The ejection of a filament near the south pole of the Sun on the morning of Sunday 16 June 2013 (Australian time). Sketch and copyright Harry Roberts ©, all rights reserved

As the Sun nears solar maximum, a variety of unusual things are expected; one is the development of so-called polar crown filaments.

As sunspots emerge in the Sun’s lower latitudes, streaks or plumes of magnetic field, the remains of earlier sunspot groups, slowly migrate towards the Sun’s poles, and these ‘streaks’ cause the reversal of polarity of the polar fields. This reversal usually occurs around solar maximum, that is, about now! And when these ‘remnant fields’ encroach on polar fields of opposite ‘sign’, polar filaments can develop. At times, this can result in a succession of filaments that encircle the sun’s north and south poles: termed a polar crown.

Indeed, we may be seeing such a ‘crown’ at the Sun’s south pole right now. For some time H-alpha logs have shown large stable prominences at high southern latitudes, between 60º to 70º south, perhaps a south polar crown. In mid June, tall prominences appeared daily in this latitude range, and on June 15-16, an impressive ejection of the western prominence was seen (Fig above).

Two tall prominences near the south pole of the Sun on the morning of Sunday 16 June 2013 (Australian time). Sketch and copyright Harry Roberts ©, all rights reserved

Peter Meadows © Helio freeware is well suited to finding the latitudes and longitudes of limb features – as it does for sunspots. Although a feature at the limb may be of uncertain longitude, the latitude can be found with accuracy. On June 15-16 two tall prominences flanked the Sun’s south pole (Fig), the western one at the high latitude of 67ºS, with an eastern one at 58ºS. Both had been present at these latitudes for four days, as though they were fixtures, despite solar rotation of ~40º over that period.

Over successive days both prominences showed activity, with material from each apparently ‘flowing’ into space: by the 15th-16th (local day June 16) both had reached great heights, with the easternmost at 110Mm, while the western one reached ~60Mm (Fig). The eastern one seemed the most active – but the western one was, in fact, soon to erupt.

The latter, the fainter of the two, was first recorded at 21:58UT to 22:40, when ‘white light’ viewing began. At 23:40 H-alpha was restarted, when changes were noted in the western prominence – in fact it was in fairly rapid motion, as the figure shows. Over the next 70 minutes the prominence ejected off the disc in a spectacular way. The earlier rounded shape changed into a tall thin plume with an attachment to the disc south of the ejection site. After just ten minutes the prominence had climbed to over 120Mm; by 00:17 (June 16th UT) it was 160Mm above the limb.

The material was faint throughout the ejection with no excitation brightening noted. The logs show the development of a connection point moving southwards (polewards) while the initial ejecta continued to rise almost vertically above the site, and it is assumed that a long polar filament was ‘peeling’ rapidly off the sun. By 00:31 the ejecta was >190Mm above the limb; the length of the rising filament now exceeded 250Mm, almost half a solar radius! By 00:50 the filament had mostly faded to invisibility except for a brighter fragment some 210Mm above the limb, and this faded from view around 01:00.

The eastern prominence remained unaffected by its neighbour’s demise: material continued to circulate up and down within the structure, and it was still present the following day. While the southern polar fields are due to reverse their polarity shortly, there is no sign yet of that happening and more prominences can be expected to reform at these high latitudes.

As Zirin says in his final remarks in Astrophysics of the Sun, “Here before our eyes is a real star (and) we can explore the world of magnetic phenomena and convection in plasmas. I invite the reader to join the fun”.

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

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.

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.

The partial eclipse of 10 May 2013 photographed in the red light of hydrogen atoms at 9:15 am AEST. North is down and west is to the right. A number of small prominences are visible along the edge of the Sun. Photo and copyright Monty Levental OAM ©, all rights reserved. Supported by the Donovan Astronomical Trust

The eclipse of 10 May 2013 that was annular along a track passing across northern Australia and partial elsewhere in the country has been and gone. It has left a legacy of fine photographs such as the magnificent mid-eclipse shot from Melissa Hulbert from whom more photos are expected. The partial eclipse from those not on the eclipse track was also most impressive with good weather over much of the country.

The Sun partially covered by the Moon just before mid-eclipse at 8:41 am on 10 May 2013. The photograph was obtained by hand-holding a compact digital camera to the eyepiece of a small telescope – parfocal photography. A Baader film neutral density filter was in front of the objective or large lens of the telescope – this filter does not change the colour of the sunlight and hence the photograph does not show the attractive but false colour provided by some other filters. Photograph Nick Lomb

Now that the 10 May 2013 solar eclipse is over we have to wait until next year 2014 for more eclipse photo opportunities. April 2014 will be a great month with both an eclipse of the Moon – on the evening of the 15th – and a partial eclipse of the Sun two weeks later. In the meantime there are other events in the sky that could be photographed with simple equipment such as Jupiter and the Moon on the evening of Sunday 12 May 2013 and the planetary groupings low in the west later in May.

Four views of rapidly changing post flare loops and other phenomena associated with sunspot group AR11726 at the edge of the Sun. Sketch and copyright Harry Roberts ©, all rights reserved

At the Sun’s limb we see events ‘side on’ and silhouetted against the blackness of space. At the limb we can measure the relative heights of H-alpha features, on the assumption that they are ‘normal’ to our line of sight.

When a large and complex spot group crosses the limb we may see ‘things’ erupt that are very hard to detect when the same group is viewed on the bright disc.

In late April AR11726, a large and magnetically active spot group, passed behind the Sun’s western limb and at the same time hosted a moderate flare; the resulting fireworks were impressive.

The figure shows the group on April 26 (UT, local 27th) when most of the spot group had gone behind the limb – only a large following (f) spot remained. In white light it was conspicuous near the limb, when Helio freeware, © Peter Meadows, and transit timings gave the spot’s latitude and longitude: +13,320, in good agreement with earlier data. As well, “Helio” gave the coordinates for the adjacent limb: +14,328, showing that the (f) spot was still some 8 degrees of longitude from the limb and that the preceding (p) spots at long. 331 were no longer visible.

The first H-alpha session showed little activity at the limb near AR11726, some small surges only, and after logging other H-alpha features on the disc, the ‘white’ or integrated light session began, to record spot groups in detail and their positional data. While this was happening the GOES flux logged a C5.7 flare at 22:25 that went unnoticed!

View 1: Resuming H-alpha at 23:03UT showed a small bright post flare loop, PFL, at the AR11726 site (23:07). Other types of H-alpha transient were also active, some flare-related. The post flare loops were 42Mm high, less than those of a great flare (~100Mm), but eye-catching nonetheless.

The loops are field transition arches (FTA) that emerge above sunspots and are usually invisible – but after a flare, coronal material ‘condenses’ along the arches and drains down to the solar disc – making the loops briefly visible. These small arches showed two brighter ‘droplets’ involved.

Two tiny flare loops (FL) close to the remaining sunspot are most likely the flare itself, seen side-on – and are ~10Mm high. Also several bright surges of various shapes rose above the limb. In the south one surge seemed to be involved with the large PFL (view 1, below).

View 2: Seven minutes later at 23:14 the large loop was fainter with bright knots and some gaps. The flare loops had evolved, but were still small and bright. Two large surges now dominated the view, a degree or so south of the (f) sunspot – one showed how surges can bend through a right angle.

View 3: the PFL’s, now twinned, were slowly fading – and the flare loops have gone – the flare probably over. The surges however are still bright and active.

View 4: The big surges had retracted, but a pair of small ones is erupting close to the penumbra of the (p) sunspot – the usual site of surges. The two ‘counter’ curves crossing the original post flare loops are surge ejecta – temporarily ‘frozen’ and soon to recoil back into their ejection site.

These dynamic transients are all typical of a complex spot group – and we may ask why they are not so often seen when the group is viewed while on the Sun’s disc? One reason is contrast – the transients, when viewed against the bright chromosphere, are faint or invisible. Another reason is Doppler shift: surges eject at ~50-100km/s and have a Doppler shift of 1-2 angstroms, making them invisible in most H-alpha filters.

