Popular Astronomy

Share to Facebook Share to Twitter Stumble It More...

Solar Highlights for April 16


Overall, low sunspot activity continues to be the norm, and we saw another down-turn in Sunspot MDF. Despite this trend, we had a naked-eye sunspot appear mid-month plus there was an small upturn in sunspot activity by the end of April. In Hydrogen-alpha there were several fine prominences, some filaments, and numerous areas of plage activity around the sunspots.
Here are the highlights for April 2016 together with a selection of images.


On the 1st the only Active Region visible was AR2526. This southern hemisphere sunspot was classed Hsx that day. There was little change as it headed towards the West (W) limb, passing over it on the 5th. We nearly had a blank Sun but two small sunspots had appeared by then: AR2527 and AR2528, both were west of the Central Meridian (CM) and both were on the northern hemisphere. They were type Axx and Bxo. AR2528 showed slight development by the 6th, but was then seen to decay the following day. It had gone over the W limb by the 11th.

We then saw on E limb on the 8th that AR2529 had appeared. This was a more substantial sunspot of type Hkx, showing the classic dark central umbra (somewhat “heart-shaped” at times) surrounded by a lighter penumbra. This northern hemisphere sunspot was several times the diameter of the Earth. By the 11th, as the sunspot headed away from the E limb, it could be seen more clearly. It had developed into a minor flare-active sunspot and had grown into a type Eki sunspot group. AR2529 was seen with the naked eye on the 13th and 14th, as it crossed the CM, and on the 17th. Two much smaller sunspots had appeared (AR2530 and AR2531) by the 14th but it was AR2529 that took centre-stage. Finally, in the early hours of the 18th AR2529, as it neared the W limb, produced an M-class flare.

On the 21st as AR2529 had departed the W limb, two small sunspots, AR2532 and 2533 remained. The first was on the northern hemisphere and near the CM, type Cao, while the second was on the southern hemisphere, type Hsx, and just over the E limb. Not much happened over the next few days. AR2533 was at the CM by the 26th with AR2532 nearing the W limb and AR2534 and AR2535 having appeared on the E limb on the 25th.

Towards the end of the month we saw a rise in sunspot numbers. By the 27th AR2536 has appeared near the E limb, while on the 28th AR2537 had emerged near the W limb. These were followed AR2538 and 2539 on the 28th and 29th respectively. Of these AR2535 appeared to be flare active.  

If you want to know more about the sunspot classification, see the guidance on our website at:

SPA Sunspot Mean Daily Frequency (MDF): 2.14 
SPA Relative Sunspot Number: 28.43

The 3rd saw two beautiful prominences, a large hedgerow type on the N limb and another tree-like prominence on the SW limb.

There was quite extensive filament and plage activity near the SE limb on the 7th together with some interesting prominence activity dotted around the limb.

Extensive plage and filament activity was seen around AR2529 on the 10th with some wide-spread but small-scale filaments scattered across the solar disk. By the 17th AR2529 showed fewer filaments than before but plage activity was still evident. By next day, as AR2529 neared the W limb, a number of prominences appeared on the S and SW limb. These were still visible on the 19th.

We then went into a relatively quiet period until the 25th when a hedgerow type prominence was seen on the E limb near the sunspot AR2525, which also showed extensive area of plage around it. The hedgerow type became a pair of arched prominences by the following day meanwhile numerous areas of plage were seen around AR2533, 2535 and 2536 on the 27th. A similar situation was seen on the 27th but a high flame-like prominence had appeared on the E limb. By the end of April we seemed to have few filaments but more small sunspots and plage activity. 

SPA Prominence Mean Daily Frequency (MDF): 4.72

Well done to Brian Gordon-States who observed the Sun on 29 days in April. Alan Heath observed on 27 days and Jonathan Shanklin was not far behind on 25 days.

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

Images and drawings were supplied by: Carl Bowron, Michael Fullerton, Mick Jenkins, Ian Lee, Cliff Meredith and Brian Woosnam.

Geoff Elston


Added by: