sarah gibson october 2005 twisted magnetic flux ropes: a breeding ground for cmes? sarah gibson...

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



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

YUV420 codec decompressorare needed to see this picture.

(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



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


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


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August 8, 2001

Gibson et al., 2005

Cavity + filament observed to bodily erupt as 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


Lynch et al., 2004



Tokman and Bellan, 2002





Lynch et al., 2004 Tokman and Bellan, 2002






(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)

sarah gibson october 2005 twisted magnetic flux ropes: a breeding ground for cmes? sarah gibson...

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