Magnetic reconnection in solar flares: Modelling and ... · Magnetic reconnection: An introduction How does it work? ‚current-centric‘ viewpoint: Tearing instability Symposium

Magnetic reconnection in solar flares:

Modelling and observations

Miroslav Bárta1

[email protected]

Marian Karlický1, Jan Skála1,2, Pavel Kotrč1 and Jörg Büchner2

in collaboration with

21

August 14, 2015Symposium 320 Solar & Stellar Flares, 29th GA IAU, Honolulu, Hawai`i 2

Outline

(Eruptive) flares – ‚standard‘ model

Magnetic reconnection: Theoretical intro

Magnetic reconnection: Application to flare physics

Energy release rateScale-gap problemFragmented energy release and particle accelerationSpontaneous plasmoid cascade – the way out?

Effects of 3D geometry

Alternative magnetic topologies

Relation to observations

Conclusions

Solar eruptive flares & CMEs – an overview

‚Standard‘ CSHKP scenario

Flare cartoons from http://solarmuri.ssl.berkeley.edu/~hhudson/cartoons/ (K. Shibata, P. Gallagher)

Excellent match between model and observed large-scale dynamics


CSHKP:

Carmichael (1964),

Sturrock (1966), Hirayama (1974),

Kopp & Pneumann (1976)

Magnetic reconnection: An introduction

What is magnetic reconnection?


Magnetic reconnection: An introduction

How does it work?

‚current-centric‘ viewpoint: Tearing instability


Magnetic reconnection: Analytical description

Sweet-Parker model

dΦ/dt ~ S-1/2

53

52

RH

Petschek model

dΦ/dt ~ 1/ln(S)


Magnetic reconnection & plasmoids

Plasmoid-rich MR

MHD simulations –plasmoid formationin L/δ>>1 regime

Ugai, 1990; Shibata & Tanuma 2001,Kliem et al., 2001, Barta et al., 2008


Plamsoid-rich MR


Laboratory plasma –plasmoid formation

in L/δ>>1 regime

Altyntsev et al. 1986



Magnetic islands a.k.a. plasmoids= non-linear stage of tearing instability

Furth, Killeen & Rosenbluth, 1963

53

52

, RHPStearing



Plasmoid mediated MR

Plasmoid instability – analytical theory

Loureiro et al., 2007 [break-down of S-P scaling at high S ]

„Classical“ tearing mode: Sweet-Parker scaling

valid for 100

L

53

52

RH

resistive timescale ~1/η

Second branch of the tearing mode: Plasmoid instability

valid for 1

L

L

Independent of resistivity – ideal (reactive) MHD instability!


MR: Emerging „phase diagram“

Yamada & Ji, 2011 Doughton & Roytershteyn, 2011



How to reach reconnection rate high enough for rapid energy release in solar

flares? Sweet-Parker model is too slow, Petchek model is unstable → need for

collision-less kinetic reconnection.

Huge scale gap between energy input and dissipative scale. Observed

signatures of “CS” much thicker than predicted dissipative CS. How is energy

transported across the scales? (Shibata & Tanuma, 2001)

‚Standard‘ CSHKP scenario – open issues I.

δ =η/(μo VA)[classical resistivity -

particle-particle collisions]δ =c/ωpe=de

δ =c/Ωpi=di

δ =β/4 di

HallMHD

Kinetic magnetic reconnection

= + +

(=0) Ideal MHD

δ=c/ωpe=de [anomalous resistivity – wave-particle interactions]

ResistiveMHD

~10-3 m~0.1m

~5m

Magnetic reconnection: Applications – Solar eruptive flares & CMEs

Lin et al., 2009, Ko et al., 2006


Volume of a single dissipative region too small to account for observed fluxes of accelerated

particles – so called number problem

Observational signatures of fragmented energy release in solar flares – how can they be

reconciled with single current-layer scenario? Alternative models proposed based on chaotic

braiding of magnetic field and SOC (e.g. Vlahos, Aschwanden)

‚Standard‘ CSHKP scenario – open issues II.



5-6 decades !

Energy accumulation

Ideal flux-rope instability(kink/torus) →

over-laying fieldstretching → current-layer

formation underneath(e.g. Toeroek and Kliem,

Fan)

Energy transport

?

Energy dissipation

Ideal Micro-plasmoidskinetic coalescence and shrinkage (Drake et al.

2005);Population mixing in

difussion region (M. Hesse);LHD or other CS instability

(V. Roytershteyn)…

What is the nature of energy transport in large-scale systems?



Karlický, 2004

Tearing-mode cascade: Mechanism for energy transport across the scales?

What is the nature of energy transport?


Fractal reconnection conceptShibata & Tanuma, 2001

Later analytical theory of ‘chain plasmoid instability’Uzdensky et al. 2010



Suggested better approach: LS-FEM with self-adaptive h-p refinement

(Skala, Applied Mathematics 2012)

Model implementation: Embedded MHD sub-systems


• MHD in 2D geometry with guiding field (2.5D approach)

• Generalised Ohm‘s law

Barta et al., 2010


Results I: Tearing cascade confirmed

Barta et al., 2011



Results II: ‚Fragmenting coalescence‘

Barta et al., 2011Even coalescence contributes to the direct cascade!



2)( LLn

Plasmoid size distribution

(Uzdensky et al. 2010):


Results III: Scaling

Tearing-mode + (driven) fragmenting-coalescence cascade towards small scales


Results synthesis



Flow-field deforms magnetic-field

structure and forms smaller-scale CSs

Karlický & Bárta, A&A (2012)

→ Self-generated turbulence

Not a full story yet!

