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The Earliest Phases of Solar 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?

• In the past, have used filaments as tracers of the erupting field. But perhaps can do without filaments!

• “Slow rise phase” --> Flare+“fast rise phase”• Examination of several individual events still

needed.

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Some Examples

Napa 2008, WG F Sterling, Moore, Thompson (2001); Sterling & Moore (2004a)

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TRACE

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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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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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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.– Heliospheric consequences may be interesting.

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Details of Eruptions

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Quiet-Region EruptionsAdvantages of quiet-region events:• Larger scale (filaments of up to a few 105 km)• More slowly evolving (pre-eruptive motions typically several hours).• Ideal for analysis with EIT (FOV = full disk; cadence = 12 min).

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).

Napa 2008, WG F Sterling, Moore, Thompson (2001); Sterling & Moore (2004a)

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Sterling, Moore, Thompson (2001)

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Quiet-region filament eruption of 28 Feb 2001 (Sterling, Harra, & Moore 2007)

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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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AR Filament Eruption – Another possible e.g. of tether weakening prior to

eruption (perhaps during slow rise).• July 11, 1998 active region filament eruption. Use TRACE, also SXT, HXT, and BATSE. SOHO offline.

• Has slow rise, then fast rise (as did previously studied events); will attempt to understand this.

• Will consider:- Morphology and dynamics.- Magnetic setting and eruption-start mechanism.- Comparison with (a specific) theory.

(Sterling & Moore 2005)

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* = SXT Flux

--- = Filament ht

--- = EUV Flux

BATSE = BATSE (HXRs)

Sterling & Moore (2005)

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What is the actual B-setup at time of eruption? Make a “guess”:

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FilamentRibbons

Neutral Line

Spot(EFR omitted)

Blue and Red = pre-existing fields

Green = Fields after reconnection

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An e.g. 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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Hinode XRT

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SOT FG V magnetogram

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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 and Lin et al. 2001.) 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) 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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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.)

Let’s leave the question of the fast-rise trigger mechanism alone (for a while), and examine an equally fundamental question...

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Another (New) Question: What Causes the Slow Rise?

All our events have a slow-rise phase. What’s the cause?

Consider quiet-region filament eruption of 28 Feb 2001 (Sterling, Harra, & Moore 2007)

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Slow-Rise, Fast-Rise, and Lightcurves. Findings:

• Slow rise, ~ 15 km/s; start accompanied by weak rise in EUV and SXRs beneath filament.

• Fast rise, up to ~ 200 km/s; start accompanied by strong EUV and SXRs beneath filament, and HXR peak.

• Deceleration after end of HXRs.

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Conclusions• What’s the trigger of eruption (fast-rise phase)?

None of the three ideas convincingly ruled out.• In many events, both tether cutting and breakout

(and maybe instability too) occur early in the eruption, and in close synchrony.

• Slow-rise phase frequently exists. Evidence is growing that it may be due to early (tether-weakening) reconnection.

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