Thursday, March 3, 2011

The Largest Sunspot Group I Ever Observed

Sunspot group designated Mt. Wilson No. 21862, taken by me with a 200mm catadioptric telescope on Nov. 4, 1980.

As Solar Cycle 24 ramps up and large area spots become more commonplace, it is interesting to take a historical perspective. So far, no spots have attained the monster size of 2000 msh (millionths of a solar hemisphere). This scale typically corresponds to linear diameters of the order of 70,000 km, which may be compared to the Earth's diameter of 12, 800 km. Thus, more than five Earths can be placed end-to-end across it.

The largest sunspot group I've ever observed (designated with Mt. Wilson No. 21862) peaked at 1510 msh in area (~51,000 km linear dia.) on Nov. 4, 1980 when the above photograph was taken using a 200mm aperture catadioptric telescope. The group itself marked the return of McMath plage region 17181, and first appeared on the sun's east limb on Oct. 30, 1980 with an area of 70 msh. In other words, the area more than a factor 20 within a span of five days. By 1st November, the group was in the complex delta class, which is determined by the presence of mixed magnetic polarities in the same region. (The Active region itself was given the designation AR2776).

Most conspicuous in the course of the group's evolution was the leader spot's pronounced, elliptical penumbra (outermost, lighter sector). This appeared to fragment from Nov. 6 which may have led to a steady decrease in total group area from that date. The emergence of a defined vortical structure in the penumbra occurred thereafter and was accentuated by the formation of a "light bridge" within the penumbra. Al through these changes, and for the duration of the solar disk transit the large group (which never went below ~ 910 msh in area) remained in the delta class.

On November 5th, the day after the photo was taken, an "intruding arm" appeared which I took to be coincident with the neutral line, or magnetic inversion line. (Line separating + and (-) magnetic polarities in the region).

As the large group's area and magnetic changes increased over Nov. 5-8 I found it useful to consult the SMS-GOES 1 - 8 Å x-ray records to assay all SID-type flares. See e.g.

My assay documented no fewer than 15 SID-flares over the period, with 4 of Importance 2+, and two of these occurring on November 7. These corresponded to 15 optical (H-alpha) flares ranging from an SB subflare releaseing ~ 1.5 x 10^22 J of energy on Nov. 8, to a significant 2B flare on Nov. 7, releasing 10^24 J of energy.

Needless to say, this observational record detailing both area and magnetic as well as fine-scale optical changes (in the penumbra) will prove immensely valuable as the new solar cycle unfolds with its own collection of large sunspot groups. How many of those will emulate the patterns (and flare frequencies) of Mt. Wilson No. 21862? We will have to wait to find out but it ought to prove exciting as this cycle may even rival solar cycle 20 in which the earlier group manifested!

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