Over the last two weeks, the Sun's outlook has been dominated by the big active region NOAA 2529. This sunspot group was relatively quiet, producing only a handful of low-level C-class flares... before erupting in a strong M6 flare early on 18 April. Despite being quiet most of the time, close examination of the available imagery showed important dynamics in the region, in particular the main spot. Note that at its maximum size, the main spot had a diameter of almost 5 times that of the Earth, and its total surface area was over 5 times that of our planet. No need to say this was an easy naked-eye object for solar observers (using eclipse glasses of course!).
This movie covers the period from 9 April (08:00UT) till 18 April (08:00UT). SDO imagery was used, and the clips were created using JHelioviewer. The beginning and end of the first clips are somewhat blurred, as the main spot was being tracked by the software and at those times the region was close to resp. the east and west solar limb. The white light clip shows the evolution of the main spot, with important movement from the penumbra (the grayish zone of the spot) into the umbra (the dark centre of the spot), as well as some outward motions. The fine, slowly moving bright structures that can be seen intruding the umbra are called light bridges and usually herald the decaying phase of the spot.
The next clip is a magnetogram laid over the white light image. Red denotes positive magnetic polarity (field lines coming out of the solar surface), while blue represents negative polarity (field lines returning to the solar interior). One can easily see the overall negative polarity of the main spot, but with small magnetic elements of both polarities streaming away from the sunspot. At the start of the clip, there's an intense reddish area to the east of the spot, becoming less bright while moving to the north, then intensifying again as it arrives to the west of the main spot (end of the clip). This is a result of line-of-sight effects when measuring the magnetic fields as the spot is close to the solar limb. See this news item for further explanations. There's also a distinct magnetic flux of positive magnetic polarity streaming outward along the main light bridge to the northwest. This is not a line-of-sight effect.
The next two clips link the surface imagery to images in the extreme ultraviolet (EUV). In "warm" temperatures of about 700.000 degrees (AIA 171 filter), a kind of fan-like structure can be seen developing, with a suggestive counter-clockwise rotation. Overlaid on the white light image, it is clear that it has its centre in the main spot. See image below. Fully grown by 12 April, it had dissolved already 2 days later.
In "hot" temperatures of several million degrees (AIA 094 filter), the fan-like structure is less obvious but nonetheless still present because part of the filter's passband relates somewhat to AIA 171's lower temperatures. Overlaid on the magnetogram, one has a good view on the (sub) flaring activity and on the magnetic field lines connecting areas of opposite polarity, in particular from the "negative" spot to the positive trailing portion of the group but also to the mostly spotless positive magnetic area in front (to the west) of the main spot. See image below.
The whole EUV structure is reminiscent of an "anemone" or a "fountain". This magnetic configuration is known for its strong flaring potential, but at the same time it is also notoriously difficult to predict the timing. A well-known example was observed in 2014, with the otherwise very quiet NOAA 2158 producing an M4 and X1 flare on resp. 9 and 10 September. See this news item , and the comparison underneath (EUV image overlaid on white light image).
NOAA 2529 was already decaying when it produced an M6 flare early on 18 April. The location of the flare was to the northwest of the main spot. In the clip, the purple hues are from AIA 094, while the green colors are from AIA 171. See image below. Material is ejected into space (but not to Earth), and the post-flare coronal loops are impressive. Solar scientists are going to have a lot of fun researching this dynamic and deceptively quiet sunspot group.
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