Solar Highlights for April 18

Solar Rotation Numbers: 2202 – 2203

A fairly high level of spotless days in April 2018 even though the average ‘MDF’ has increased a little. When sunspots did appear, they looked to be part of the current Sunspot Cycle. Why do I say this? Well, broadly speaking, the sunspots we had were of mid-to-low latitude, whereas new Sunspot Cycle sunspots tend to appear much a higher latitude. AR2707 seemed to last a bit longer than others. The last high latitude sunspot we saw was in January (AR2694 at -32°). As we pass through Sunspot Minimum we should start to see more high latitude spots appear.

Here is a summary together with a selection of images made by our observers.


AR2703 was still visible from last month. This tiny southern hemisphere sunspot at latitude -08° and still sunspot type Axx on the 1st of April. It disappeared the following day, having survived about 3 days.

The Sun was spotless from the 2nd until about the 13th.

On the 10th another tiny sunspot appeared roughly in the middle of the solar disc. However, it didn’t really develop into anything and so the Sun remained devoid of sunspots a few more days until the 13th.

AR2704 appeared on the 13th at latitude +10°. It did not change much but seemed to shift slightly north to +12° before decaying rapidly on the 15th and disappeared by the 16th.

The Sun was spotless from the 16th until the 18th.

There seemed the chance of something developing at latitude +10° (where it would have been designated AR2705) but it never really materialised into anything clear-cut.

2018 April 19 @1406UT AR2706 white light. Image taken by Mick Jenkins.

AR2706 came over the eastern solar limb around the 18th and 19th. It was close to the solar equator at +04° and of type Bxo. This sunspot developed into type Cao on the 21st and finally into type Dao on the 22nd. Its growth was short-lived, and it was gone by the 24th.

2018 April 20 @1532UT whole disc white light. Image taken by Cliff Meredith.
2018 April 22 @1438UT AR 2706 white light. Image taken by Mick Jenkins.

AR2707 was the second southern hemisphere of the month at latitude -10°. It was type Axx and was nearby AR2706 from the 22nd to the 24th. AR2707 lasted several days until the 29th, when it disappeared as it approached the western limb of the Sun.

The Sun was spotless from the 29th onwards.

SPA Sunspot Mean Daily Frequency (MDF): 0.42 (was 0.02).
SPA Relative Sunspot Number: 5.99. (was 0.21).

Solar Prominences, Plage, Filaments and Flares

Despite the small increase in the average ‘Prominence MDF’ this month compared to last month, there were quite weak levels of prominence activity in April. Most prominences that appeared were either small or quite dim but there were the occasional exceptions.

There was an impressive ‘hedgerow’ type prominences on the south east limb (the edge of the Sun’s disc) on the 5th and the 21st when a sizable ‘smoke-stack’ prominence on the southwest limb. A few days later, on the 25th, we had at least five small prominences were seen around the solar limb.

2018 April 5 @0847-0849UT Ha prominences. Images by Carl Bowron.


2018 April 14 @1300UT Ha AR2704. Image by Carl Bowron.

Bright plage activity did not pick up until the 20th, when AR2706 appeared and was surrounded by it. This plage activity only began to fade away as the sunspot group neared the western limb at the end of April.

2018 April 25 @0845UT Ha whole disc drawing by Ian Lee.

The same was largely true of filaments. They either were not there, or small or faint. Again, there was an exception, this being on the 28th when AR2706 (by then much faded in white light but still very much visible in Hydrogen-alpha light) had a single dark curved filament reaching out towards the northeast.

2018 April 26 @1124-1129UT Ha prominences. Image taken by Carl Bowron.
2018 April 26 @1142UT Ha AR2706. Image taken by Carl Bowron.

No solar flares were reported.

SPA Prominence Mean Daily Frequency (MDF): 1.71 (was 1.11).

Brian Gordon States observed 24 days. Alan Heath and Jonathan Shanklin both observed 19 days each and Bob Steele 15 days.

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

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

Geoff Elston