sarah gibson

Sarah Gibson AGU, Spring 2005

The calm before the storm: The link between quiescent cavities and

CMEsSarah Gibson

David Foster, Joan Burkepile, Giuliana de Toma, Andy Stanger

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Sarah Gibson

Mauna Loa Mk4 white light coronagraph image showing quiescent cavity: November 18, 1999

CME eruption of cavity: November 19, 1999

OutlineNew, quantitative observational analysis of quiescent white light cavities

– Frequency

– Morphology

– Intensity contrast

– Correlations

Relation between quiescent cavities and CMEs

Relevance to structure of pre-CME coronal magnetic field

AGU, Spring 2005

Theoretically compelling:

• Coronal magnetic helicity very nearly conserved as a global quantity (Berger and Field, 1984)• Magnetically dominated plasma relaxes to minimum energy conserving helicity (Taylor, 1974, 1986)• Free energy stored in still-twisted magnetic fields is plausible CME driver (Low, 1999 ) • Flux ropes “fundamental building blocks of magnetism in the solar atmosphere”? (Rust, 2003)

Magnetic flux ropes: What’s the attraction?

Observationally compelling:

• Magnetic flux ropes models have been applied to solar regimes from the interior out to 1 AU• Distinct observational signatures of flux ropes identify regions of stored magnetic energy• Evolution of these flux rope signatures clues to predicting eruptions?

Magnetic flux ropes

If they exist quiescently in the corona, are they formed from emerging twisted magnetic flux, or from photospheric motions?

Controversial

Are there flux ropes in the corona during CME eruption?

Also quite well accepted

Are there flux ropes in interplanetary space?

Pretty well accepted

What’s a magnetic flux rope?

Suggested definition: A set of magnetic field lines winding about an axial field line in an organized manner

Do flux ropes exist quiescently in the corona prior to the CME?

Controversial

Magnetic flux ropes

Do flux ropes exist quiescently in the corona prior to the CME?

Controversial

CMEs-3 part structure: Observations

(Illing and Hundhausen, 1986)SOHO/LASCO C2/C3 white light coronagraphs

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CMEs-3 part structure: Models

Uniting theme: Magnetic flux rope -- the question is, when does it form?

Low and Hundhausen, 1995

Lynch et al., 2004



Krall et al., 2001

Linker et al., 2003

Quiescent cavity-3 part structure

Eclipse observations have long (as early as eclipse of Jan 22, 1898!) demonstrated the presence of non-eruptive 3 part structures -- prominence/cavity/helmet

• Prominence cavity = filament channel viewed at limb

• Prominence cavities also sometimes visible in H- and radio (Straka et al., 1975; Kundu et al., 1979) as well as EUV (Sterling and Moore, 1997) and soft Xray (Serio et al., 1978; Hudson et al. 1999)

• White light observations have some advantages

1. Only sensitive to density2. Can see cavities higher above limb3. Quantitative comparison to 3 part CME in white light possible

NCAR/HAO Newkirk WLCC telescope, March 18 1988 eclipse, Philippines

Innermost to outermost: EIT 284, Mark IV, Lasco C2, Lasco C3Quiescent cavities occur in the low corona

Observational properties of cavities: Frequency

1. First noted all days November 1999 -- September 2004 where HAO/MLSO Mark IV coronagraph showed clear cavities: 206 days with clear cavities

• Solar cycle effect -- more cavities visible during descending phase• Correlated with polar crown filament visibility• Lower limit on days with cavities -- many more identified if cavity systems examined in detail


2. Then chose most visible cases: 25 cavities








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3. These are not independent cavities -- together represent 12 systems

























• Polar crown filament systems span more than four months• Cavities visible for as many as nine days

4. In order to study evolution of systems, also analyzed adjacent days and reappearances as the system rotated past solar limbs: 88 cavities total quantitatively analyzed

Observational properties of cavities: Morphology

(all values Minimum/Mean/Maximum, 12 system best only, full dataset of 88)

