Regular solar observer & correspondent Harry Roberts reports on his observations of a Dragon shaped prominence on the Sun.
2016 December, “Here be Dragons!”: Back in 2007 most Heliophysicists predicted that solar cycle 24 (SC24), the current cycle, would be a strong one: “the strongest for a century”, asserted one well known agency! Only a few ‘lesser-known’s’ correctly predicted the current state of affairs. Why?
Basically, because the modelling of solar activity is still in its infancy and many were misled by the strong activity cycles of the second half of the 20C. Flaring, for instance, showed a steady rise in peak flare power (GOES Xray flux) over cycles SC21, 22 and 23. This trend, if extrapolated to SC24, suggests many flares with fluxes > X20. In reality, thus far, only one flare in SC24 has reached GOES X5 – just 25% of that trend level. All other measures of solar activity, and there are many, tell the same story: it’s the weakest cycle in a century!
Yet the Sun remains an exciting place for ‘explorers’ with narrowband ‘scopes. Here we consider some events in early December – and ponder what they tell us about current activity.
“Dragon”. Dec 8 at 21:15 UT(9th local date, Eastern Australian time) showed a spectacular prominence on the Sun’s NW limb. Though faint in parts, it looked like the ‘classical’ Chinese Dragon. Timings showed the ‘beast’ stretched around the limb for some 20deg of solar latitude, or 220Mm (220,000km), with detached ‘bits’ covering >300Mm, and the dragon’s ‘head’ towered 82Mm above the limb, with most of the ‘body’ ~40Mm high. Earth is shown to scale. A detailed sketch was made (Fig 2). Pity it wasn’t the ‘Year of the Dragon’!
Fig 1 maps the feature (schematic) at the solar limb – and also shows a disc filament logged on the 3rd (UT), five days earlier. A single timing at the filament’s following (f) end sited it at +35°lat. and 163°long. (+35,163). The preceding (p) end is interpolated to lie at about +17,185. When this ‘snaky’ filament rotated to the limb 5d later – it became the fearsome ‘Dragon’!
The ‘Dragon’s’ many footpoints were used to assign coordinates as shown (Fig 2), but note that the longitudes are all the same (157°) since we see it against the NW limb. In fact, we will assume the footprint of the prominence would match the filament of the 3rd – and stretch across ~20deg longitude (163 to 185). These data suggest the ‘Dragons’ head was, by the 8th, sited more than 20deg behind the limb.
The Dragon’s ‘head’ showed changes during the session and GONG images suggest it was slowly ejecting, while much of the ‘body’ was stable.
Causes. What causes such strange things? The Sun has distinct zones or ‘regions’ of activity that differ greatly. Note the small spot group AR12612 (Fig 1). Here ‘AR’ means ‘active region’ – and current northern spot groups arise mostly at ~10degN lat. Spots are the product of emerging ‘strong’ magnetic flux, ~2000G. The filament on the other hand is termed QRF, quiet region filament, and obviously arises in a ‘quiet’ region, as the product of a ‘plume’ or ‘streak’ of ‘old’ flux from earlier spot groups, like AR12612 and others. In such ‘steaks’ the surface flux is <50G, much weaker than in the spots. Somehow this very low flux is sufficient to ‘entrap’ material and sculpt structures that can exceed 50Mm in height. Above this height the filament, or prominence, becomes unstable and ejects into space – though all the material is eventually pulled back to the Sun by gravity. In the meantime we may be treated to spectacular sights when something like the ‘Dragon’ appears!
Magnetic Flux. A synoptic (i.e. flattened) map of the solar surface magnetic flux, (c) NISP SOLIS, cropped to the area cited (Fig 3), shows the features we are discussing. The grey background is magnetic ‘neutral’, and BLACK and WHITE are polarities of opposite ‘sign’. Such maps are compiled from daily magnetographs and reveal the slow evolution of activity regions and the effects of solar differential rotation.
In Fig 3 ‘our’ filament – the one destined to appear ‘dragon-like’ at the limb – is plotted as a bent white line from points (p) to (f), between a streak of black polarity and a white one of opposite sign: a ‘trap’ for ionized material. A dotted line X-X shows the limb on the 8th, arrows show our line of sight, and confirm that the QRF ‘head’ was by then >20° behind the limb and the ‘tail’ just on the limb.
Viewed with “magnetic” eyes, solar activity is unevenly spread across the disc – with concentrations of strong field at low latitude sunspot (i.e. active) sites, and large faint ‘plumes’ or ‘streaks’ trailing behind (i.e. east of) the active sites at higher latitudes. Streaks are steeply inclined toward the Sun’s poles: these are the ‘quiet’ regions where filaments, like ‘Dragon’, may form. Overall, it’s an amazing process to witness – even at times of lower activity, like now, in SC24!
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.