Upload
eileen-hartle
View
214
Download
0
Tags:
Embed Size (px)
Citation preview
CME/Flare Mechanisms
• Solar “minimum” event this January • For use to VSE must be able to predict CME/flare
Spiro K. AntiochosNaval Research Laboratory
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
Where does Eruption Occur?
07/14/00 event – from Schrijver et al 08/16/05 NASA Science Update
• Filament channel provides necessary energy for eruption
• For long range prediction (groups A & B), need to understand how they form (and what they are!)
Why does Eruption Occur?
(e.g., T. Forbes)
+
+
-
-
(DeVore et al)
+- 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
• 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
Models for CME Initiation
Breakout Model
(from Lynch et al )• Multi-polar field & footpoint shear• Reconnection removes overlying flux• CME due to run-away expansion, accelerates when flare turns on
Twisted Flux Rope Model
• 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
(Fan 2005 – flux “emergence”)(Amari et al 2003 – flux “cancellation”)
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
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)
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
Observational Signatures of Breakout
• Initial potential state • Filament channel grows• Stressed field and null
• Onset of breakout reconnection• Null and distant brightenings?
• Footpoint signatures move toward neutral line
• Onset of flare reconnection
• Flare ribbons move apart as usual
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
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)
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