Prominences on the edge of the Sun on 5 & 6 June 2011. Image and copyright Harry Roberts ©, all rights reserved
2011 June 5 and 6 saw some fine prominences at both the east and west limbs (Figs) – and we see the trapped plasma [a plasma is a gas that has been heated and its atoms have lost or gained electrons so that they are ionised and have an electric charge] (almost) any time we view the Sun in H-alpha. Yet science struggles to confine plasma, despite decades of effort. How does the Sun do this trick so easily?
Confusingly, trapped material is called a filament on the Sun’s disc – and a prominence when seen above the limb. Either way it’s trapped plasma. The first views must have been during total eclipses – and over past millennia many a giant prominence must have drawn attention.
The temperature inside a quiet filament is about 5,000K, much less than the chromosphere’s 8,000K – that’s why the same thing looks different depending on our line of sight: dark on the disc, bright above the limb. Filaments can be stable for weeks – so how does the Sun entrap material for so long?
The plasma “trap” is formed by magnetism, and Zirin (“Astrophysics of the Sun”) lists the main parts –reviewed below.
Fields: quiet prominences arise where surface fields are rather weak, about 10 times Earth’s field (i.e. 5-10 gauss). Such fields occur in bands on the Sun at latitudes above the current spots, where large unipolar regions are found. Where two such regions adjoin, a plasma “trap” can form. Between the ‘unlike’ regions a narrow “neutral line” arises, and a “magnetic channel” forms there.
Magnetic channel: Most emerging field is vertical, but at the neutral line the fields connect across the zone as loops. However, below these loops the fields do not connect across the gap, but instead turn 90º from vertical to horizontal, and run in opposite directions along the channel – forming parallel “fences” of horizontal field that can entrap material. This “makes sense because the field can hold up the material only if it is horizontal. (Zirin, p274)”.
“Plasma cannot be contained by field lines convex to it” (p278), – i.e. plasma will escape unless the “trap” is everywhere concave.
Exact details are still under study – and how material gets in is unknown. The captured material grows into a sheet 50Mm high, and sometimes as wide. And while the filament (or prominence) can be 50 Mm high, above that height it becomes unstable – and soon erupts, apparently as the transverse loops are broken. Yet the “trap” survives, and often refills in a day or so.
The “caged” filament can grow so dense it “blankets” the Sun’s surface, and a bright band below the filament is often seen in H-alpha.
Coronal heating: “Since the prominence is immersed in the million-degree corona, why doesn’t it heat up? (Because it) is supported by magnetic field lines that go to the surface, so hot coronal ions would have to cross the field lines to heat it (p280)” – this is something ions can’t do.
Prominences: they are wonderful things, and may grow much larger than the ones shown – it’s a good reason to often view the Sun in H-alpha – even every day – if it’s clear.
Harry Roberts is a frequent contributor to this blog and a member of the Sydney City Skywatchers.