1. Cavity distance from equator: 5/56/77 5/56/90 degrees

2. Cavity width: 6/18/36 4/19/40 degrees

3. Cavity height: 1.25/1.47/1.60 1.24/1.46/1.64 Rsun







Observational properties of cavities: Contrast

(all values Minimum/Mean/Maximum 12 system best only, reduced dataset of 69)

1. Cavity intensity depletion at 1.2 Rsun: • Equatorward: 1 %/ 27% / 47% 0 %/ 26% / 54%• Poleward: 0 %/ 16% / 34% 0 %/ 14% / 34%

2. Cavity sharpness (slope at edge):• Equatorward: 8.8e-9 / 2.5e-8 / 3.9e-8 2.1e-9 / 2.0e-8 / 6.7e-8 (Bsun/degree)• Poleward: 1.7e-9 / 1.8e-8 / 4.2e-8 3.4e-10 / 1.4e-8 / 4.9e-8 (Bsun/degree)




prominence

Observational properties of cavities: Correlations

1. Distance from equator vs. time follows streamer belt :

• near equator around minimum• all latitudes during descending phase

2. Width vs. height:

• Fatter tends to be taller

3. Intensity depletion vs. cavity sharpness:

• darker tends to have sharper boundaries

4. Intensity depletion vs. width --Two conflicting effects:

• Morphology -- parallel to line of sight ---> smaller, sharper boundary cavities

• Evolution -- more “mature” filament cavities tend to be higher/fatter and sharper boundaries

CMEs from cavities

Of the 12 systems:

3 were observed by MLSO Mark IV to erupt outwards in a CME at least once in their lifetime. Two of these were Polar Crown Filaments (PCFs)

5 more systems could be associated with CMEs observed by SOHO but at times of no Mark IV cavity data, either due to Mark IV not observing, or because the system was not near a limb. All but one of these were PCFs, the other was a high latitude CF.

4 systems could not be associated with CMEs. 2 was a PCF, 2 were not. Both PCF cases had a lot of missing data days around it, and at the times when it might be expected to be visible at the opposing limb.

..or to put it another way -- 9 out 12 were PCFs (or high latitude CFs) -- 7 of these erupted at some time, and the others had significant data dropouts for both Mark IV and SOHO.

Example of cavity --> CME: November 19, 1999

November 18, 1999 QuickTime™ and aCinepak decompressor


Example of cavity --> CME: February 5, 1999

February 4, 1999

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CMEs from cavities

Total of Mk4 observations of 6 cavities --> CMEs (one system had two cases, another had three, separated by a month or more).

We then searched Mark IV observer’s logs for good 3-part CMEs, and examined data prior to these for precursor cavities. This gave us an additional 8 CMEs <-- cavities.

How do the cavities-->CMEs differ from the CMEs<--cavities?

Cavities --> CMEs -- PCFs, slower speeds

• 5/6 were PCFs• Speed min/mean/max: 155/334/660 km/sec

CMEs <-- cavities -- active region and some quite fast

• 7/8 associated with (or at least near) AR • Speed: min/mean/max: 498 / 836 /1280 km/sec

Example of CME <-- cavity: August 9, 2001

August 8, 2001



Example of CME <-- cavity: May 15, 2001

Active region/Fast CME case! see Maricic et al., 2004 for detailed analysis



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Example of an EIT cavity --> CME: December 28, 2002

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CMEs from cavities

How are the cavities that erupt as CMEs distinguished from the general set of cavities?

Historically, cases have been found where a quiescent cavity system was observed to gradually rise, swell, and ultimately be released in a CME (Fisher and Poland, 1988; Illing and Hundhausen, 1985; Hundhausen, 1999)

This is part of the same phenomenon of streamers swelling before eruption (e.g. “bugles” in synoptic maps as pointed out by Hundhausen, 1993.)