When a big spot group rounds either limb it is the time to search for rare H-alpha transients – particularly if the group is hosting regular flares. And you need a little bit of luck too!

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

The large flare associated with sunspot group AR11718 sketched on the morning of 13 April 2013 (Australian time). Sketch and copyright Harry Roberts ©, all rights reserved

Nothing on the sun is as exciting as a big flare. In H-alpha the surface of the sun within a spot cluster will suddenly brighten to at least twice that of the background, usually as several narrow curved ‘ribbons’. These soon ‘double’ themselves, then separate rapidly while still brightening. Often there are chains of bright points that grow into ribbons. Sometimes, with good ‘seeing,’ bright arches can be detected connecting the expanding ribbons – an ‘arcade’ of flare arches.

Simultaneously, the X-ray flux from the GOES satellites will ‘spike’ up, as a burst of x-rays hits their detectors. The satellites can’t say where on the sun the flare has erupted, we must use our H-alpha ‘scopes for that.

Visual Classification. The strength of flares depends much upon the total power released during the event. Flares can be classed by a visual ’rule of thumb’ that combines the area of the flare with its brightness compared to the chromosphere (IAU Commission 10, 1966, see Zirin “Astrophysics of the Sun”, 1986, p347).

Flare brightness. Flares begin as areas of plage, which is 1.4 times brighter than the chromosphere. When the plage brightens to twice the background it can be called a ‘faint’ flare of class ‘f’. Classes ’n’ (normal) and ‘b’ (bright) are successively brighter flares. I know of no exact definition of these two classes – except to say the ‘n’ is the commonest, about five times chromosphere brightness. Class ‘b’ flares, up to ten times background, are rarer.

Flare ‘importance’. This is, in effect, the area of the flare, and has five classes: S (a sub-flare) followed by classes 1 to 4. These are defined as the area of the flare in square degrees thus:

Class S = <2 square deg.; 1=2.1 – 5.1; 2=5.2 – 12.4; 3= 12.5 – 24.7; and 4 = >24.7.

This assumes the viewer can measure the flare’s area in fractions of square degrees: an uncommon ability. These areas however convert to millionths of the visible hemisphere – and Peter Meadows’ © “Helio” freeware calculates this with ease, assuming you can locate the flare on a Stonyhurst disc. Visual flare classification, though useful, is superseded by the GOES X-ray flux measured in Earth orbit.

The detailed anatomy of sunspot group AR11718. Sketch and copyright Harry Roberts ©, all rights reserved

M3.3 flare. Sunrise on April 12 was clear, and as the ‘scope was assembled the GOES website revealed a class-M flare in progress. The eyepiece showed it in AR11718. The event was hastily logged with timings through key spots of the group – and the flare was mapped to scale.

“Helio” gave it a large area, 450 units – the largest for the writer has seen in SC24, so far.

As noted, the start of the flare preceded sunrise – so its early growth was not seen, and it ‘peaked’ ten minutes before viewing began. At 20:49UT however the flare was of ‘normal’ (i.e. n) brightness, although the event was ten minutes past its peak – and was bright enough to obscure the sunspots below the flare ribbons (Fig).

At its peak, this flare was probably of brightness ‘b’, greater than five times background!

In the eyepiece bright flare ribbons spread SW from the many following (f) spots at +23º, 104, and also between the (f) spots and the more complex preceding (p) spots, covering much of the latter. It looked like a crude letter ‘N’ splashed in white on the sun’s red ‘face’.

Flare Area. “Helio” showed the spot group covered some 500 area-units and the flare 450 units – they were almost the same size! At 450 units (millionths of the visible hemisphere) the M3.3 was just below the ‘area’ (or ‘importance’) class 2 (i.e. 500-1200 area units). Perhaps at its peak it was a 2b flare – but my log shows a 1n.

