The Formation of Eruptive Structures in the Solar Corona

Brian Welsch, Space Sciences Lab, UC BerkeleyObservations and numerical simulations suggest that flux ropes emerge from the convection zone into the corona to form active regions. Flux ropes are also inferred from in situ magnetic field measurements in a subset of interplanetary coronal mass ejections (ICMEs). Are emerged flux ropes (EFRs) and interplanetary flux ropes (IFRs) the same physical entities? Or are IFRs formed in the corona, either before the CME, or during the eruption itself? Recent studies suggest that flux emergence can play a role in triggering CMEs, but do not support a model in which “that which emerges is that which erupts.” Instead, the “storage and release” model, in which pre-existing coronal structures --- some of which are probably flux ropes --- are rendered unstable by some mechanism(s) is favored. Possible destabilizing mechanisms include the emergence of new flux, cancellation of existing flux, and shearing/rotation of existing flux. Evidence supporting this model includes: formation of eruptive filaments between active regions formed by different EFRs; lack of correlation between the properties of emerging flux and properties of the coronal response; and lack of discernible flux rope structure in the majority of IFRs. The “storage and release” model implies that dynamic models of coronal magnetic fields are necessary to understand the coronal response to processes that can destabilize magnetic structures in the corona.

Background• I was asked to give a talk on “flow convergence

with respect to CME initiation, as a complementary talk to one describing a flux-rope model for CME initiation.”

• Implict assumption: flux rope converging flows – But I don’t think this distinction is valid!

• The difference isn’t in the eruptive structure, but formation of that structure.– Is “that which emerges” also “that which erupts”?

Flows are complex: we have seen shearing, convergence, and emergence in the same active region at the same time.

LCT of MDI of AR 8210 on 01 May 1998

Which flow patterns are common? Which “matter”? We’re conducting an Active Region (AR) flow study to find out.

• MDI full-disk, 96-minute cadence magnetograms from 1996-98 have been compiled.

• NAR = 64 active regions have been selected.– Each had a single, well-defined neutral line.– Hence, most were bipolar.– ARs both with & without CMEs were selected.– Several ARs were followed over multiple rotations; some

lacked NOAA AR designation.– Our sample includes Nmag = 4062 AR magnetograms.

• But they have only recently been tracked! – No statistical results to discuss yet…

I’m still going to talk about flux ropes, to address formation of these structures; but won’t discuss flows.

• Punchline - Most CME models have “quasi- flux rope” structure. They really only differ in:– modes of formation.– modes of eruption.

• “Mix & match” is possible: A given model’s formation mechanism can be coupled with a different model’s eruption mechanism.– e.g., emergence could drive breakout CMEs, though

breakout simulations have been driven with shearing.

Define Terms: EFR = Emerged Flux Rope

Forms active regions, or emerges into pre-existing active regions.

MDI move of AR 9244,courtesy SXT Nuggets Page

From Abbett et al. (2003)

Define Terms: IFR = Interplanetary Flux Rope

ICMEs with “high magnetic field, smooth field rotation, and low proton temperature (Lepping et al., 1990)” – Gopalswamy, SSRv 124, 145 (2006)

Most ICMEs have no clear flux rope structure.

(Cane & Richardson, 2002, JGR 108108, Issue A4, 6-1)

Define Terms: QFR = Quasi- Flux Rope*A bundle of roughly parallel field lines, with a range of possible twists, not necessarily enclosed by a “flux surface” (on which Bn would vanish).

DeVore & Antiochos, ApJ 539 954 (2000)

Lionello et al.,ApJ 581 718 (2002)

Given this definition, filaments are QFRs; many can exist in the corona. * made up especially for this talk!

Martens & Zwaan (2001)

Are QFRs – which erupt – formed directly from EFRs?In many cases, clearly not.

Filaments (QFRs) often erupt, and How can EFRs form circular QFRs? can form between EFRs.

Blos over H-α

TRACE movie of circular filament eruption.

Are pre-eruptive QFRs the same as IFRs?IFRs probably form during eruption.

Images from Moore et al. (2001) & Qiu et al. (2007)

Schrijver (2007) asserts: 1) strong-field polarity inversion lines (SPILs) are associated with flares; and 2) SPILs form by emergence.

The emergence of a current-carrying flux tube would lead to parallel, opposite-flux fibrils in close proximity in magnetograms.

MD

I, AR

8100

Indeed, Welsch & Li (2006, submitted) do see evidence that SPILs form by emergence.

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Falconer et al. (2003, 2006) and Schrijver (2007) have correlated strong-field polarity inversion lines (SPILs) with CMEs and flares.

Q: How do these gradients arise? Emergence? Cancellation? Other?

Strong gradients are regions where (+/-) fields with |Bz| > 150 G lie in close proximity.

R is the total unsigned flux near strong gradients.

Are increases in R correlated with increases in unsigned flux --- a signature of new flux emergence? YES.

But the response of the corona – measured by flare class – to a SPIL of fixed “strength” R varies.

Muglach & Dere (2005) studied pre-eruptive MDI magnetogram evolution. An excerpt:

Muglach & Dere (2007) ask: “When flux emergence ‘causes’ an eruption, is the new flux a spark, or the fuel?” Their conclusion: it’s a spark.

Conclusions Re: Affect of EFRs on the Corona

• Many structures that erupt (QFRs) or that are observed in situ (IFRs) are not “intact” emerged flux ropes (EFRs). Instead, many QFRs form in the corona, and many IFRs probably form during eruption.

• It appears the EFRs that form SPILs do not drive flares directly in many cases. Instead, SPIL-producing EFRs probably destabilize pre-existing coronal structures.

Overall Conclusions• “Storage & Release” paradigm is consistent with observations; driving of CMEs by surface

motions or flux emergence is not.

• Real-time modeling of pre-event energy storage, and the coronal response to photospheric field evolution --- including flux emergence --- is probably necessary to predict flares / CMEs.