observations morphology quantitative properties underlying physics aly-sturrock limit
DESCRIPTION
Observations Morphology Quantitative properties Underlying Physics Aly-Sturrock limit Present Theories/Models. Coronal Mass Ejections (CME). S. K. Antiochos, NASA/GSFC. Recap of CME Physics. For some reason magnetic shear concentrates at PILs producing filament channels - PowerPoint PPT PresentationTRANSCRIPT
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• Observations– Morphology – Quantitative properties
• Underlying Physics– Aly-Sturrock limit
• Present Theories/Models
Coronal Mass Ejections (CME)S. K. Antiochos, NASA/GSFC
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Recap of CME Physics
• For some reason magnetic shear concentrates at PILs producing filament channels– Exact topology unclear (especially twist)
• Filament field held down by overlying non-sheared coronal field– Need some mechanism to disrupt force balance
catastrophically– Simply continuing the shearing does not do it! As
shown by many simulations– Agrees with Aly-Sturrock limit
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• Bipolar (one polarity inversion line) initial magnetic field • Filament-field formation by shearing and reconnection• See pronounced expansion & kinking – but no eruption
Demonstration of Non-Eruption
(from, DeVore et al, 2005; Aulanier et al, 2005)
+-
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Non-Eruption
Underlying physics:
• Corona has no lid
• Magnetic field lines can stretch indefinitely without breaking– Free to open slowly in response to photospheric
stress and gas pressure (rather than erupt as CME)
• Slow opening (not associated with filament channels) observed to occur continuously in large-scale corona
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Aly – Sturrock Limit
Force-free field in an infinite volume (Aly 1984)– From div·T = 0, derive that:
∫ (Br2 - Bt
2) dA= 0, at limiting bdy
– Implies that transverse component cannot increase indefinitely
– Virial eqtn:
∫ B2 dV= ∫ r ( -2 Br2 + B2) dA
– Energy in interior related to field at bdy– Implies upper bound on energy– Aly-Sturrock conjectured lub is fully open field– Agrees with many simulations
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Test of Aly – Sturrock Limit
• Roumeliotis et al, 2.5D sheared dipolar force-free field in spherical geometry
• Beyond certain shear, field expands outward exponentially
• Energy saturates at open limit
• Only certain BC are physical
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Present Theories for Eruption
• Non-ideal evolution – reconnection
• Partial opening – ideal instabilities
• Non-quasi-static evolution – flux emergence?
• All appear to “work” in numerical simulations
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NUMERICAL SIMULATIONS
• Solve 3D or 2.5D ideal/dissipative MHD with variety of numerical schemes– Both explicit and implicit– Both fixed and fully amr grids– Both Cartesian and spherical grids
• Initial conditions:– Usually equilibrium with varying degree of
complexity– Simple dipole to observed photospheric fields with
solar wind• Boundary Conditions:
– Open conditions at outer boundaries– Photospheric conditions main discriminator
between models– Simple shear to incomprehensible contortions
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NUMERICAL SIMULATIONS
• Term “simulation” is misnomer
• Simply method for obtaining approximate solutions to standard equations
• Drastic change in theory techniques, but still comes down to physical insight
• Hopefully numerical simulation will turn into user-friendly community tools
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ARMS NUMERICAL SIMULATIONS
• Ideal MHD eqtns. (but numerical resistivity)– Use non-conservative energy equation for low-beta systems– Spherical grid with adaptive mesh refinement
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Reconnection-Driven CME Models
• Breakout:– Field erupts to a state that it cannot get to by any ideal
evolution– Magnetic reconnection removes overlying field,
decreasing downward pull– Need topologically complex field
– More than 1 dipole– Generally present on Sun
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Non-Dipole Coronal Topology• Field of two dipoles – axi-symmetric
– Large global at Sun center, weaker near surface– Must have 4-flux system with separatrix bdys, and null
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Magnetic Reconnection• Frozen-in condition:
– B-field lines ~ constants of the motion– Produces topological complexity and all solar
activity
• Even in corona have finite diffusion, t ~ L2 /η >> 106 years, for L ~ 1 Mm– If L sufficiently small, field lines lose identity and
can “reconnect” on short time scales, but only over localized region
– Need to develop significant magnetic structure on small scale for reconnection to be effective
– Magnetic topology plays critical role
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Breakout Model
• 2D multi-polar initially potential field
• Create filament channel by simple footpoint motions
• Outward expansion drives breakout reconnection in corona
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• Breakout reconnection allows for explosive eruption• Flare current sheet, flare reconnection, and twisted flux rope all
consequences of ejection• CME with no flare possible for slow eruptions
Breakout Model
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Breakout Model
• 3D simulation using 3D AMR code Lynch et al
• “Create” prominence by simple boundary flows
• Reproduces standard features of CMEs/flares
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Breakout Model
• 3D simulation by Roussev et al (2008) of 04/21/02 event
• Complex topology with flux transfer prior to eruption
• Generalized breakout process
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Cancellation Model
Loss of equilibrium/ flux cancellation:• Reconnection/emergence at photosphere converts
downward to upward tension
• Produces twisted flux rope in corona, prior to eruption
• Rope loses equilibrium, jumps upward
• Subsequent flare reconnection accelerates ejection– Van Ballejooigen, Forbes, Mikic/Linker, Amari, …
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Flux Cancellation Model• Analytic
model by Forbes et al
• Detailed simulation of 05/12/97 event (Titov et al)
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Aneurism Model
Ideal instability (kink-like)
• Part rather than remove overlying field
• Need twisted flux rope– Sturrock, Fan, Kliem, …
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Aneurism Model
• 3D simulation by Fan et al.
• System driven only by flux “emergence”
• Kink or torus instability depending on overlying field
• So far only idealized configurations
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– Apparently have three mechanisms that can produce explosive CMEs in 3D simulations:
• Reconnection (Breakout), loss-of-equilibrium (flux cancellation), ideal instability
– All require sheared prominence field– All produce twisted flux rope as a result of eruption– Flux cancellation and ideal instability require
twisted flux rope before eruption
Models for CME Initiation
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64K Question• What is the pre-eruption structure of the
prominence field?– Clearly has strong shear– Does it have twist (twisted flux rope topology)
• NRL VAULT image of 06/16/02, 20K material, spatial resolution < 200 km
• Little evidence for twist in either structure or motions, but exact topology still unclear
• See Rob’s movies!