Attempts have been made to reconcile visual classifications of flares with the GOES soft x-ray flux. In “Astrophysics of the Sun” (P 347) we see a 1n flare approximates a GOES M3 – in good agreement with our case. However, experience suggests that visual classification is an unreliable guide to the x-ray flux for most flares.

In H-alpha, 70 minutes after the flare peak, it was still twice the chromospheric brightness i.e. visual-class 1f.

Proton Effects. This M3.3 flare produced only a slight lift in the already elevated proton effects due to an earlier and stronger M6.5 flare in AR11719, 36 hours before. The lack of a proton effect may be due to the relative alignments of the solar eruption and the tiny ‘target’, Earth, one hundred solar diameters away.

This was a ‘great’ flare, the largest I have logged this cycle. And, hopefully, there are many more to come – just around the corner!

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

A dark filament at the north-east edge of the Sun sketched on three dates in March/April 2013. Sketch and copyright Harry Roberts ©, all rights reserved

As new sunspots develop over time they slowly migrate towards the sun’s equator. Simultaneously, ‘remnant’ fields from earlier sunspots drift towards the sun’s poles: a kind of ‘polar current’. These spotless plumes of weak field are termed ‘streaks’ by some researchers and they are hard to detect even on sensitive magnetograms. However, h-alpha users can expect to find “quiescent” filaments forming where streaks of unlike polarity come into contact.

Yet there is a puzzle here: while streaks of unlike polarity are often found side by side, a filament does not always form; it seems something extra is needed for that to happen. What is it?

Big Filament. On March 27 a large dark filament adorned the sun’s north east limb (Fig, top) – and timings put the leading end, or ‘head’, at forty degrees north latitude, longitude 239 degrees, (i.e.37º east of the central meridian). The filament’s ‘tail’ passed behind the eastern limb at 44ºN, 194º longitude. Already forty degrees onto the disc, it had been overlooked a day earlier.

New timings on March 29 sited the filament’s ‘head’ at +40, 236, in good agreement with the earlier ones. Now a tall prominence above the northern limb was clearly part of the dark filament – and the prominence was over 60Mm high. These features were also seen on the 28th (not shown here).

Recalling Zirin’s maxim that “virtually every prominence that rises above 50Mm will erupt in 48 hours”, it seemed an ejection of the filament was imminent.

Big Ejection. The writer missed the main event when the whole filament, by then crossing some 90 degrees of longitude, peeled off into space, appropriately at 02:00 on April first (!) when my attention was elsewhere. The map (Fig, bottom) shows the filament erupting (dotted, red), and the filament ‘channel’ (dotted, grey). The SDO satellite recorded the filament ejection in the helium 304Ǻ band of EUV.

Active Prominence. However, when next viewed on April 1st at 20:40UT(18h later), the ejection was still, apparently, in progress! The dark filament of the previous five days was gone, but at the site a large prominence was slowly rising into space (Fig).

It appears the ejection event of April 1 (02:00) was not ‘complete’, and that a remaining fragment was slowly rising and rotating clockwise at the same time (Fig, three views). The SDO record showed that the prominence did survive the main ejection – and seemed to be rearranging itself to reflect the new ‘reality’.

One part of the complex magnetic structure that shapes a filament – i.e. the ‘filament channel’- usually survives an ejection and a new filament may quickly reform at the same site. We will soon find out, given clear skies.

Three views of the active prominence on 1 April 2013 (Universal time). Sketch and copyright Harry Roberts ©, all rights reserved

The second figure shows three views of the active prominence and its subtle gyrations over forty minutes. Next day (cloudy) H-alpha patrol images showed the large prominence had gone.

Polar Crown. As the ‘streaks’ of old field drift pole-wards during the cycle, filaments occur more often at higher latitudes. Termed quiet region filaments, they can form a ring of filaments (i.e. prominences) at these higher latitudes – often called a “polar crown”. While its ‘early days’ yet in SC24 – this big filament may be a harbinger of bigger ones to come.

