The Enigmatic Threads of Filaments What can be Inferred from Current Observations? Oddbjørn Engvold...
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The Enigmatic Threads of Filaments What can be Inferred from Current Observations?
Oddbjørn EngvoldInstitute of Theoretical Astrophysics
University of OsloNorway
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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).
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Hedgerow prominenceR.B. Dunn, September 12, 1956
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’Suspended cloud’prominenceR.B. Dunn; October 2, 1957
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Prominence of February 22, 1974Solar Tower Telescope, Sac Peak
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Large Prominence observed with Hinode30 Nov, 2006
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Quiescent filament of August 2, 2007; SST La Palma
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High resolution Hα filtergram of quiescent filament observed with SST, La Palma (2005)
Hα line center Doppler (±0.3Å)
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End section of a quiescent filamentSwedish Solar Telescope, October 2005
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Filament barbs some times have the appearance of a curtain
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Length : 37 arc sec
H, 2004-08-22, SST
Long filament threads
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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.
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Varying brightness in a solar filament
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Movie showing swinging threads
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”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.
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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)
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Ubiquitous, low amplitude oscillations in section of a quiescent filament (Yong Lin,PhD Thesis 2005)
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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
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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
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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)
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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
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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.
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A thread model of filament magnetic fields(Lin, Martin and Engvold, 2007)
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White light eclipse image illustrating the
location of a prominences within a dark cavity
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Spicules - a fundamental structure of the solar chromosphere
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TRACE
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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
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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.
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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?
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Partly ’filled’ threads
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”Swaying” threads in quiescent filamentsBefore sharpening and smearing on one direction
After