Perhaps the most exciting event amateurs can hope to see is the eruption of a great flare. At first the large sunspot group seems normal, then a few isolated points quickly brighten, and as the amazed amateur watches, a brilliant flare lights up across a vast area. While the sun in H-alpha normally glows dull red, an erupting flare seems almost white in intensity; an amazing sight never to be forgotten. In this piece on H-alpha astronomy I revisit a great flare that occurred in sunspot group AR720 on January 2005.
In summary, AR 720 hosted a total of 85 flares of all classes, and five were the most powerful X-class. The drawings show an X 2.6 flare in progress.
Magnetograph data before the flare (image 1) suggested a magnetic boundary as shown dotted in red. Fast growth of “preceding” sunspots (arrowed p in fig) over the 48 hours prior to the flare pushed a large “following” f polarity spot into preceding position, causing REVERSED POLARITY for the whole group. This, and its complex Beta-Gamma-Delta magnetic class may explain the huge flares.
Image 2: AR 720 at 22:11 UT when flaring begins, drawing by Harry Roberts
Image 2 shows the flare starting at 22:11 UT, as a narrow bright ribbon following the magnetic boundary. Image 3 shows the peak of intensity at 23:08 UT, when the ribbons had spread away from the original line to a separation of 40,000 Km. The flare first arises along the polarity boundary, then spreads away from that line. The E-W length of the flare is about 100,000 km.
Image 3: AR 720 at 23:08 UT with the flare ribbons spreading and peaking at X 2.6. Note at 4. the polarity ribbon spreading along polarity boundary, drawing by Harry Roberts
Flares as powerful as this one (X 2.6) result in the ejection of billions of tonnes of matter (called CME’s) as well as vast webs of magnetic field from the sun into the interplanetary environment.
What’s needed to see flares? A hydrogen-alpha filter can be fitted to an existing telescope, or a dedicated H-alpha (only) ‘scope can be used; costs are $1400 and upwards depending on ‘specs’ The writer uses an old C8 fitted with a Lumicon H-alpha filter, and is VERY HAPPY with that remarkable ‘combo’. The drawings are three from a set made at the telescope eyepiece during the flare, over a period of about two hours.
Harry Roberts (expert Sun and Moon observer and member of the Sydney City Skywatchers)