Energy Release and Particle Acceleration in

Flares

Siming Liu

University of Glasgow

9th RHESSI Workshop, Genova, Italy, Sep. 2009

The Zoo of Flare HXRs

Aschwanden et al.1995

Flares are Multi-Scale Phenomena

Bai 1993; Lee et al. 1995; Veronig et al. 2002

So Are the Energy Release and Particle Acceleration Processes

Aschwandon et al. 1995

Elementary Flare Bursts Have Variable Timescale

De Jager & De Jonge 1978; Liu et al. 2003, 2006

How Many Elementary Bursts Are There?

Aschwanden et al. 1996, 1998

The Zoo of Flare HXRs

Aschwanden et al.1998

High Degree of FreedomInitial and Boundary Conditions

Grigis & Benz2005; Chernov 2008

Physics is Scale-Dependent

Chernov 2008

Injection and Trapping

Aschwanden et al.1996, 1998

Injection is an Artificial Process

Aschwanden et al.1998; Chernov 2008

• Release of Large Scale Magnetic Field Energy• Wave Generation, Transport, and Dissipation • Plasma Heating: Energy gain of the low-energy background particles• Particle Acceleration: Energization of high-energy particles.• Particle Transport: Beam• Chromospheric Evaporation driven by energy deposition through particle beam, conduction, and wave flux.• Radiative Processes.

Possible Justification for an Injection

Aschwanden et al.1998; Chernov 2008

Particle acceleration timescale is short and nonthermal particles decouple from the background plasma. But solar flare studies are more concerned with the energetics and artificial injection is only appropriate for test particles or energetically unimportant component.

The Key: Energetics

Aschwanden et al. 1996

Flares are multi-scale phenomena with high degree of freedom

The physical processes are likely different on different scalesEnergetics is the key for the study of energy release of

particle acceleration.

Generic Particle Distribution of Stochastic Particle Acceleration

Liu et al. 2009

Low Density High Density

High Temperature Low Temperature

Evolution of Elementary EventsAppropriate for studies of the

energetics

Liu et al. 2009

Low Density High Density

High Temperature Low Temperature

Weakness of Stochastic AccelerationNo spatial structure and the relevant physical processes have to be treated

approximately

Kontar et al. 2005; Liu 2006

Extended Bursts

Warren & Doschek 2004

Conclusions• Energy release and particle acceleration in solar flares are multi-scale processes with high degree of freedom and different physical processes dominate on different scales.• Their studies should follow the energetics, i.e. modeling the energetically dominant component first and treating other phenomena as “perturbations”.• Stochastic particle acceleration provides an appropriate frame work, which can be combined with small scale plasma processes and large scale MHD processes.

Sub-second scale features are likely

caused by transport and

plasma physics processes

Liu et al. 2009

Multiple Loops

So Are the Energy Release and Particle Acceleration Processes

Hoyng et al 1976; Liu et al. 2004, 2006; Lin et al. 2003

Impulsive HXR Bursts

Extended HXR Bursts

General Constraints on the Particle Acceleration: Fast

De Jager & De Jonge 1978

General Constraints on the Particle Acceleration: Selective

Energetically(Stochastic Acc., Sub Dreicer Field, Shock Injection)

Kontar et al. 2005; Eichler 1979

General Constraints on the Particle Acceleration: Selective

Spatially (Intermittency, Super-Dreicer)

Dauphin et al. 2007

Which one looks more like an HXR flare?

Aschwandon et al.1998

Stochastic Acceleration

Liu et al. 2009

flare ribbonschromosphere

UV loops (10 K)5

H-alpha loops (10 K)4

X-ray loops (10 K)7

magnetic field lines

conduction front

reconnection inflow

current sheet

Superhot hard X-ray region(>10 K)8

(standard flare configuration adapted from Forbes & Acton, 1996)

Extended HXR Bursts

Impulsive Well-Observed Bursts

Liu et al. 2003, 2006

Asymmetry

Kontar et al. 2005

Extended Bursts

Lin et al. 2003

Statistical Studies

Battaglia & Benz 2005

2.3-4.24.2-5.65.6-8.3

Statistical Studies

Krucker et al. 2007

EUV at 171A(by TRACE)

H-alpha 6563A(by BBSO)

soft X-ray 1-8A (GOES)

hard X-ray 20 keV (Yohkoh)hard X-ray 100 keV (Yohkoh)

microwave 6.6 GHz (OVSA)

Most flares have impulsive non-thermal and gradual thermal emission components

Measuring the electron acceleration efficiency?

Measuring the electron acceleration efficiency?

Model the thermal X-ray emission and find the component correlated with the

non-thermal X-ray emission.

Challenging, if not impossible!

The particle transport, chromspheric evaporation, and radiative cooling processes are difficult to

model for complex flares.

If the particle acceleration process is universal for all flares, as we usually assume, we should study the particle acceleration efficiency with relatively

simple flares.

Goals

Theory How does the efficiency depend onproperties of the background plasma: B, T, n,

size of the flaring region, energy release rate,

…

Observations Measure (constrain) the efficiencyExplore its dependence on the emission

characteristics: spectral features, flux density, size of the emission region, variation time scale,

…

Particle Acceleration Efficiency measures the energy partitionbetween the emerging thermal and non-thermal particles.

Energy Release Event

Turbulence

Heating

Acceleration

Radiation(non-

thermal)RADIATION(thermal)

Radiation(non-

thermal)

Heating

Heating

Heating

Turbulence

Acceleration

IMPULSIVEIMPULSIVE

GRADUAL

Particle Distribution Function

E

E f(E)

fit ( th. +nth. ) = - 4.5

3-4 keV6-9 keV

13-20 keV20-30 keV

GOES 1-8 A

observed

7.8 GHz9.4 GHz

11.8 GHz14.8 GHz

= - 3.0

= - 3.1

= - 4.4

Hard X-ray

microwave

(Qiu et al. 2005)

Measuring the electron acceleration efficiency?

Particle Distribution Function

Multi-Processes:reconnection geometry

wave/turbulence generation and transport, particle generation and transportplasma evaporation and cooling

Theoretical Considerations

Solar flares are multi-scale phenomena in terms of energy, duration, and spatial scale,so are likely the particle acceleration events.

Thermal vs. Non-thermal

Thermal Non-thermal

Theory Particle Distribution Maxwellian Power-law

Energy Low High

Interactions Coulomb Collisions

Particle-FieldCollisionless

Observations

Emission PropertiesTemporalSpectralSpatial

Energetics

GradualNarrow

Extended??

ImpulsiveBroad

Compact??

Particle Acceleration Efficiency measures the energy partitionbetween the emerging thermal and non-thermal particles.