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

A sketch of the sunspot group AR11692 with the locations of two sunspots seen at the previous rotation of the Sun marked at α and β. Sketch and copyright Harry Roberts ©, all rights reserved

While much effort is spent on flare prediction models, big flares still erupt in spot groups with no signs of flaring potential. When AR11692, a large round, simple sunspot, rotated into view on March 10th it seemed an unlikely flare candidate – yet, on March 15 it hosted a strong M1.1 class flare. Why?

The older logs showed the new spot had made an earlier transit of the sun in February, maybe as AR11671, when it was ‘as quiet as a lamb’. That is, up until Feb 18 when a new sunspot group (i.e. dipole) arose close by: AR11678. The two close groups then ‘blended’ in an unusual way as earlier reported.

Briefly, the new preceding (p) spot of 11678 grew rapidly and pushed closer to the old single 11671 spot: these ‘blended’ groups produced many flares in the range GOES B8 – C2 in February.

As they rotated out of view (Feb 23) we asked if any of the main spots would return in early March? And one of the three seems to have done so.

But which spot we wonder? The logs show that 1671 was located at +14, 76, and the (p) spot of 1678 arose about +11, 69 (Feb 18), but moved rapidly westwards, reaching +10,74 by the 20th. Meanwhile, the older 1671 spot shrank as the ‘intruder’ grew larger.

AR11692. Based on this ‘history’, it seems the new AR11692 spot is, in fact, the old (p) spot of AR11678 – it has the ‘right’ polarity (violet 2400G) and right position (+10,75 see Fig); in addition, it is followed (f) by a large area of opposite (i.e. ‘red’) polarity – now devoid of following spots (Fig, magnetogram, white area). The whole region, it seems, is a long-lived ‘activity nest’ . (Schrijver, C and Zwaan C. “Solar and Stellar Magnetic Activity, Cambridge Uni Press, 2008, pp142-143).

The now vanished spots of old blended groups 1678/1671 are dotted in the figure – alpha is the old 1671 spot above the ‘survivor’, and beta, the complex (f) spot of old1678 – east of the ‘survivor’ at +10,66.

M1.1 Flare. This complex history was not at first obvious – and it was a surprise when the simple AR11692 spot produced the second M-class flare for March, a GOES M1.1 on Mar 15 at 07:00UT.

Only then did the complexity of the site become clear. The flare is the product of the strong active region filament, or inversion line (X-X’), produced by February’s ‘collision‘ of the two earlier sunspot groups.

Flares this strong are uncommon at present; with one on March 7 and another Feb 17th and it erupted in AR11692 some 14 hours before the Mar 15th log (Fig). An NSO-GONG image made at Learmonth in WA showed the large flare – outlined red in the figure. It arose along the filament X – X’ (dotted) and the ribbons spread to sites R1 and R2 at the 07:00 UT flare peak.

March 15 log (Fig). This combines the WL and H-alpha records of Mar 15, 20:39 – 22:03UT with the GONG flare record of 14 hours earlier added (Mar. 15th, 07:00UT, not seen by the writer). The sites of February’s two other spots from the 11671/11678 blending are dotted, alpha, beta (Fig).

The inversion line X – X’ between regions of opposite polarity has persisted since February. An unstable filament was recorded at the site (arf1) – perhaps it had reformed since the M1.1 flare. More strong flares at the site are likely.

Some new groups are also shown (Fig) – these are AR11695, a single spot with a strong V23 field some 20º to the east; and to the west the following (f) spot of AR11696. Both groups are unrelated to the ‘survivor’ from February.

Why a strong flare? Zirin notes the longevity of the inversion line (“Astrophysics of the Sun”, Cambridge Uni Press, 1986 pp333-339, 7) after Delta class spots decay – and their tendency to host repeated ‘spotless’ flares. In our case the flaring at the site X – X’ has strengthened since the modest ones of February.

Where has the energy come from? Presumably a new dipole or two has augmented the ‘survivor’ spot –it now has stronger V2400 polarity – yet the only new spots seen are a scatter of tiny violet ‘pores’ around the old spot on the N and W sides (Fig, s1-s4). Perhaps this was all it needed for an M1.1 flare.

