Harry finds a study in contrasts as he outlines the sunspots visible on the Sun in April 2012

ar 11459_465_fig1

Two southern sunspot groups. Sketches and copyright Harry Roberts ©, all rights reserved

Activity on the sun can be very unevenly distributed, both in location on the disc, as well as in time. Waves of activity come – and go. For the whole of SC24 (so far) northern hemisphere spots have greatly outnumbered southern ones. Why is this so?

Recently a burst of southern activity produced some interesting spots, interesting due to their differences rather than their similarities. Let’s look at three southern groups, 11459, 62, 65 and one northern, 11467. These groups were all so different it is hard to believe the same process produced them; and all were on the sun during the last half of April 2012.

AR11459 arose mid-month as an open grouping of scattered nuclei with very little penumbra, stretched across a large solar latitude as well as longitude. Emerging spots mostly spread east-west (due to the Hale-Nicholson force) with little north-south spread; but this group covered more than 5 degrees of south latitude. While it grew somewhat, it remained by far the most open and scattered of our examples (Fig1, lhs).

Umbral fields in its main spots were weak, with R21 (red 2100G) in the preceding spots, and V21 and V20 in the following ones. Fields > 2000G are needed to form penumbrae – and this group’s penumbrae were faint and hard to see. Almost 40 tiny spots could be counted in this group.

AR11459 looked like the “skeleton” of a major spot group, one that needed stronger fields to put “flesh” on its scattered “bones”.
ar 11462two_viewsFig2

Two views of sunspot group AR11462 with the second showing the group as it reached the edge of the Sun. Sketches and copyright Harry Roberts ©, all rights reserved

AR11462 by contrast, looked much healthier (Fig2, lhs). This was a classic bipolar group with fields in the range R22 to V23, resulting in large penumbrae with multiple umbrae, some elongated, in both the preceding (p) and following (f) spots. It emerged on the 18th and grew to its impressive size in little more that 24 hours. Despite this growth and strong umbral fields flaring was modest, no more than GOES C2.

This group was a fine sight at the SW limb April 23(Fig2, rhs) with several bright surges above it. Surges a and b are the type that appear near large penumbrae where emerging fields turn almost 90 degrees, and c is perhaps also a surge, tightly collimated, in more vertical fields of the following spot. Prominences x and y may be ejecting filaments unrelated to the spot group.

AR11465 emerged on April 19 and by 23rd had the appearance of a major active group (Fig1, rhs). It was compact with a dark penumbra holding many elongated umbrae and chains of smaller spots. And by the 24th the following V20 violet spot to the NE had joined with the main mass – promoting the group to Hale delta class: a sure predictor of strong flares (Zirin “Astrophysics of the Sun” Cambridge Uni Press. P402). Yet they did not occur. The strongest flare for this group was C2.5, only a bit stronger than those of tiny group AR11467, below.


A flare from the northern sunspot group AR11467. Sketch and copyright Harry Roberts ©, all rights reserved

AR11467, a northern group, was one that flared during the writer’s “watch”. This group is the easiest to describe: it was just a tiny dark speck or two, almost without penumbra. Despite its puny size it had three GOES X-ray class C flares (all Sun_4 May 8_42 am AEST_Nick Lomb

A preview of May’s activity – the large southern sunspot group AR11471 on the morning of 4 May 2012. Photograph and copyright Nick Lomb ©, all rights reserved

Overview. These active regions give a sense of April’s activity. There were many new groups, often short-lived, and they remained magnetically simple. AR11465, April’s only delta group, hosted some modest flares, but nothing like the month before. The contrast with March’s multiple X and M class flares was striking- but giant northern group AR11429 had dominated March’s activity. What will the month of May reveal?

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


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