Harry observes a long-lived sunspot AR 11084 and considers if it is the return of a previously seen spot
Two sketches of sunspot AR11084, drawing Harry Roberts
While many Cycle 24 (C24) sunspots are hard to see and short lived, AR11084 was easy to see and long-lived. In fact, it is probably the return of AR11078 that first appeared in the sun’s SW quadrant on June 7th, when it emerged as a pair of tiny spots preceding (p) a pair of larger spots with some penumbra around them – a bipolar spot group.
It was next recorded on June 11 close to the solar SW limb – now both (p) and following (f) spots had grown larger and developed penumbrae – Helio freeware sited the group at –19/140 (i.e. solar latitude 19º south, longitude 140º) Bright faculae threaded between the (p) and (f) components. Umbral fields showed the bipolar group “broke” the Hale Nicholson Law – having reversed polarity for a C24 group, with violet polarity preceding red – the second reversed group of C24. Would the group survive a transit of the sun’s far side? Partly, it did!
June 26 showed a fair sized single spot at the sun’s SE limb with very bright faculae following 8º behind (Fig 1) – there was only the one penumbral spot present. H-alpha showed a low bright prominence above the ‘new’ spot and filaments and plage surrounding it. The ‘new’ spot was sited at –19/146. It slowly “dawned” on the writer that the new spot was the return of AR11078!
Two clues prompted this: one is positional. AR11078’s preceding component had a longitude of ~143º, (as well as the same latitude as the new spot 19ºS). Allow a degree or two of western drift that all (p) spots undergo and we see that the ‘new’ spot matches 11078’s western component exactly.
Another clue was polarity – the two parts of 11078 had reversed polarity, with the (p) spots having violet polarity (as stated) and the single monopolar spot of 11084 had violet polarity also, with bright faculae following. This makes AR11084 Hale class “Alpha preceding” – the bright faculae having persisted since first seen in AR11078 14 days earlier. Mt Wilson magnetograms showed that it was the following spots of the earlier group that had disappeared, leaving the (p) spot and faculae intact.
An extreme ultraviolet view of AR11084 from NASA’s Solar Dynamics Observatory, annotated by Harry Roberts
Alpha class sunspots have only one polarity with no other visible spots nearby. Hale first suggested (1920’s) they were bipolar BUT the spots of opposite polarity were invisible. This is in fact the case; and field transition arches (fta in Figs) from the single spot fan out and connect to multiple small polarities in the associated faculae; polarities too “warm” to be visible as spots in any ‘scope. A fine image from the SDO satellite shows exactly this in the corona above AR11084 (Spaceweather Archive for July 2nd – take a look. Note: the fta are NOT visible in H-alpha unless a flare has occurred when they show as post flare loops. SDO can detect them in EUV)
A sketch of spiral filaments around sunspot AR11084, drawing Harry Roberts
AR11084 was remarkably stable – growing a little to just over 100 area units as it crossed the sun’s face, followed by extensive dark filaments and plage –at one stage dubbed the “spiral” sunspot due to filaments that like the arms of a galaxy encircled the spot (Fig 2). The spot produced very weak flares– despite its umbral field slowly rising to V24 on June 30 15:00UT. The spot will surely survive to pass behind the sun’s SW limb a second time – and may yet reappear at the SE limb around July 23.; who knows?
Keep a close watch on the sun – anything can happen!
Harry Roberts, a frequent contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.
