Harry sees a short-lived sunspot group – AR11085

Sketches of short-lived sunspot AR11085

These are strange days for sun watchers: many emerging spots are faint and short-lived – some last less than 24 hours; here today, gone tomorrow! ‘Old hands’ suggest we are still in sunspot minimum – but most measures of activity show a slow rise from the deep deep minimum of 2008 –2009.

A good example is AR11085, that was briefly seen on June 29 when the 4”’Mak.’ (white light) was pointed sunwards. The large single spot AR11084 was clear to see on the sun’s disc – and well west of it a small bipolar spot cluster was visible too –“AR 11085?” my log suggests. Nobody else reported this new group – Monty saw nothing at the site only 5 hours earlier; Mt Wilson’s 150’ ‘scope had nothing 18 hours before. Timings sited the new group at –23/203; I expected the Net would soon bristle with reports.

I was to be disappointed; apparently nobody else saw the new group. And on June 30 I was surprised when it got a NOAA active region number AR11085 – clearly someone had seen it! (Sadly, NOAA does not reveal its sources.) Next day Mt Wilson reported one tiny spot at the 11085 site, with a weak umbral field of R17 (i.e. red 1700 gauss. Remember, around 1800G is the minimum for visibility).

Since the group had not been seen by Monty L six hours earlier, and when logged by Mt Wilson ten hours later, had almost gone, it seemed that AR11085 had had a very short life indeed.

Timeline of observations of short-lived sunspot AR11085. Drawing Harry Roberts

To get an Active Region number a spot has to last at least 24 hours – shorter-lived groups being so much statistical ‘noise’. How long did AR11085 actually last? Presumably NOAA knew it had a life of over 24 hours – at this point a timeline was created to find the probable lifetime of the group (Fig).

The timeline plots observations both negative (not sighted) and positive (sighted) for the spot group by the following: MtW is Mt Wilson’s 150’ solar ‘scope; ML is Monty Leventhal; HR is the writer; and NOAA the well-known US Authority.

Although Solarmonitor’s website asserts the group persisted through June 30th, the evidence (MtW) shows it was almost gone by 14:15UT on the 29th – and its persistence through the 30th is unlikely. Unless other records can be added we may conclude that AR 11085 lasted less than 24 hours – say 18 hours. Perhaps the group should not have gained a NOAA active region number – and it illustrates one of the challenges now facing sun watchers: recording current sunspots before they disappear!

Harry Roberts, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers