Two views of AR11476 sunspot group at different times on 5 May 2012 (UT). Sketches and copyright Harry Roberts ©, all rights reserved

A big spot group emerging on the sun can host a wide range of amazing phenomena when viewed in hydrogen-alpha. While attention is mostly on any big flares there is a lot more to record that, over a two-hour session say, can be almost overwhelming. To show this let’s review the logs for May 5 and May 9, 2012.

May 5 summary: First views of AR11476 (476 for short) showed remarkable surges erupting from the group (Fig1). These arose as narrow jets near 476’s central spots and bent almost horizontal to travel north for 60Mm. There they turned upwards for a further 40Mm – in a flattened ‘S’ shape 100Mm in length! New surges joined the earlier ones and engaged in an “out there and back again” display of plasma physics. Several records of these were made and two are shown. Some surges were dark (in absorption) against the disc, while a smaller surge at the site was bright against the disc; some were both bright and dark against the disc: a rare display!

Flare M1.3: At 23:02 the surface (i.e. chromosphere) between the large preceding (p) spot (+9, 189) and the intermediate spots was lit-up in a scatter of brilliant points – a flare that, I later found, was a short-lived M1.3 that peaked just sixty seconds earlier as I switched from WL back to H-alpha (!).

White light: This showed the group stretched across 14 degrees of longitude from a large double (p) spot sited at +9, 189 (twenty degrees onto the disc) to a single following (f) spot at +10, 175, just six degrees from the sun’s limb. Smaller spots lay between the two. This was a very big group and contained ten spot umbrae. Helio freeware gave an aggregate area of 500 units – it was another northern ‘supergroup’ arising (cf 11429 in March)

Magnetic class: at this stage 476 was a relatively simple Beta-preceding group, with a well-defined separation of violet spots in front and red in the rear.

White light image of the whole Sun on 10 May 2012 at 0:13 UT. Photo and copyright Nick Lomb, all rights reserved

May 9 summary: From the fifth to the ninth the logs shows a slow increase in magnetic complexity. On the 7th (not shown) a single red polarity spot arose in the large “violet” (p) spot on the south side, promoting the entire group to Hale class Gamma-Delta (i.e. opposite polarities in a single penumbra: the most complex type).

Sunspot group AR11476 on 9 May 2012 (UT). Sketches and copyright Harry Roberts ©, all rights reserved

By the 9th this was well advanced (Fig3) with the huge and complex “violet” (p) spot sprouting “red” spots on its south side. As expected the group produced a burst of GOES Class M flares that peaked with three on the 9th.

The fit between the WL spots and the Mt Wilson magnetograph on 9 May 2012. © Regents of University of California

Fig 4 suggests the fit between the WL spots and Mt Wilson’s magnetograph of the ninth of May. The possible inversion line is marked in black with triangle arrows.

Flaring: No large flares were logged on the 9th during the two-hour session but three small one were. These are coloured in the figure; the brightest, a C1.5 at 22:45, is orange. All were small with only the latter being ‘bright’, and they arose near the “inversion line” cited – the boundary between opposite polarities in the group (outlined, Fig4).

Surges: as on the fifth surges were very active. The largest (Fig3, partly shown upper left) was 60Mm long and showed Doppler ‘blue-shift’ in approach at 21:30, presumably during retraction. On the west side of the (p) spot are several smaller surges (arrows in Fig) that emerged and retracted during the session. Some faint active region filaments were also present, captioned ‘arf’, but such filaments were unusually faint in Group 476. Why?

Summary: These two records of AR11476 are not meant to be a comprehensive history of the group. Developments in the huge preceding spot, as well as the fantastic surges of the fifth, will need to be treated in more detail (Ed willing!)

At 80 magnification the sun’s disc is a little larger than the eye-piece FOV, and the image is full of detail that changes moment by moment. Recording everything is at times impossible – but it sure is fun trying. Keep your h-alpha ‘scopes at the ready!.

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

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”.

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

The flare had some ribbons and two bright flare loops (Fig). The scale bar shows the loops were <20”arc in length: typical for a small event. The flare peaked at 22:14 and by 22:19 began fading. Only one tiny spot was seen- showing that small groups can flare like big ones, at times.

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

A bright prominence at the edge of the Sun that reached a height of 149 000 km on 3 April 2012 (UT). Image and copyright Monty Leventhal OAM ©, all rights reserved

Serious observers of the Sun like Monty Leventhal OAM of the Sydney City Skywatchers use special filters called hydrogen alpha filters. These are safe to use as they cut out all light from the Sun except for the red light of hydrogen atoms. Hence these filters emphasise features that radiate at that wavelength, which are those composed of hot hydrogen atoms. Features on the Sun that can be seen with a hydrogen alpha filter includes prominences, filaments and flares.

A prominence at the edge of the Sun reaching 93 000 km on 19 March 2012 (UT). Image and copyright Monty Leventhal OAM ©, all rights reserved

Prominences are hot clouds of gas travelling along lines of magnetic field. They can exhibit all sorts of shapes such as arches and loops and can sometimes stay above the edge of the Sun for days. Others can detach from the Sun’s visible surface and float away.

It should be noted that the Sun’s visible surface is not solid. Nothing on the Sun is solid as it is a gas even towards its central regions. The visible surface is a region of surface temperature around 5500°C with the deeper regions beyond it too hot and opaque to be visible.

A long filament viewed in the red light of hydrogen atoms stretching 297 000 km across the Sun on 3 April 2012 (UT). Image and copyright Monty Leventhal OAM ©, all rights reserved

When prominences are seen against the bright solar disc instead of the darkness at the edge of the Sun they appear as filaments – long dark lines snaking across the Sun.

The most exciting and the rarest events on the Sun are flares. These are explosions on the Sun that can be seen as the brightening of regions of the Sun near sunspot groups. They can last from for just a few minutes to a few hours. Satellites such as the GOES satellites provide continuous measurements of the X-rays emitted by the Sun and so provide complementary information to what can be seen visually.

Three sketches of sunspot group AR11429 together with images from the Solar Dynamics Observatory. Please click on image to see readable size version. Sketches and copyright Harry Roberts ©, all rights reserved

While Part I reported on the early stages of this group – here we discuss its very unusual magnetic class.

When AR11429 appeared at the sun’s east limb it was already a compact delta group, a blend of opposite polarities in one big penumbra (Fig. Pt1). Clearly it had emerged that way, and the polarities were all reversed! A real rarity!

“Reversed” means it had spots with following (f) polarity in front, and preceding (p) spots in the rear –a fully reversed group! This made it one of the rarest of groups – those that are “born” complex – that Zirin and Liggett call “island delta” groups.

“Island delta” groups don’t arise from chance collisions of two or more spot groups, instead they emerge on the sun’s surface already – “with dipoles intertwined and polarities reversed from the Hale-Nicholson rules – (that) we will call the “island delta”” (Zirin, H. “Astrophysics of the Sun”. P337).

Mt Wilson drawings for March 3 and 4 (© Regents of University of California) show that the new group fitted this description, and all authors agree that “island deltas” are the most active sunspots known (Part 1, Fig).

Cause: What causes “island delta” groups? Sunspots with opposite polarities in one penumbra are called delta groups (Kunzel, 1960). They can arise from three processes: some when new spots (dipoles) emerge inside an existing group, and most commonly when two close groups grow rapidly forcing opposite polarities into contact. Paradoxically, the “island” delta groups emerge with their fields already entangled.

That the “island delta” type can occur at all is puzzling: “why didn’t it simplify below the surface? Is it possible all active regions are formed complex and most are simplified before we see them” (Ibid, p339)?

Leaving this unsolved, Zirin adds: “No matter, the Hale-Nicholson force pulls the p spots forward”.

In the case of AR11429 this force was not strong enough to pull the (p) spot through the larger (f) spot, that held onto the lead – though some forward motion occurred (Figs).

Why are Delta groups so active? In normal spots – “pairs of opposite polarity umbrae are some distance apart, so that field lines form a graceful arch across the neutral line. Delta spots (by contrast) squeeze together so closely that the magnetic field must make a sharp 180º turn to connect the two. The problem is resolved by the formation of a shear boundary … so considerable energy is stored in the magnetic field” (Ibid, p337)

The energy built up in the “narrow channel of the inversion line means the sheared field may be very strong, perhaps stronger than in the umbrae” (p 339) – and powering extreme flares, it seems. “Delta groups are responsible for almost all great flares” (p 335).

Area: Tabulate the size of the group and we find it was ~800 units on Mar 5 and peaked at ~1200 on Mar 7, after which a steady decline began.

In Summary: AR11429 started life as an “island delta” group with all its mixed umbrae in a single penumbra; the first such group of SC24 (Fig, Pt1). It emerged with (f) polarity well in front of a smaller (p) spot, and also with (p) polarity in the rear- a fully reversed group! (The first I’ve seen).

As predicted, the (p) spot should have attempted to push past the (f) spot into preceding position – but that didn’t happen (it was spectacularly seen in AR930 2006 for example). Why? Maybe the Hale-Nicholson force is now weaker than normal – so the fully reversed polarity persisted throughout, as the logs show (Fig, a, b and c).

SDO satellite images show that umbral fields of its spots connected to remote groups like AR11430 to the west, and AR11431 way off to the south across the sun’s equator (Fig, e); due to that “180º bend” aversion cited.

And consider AR11429’s neutral line (dotted in Part 1 Fig) that separates (p) and (f) polarities – it’s there that fields are strongest, and the SDO image (Fig d) show low bright loops along that line – in accord with Zirin’s above remarks.

AR11429 evolved mainly by increasing in longitude length (Figs) – but remained a “reversed” group, with (f) polarity in front and (p) following – no spots “shouldered” others aside, and hence flaring was modest

The P1 spot (main p polarity, Fig) moved forward to briefly connect with the large F1 in front – but no rotation occurred (as in AR930, 2006).

The flaring? AR11429 had some large flares, the biggest a GOES X5.4 on March 7, as well as two X1’s and some M class. Yet this was less than expected of an “island delta” group, and the strongest SC24 flare so far is the X6.9 in AR11263 (2011 July) that was not an “island delta” group.

After Mar 7 the group’s spots began to separate, and its magnetic class fell from Beta-Gamma-Delta to Beta-Gamma: more M class flares erupted (Fig, c, late stages M8) but the X class were finished. It remained a reversed group, however.

Yet AR11429 remains a “record-breaker” – as the largest fully “reversed” group of SC24 and briefly a rare “island delta” type that, usually at least, host the greatest of all solar flares. The “great” flares of SC24 must still lie somewhere in the future; we must wait and see.

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 constellation of Orion imaged on the evening of 12 March 2012 with the main stars labelled. Image and copyright Nick Lomb ©, all rights reserved

There is a lot to see in the evening sky this March 2012. The Moon is out of the way until near the end of the month and the two brightest planets Venus and Jupiter are close together in the north-west. On the other side of the sky Mars is still bright in the east after its recent opposition. The planets are augmented by the International Space Station making bright evening passes and there are occasional bright flashes from Iridium satellites.

Among all this activity it is also worth looking at some old favourites such as the constellation of Orion that is prominent high in the western sky. Orion is one of the easiest constellations to recognise with four bright stars in a rectangle and three stars in a row in the middle. In Australia many people refer to part of the constellation as the Saucepan – the three stars of the belt form the base and the dagger with the Great Nebula of Orion in the middle represent the handle.

The international GLOBE at night project wants people to observe one of two or three constellations in the evening sky to report on the brightness of their sky. Orion is one of the two southern hemisphere constellations available for this purpose. There are two opportunities left this year to contribute, until 22 March 2012 and 11 to 20 April 2012. Contributing an observation is easy to do and there is a cool webapp so that observations can be submitted in real-time.

The brightest star in the constellation is the blue supergiant Rigel that represents the left foot of the giant Orion according to Greek mythology. Strangely, the star is named Beta Orionis even though it is the brightest star. It is at a distance of 860 light years from us and radiates 85 000 times as much energy as our Sun. There is a faint companion that is itself double and is so far from the main star that it takes over 20 000 years to make one circuit.

The Alpha star in the constellation and the second brightest star is the huge red supergiant Betelgeuse. It is so huge that if it replaced the Sun it would engulf all the four inner planets, Mercury, Venus, Earth and Mars. It is at a distance of about 570 light years and radiates 85 000 times as much energy as our Sun.

Another one of the four stars forming the outer rectangle of Orion is Saiph. This star is a blue supergiant like Rigel, but even hotter. This high temperature means that more of its energy is radiated as ultraviolet and so it appears fainter than Rigel to our eyes.

The fourth of the stars forming Orion’s rectangle is Bellatrix. This is again a hot blue star that at a distance of 240 light year is closer to us than most of the other stars in Orion.

Finally, let’s mention Meissa that according to the old mythological drawings is the head of the giant Orion. This is a double star with one component being a rare O-class star with the extreme temperature of about 35 000 Kelvin while its companion is a little cooler 27 000 Kelvin. (The Kelvin temperature scale used by astronomers is the same as the ordinary Celsius scale, but with 273 added so that the freezing point of water is at 273 K.)

So on these dark autumn evenings once you had your fill of the bright planets and satellites, have a look at the giant Orion and then maybe report what you see to Globe at Night.

Reference: Stars by Jim Kaler

Detailed sketches of reversed sunspot group AR 11429 on 4 & 6 March 2012 (UT). Image and copyright Harry Roberts ©, all rights reserved

Sunspots are fairly predictable when they first emerge on the sun and grow steadily bigger. Most are bipolar structures with the magnetic fields in their western (preceding) spots opposite that of the eastern (or following) spots. And the preceding (p) spots in a given solar hemisphere are also magnetically opposite to the pole of that hemisphere. The sun’s poles reverse every eleven years when sunspot cycles reach their activity peak. In this the sun is very dynamic compared to Earth: we haven’t had a pole reversal for 100,000 years or so.

Hale and Nicholson in the early 20th C discovered these polarity “laws” – and most spots are happy enough to obey them. Yet every now and again along comes a “non-conformist sunspot” with other ideas!

These are called “reversed groups” and they are pretty rare; “reversed” since their preceding spots have the same polarity as the pole of the hemisphere they inhabit – the opposite of normal groups.

Reversed groups are usually small and short-lived: consider AR 11078 the second such group for SC24 on June 10, 2010, a small bipolar group with V10 (p) and R15 (f): a reversed group since red normally precedes in the southern hemisphere (for cycle 24). The first SC24 reversed group was perhaps AR 11030 a tiny group in the sun’s north 2009 Nov 5 that lasted only a few days.

More recently on the disc in early March was AR 11423 a small pair of reversed spots in the north (at +15, 42) – it soon faded.

Big reversed sunspots are quite uncommon: I recall some from SC23 – but have yet to find them in the logs. Yet despite their rarity, there is truly large and fully reversed group on the sun right now, this is AR11429 – and it’s one for the record books!

“So strong is the Hale-Nicholson law that occasional new spot groups with the wrong polarity will die out rapidly or else undergo great activity as the p spot struggles to get from the back to the front”. (Zirin, H. “Astrophysics of the Sun” P308).

This last comment relates to delta class sunspots, where opposite polarity spots are found within one penumbra, and AR11429 is just such a group. As well it has a large area, above 1000 units – amongst the larger size spots groups. Such a large reversed group is somewhat unexpected given recent weak sunspot activity.

Image of sunspot group AR 11429 in the light of hydrogen atoms taken on 5 March 2012 (UT). Note that this image is orientated upside down to the sketches above. A flare of X-ray class M1.5 started 10 minutes after this image was taken. The sunspot group produced even larger flares such as the M8 class flare on 10 March 2012. Image and copyright Monty Leventhal OAM ©, all rights reserved

Readers are urged to closely monitor this group, particularly in H-alpha, as large flares are expected for such a delta group (Fig). Brace yourselves for fireworks!

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

Your chance to enter Australia’s premier astrophotography competition – The David Malin astrophotography competition
Entries open on 2 April and close at midnight on Friday 15 June 2012 (AEST)

'Curves' by Geoffrey Wyatt; Overall Winner, Winning Sky Photos: David Malin Astrophotography Awards 2011

The competition this year will have three sections of entry – General Section, Open Themed Section (the theme this year is ‘Symmetries’) and a Junior Section (18 and under). The general section is divided into five categories; Wide-field (camera shots), Deep Sky (telescope shots), Solar System Objects – Hi-Resolution (<30') and Wide Field (> 30′) – and a category for Animated Sequences. The Junior Section will have one open category and entries can be of any astronomical subject.

The ‘David Malin Innovation Prize’ may be awarded, at Dr Malin’s discretion, for a striking astronomical image that shows exceptional imagination, innovation or an unusual approach in any of the categories.

The photographs will be judged by world-renowned astrophotographer, Dr David Malin. It is not just technical skill that Dr Malin will be looking for, but a memorable picture that reflects and captures the beauty, inspiration and interest of astronomy. All images will be judged by this criteria.

A selection of the finest astrophotographs received will be professionally printed and exhibited for a year at the CSIRO Parkes Observatory’s Visitors Centre. In addition, a second set will tour the country in a travelling exhibition, organised by the Powerhouse Museum, to selected venues beginning with Sydney Observatory in August.

The presentation ceremony for the 2012 CWAS ‘David Malin Awards’ will be held during the 2012 CWAS AstroFest Conference Dinner at 7:00pm on Saturday 14 July 2012.

The 2012 Central West Astronomical Society ‘David Malin Awards’ are proudly supported by CSIRO’s Astronomy and Space Sciences, the Powerhouse Museum and Canon Australia Canon which is providing significant prizes for both the category winners and honorable mentions.

More information about the David Malin Astrophotography Awards.
Entries open on 2 April 2012 – but they will accept entries taken up to two years before the closing date of entry.

Sketches of a flare on the edge of the Sun on 12 February2012 (AEST) associated with sunspot AR11419. Image and copyright Harry Roberts ©, all rights reserved

The sun is a big place, and active regions (AR) where sunspots, flares and surges occur, are quite small, often less than one thousandth of the visible disc. And searching the disc for new active regions is a big part of any viewing session. Once a new spot has emerged at an activity centre the NOAA will assign an active region number to the site (but the spot must last 24 hours for that to happen.)

Solar rotation means that fully-grown spots often appear without warning at the eastern limb – in white light that is.

In H-alpha, however, we get some warning of approaching sunspots, due to the many tall features that surround a sunspot.

In fact, a good-sized sunspot may have a veritable “crown of thorns” appearance when viewed side-on. This is due to a variety of features that arise when magnetic flux breaks the sun’s surface, including surges, filaments, flux loops and sometimes, flares.

Surges in particular arise within or near sunspot penumbrae and as magnetic entities they can be curved, and material in them can emerge and retract many times at a given site. And surges may emerge at any angle from horizontal to vertical. Since several may be present at one time they can result in the “crown of thorns” appearance just mentioned.

When an active “far-side” sunspot nears the sun’s eastern limb the spiky cluster of surges and bright arch filaments is seen well before the spot itself. While surges can reach great heights (>150Mm) most are 10 to 30Mm long, as are the arch filaments (D.Y. Chou and H. Zirin, “The Vertical Structure of Arch Filament Systems in Solar Emerging Flux Regions”, October 1, Astrophysical Journal, 333, 420.(1988)).

These features, when seen above the sun’s limb appear bright, but this is mostly a contrast effect, the same things seen against the bright disc (chromosphere) will be dark; that is, surges are typically fainter than the h-alpha disc.

Flares, on the other hand, are brighter than the h-alpha disc, typically twice as bright. As a “rule of thumb” h-alpha plage is 1.4X to 1.6X the disc brightness, and anything brighter than 1.6 is a growing flare. For those who may not have the GOES plot handy this is a useful way to tell if a bright area is plage or an erupting flare. The same “rule” applies to anything seen above the limb also, as the Figure shows.

Early on Feb 12 (Feb 11 20:27UT) when the h-alpha disc was searched for activity a dozen or so sites were found, ranging from current spots to plages and prominences; among the latter was a small cluster of “spikes” on the NE limb, where no sunspot was seen. The spikes were fainter than the disc, and identified as surges at a spot site behind the limb (not shown in Fig).

Unexpectedly, at 20:43UT a bright loop and ribbon on the limb erupted against the fainter spikes mentioned. The new features were twice the brightness of the h-alpha disc – and were clearly a moderate flare at the unseen sunspot (Zirin, H “Astrophysics of the Sun” Cambridge Uni Press. 1986. Fig 9.10, P 277).

Flares arise as bright ribbons on either side of a neutral zone where field arches reconnect. The arches and the ribbons rapidly achieve flare brightness, in this case over about ten minutes, due to a temperature spike of > million degrees.

As flare arches continue to rise they cool and finally fade in brightness to equal the disc. They are then termed post flare loops and can last for hours after a big flare. The GOES X-ray flux showed this limb event was a moderate C2.5 flare.