In the same work Zirin notes that Delta-class spot groups most commonly arise when – “a growing bipolar spot collides with another dipole so that opposite polarities are pushed together” (Zirin, H. “Astrophysics of the Sun”, Cambridge Uni Press, 1986, pp333-339, 7).

Presumably this is how the original blended groups of February arose – and the site is far from dead yet!

M1.1 Geo-effect: a moderate proton storm resulted from this unexpected flare – striking Earth in the early hours of March 16 – and high latitude aurora watchers rejoiced!

On the sun, you never know what will happen next! Even quite simple sunspots can produce strong flares, at least if their history is as complex as that of AR11692.

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

Sketches of the adjacent sunspot groups AR11671 and AR11671 in white light and in hydrogen alpha – the red light of hydrogen atoms. Sketch and copyright Harry Roberts ©, all rights reserved

On Feb 10th a mid-sized sunspot came around the sun’s northeastern limb; dubbed AR11671, it was a single round spot sited at +14,75. It rotated across the sun, pretty much ‘minding its own business’, for a week or so. Now and then a tiny following (f) spot appeared briefly, a few degrees behind it – but they didn’t last.

This ‘placid’ scene changed abruptly on Feb 18, when a new sunspot group (or dipole) burst into view close by and grew rapidly bigger – it was centred at ~+9,70. Dubbed AR11678 it showed some odd behaviour. As the new spots emerged, looking much like a normal bipolar group, two new spots also emerged to the NW of old group 1671. SDO continuum images showed these new spots emerged simultaneously with AR11678 – as if part of it – in effect, blending the new group with the old: unusual sunspot behaviour. The new spots (Fig, at top, arrows) had the same polarity as the old 1671 spot.

This behaviour seemed to be a variant on the ‘standard model’ of sunspot growth, discussed previously, whereby a sunspot dipole may grow to a maximum of ~50Mm, and any further growth is the result of a new dipole emerging nearby and joining ‘end to end’ with the earlier group (Zirin “Astrophysics of the Sun” Cambridge Uni Press (1986) P. 320). Rearrangement of sunspot fields is the usual result, and this causes flares, ejections, surges etc.

In the 1671/1678 example the new dipole was far enough away to be termed a new group, albeit very close. Older systems of sunspot classification treat close groups as separate if they are more than 5º of latitude or 10º longitude away from each other – a useful ‘rule of thumb’, but not always correct. SDO logs show sunspots freely interconnect their fields even to groups many times further away.

Old and new. The blending of old and new groups in our case did produce a strong rise in activity – much of it centred between the two groups – and flare ‘stats’ attributed events to one or other of the groups accordingly. None were ‘great’ flares – but many GOES C class and B class erupted, almost hourly. The Figure shows a very pretty C1.7 flare (visual class 1N, Feb 20, 21:58UT) close to the AR11786 (p) spot, attended by a mass of brief surges and some active region filaments. This one example was typical of the many low-amplitude short-lived flares and dark surges that, in my case, were mostly hidden by cloud. An M1.9 on Feb 17 was in an unrelated spot group, AR11675.

Umbral fields. Nothing seemed abnormal in the umbral fields of AR11678 – in fact Mt Wilson assigned Hale-class beta (low complexity) throughout – but NOAA (using other data it seems) quickly assigned Hale class beta-gamma-delta to the group: the most active class. There are hints in the Mt Wilson data of ‘mixed’ (or delta) polarity in the two lesser spots sited at +9, 66 (Fig, below).

Possible return. The possible return of blended groups 1671 and 1678 around March 10 may shed more light – even as they went behind the NW limb the presumed delta spot had gone – as delta spots do.

While there’s many a small flare, where are the great ones?

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

A compass needle is an example of a magnetic dipole – two separated magnetic poles of opposite polarity. In a magnetic field like that of the Earth the needle pivots to align itself with the field. This is a detail from a surveyor’s compass made and/or marketed by Angelo Tornaghi of Sydney in the 1860s. Courtesy Powerhouse Museum

All sunspots start out as a magnetic dipole – but what is a dipole? Wiki says “a magnetic dipole is a closed circulation of electric current” and on the sun it’s agreed the source of the current is a dynamo at the sun’s core. Despite the strong fields that are said to exist in the core (>100,000G) I am not aware that the dynamo has yet been imaged in any way.

We do, however, have seismic images of the buoyant ‘tubes’ or ‘ropes’ of magnetic flux as they emerge at the sun’s surface. Some twisting of the flux tubes below the surface is seen – but only to a depth of 100Mm or so. Are they twisted all the way down?

Certainly after emergence the dipole’s arches are briefly visible in H-alpha and are no longer twisted. Cut a flux ‘rope’ and we would find a ‘south’ polarity at one end and a north at the other, like an electric circuit – and current flows through the flux ‘ropes’.

Dipole emergence: Here’s how Zirin (“Astrophysics of the Sun”, Cambridge Uni Press, 1986) describes the emergence of a sunspot dipole: “Pores (tiny spots first) appear, clumped in bipolar form, and intense X-ray or H-alpha emission is seen”. P317. And “The bright H-alpha or CaII K plage is crossed by groups of dark fibrils connecting the two ends. The fibrils, called an arch filament system (AFS) by Bruzek (1967), always show upward motion”. The fibrils appear dark against the bright plage.

Most observations of sunspots show groups that are already days old – catching the first moments of spot emergence is rare. This is partly because the emerging dipole is only visible briefly as arches within the H-alpha fibrils and they are small and of low contrast.

Bright plage will be present too, but may be small in area. Tuning the filter while watching closely may reveal the arch tops and footpoints – they have strong and opposite Doppler shifts. Also ‘baby’ spots or pores are tiny (<1000km dia.) and close to the limit for a ‘scope on Earth.

Sunspot group AR11670 between 6 & 9 February 2013 as an example of the development of a sunspot dipole. Sketch and copyright Harry Roberts ©, all rights reserved

The AR11670 dipole: The figure shows the history of AR11670, a case in which the first hours of the group were logged (Feb 6th, 20:18UT) as well as the developments over following days (Fig, top, time in hours since the first log).

0 hours: We see several features of a nascent spot group, with three tiny parallel arch filaments (AFS) about 10Mm in length. Also a chain of small spots (seen at steady moments in WL) is steeply inclined to the E-W equatorial plane.

As well, two short-lived surges appeared on the east side while bright Ellerman Bombs and H-alpha plage is also present. On the west side a small active region filament was logged.

+24 hours: ‘What a difference a day makes!’ Two small spot clusters are now aligned E-W (the Hale-Nicholson force at work). They are about three degrees in longitude length (LL), and have meagre penumbras; the arch filaments are gone because – “When the first spots form penumbras the arch filaments connected to them disappear”. (Zirin, P320.)

+48 to 72 hours: Development progressed normally over the next 48 hours – with the spots growing larger: the (p) spots moving westwards a few degrees while the (f) spots moved a bit eastward.

The third day: “By the third day the (p) spot has slowed its motion: few single EFR dipoles ever exceed 50,000 km in length. Extended groups can result from the eruption of several dipoles end to end” (Ibid. P320).

We see that AR11670 behaved much in the way described: its six degrees LL equated to about 65Mm (at lat. 18ºN).

Multiple dipoles: By contrast, we recently looked at two other active groups where, it’s surmised, newer dipoles ‘end to end’ extended both groups lengths and magnetic complexity (AR11654 and AR11640) and resulted in more transient activity than our present subject, AR11670. This latter group reached its greatest length by day 3 or 4 and it grew no further. A second dipole might now have appeared – but didn’t, it seems.

The sun is an astrophysics test-bed right on our ‘doorstep’ – and events there are well within reach of amateur ‘scopes. With luck we can witness marvellous things, some needing higher magnification and good seeing. Zirin urges “persistence!”

Take a closer look at our star- and see, maybe, sunspot dipoles emerging!

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