days of April the development of AR2333 seemed to come to a gradual halt and as the recent sunspot activity we had enjoyed went over the W limb we ended the month with blank solar disks on the 29th and 30th. 

SPA Sunspot Mean Daily Frequency (MDF): 3.24  
SPA Relative Sunspot Number: 49.86


In stark contrast we saw some very impressive activity in H-alpha. The 1st saw a lot of filament activity on the solar disk but of particular interest were the two large hedgerow type prominences seen on the W limb. By the 2nd these were slightly lower in extent but a very tall prominence had by now appeared on the limb near AR2305 and there were three long filaments on the disk.

There was extensive plage activity around AR2320 as it moved away from the E limb on the 6th. Plage activity appeared to become more scattered behind the sunspot group by the following day. Also seen on the 7th was a substantial tree-like prominence on the NE limb (following AR2316) that could be traced on to the disk where it appeared as a filament. On the 8th we saw a lot of smaller prominence right along the E limb and by this time AR2320 was over the CM and the number of plages around it had reduced. The 9th saw a multitude of hedgerow type prominences on the NE, E and W limbs while on the disk we saw quite a collection of filaments some with plages. AR2320 was right in amongst these.

The 12th saw another highly active disk covered in plage and filament activity largely connected with the sunspots visible then which were AR2320 and AR2323 nearing the W limb and emerging sunspots AR2321 and AR2322. There was not much in the way of prominences except for one on the E limb.

As AR2322, AR2321, AR2324 and AR2325 came away from the E limb there was a lot of plage activity, and some filament activity too, seen around them on the 15th. Also visible was a tall hedgerow prominence on the NE limb. From the 16th to the 18th we saw a tremendous amount of H-alpha activity clustered around the NE limb associated with AR2321, AR2324 and AR2325. By the 21st this activity had spread right across the disk carried westwards by solar rotation.

The 22nd saw not only a highly active disk with plages and filaments but two large prominences, one on the E limb near AR2333 and one on the W limb near AR2321. The eastern limb prominence was larger still over the following two days as the Sun’s rotation brought it over the limb forming a huge highly intricate hedgerow-type prominence that dominated the view until the 25th.

Over the 26th to the 28th a tall prominence was seen on the NE limb that could be traced onto the disk as a filament.

On the last two days of the month despite no sunspots to be seen there were a few small prominences visible and some plages and filaments to be seen especially on the 30th.    

SPA Prominence Mean Daily Frequency (MDF): 5.07  

Two solar flares were reported by Alan Heath:
11th   0850UT
          0920UT     same flare, still present
          1000UT    same flare, still visible
          1100UT    flare much weaker now, looks more like a plage

21st    0830UT    Flare at preceding (W) limb
          1030UT    same flare, still visible.

Well done to Brian Gordon-States, Alan Heath and Jonathan Shanklin who all observed on 29 days this month. Ian Lee was close behind at 25 days observed in April.

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

Images and drawings were supplied by: Richard Bailey, Mark Beveridge, Carl Bowron, Paul Brierley, Michael Fullerton, Mick Jenkins, Ian Lee, Cliff Meredith, Julia Wilkinson and Brian Woosnam.

Geoff Elston
SPA Solar Section Director

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