At 20:56 (Fig, c) the site showed a few post flare loops and surges, all below disc brightness. At this time H-alpha ended and white light viewing began, during which it seems a small filament erupted at the flare site. When H-alpha recommenced at 21:38 the filament was seen to be 33Mm above the limb (Fig, d). This was possibly the ejection that triggered the flare an hour earlier – or a new event also beyond the limb.

Helio freeware sited the flare at heliocentric 25ºN, longitude 201º.

New spot: In white light next day a new spot group was seen at the expected location (Fig, e, RHS, WL). This time “Helio” sited the spots at 28ºN, Ln201º, and the adjacent limb at Ln187º. The new group was now 14º inside the limb, in good agreement with the flare site of the day before (solar rotation is ~13º per day). The new spots were soon dubbed AR11419, and while GOES had logged many flares at the far-side site before the 12th, some of class C8, all flaring ceased about this time.

The new group was identified as the remains of active region AR11402 from January, a rather simple spot group but one that had major flares, the strongest an X1.7 on Jan 27.

The limb flare shown (Fig) is just about the last flare that AR11419 produced. The brightness of the loop and ribbon (Fig, a and b) was the only sign of a flare, and neither feature was large, the loop in a being about 10Mm high, typical for a flare “arcade”.

Catching a flare at the limb is uncommon. While not a major event, the C2.5 showed the flare from side-on, in elevation – a rare vantage point from which to follow the high temperature physics, something that is mostly logged from above, when flare arches are seldom seen.

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 sunspot group AR1146. Image and copyright Harry Roberts ©, all rights reserved

AR 11416 first appeared in the sun’s SE quadrant on Feb 8 around 16:00 UT, as a small grey pore with a magnetic field of 1500G; just 24h later it was a cluster of maybe a dozen spots with two compact penumbral spots preceding (p) – an impressive rate of development. The stage was set for fireworks, it seemed.

These early stages were not logged in Sydney due to cloud – unusually persistent cloud – but they were recorded in drawings atop Mt Wilson, using the 150’ solar telescope.

My first view of AR11416 was on Feb 11 (05:30 UT, Fig, top) in WL [white light] during a brief clearing when a mid-size bipolar group was recorded – with an area about 100 units and Hale class Beta-Gamma, that is – Bipolar groups with no marked north-south inversion line (Zirin, H. “Astrophysics of the Sun”. Cambridge Uni Press. 1986. P316).

Note that both (p) and following (f) spots had fields of 2300 gauss (R23 and V23kG). This growth from a pore to a complex group in only three days was remarked upon by many (see Spaceweather.com for Feb 12), and the probability of an M class flare in the group was put at 50%, and of an X-class at 10%.

While earlier northern group AR 11410 (by now gone) had hosted flares to GOES X-ray class M1, the expected fireworks in AR 11416 did not materialise, despite the rapid growth and strong fields – its strongest flare was a meagre C1 on Feb 9th when it was just 24h old.

My second sketch shows 11416 on the 12th (Feb 11 21:00UT, only 15h after the first, Fig, below) when big changes were seen. At least 23 spots were mostly concentrated in the large (p) and (f) penumbrae, with a few scattered between. The (p) spots still had 2300G umbral fields, but the following spot’s umbrae had split into many smaller ones, and the field was commensurately weaker, V20 maximum. Despite this, there was much interesting complexity in the group– with spot chains and irregular penumbral outlines that usually denote strong flares – yet very little occurred. In H-alpha several small active region filaments were seen (arf) but little plage.

By the 15th the (f) spot had faded to scattered tiny spots with a few small penumbrae and on 16th the (p) spot still retained a large oval penumbra holding two round umbrae (not shown). It would be good to know the field strengths but MT Wilson had been under cloud since the twelfth. Their available umbral fields have been added to the sketches, but note they are not co temporal with my logs due to time zones, and the spot group had rapid changes of shape over the period.

My last view of AR 11416 was on the 17th 22:00UT (Fig, RHS) with it only 6º from the sun’s SW limb. The two round umbrae in an oval penumbra had survived it seemed, and might still have had fields of R23, if only Mt Wilson had clear skies! A patch of H-alpha plage was seen, but no activity was logged above the limb – all remained quiet.

