Today is the first day of winter in Australia and the rest of the southern hemisphere. Conversely, it is the beginning of summer in the northern hemisphere. In some countries people occasionally use the astronomical definition of the season that starts from the solstice on 21 June, but fortunately that is not done in Australia.
Winter surprise. Courtesy MS Clip Art
German-born American astronomer Martin Schwarzschild was born on this day in 1912. His father Karl had been a renowned director of an observatory in Potsdam. Martin Schwarzschild became a professor of astronomy at Princeton University in the United States and is best known for his work on the structure and evolution of stars.
The cover of the Dover publication of Martin Schwarzschild’s famous book on the Structure and Evolution of the Stars. Nick Lomb collection
Canopus, the second brightest star in the sky, can be found in the early evening by extending an imaginary line through the top two stars of the Southern Cross to the right or west. It is a supergiant star that has 100 times the width of our Sun and 10,000 times its energy output.
Finding Canopus with the Southern Cross (Crux on sky map). Courtesy 2013 Australasian Sky Guide
Three views of the flaring sunspot group AR11748. Sketch and copyright Harry Roberts ©, all rights reserved
On May 13th a sudden burst of class-X flares just behind the Sun’s east limb took sun watchers by surprise: CME’s were emerging from the site – then post flare loops adorned the eastern limb around latitude +10º. This sun watcher expected a giant spot group would soon rotate into view.
When the group dubbed AR11748 finally arrived it left no doubt of its credentials: it was in the midst of an X3.2 flare – with bright flare ribbons, flare loops, and a spectacular array of post flare loops, as reported earlier.
Yet the group was of very modest size, just ~200 area units, with only 5 or 6 spots in some meagre penumbrae. How could this sparse group be hosting its third class-X flare? And why do some spots flare much more than others?
These are good questions – and the answer is perhaps a complex one. It’s often wrongly thought that bigger and more complex sunspots have bigger flares – and big spots certainly draw the most attention.
The explanation for this group’s extreme flares lies perhaps with two key points: firstly, the most magnetically complex groups produce the strongest flares –i.e. they don’t have to be the largest groups.
Umbral fields: Secondly, as Livingston and Penn announced some years ago, this cycle, SC24, is unusual in having the weakest sunspot fields for about a century. Most earlier sunspot science was concerned with the size of sunspot umbrae and penumbrae: historic statistics hinge on this – but we know that when spot umbral fields fall below 2000G the spot’s penumbra fades away and that the umbrae are also much diminished.
The umbral fields of AR11748 clearly show this effect (Fig), with most spots well below 2000G: hence the penumbrae are much smaller. With stronger fields than 2000G we would have seen a much larger group with, perhaps, more powerful flares.
Magnetic complexity: the Figs show that the group’s spots had a disordered mix of opposite polarities (Hale class Beta-Gamma), and in view 1 we see unlike fields in the penumbra of the preceding spot – a Delta configuration – a predictor of the strongest flares, as witnessed. Magnetograms images at the time showed the group to be a blending of three or more dipoles, one of which was reversed it seems – a very complex magnetic brew!
Image 3 (Fig) shows the group on May 16 following an earlier M1.3 flare, when the flare’s brightness has fallen to GOES C3.8. Note the dark filaments winding between the complex mix of violet and red polarities.
Proper motion: We know that in most bipolar sunspot groups it’s the preceding (p) spots that move westwards while the following (f) spots ‘stay put’. However, AR11748 did things differently: the (p) spots stayed static at longitude 300º while it was the (f) spots that moved eastwards roughly 1º a day from May 14th to 17th.
All told, AR11748 was an unusual spot group, one that may have looked bigger and more active if only its umbral fields had been more like those typical of 20th century sunspots; but now in the 21stC we must adjust to a new reality. And despite the diminished sunspots, the Sun has shown us that it still knows how to create great flares, and sometimes they erupt with very little warning!
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.
In the early evening the bright star Canopus is high up in the south-west. It is the brightest star in the constellation of Carina the Keel and the second brightest star in the sky after Sirius.
Canopus in the constellation of Carina and its neigbours. Courtesy 2013 Australasian Sky Guide
A time sequence of flare loops associated with sunspot group AR11748. Sketch and copyright Harry Roberts ©, all rights reserved
“Post flare loops are an elegant feature of most large flares”.
I have often used this quote from Zirin’s “Astrophysics of the Sun” (P396) when such features have erupted; and the sun has done it again -so here is another attempt to describe these ‘elegant’ loops. On the same page we read: “they [the loops] are absent from some explosive flares, and may require a particular [magnetic] geometry”.
It appears that conditions inside a hot bubble blown in the corona by a strong flare permit material to condense on the magnetic arches connecting different parts of a spot group. Such arches have been abruptly changed by ‘reconnection’, triggering the flare. In the aftermath, material rains down from the apex of the arches to the sun’s surface (Figs).
The flare: the modest sunspot group, AR11748, hosted a very strong GOES X3.2 flare on May 14, 01:11 UT (the peak) that then caused the rapid growth of the loops. This was a big flare for such a modest group, one of four X- class events it hosted – a puzzle in itself. Why was it so active?
Flare loops: Flares start as bright low arches connected to foot points or ribbons of the flare ‘arcade’ and these rapidly separate as the flare reaches peak brightness. They are bright, at least twice the chromospheric background, often much more. These tiny flare loops (~10Mm high) are the brightest parts of the flare ‘arcade’ and the fainter parts are unseen. Fig1 shows both the two flare ribbons (r1 and r2) and several flare loops – some taller flare loops are rising above the limb. Several logs were made of their growth but only four are shown for clarity.
Post flare loops: as the flare ribbons (i.e. the foot points) separate, the loops grow taller (Fig2) and their brightness falls, typically to equal the chromosphere. When that happens (Fig, 3) the loops can no longer be seen connected to the flare ribbons (the disc is too bright) – although they may be visible off the H-alpha centre line at 6563Å and appear darker than the chromosphere. They are now termed post flare loops.
By 02:15 (Fig3, the flare is by now GOES M2.3) a dense mass of concentric post flare loops rises above the Sun’s limb. Their height, measured above the flare ribbons on the disc, is about 40Mm – not especially high for such a strong flare: at times they can be over 100Mm high.
By now, 65mins after the flare peak, they were an elegant sight indeed – much like a piece of modernist sculpture. An odd curved spike projected from the topmost arches – perhaps a lesser arch in the plane of our line-of-sight, the rest of that arch being invisible.
Fig4 (02:51) shows the fading stages of the display, with several incomplete arches visible, now mostly of chromospheric brightness or less, but still with ‘hot spots’ at the arch tops.
This was a wonderful display of transient solar activity – activity that is localised at the sunspot group that hosted the flare. While flare loops are mostly seen above much bigger spot groups than AR11748, still it hosted several great flares. What was the source of such extreme activity? For an answer we must examine the magnetic signature of the group, where we will find a complex blend of several solar dipoles: the subject of a future article.
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.
Pioneering Australian radio astronomer Ruby Payne-Scott was born on this day in 1912 at Grafton in NSW. She studied at Sydney Girls High School and then at Sydney University. During the War she was engaged in secret work on radar and afterwards turned her attention to studying radio noise from the Sun.
A new paperback book about the life of Ruby Payne-Scott, Making Waves by Miller Goss, will be released at the end of July.
Doodle on the 28 May 2012 centenary of the birth of Ruby Payne-Scott. Courtesy Google
To help you learn about the southern night sky, Sydney Observatory provides an audio guide/podcast, transcript of that audio, and a sky map or chart each month. This month’s guide is presented by Geoffrey Wyatt, Astronomy Educator at Sydney Observatory.
Among topics Geoff ranges through this month is ‘the irreversible process of spaghettification’ – Geoff’s inimitable way of describing what would happen should you chance into a black hole. Geoff’s enthusiasm and humour are captivating but based on sound science, and enriched with stories of historical and cultural aspects of astronomy, including Indigenous and Greek myths.
His highlights for the month include a conjunction in the west immediately after sunset of the planets Jupiter, Venus and Mercury. You also have the chance to see the Theta Ophiuchid meteor shower (from late May through to mid June). The peak is typically around the 10th to 11th June, early in the morning. For this and much more, listen to the audio, or read the transcript below for more details.
HEAR THE AUDIO
You can subscribe with iTunes or upload the (54 mins) audio to your iPod or mp3 player, or listen to it on your computer.
SEE THE SKY CHART
We provide an embedded sky map (below) and a June 2013 night sky chart (PDF) which shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.
BUY THE BOOK
Our annual book, ‘The 2013 Australasian sky guide’, by Dr Nick Lomb has more information and star maps for months from December 2012 until December 2013 inclusive, plus information about the Sun, twilight, the Moon and tides, and a host of other fascinating astronomical information. You can purchase it ($16.95) at Sydney Observatory and Powerhouse Museum shops or other good bookshops, or online through Powerhouse Publishing (additional packing/postage costs apply).
READ THE TRANSCRIPT (after the jump)
The brightest planets in the night sky are Venus and Jupiter. Early tomorrow evening and the following evening we have the spectacle of these two planets close together in the sky with a separation of just over one degree or two moon-widths. Unfortunately, they are close to the horizon in the north-west with Venus setting just as the sky darkens.
A previous Venus and Jupiter conjunction, the conjunction of 2 September 2005. Photo Nick Lomb
NASA’s Phoenix spacecraft landed in the north polar region of Mars on this day (Australian time) in 2008. Phoenix landed in the late northern hemisphere spring on Mars and continued to function until winter when there was no longer sufficient power collected by its solar panels.
A selfportrait of the Phoenix Lander on Mars. Courtesy NASA/JPL-Caltech/University Arizona/Texas A&M University