Also associated with a trend for filaments to be more likely to erupt in a CME if they extend high enough up in the corona (between 1.2 and 1.35 Rsun -- the lower bounds of the Mark IV coronagraph) (Munro, 1979; Gilbert et al., 2000)

CMEs from cavities

We therefore considered whether cavities

1. had filaments high enough to be seen in white light

2. exhibited “necking”

Of the 10 cases where we had cavity observations within 24 hours of eruption:

• 6 out of 10 had high filaments (vs. 25/99 of entire sample)

• 10 out of 10 had necking (vs. 63/99 whole sample)• of these 7 out of 10 are truly bulging (vs. 27/99 whole sample) • 2 out of 10 are within error bars straight up and down (vs 36/99)• 0 out of 10 have no necking (vs 36/99)





CMEs from cavities

There are also good examples of the height of the cavity, degree of necking, and presence/height of the filament increasing in the days leading up to the CME:




are needed to see this picture.July 21, 2002 July 22, 2002

(CME seen by LASCO early July 23, 2002)

CMEs from cavities

…and in a system as a whole

Oct 1 Oct 19 Nov 2 Nov 19 Dec 13 Dec 26 (CMEs Nov 19, Dec 31)











CMEs from cavities

We also considered whether cavities erupting as CMEs were darker or had sharper boundaries than those that did not immediately erupt

• No clear evidence that they are darker (min/mean/max polar-edge depletion = 4% / 16% / 43% vs. 1 %/ 14%/ 34% for non-CMEs)

• May be more sharply defined (min/mean/max polar-edge slope = 8.6d-9 / 2.9d-8 / 1.1d-7 vs. 3.4d-10 / 1.3d-8 / 4.89d-8 for non-CMEs)

• There are a few cases where depletion and especially sharpness may increase in the day or days leading up to the CME

• These are preliminary results -- need more data!

CONCLUSIONS

Cavities are ubiquitous, and undersampled due to line of sight effects

• Quite possibly whenever there is a filament channel, there is a filament cavity

• Visibility depends upon morphology (particularly height), intensity contrast, and degree of obscuring features along the line of sight

• STEREO will be essential to disentangling morphology vs. evolution effects!

Cavities are parts of filaments:

• can extend higher than 1.6 solar radii

• models of filament systems (particularly polar crown filaments) should consider observed properties of cavities

White light quiescent cavities are biassed to Polar Crown Filament Systems

• Extremely long-lived, and stable

• The presence of a quiescent cavity is not by itself a good indicator of an impending CME

• However, given enough time a PCF-cavity system will erupt, and probably more than once

• A cavity that grows higher, more bulging (and/or contains a filament) and possibly more sharply defined is a good bet

Small, non-PCF (even active region!) cavities do exist

• We found some when we worked backwards from 3 part CMEs.

And finally -- we have demonstrated 3-part quiescent structures appear to erupt bodily as CMEs.

Which brings us back to magnetic flux ropes

So do observations imply a precursor magnetic flux rope? I say, yes! For three reasons:

1) Cavities can be formed by increased pressure of sheared, expanding fields

• Flux ropes are certainly consistent with this

• But so are other more generally sheared fields: and the presence of sheared fields prior to a CME is not controversial.

2) But cavities also exist for days/weeks/months -- what prevents the sheared field from refilling?

• Detached fields of magnetic flux ropes will enhance cavity longevity, thermally isolating them and suppressing their being refilled from below

3) Cavities have sharply defined, circular boundaries, and cavities-->CMEs exhibit necking

• This is a natural consequence of an organized winding of field lines about axial field line (my definition of flux rope)

• A sharp boundary is consistent with a magnetic flux surface at the outer boundary of a flux rope

• Necking is consistant with a flux rope high enough in the corona to show its boundary at levels lower than its axis

Throwing down the gauntlet…

The challenges for CMEs models, with or without pre-existing flux ropes, are:

1. To reproduce cavity intensity contrast and sharp edges consistent with observations

2. To reproduce cavity sizes (heights/widths/lengths) consistent with observations

3. To reproduce cavity evolution (necking, high filament, sharpenimg boundary) prior to CME

4. To reproduce cavity longevity (months!) and stability

sarah gibson

Documents

magnetic flux ropes

emerging twisted magnetic

organized mannerdo flux

flux rope signatures

cavities visible

coronal magnetic helicity

twisted magnetic fields

cavities higher