AR11416 was notable in that it failed to develop the complexity needed for flaring- despite having flux emergence or sunspot motion due to flux emergence (Zirin P343). Plenty of new spots had emerged and the preceding spot had rotated maybe 40º clockwise in just 15h – but still no flares! The reasons for its placid demeanour must remain an interesting puzzle.

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 prime spectrum (the linear spectrum seen when the broadening optic is removed, giving a brighter spectrum) of Eta Carinae. Top- prime spectrum with emission bands absent (speculative). Below – the usual appearance. Image and copyright Harry Roberts ©, all rights reserved

When the Baader Spectroscope was new, about twenty years ago, I viewed the spectrum of Eta Carinae, but nothing was seen apart from the continuous spectrum. Follow-up views confirmed the disappointing lack of spectral detail in this most active star.

Surprisingly however, in 2010, a view with the ten-inch ‘Dob’ clearly showed a bright emission line, H-beta of the Balmer Series! Detail was now seen, but what had changed?

When later viewed as a prime spectrum (the linear spectrum seen when the broadening optic is removed, giving a brighter spectrum), both blue H-beta and red H-alpha were seen as bright emission points, with possibly violet H-gamma also. This was a big surprise – and was attributed (wrongly, it seems) to the steady brightening of Eta Car over decades.

More recently however, in a 2004 Oct “S&T” article by Robert Naeye I read: “The binary idea received a boost in 1996, when Augusto Damineli pointed out that Eta Carinae’s optical spectrum changes every 5.5 years, when emission lines from highly excited atoms disappear for several months”.

This led Damineli to suggest an extreme UV binary companion that excites surrounding gas to emit in the optical band, but which, on approaching the primary’s stronger stellar winds, is suppressed by them – and the emissions temporarily cease.

This “idea was bolstered in late 1997 and early 1998, when previous spectral changes recurred on cue” (ibid).

Calibrated and annotated spectrum of Eta Carinae produced by Dr. Paddy McGee, High-energy Astrophysics Group, University of Adelaide, South Australia. Copyright University of Adelaide

The next “vanishing” was due in 2003 when the emission lines again faded in July, to reappear some months later. Yet they were still clear in a University of Adelaide plot just two months earlier (above). This, and other data including X-rays, “proved consistent with a binary model”.

Presumably in late 2008 to early 2009 the lines again vanished. Yet they were strong in 2010 April in the ten inch Dobson, when I was amazed to first see them. It now seems my earlier negative observations were (by chance) made at times when the emission lines had vanished.

“Even though the spectral and x-ray events recur with regularity, no two cycles are exactly the same” (ibid). And others in the field found the shape of the strong H-alpha line was much changed since 1997 and that Eta Carinae had brightened threefold since that time. This was in 2003, and the star has brightened further over the last 8 years, it’s currently at 4.5 mag. This brightening is attributed to the rapid clearing of dust shells around the stars – for unknown reasons.

It’s noteworthy that the prime spectrum of star Eta (Top fig, lhs) shows the emission points are no bigger than the Airy disc of the star (~1 second of arc). That is, they are not due to the large “Homunculus” nebula, but to a small dust shell around the binary pair. A one arc second shell 7000Ly away would be at most 10 light days (“back of envelope” calculation – needs checking) or ~2000AU diameter, but probably much smaller. The very eccentric orbit of companion Eta Car B is, Damineli suggests, ~ 50 AU in diameter (ibid). The twin spheres of dust called the Homunculus are each ~¼Ly diameter, or 16,000AU. The source of the bright spectral lines is at most one eighth the size – and may be the equatorial disc between the two spheres, or a part of it.

That the emission lines can vanish implies they are products of the disc (i.e. a nebula) and not of either star. The lack of OIII lines however shows the nebula may be low in “metals”, unlike say the Orion Nebula.

Eta Car is likely the most interesting star in the sky and the spectroscope shows unique details of processes inside its strange dusty nebula. It is now almost three years since the last event – will the lines again vanish in two years’ time?

We must wait and see. Around mid-2014 the emission bands in Eta Carinae’s spectrum will, most likely, disappear again as the companion nears the primary. Between times the star will be monitored as it brightens – and I hope to watch the “incredible vanishing spectrum” as it actually happens – and that will be a truly amazing thing!

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