Sarah Gibson October 2005 Twisted magnetic flux ropes: A breeding ground for CMEs? Sarah Gibson borators: Yuhong Fan, Joan Burkepile, Giuliana de Toma, B.
Sarah Gibson October 2005
Twisted magnetic flux ropes: A breeding ground for CMEs?
Sarah Gibson
Collaborators: Yuhong Fan, Joan Burkepile, Giuliana de Toma, B. C. Low
Magnetic flux ropes
What’s a magnetic flux rope?
Suggested definition: A set of magnetic field lines that wind at least one full turn about an axial field line
Roussev (2003) (after Titov and Demoulin (1999))
After Gibson and Low (1998)
Amari et al. (2003)
Loosely wound
Tightly wound
Spheromak-based
Magnetic flux ropes
Are there flux ropes in interplanetary space?
Pretty well accepted
Lepping et al. 1995
Magnetic flux ropes
Are there flux ropes in the corona during the eruption of a coronal mass ejection (CME)?
Also quite well accepted
Do flux ropes exist quiescently in the corona prior to the CME, or are they formed during eruption?
Controversial
Lynch et al., 2004
Amari et al., 2003
Examples of models where rope forms during eruption
Theoretically compelling:
Precursor flux rope stores magnetic energy:
o Coronal magnetic helicity very nearly conserved as a global quantity (Berger and Field, 1984)
o Magnetically dominated plasma relaxes to minimum energy conserving helicity (Taylor, 1974, 1986)
o Flux rope represent such a minimum energy state given enough helicity (Low, 1994)
o Flux ropes “fundamental building blocks of magnetism in the solar atmosphere”? (Rust, 2003)
Precursor flux rope possesses the seeds of its own destruction:
• Free energy stored in still-twisted magnetic fields is plausible CME driver (Low, 1999 )
• Instabilities and loss of equilibria occur as thresholds are crossed for maximum flux or twist (Lin et
al., 1998; Roussev et al., 2003; Linker, 2003; Fan, 2005; Toeroek et al., 2004)
• Is there a threshold for maximum global helicity? (Zhang et al., 2005)
Precursor magnetic flux rope
Precursor magnetic flux rope
Observationally compelling:
Coronal plasma gives us clues to field structure (flux frozen in highly conductive corona)
Many twisted-appearing structures in corona
Three-dimensional MHD models needed for meaningful comparison
I will present the case for a precursor flux rope, by comparing models to observations before, during, and after the CME
Evidence for a precursor flux rope: Before the CME
1. Observations of quiescent corona are consistent with flux ropes as pre-CME magnetic configuration
• Filaments
• Cavities
• X-ray sigmoids
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Before the CME: Filaments
Cool, dense, filament material supported by dips of winding rope
Priest et al., 1989
Before the CME: Filaments
Three-dimensional flux rope model using photospheric boundary
-- location of dips match observed filament location well.
Van Ballegoiijen (2004)
Before the CME: Cavities
Filament viewed at the solar limb is surrounded by dark cavity
Before the CME: Cavities
Flux rope models predict such a cavity:
• Enhanced magnetic field in rope compensates for low gas pressure of cavity
• Requires thermal isolation from photosphere for longevity
• Circular cross-section, sharp boundary (magnetic flux surface/tangential field discontinuities)
Low and Hundhausen, 1995
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Gibson et al., 2005
Before the CME: Sigmoids
Active regions are significantly more likely to produce flares or CMEs when associated with sigmoid structures. (Canfield et al., 1999)QuickTime™ and a
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(Gibson et al., 2002):
Before the CME: Non-eruptive sigmoids
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Dynamic evolution of flux rope field lying above and within separatrix surface relative to the shorter, arcade-type field below and external to it, could result in tangential discontinuities, leading to the the formation of electric current sheets along the separatrix surface (Parker, 1994; Titov and Demoulin, 1999; Low and Berger, 2000).
2. Observations of CME in eruption are consistent with flux rope precursor
• 3 part eruption of CME (bodily eruption of filament and cavity)
• Transient sigmoid
• Transition to cusped post-flare loops behind eruption
Evidence for a precursor flux rope: During the CME
3-part CME a consequence of flux rope in many models
Lynch et al. (2004)
Linker et al., 2003
Gibson and Low, 1998; 2000
During the CME: 3-part CME
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Krall et al., 2001 Fan and Gibson, 2005
Cavity + filament exists before eruption (for days, weeks, or even months)
-- evidence that flux rope also pre-exists CME
During the CME: 3-part CME
Mauna Loa Mk4 white light coronagraph image showing quiescent cavity: November 18, 1999
CME eruption of cavity: November 19, 1999
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Gibson et al., 2005
Cavity + filament observed to bodily erupt as 3 part CME
During the CME: 3-part CME
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August 8, 2001
Gibson et al., 2005
Cavity + filament observed to bodily erupt as 3 part CME
During the CME: 3-part CME
February 4, 1999
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Gibson et al., 2005
Current sheets form during eruption --> sigmoidal reconnecting field lines
During the CMEs: Transient sigmoid
Kliem et al., 2004
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Fan, 2005; Gibson and Fan, 2005
When X-type point(s) exist prior to eruption below rope, simulations demonstrate a total eruption of the rope
Roussev (2003)
3D model (Titov & Demoulin, 1999)
(Toeroek and Kliem, 2005)
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2D cartoon (Gilbert et al. 2001)
During the CMEs: Sigmoid-to-cusp
Adapted from Gilbert et al. 2001
• Forms two ropes separated by cusped, sheared arcade
• Sigmoid-separatrix-surface survives
• Some dipped field erupts with upper rope, some shrinks back down with lower rope
• Can break in this manner because
• 3D
• NO X-line
During the CMEs: Sigmoid-to-cusp
Gibson and Fan, 2005
3. Some post-CME observations appear inconsistent with total eruption of pre-existing flux rope
• Partially or non-erupting filaments
• Immediate reformation of long-lived X-ray sigmoid (sigmoid-->cusp-->sigmoid)
• Magnetic cloud charge state signatures
Evidence for a precursor flux rope: After the CME
(Gibson et al., 2002): Gloeckler et al., 1999
After the CME: Partially or non-erupting filaments
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Filaments are often observed to either not erupt, or only partially erupt with a CME (Gilbert, 2000; Pevtsov, 2002; Gibson et al., 2002)
After the CME: Immediate reformation of sigmoid
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Sigmoid-->cusp-->sigmoid (Gibson et al., 2002)
After the CME: Magnetic charge states
• Charge states of solar wind ions frozen in at coronal temperatures
• Most magnetic clouds indicate only relatively hot coronal material -- no clear signature of cool filament material
• Some cases that do imply cool material (He+ events) imply coexisting mixture of cool and hot material (Skoug et al., 1999; Gloeckler et al., 1999). These events are only seen during the rising phase of the solar cycle.
• A recent case (Zurbuchen, private communication) demonstrates purely cold material -- this event had no associated flare
First alternative: Flux rope formed during eruption
• In three-dimensions combined with embedded sheared-field filament model could explain pre-CME quiescent filament (but what about long-lived cavity and non-eruptive sigmoid?)
• Can explain 3-part CME (but what about the bodily erupting cavity?), (eruptive sigmoid?)-->cusp, and depending on location of pre-CME filament relative to reconnections, could explain either totally erupting, partially erupting, or non-erupting filament (but what about reformation of non-eruptive sigmoid?)
• Explains why cold filaments rarely seen in magnetic cloud, since field lines making up the erupting rope were heated by reconnection during eruption (but what about observations of cold or mixed material in magnetic cloud?)
Tokman and Bellan, 2002De Vore and Antiochos, 2000
First alternative: Flux rope formed during eruption
Lynch et al., 2004
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Tokman and Bellan, 2002
• In three-dimensions combined with embedded sheared-field filament model could explain pre-CME quiescent filament (but what about long-lived cavity and non-eruptive sigmoid?)
• Can explain 3-part CME (but what about the bodily erupting cavity?), (eruptive sigmoid?)-->cusp, and depending on location of pre-CME filament relative to reconnections, could explain either totally erupting, partially erupting, or non-erupting filament (but what about reformation of non-eruptive sigmoid?)
• Explains why cold filaments rarely seen in magnetic cloud, since field lines making up the erupting rope were heated by reconnection during eruption (but what about observations of cold or mixed material in magnetic cloud?)
First alternative: Flux rope formed during eruption
Lynch et al., 2004 Tokman and Bellan, 2002
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• In three-dimensions combined with embedded sheared-field filament model could explain pre-CME quiescent filament (but what about long-lived cavity and non-eruptive sigmoid?)
• Can explain 3-part CME (but what about the bodily erupting cavity?), (eruptive sigmoid?)-->cusp, and depending on location of pre-CME filament relative to reconnections, could explain either totally erupting, partially erupting, or non-erupting filament (but what about reformation of non-eruptive sigmoid?)
• Explains why cold filaments rarely seen in magnetic cloud, since field lines making up the erupting rope were heated by reconnection during eruption (but what about observations of cold or mixed material in magnetic cloud?)
(Toeroek and Kliem, 2005)
Second alternative: pre-existing flux rope totally erupts
• Pre-eruption flux rope consistent with pre-CME quiescent filament and cavity (but what about non-eruptive sigmoid?)
• Consistent with a totally erupting filament and eruptive sigmoid->cusp (but what about partial/non-erupting filaments, and reformation of long-lived sigmoid?)
• Consistent with (rare) observations of cold filaments in magnetic cloud (but what about observations of hot or mixed material in magnetic cloud?)
2D cartoon (Gilbert et al. 2001)
Third alternative: pre-existing flux rope that erupts only partially
• Pre-eruption flux rope consistent with pre-CME quiescent filament and cavity, as well as non-eruptive sigmoid
• Consistent with a totally erupting filament, partially eruptive filament, and non-eruptive filament, depending where filament mass lies relative to reconnection point, and also with an eruptive sigmoid-->cusp.
• Consistent with reformation of long-lived sigmoid
• Consistent with hot, mixed, or cold filaments in magnetic cloud, depending upon where reconnections take place
Consistent with observations, before, during, and after CME!
Adapted from Gilbert et al. 2001
Conclusions
Quiescent, pre-eruption magnetic flux ropes in the corona
• Are a natural equilibrium state for a coronal magnetic field that is minimizing energy while conserving helicity (Taylor, 1974, 1986; Berger and Field, 1984; Low, 1994; Rust, 2003)
• May form as an equilibrium state in the corona via photospheric motions (Amari, 2000; Linker, 2003) or flux emergence (Fan and Gibson, 2004, 2005; Fan, 2005; Manchester et al., 2004b)
• Can produce a CME by losing equilibrium when flux or twist thresholds are crossed (Lin et al., 1998; Roussev et al., 2003; Linker, 2003; Fan, 2005; Toeroek et al., 2004)
The partial eruption of such a quiescent flux rope
• Can explain observations of the evolution of filaments, cavities, and sigmoids from their pre-eruption quiescent state, through the eruption, and to their post-eruption state.
Doesn’t happen in 2D• In 3D, when the flux rope extends down to the
photosphere so there is no pre-existing X-type point below the rope, the rope can break in two via internal reconnection
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• However, 2D simulations show that the loss of equilibrium of the rope leads to its total eruption with current sheets forming below the rope
(Lin, 1998) Fan and Gibson, 2005)
Good-bye two-dimensional cartoons!
Roussev (2003)
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http://solartheory.nrl.navy.mil/solartheory/erupt.html
Van Ballegoiijen (2004) Manchester et al. (2004a)
Amari (2003)
Tokman and Bellan (2004)
Good-bye two-dimensional cartoons!
Roussev (2003)
http://solartheory.nrl.navy.mil/solartheory/erupt.html
Van Ballegoiijen (2004) Manchester et al. (2004a)
Amari (2003)
Tokman and Bellan (2004)