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Early Stages of Eruptions

Alphonse C. Sterling1

NASA/MSFC

1 Currently at JAXA/ISAS, Sagamihara, Japan

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Introduction• What happens at the start of solar eruptions?

– Preflare phase.

– Hints about the trigger mechanism?

• We can’t see the coronal field, but have been attempting to use filaments as tracers of the erupting field.

• “Slow rise phase” --> Flare+“fast rise phase”• We have examined several individual events, but

more still needed!

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Quiet-Region vs. AR EruptionsSome aspects of quiet-region events:• Larger scale (filaments of up to a few 105 km).• More slowly evolving (pre-eruptive motions typically several hours).• OK for analysis with EIT (FOV = full disk; cadence = 12 min). Easy to find in EIT. (STEREO will be better, of course.• Possible cavity connection.• If we assume quiet-region eruptions and active-region eruptions are essentially the same, except for (basically) magnetic-field strength, can learn about all eruptions from quiet-region events (cf. Sterling & Moore 2005; Williams et al. 2005).

AR eruptions:• Faster, more energetic.• Easier to identify heatings resulting from reconnections, etc.• Study AR eruptions when possible (TRACE, Hinode, etc.)

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First, an Example

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Sterling, Moore, Thompson (2001)

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Some Theories for Eruption Onset:

• Tether cutting. • Breakout model. • MHD Instability.

(Generally, this applies to “fast eruption” onset, although slow rise might be part of the process.)

These theories are testable (at least to within some limits) by our observations.

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Some Theories for Eruption Onset:• Tether cutting. Moore & Labonte (1980); Sturrock (1989); Moore et al. (1997; 2001).

(Also Chen & Shibata 2000; Lin et al. 2001; van Ballegooijen & Martens 1989 (?)) Fundamentally bipolar. Energy release via reconnection deep inside the “core field.”

• Breakout model. Antiochos (1998); Antiochos et al. (1999). Fundamentally multi-polar, with bipole core fields and restraining overlying fields. Earliest energy release via slow reconnection at interface.

• MHD Instability. Sturrock et al (2001); Rust & LaBonte (2005); Fan & Gibson (2007); etc.

Rapid rise prior to reconnection onset.

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(Moore et al. 2001)

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Runaway Tether-Cutting Reconnection

(Moore & Sterling 2006)

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Breakout Reconnection

Moore & Sterling 2006

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Ideal MHD Instability

Moore & Sterling 2006

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First example, revisited:

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Cavity Examples

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HXT Lo

14 – 23 keV

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Perhaps tether cutting has trouble explaining observations of this event. In general however, fast rise and onset of brightenings in SXRs occur in close synchrony. Breakout signatures can occur earlier, but this is not definitive.

Do we see the chewy n?

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Observed Characteristics• Sample size. So far, we have examined about 12 (including 2

AR) events “in detail” (e.g., motions and intensity changes); mapped trajectories of about 25 additional events.

• Trajectory. Filament eruptions often undergo two stages: slow and fast (e.g., Tandberg-Hanssen et al. 1980, Bong et al. 2006; cf. Ohyama & Shibata 1997).

• Slow-rise linearity. In several events, slow rise is fit better with line than with a polynomial or exponential. (Slow-rise is sometimes complex, however; Akiyama & Sterling.)

• Flaring at start of fast eruption. Onset of SXR and HXR flaring coincides ``closely’’ with start of fast eruption.

• Breakout signatures. Occur close to time of start of fast eruption. (Moore & Sterling 2006; Bong et al. 2006)

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Bong et al. (2006)

Breakout after start of fast eruption(?)

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Observed Characteristics – Cont.• Dimmings. (Discussed by many workers; hard to generalize,

e.g., Howard & Harrison 2004.)

– Local dimmings. Intensity dimmings next to and along neutral line begin weakly during slow-rise phase (stretching of field lines), and become strong dimmings at start of fast-rise phase.

