Understanding CME and Solar Flare Mechanisms: Key Insights

1. CME/Flare Mechanisms Spiro K. Antiochos Naval Research Laboratory • Solar “minimum” event this January • For use to VSE must be able to predict CME/flare

2. Present State of Understanding on CME/Flare Mechanisms • Know where eruption can occur • Sheared filament channel • Very robust indicator • Promising ideas for why eruption occurs • Reconnection and twisted flux rope models • But not yet able to determine when • Essential for predicting possible geo-effectiveness • Observationally constrained • Also need to determine how will erupt • Essential for predicting SEPs

3. Filament channel provides necessary energy for eruption • For long range prediction (groups A & B), need to understand how they form (and what they are!) Where does Eruption Occur? 07/14/00 event – from Schrijver et al 08/16/05 NASA Science Update

4. Why does Eruption Occur? - PIL - • Strongly non-potential field forms in narrow region over polarity-inversion line • Exact topology still unobserved • critical for eruption mechanism • Held down by ~ potential overlying coronal field • Force balance breaks down and field expands outward explosively producing CME, shock, particles, etc. (see following talk by Roussev) • Field reconnects below eruption to a more potential state producing flare, X-rays, etc. • Generic to all models + + (DeVore et al) - + (e.g., T. Forbes)

5. Models for CME Initiation • Reconnection models(Resistive): • Sheared 3D arcade topology (but not essential) • Reconnection removes overlying field • Tether-cutting: reconnection inside filament channel • Breakout: reconnection outside filament channel • Needs multi-polarity system • Twisted flux rope models (Ideal): • Twist is essential to pre-eruption topology • Generally bipolar polarity region (not essential) • Ideal (kink-like) instability/loss-of-equilibrium moves aside overlying field

6. Breakout Model • Multi-polar field & footpoint shear • Reconnection removes overlying flux • CME due to run-away expansion, accelerates when flare turns on (from Lynch et al )

7. Twisted Flux Rope Model (Amari et al 2003 – flux “cancellation”) (Fan 2005 – flux “emergence”) • Bipolar field with some process to form twisted rope • Bulk of energy still in shear • Rope lifts/kinks for some critical twist, overlying field moves aside

8. Why does Eruption Occur? • Both breakout and twisted flux rope models shown to produce fast eruption in idealized 3D simulations • Role of tether-cutting still unclear • Now testing with observed magnetic fields • Beginning to incorporate better plasma energetics • Need to incorporate better photosphere-corona interaction • Flux emergence and cancellation • But, overall, impressive progress has been made in recent years

9. When will Eruption Occur? • Breakout: onset of fast reconnection at coronal null • Current sheet thins to critical scale • Flux rope: system reaches critical twist or energy • Question needs more theoretical and numerical study • Given answer, then in principle, could use observations to determine coronal B • Effective extrapolation methods in use • But B not measured in force-free region • Perhaps some combination of observations will work (need STEREO, SOLAR-B, and SDO)

10. When will Eruption Occur? • Given sufficiently accurate field (and driver), could use numerical models to predict eruption • Reconnection: calculate free energy – see DeVore poster calculate growth of current sheets • Twisted rope: calculate equilibrium and ideal stability • But photospheric driving (emergence/cancellation) may be difficult to predict • For near term, need to find pre-eruption observational signatures • For breakout, pre-eruption reconnection

11. Observational Signatures of Breakout • Filament channel grows • Stressed field and null • Onset of breakout reconnection • Null and distant brightenings? • Initial potential state • Footpoint signatures move toward neutral line • Onset of flare reconnection • Flare ribbons move apart as usual

12. Signatures of Breakout in July 14, 1998 Flare • Extrapolated potential field (Aulanier et al 2000) • Two-flux system embedded bipole, topology identical to 3D breakout simulation

13. Signatures of Breakout in July 14, 1998 Flare • Overlying loops disappear before flare and see disturbance along spine, distant brightening (see also papers by Sterling & Moore et al)

14. Prospects for Future • Theoretical/numerical work proceeding at good pace • Have effective mix of groups, codes, expertise, … • Need to concentrate on resolving basic physics questions • Will have revolutionary observations in few years • Solar-B (and SOLIS) vector B fields – ultra-high resolution • 90◦viewing from STEREO • Multi-T images from SDO, spectroscopy from Solar-B • Need “campaign-style” attack on: • How do filament channels form? • What is their magnetic structure? • Both theory and observations should be ready by Sentinels • Provide in situ tests for CME/flare mechanisms • Determine structure of eruption near Sun • Connect eruption to particle production