Harry watches and ponders a long-lived filament on the Sun

A long-lived filament progressing across the Sun. Sketch and copyright Harry Roberts

A long-lived filament progressing across the Sun. Sketch and copyright Harry Roberts ©, all rights reserved

While a ‘prominence’ is “a bright feature seen above the Sun’s limb in a strong chromospheric spectral line” (H-alpha in our case)”,…a filament is a similar feature projected as a dark, usually elongated structure on the disc”.

“The filament-prominence configuration contains relatively dense and cool (5,000<T<8,000K) plasma, which is partly visible in the light of H-alpha”.

These quotes are from Solar and Stellar Magnetic Activity by Schrijver and Zwaan (2008), page 196 – a valuable text for students of our star.

They add: “all filaments reside over zones of polarity inversion, between two strips of (dominant) opposite polarity”, the filament channel.

The Sun has two kinds of filament: those within active regions like sunspot groups; and those in quiet regions: “over polarity inversion zones between weak magnetic network” (P197). The latter are called quiet region filaments (QRF) from Zirin’s usage in Astrophysics of the Sun. The fields that form QRF are ‘weak’, about 5 – 10G or <1% of sunspot umbral fields. Yet these weak fields are able to form long-lived filaments of great length.

We know that the Sun’s radiative zone rotates as a solid body (Zirker et al), while the convection zone, the upper third of the solar radius, rotates differentially. The magnetic shears or twists of this zone cause not only sunspots but also the streaks or plumes of ‘old’ magnetic flux in the wake (i.e. eastwards) of sunspot groups. It is there, that the inversion boundaries between flux of opposite ‘sign’ give rise to the QRF. Yet not all such boundaries (or channels) host a filament: for unknown reasons.

What is the role of filaments? Schrijver and Zwaan suggest that: ”filaments probably indicate zones that are important in the flux removal from the solar atmosphere and in processes in the solar dynamo mechanism”. P198.

Filaments as markers. “Filaments are a good way to measure the rotation (of the Sun) because they are long-lived (my emphasis). However, it is difficult to measure their position when they are not on the meridian because we do not (accurately) know their height” Zirin: P116.

Currently most filaments seem faint and indistinct in comparison with those of SC23. Positional timings of a given filament can be made only if it has a well-defined point that can be timed.

On July 31 a dark QRF at the sun’s NE limb was logged, and on August 1 its preceding (p) end was used for a timing. Helio freeware sited that point at +10,229(Fig).

This filament proved to be long-lived, and was logged several times until it was no longer visible on Aug 8, when about 55ºW of the central meridian (CM). However, it survived, to become a tall prominence at the western limb on Aug 10 (Fig, rhs). The Figure shows some of the timed logs as well as the ‘untimed’ one of July 31; all other detail is omitted for clarity.

Zirin’s caution (above) applies, it seems, to the logs of Aug 6 and 7. In both cases the filament is inclined to our line-of-sight by >40º and it seems that the darker upper parts of the filament were logged, rather than the ‘foot-points’ at the surface. Perhaps only the logs between ±30º of the CM are reliable.

Filament’s Return! On Aug 24 it was good to see, after the filament’s two weeks behind the Sun, that the foot-points of a big prominence at the eastern limb agreed closely with those of the filament three weeks earlier (Fig).

At the west limb on the 10th and the east on the 24th, we see the filament projected against the darkness of space: as a prominence. The longitudes of the ‘foot-points’ are in fact those of the limbs, not the filament; the full longitude extent of the filament is actually >20 degrees.

Rain prevented views of the returned filament on the disc until Aug 30UT, when its west and east foot-points showed it remained at the same longitude (Fig, in red) as its earlier rotation, but had migrated somewhat to the north. Various authors note the slow migration of QRF’s towards the relevant pole, citing the meridional current as the cause.

Will the long-lived filament complete another transit of the Sun? We hope to find out soon, clear skies permitting!

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

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