PIC simulations at small scales


Magnetic reconnection: Effects of 3D geometry

Priest and Pontin, 2009


Savage & McKenzie, 2009

2D vs. 3D reconnection

Tearing – kink-mode interactions

2D: Discontinuous field-line mapping 3D: Continuous field-line mapping – Slipping reconnection

Modulation of the

reconnection rate along

the arcade-axis/PIL:

Structuring of the

arcade and downflows

in the ‚invariant‘

direction

Size of the HXR

sources vs. Hα ribbons

Galsgaard & Nordlund, 1996

Gordovsky, 2014

Priest and Pontin, 2009


Magnetic reconnection: Alternative geometries for energy release

Null-point reconnection – complex MF events

MR inside a twisted loop – compact flares

Q: Are plasmoids really relevant for actual solar flares?


Multiple, small-scale short-living dissipative regions (multtiple X-lines) → fragmented energy release

Barta et al., 2011

Relations to observations

Fractal-like current-sheet structure: Mapping to the structure of flare ribbons


Predicted from simulations:

Barta,Karlicky,Vrsnak, A&A 2008

Barta et al., ApJ 2011b

Searched in observed Hα data:

Miklenic et al., ApJ 2010

(negative)

Found in Kanzelhoehe

archive?

Kotrc et al 2015. (in preparation)


Flare-ribbons structuring/splitting: Model

Hα flare ribbons


Flare-ribbons structuring/splitting: Observations

Plasmoid?


Hα flare ribbons



Nishizuka et al., 2009

Flare-ribbons structuring: observation


Flare ribbons


EUV spectral lines of jets



EUV spectral lines of jets

Schmit, Innes, & Barta, 2013



Radio signatures of plasmoids

Decimetric Pulsating Structures: Radio emission from plasmoids

Karlicky 2004, Barta et al., 2008


Ohyama & Shibata, 1998


Radio signatures of plasmoids

Decimetric Pulsating Structures: Radio emission from plasmoids

Nishizuka et al., 2015



Above-the-loop-top HXR sources

Milligan et al. (2010)


Further signatures

SXR blobs – Ohyama & Shibata (1998)

Heliospheric CS behind CME – Vrsnak (2009), Bemporad (2008), Ciaravella et al. (2013), Riley et al. (2007)

…

Magnetic reconnection in large-scale systems (e.g. flares): Summary I

Conclusions


There is a long-standing question (despite not often clearly formulated) in a physics of magnetic reconnection in large-scale systems (e.g. solar eruptive flares) on how is the free magnetic energy accumulated on a large scales transferred to the (micro)scales where kinetic dissipation takes place.

It is clear that some mechanisms of consecutive fragmentation of the current density (and

corresponding magnetic field) structure have to play a role.

High-resolution MHD simulation identified two processes of this fragmentation: Cascading

tearing (a.k.a. chain plasmoid instability) and fragmenting coalescence of plasmoids. Interplay

between those processes represent a possible mechanism for energy cascade in a large-scale

reconnection. Multiple small-scale reconnection sites are consistent with the observed

fragmented energy release in flares.

Magnetic reconnection in large-scale systems (e.g. flares): Summary I

There is a long-standing question (despite not often clearly formulated) in a physics of magnetic reconnection in large-scale systems (e.g. solar eruptive flares) on how is the free magnetic energy accumulated on a large scales transferred to the (micro)scales where kinetic dissipation takes place.

It is clear that some mechanisms of consecutive fragmentation of the current density (and

corresponding magnetic field) structure have to play a role.

High-resolution MHD simulation identified two processes of this fragmentation: Cascading

tearing (a.k.a. chain plasmoid instability) and fragmenting coalescence of plasmoids. Interplay

between those processes represent a possible mechanism for energy cascade in a large-scale

reconnection. Multiple small-scale reconnection sites are consistent with the observed

fragmented energy release in flares.

Conclusions


Magnetic reconnection in large-scale systems (e.g. flares): Summary II

PIC simulations confirmed that these two fragmentation processes continue down to the kinetic scales.

However, at the small and medium scales also flow-field driven instabilities play a role: Flows driven by the reconnection can deform magnetic field in a such a way that smaller-scale CSs are formed – dynamo action towards smaller scale.

At the even smaller scale these reconnection driven flows provide 'turbulent environment' for reconnection a la Lazarian & Vishniac (2001). However, the turbulence/cascade is natural consequence,not assumption of the model.

The process of current-layer fragmentation thus should be seen as an interplay between magnetic-field and flow-field driven instabilities.

Observations support this scenario to be in action in the solar eruptive flares.

Full 3D geometry is important for realistic modelling of solar eruptive flares.

Alternative magnetic geometries (e.g. MR in a twisted loop, null-point MR) can play a role in some events.

Conclusions


Large scales Medium scales Small scales

Energy cascades in magnetic reconnection


Conclusions



Conclusions


PIC simulations confirmed that these two fragmentation processes continue down to the kinetic scales.

However, at the small and medium scales also flow-field driven instabilities play a role: Flows driven by the reconnection can deform magnetic field in a such a way that smaller-scale CSs are formed – dynamo action towards smaller scale.

At the even smaller scale these reconnection driven flows provide 'turbulent environment' for reconnection a la Lazarian & Vishniac (2001). However, the turbulence/cascade is natural consequence,not assumption of the model.

The process of current-layer fragmentation thus should be seen as an interplay between magnetic-field and flow-field driven instabilities.

Observations support this scenario to be in action in the solar eruptive flares.

Full 3D geometry is important for realistic modelling of solar eruptive flares.

Alternative magnetic geometries (e.g. MR in a twisted loop, null-point MR) can play a role in some events.

Thank you / Mahalo !

Questions?


Documents

Magnetic reconnection in solar flares: Modelling and ... · Magnetic reconnection: An introduction How does it work? ‚current-centric‘ viewpoint: Tearing instability Symposium