– Remote dimmings (and brightenings). In some cases, dimmings and brightenings occur at locations far removed from main neutral line, consistent with breakout-type reconnection.

• Magnetic cavities. These show that filaments belong to a more extended magnetic environment; interaction with overlying fields can lead to remote brightenings/dimmings. (Cf. Gibson et al. 2006.)

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We have made progress on the fast-eruption-onset question, but issue is still not settled.

Hard to say for sure signatures for which mechanism “come first.” (And perhaps some operate in combination.)

An equally fundamental question however, is: What triggers the slow rise? This question may be easier to address.

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An AR-event example from Hinode

• On-disk filament eruption of 2 March 2007, seen with TRACE, STEREO.

• Hinode:– SOT (FG V magnetogram), etc.– SXRs from XRT

• Also use MDI magnetogram

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TRACE

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TRACE on MDI

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Hinode XRT

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XRT on MDI

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XRT on MDI

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XRT on MDI

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SOT FG V magnetogram

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UT Time on 1-2 Mar 2007

Magnetic Flux in box ( )

3.0

3.4

1.0

19:00 00:00 05:00

Mx 10 19×

(Cf. flux of whole region: ~ Mx)2110

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This initial observation from Hinode of a filament eruption supports the idea that flux changes in or near the eruption site are responsible for pre-explosive phase (e.g., slow-rise phase) dynamics.

Flux changes = flux emergence and/or flux cancelation (also: tether weakening, slow tether cutting).

(E.g., van Ballegooijen & Martens 1989, Moore & Roumeliotis1992, Rust & Kumar 1996, Lin & Forbes 2000, Chen & Shibata 2000, Feynman & Ruzmaikin 2004, Williams et al. 2005, Sterling, Harra & Moore 2007, Mikic et al. 2007.)

Discussion

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Conclusions from this Hiinode Event• Supports that pre-eruption (pre-flare) filament slow-rise phase

is due to early flux changes (in this case: cancelation; slowly-driven tether-cutting reconnection).

• During pre-eruption period, = ~ 5% of flux of total erupting system. Therefore, the cancelation triggers release of the energy contained in the sheared field; it does not power the eruption.• A question: What triggers the fast-rise phase? Hard to

determine; see, e.g., Moore & Sterling 2006, Chifor et al. 2007. (Also, Liu et al. 2008, ApJ; suggest kinking plus internal tether cutting.)

Mx 10 ~ 19ΔΦ

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Quiet-region filament eruption of 28 Feb 2001 (Sterling, Harra, & Moore 2007; same event as Marque et al. 2002)

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Sterling, Harra, & Moore (2007)

EIT SXT

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Cf. Feynman & Ruzmaikin (2004)(also, e.g., Li et al. 2006, and many others).

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Sterling, Harra, Moore (2007)

Slow-rise phase: Tether-Weakening Reconnection?

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Some Questons, and Objectives• How common is the slow-rise phase?• What triggers the fast-rise phase? (TC, breakout, instability, something

else?)• What triggers the slow-rise phase? I suspect B cancelation and/or

emergence.• Examination of several more good events needed.• More broadly:

– Larger-scale consequences of slow rise phase (e.g., hints for breakout?).– Dimmings and remote connections (dittio).

• Need:– More good e.g.s (AR or QS).– B data.

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Cavity Connections• More examples are needed, of course! If there is a particularly good

e.g., then it may be ripe for modeling.• Rise of cavity with time: Over the long term, could be easier to see

than with filaments alone. (Gibson et al. 2006; S. Martin, some talks and priv. com.) That is, could help understand the long-term evolution toward eruption.

• Observations of cavities in (probably) EUV with SXRs could help understand the role (if any) of tether cutting at time of eruption.

• Chewy N?• Comparing with B evolution may help understand long-term

evolution. (van Ballegooijen et al.?) This may be difficult in practice, however: B measured on disk while cavity height measured at limb; cavity-related fields may be too weak to measure changes do to cancelation, etc., accurately.

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01 August 2008 Eclipse from Jinta, Gansu, China

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