The Enigmatic Threads of Filaments What can be Inferred from Current Observations?

Oddbjørn EngvoldInstitute of Theoretical Astrophysics

University of OsloNorway

The Structure of solar prominences

• The main structural components of solar prominences and filaments, their spines, barbs and legs at the extreme ends of spine, are demonstrated from recent high-resolutions observations.

• The therad-like structures appear to be present in filaments everywhere and at all times. They are the fundamental elements of solar filaments/prominences.

• Spines, barbs and legs are interrelated (see following talk by Sara).

Hedgerow prominenceR.B. Dunn, September 12, 1956

’Suspended cloud’prominenceR.B. Dunn; October 2, 1957

Prominence of February 22, 1974Solar Tower Telescope, Sac Peak

Large Prominence observed with Hinode30 Nov, 2006

Quiescent filament of August 2, 2007; SST La Palma

High resolution Hα filtergram of quiescent filament observed with SST, La Palma (2005)

Hα line center Doppler (±0.3Å)

End section of a quiescent filamentSwedish Solar Telescope, October 2005

Filament barbs some times have the appearance of a curtain

Length : 37 arc sec

H, 2004-08-22, SST

Long filament threads

Observable matter appears unevenly distributed in filament threads (T=7-10 000º K)

• Filaments are usually recognized as dark, absorbing structures on a brighter background chromosphere.

• Thin threads are generally not uniformly dark all along their lengths, but they are usually seen as chains of flowing dark sections.

• Doppler images tend to show filament threads more clearly than in line center intensity images.

• Some filaments show both dark and relatively bright parts.

• Remark: It is not yet known how much temperature and density vary within individual fine threads.

Varying brightness in a solar filament

Movie showing swinging threads

”Swaying” threads in quiescent filaments

• The ’polluting’ signals from the background chromosphere signals may be suppressed via smearing in the directions of the threads.

• Threads must be treated individually.

• Several x-time cuts provide information about phase differences along the threads

Preliminary measurements have given:Periods: 3-4 minAmplitudes: ~90 kmPhase speed: >100 km/s

x = 20 000 km, t= 9.4 min & image cadence is 4 seconds.

Spatially coherent oscillations

• Period ~ 26 min.

Long period oscillations are common over the whole filament, whereas individual threads oscillate independently at shorter periods.

Is there a weaker magnetic field that enables the threads to interact?

(Lin, Wiik and Engvold, 2003)

Ubiquitous, low amplitude oscillations in section of a quiescent filament (Yong Lin,PhD Thesis 2005)

Wave propagation along threads

Vph1 = 8.8 km/sVph1 = 8.8 km/sP1 = 5.2 minP1 = 5.2 min

Vph2 = 10.2 km/sVph2 = 10.2 km/sP2 = 5.6 minP2 = 5.6 min

Oscillations, waves and flows(Lin, Engvold, Rouppe van der Voort and van Noort, 2007)

Phase velocity: 15 km/s; period 3.6 min & wavelength: 3300 km

Flowing and oscillating structures (Lin, 2004)

• Small-scale absorbing structures oscillate as they flow along filament threads

• Velocity amplitude: 0.4 – 1.4 km/s

• ”Periods”: 12–27 min

• Phase velocity = 60 km/s (could be measured in one case)

Damping of oscillations• The damping times are usually between 1 and 3

times the corresponding period (Molowny-Horas et al. 1999; Oliver and Ballester, 2002)

• Damping possibly due to ion-neutral collissions (Pécseli and Engvold, 2000; Forteza et al. (poster this symposium), and/or by non-adiabatic damping of magnetoacoustic waves (Carbonell et al. (poster this symposium)

Large amplitudeVrsnak et al. 2007

Low amplitude Lin et al. 2003

Threads

• Fine threads are the building bricks of filaments and prominences.

• The streaming (and counterstreaming) of matter in threads at speeds 8-10 km/s, and higher, must inevitably be field-aligned. That demonstrates the magnetic nature of the threads.

• The threads

- are constantly on the move,

- they appear and disappear in the course of minutes, and less. The latter is partly an effect of the flowing of matter within them.

• Observations of the fine threads of solar filament are disturbed by the highly fluctuating background chromosphere.

A thread model of filament magnetic fields(Lin, Martin and Engvold, 2007)

White light eclipse image illustrating the

location of a prominences within a dark cavity

Spicules - a fundamental structure of the solar chromosphere

TRACE

Conclusions

• Understanding the physics of threads is central to understanding filaments and prominences, overall.

• Observed features and characteristics of filament/prominence threads underlines their magnetic nature:

- Connection to the magnetic photosphere below

- Counterstreaming

- Oscillations

- Overall dynamics

Acknowledgments

The speaker thanks Yong Yin, Sara F. Martin and Luc Rouppe van der Voort for inspiring and helpful discussions as well as for various input to this talk.

Questions to be addressed: • How can thin magnetic threads be formed and maintained in a

low-β plasma within filament channels?

• What controls the thermodynamic conditions within the magnetic threads?

• How do threads connect/interact with photospheric magnetic fields?

• What causes the ubiquitous flowing (counter-streaming) of the plasma?

• What are between the ’cool’ threads (TR?) ?

• What is the significance of the oscillatory nature of filament?

• Are conditions for support of the highly dynamic plasma different from that of static cases?

Partly ’filled’ threads

”Swaying” threads in quiescent filamentsBefore sharpening and smearing on one direction

After