Institute of Astronomy,Radio Astronomy and Plasma Physics GroupEidgenössische Technische Hochschule Zürich

Swiss Federal Institute of Technology, Zürich

Flare Electron Acceleration

Arnold Benz

1. RHESSI Observations

Spectral evolution of flares

thermal

non-thermal

RHESSItwo component fits:T, EMγ, F35

Grigis & B.

flux

spectral index

P. Grigis

P. Grigis

Battaglia et al. 2005

Δ

●

< C2

> C2

ΔΔ●

Battaglia & B., 2005

FHXR ─ γ Relation

1. "Pivot" point at about 9 ± 3 keV (soft-hard-soft)

2. Consistent with constant acceleration rate above threshold energy (13.9 keV)

3. Consistent with constant total power in particles above threshold energy (13.6 keV)

4. Consistent with stochastic acceleration beyond 18.1 keV

5. Inconsistent with pure "statistical flare" scenario

Approximation for d << dc:

f(E) fo E-

fo (WL) 7/8 anti-

(WL) -1/2 correlation !

E1/2 f(E) f(E)

t + z E E E1/2 + t

(

D Ecoll

Diffusion by stochastic wave turbulence

(

( ((f(E) =

Assume steady state => Bessel equation

Solution: f(E) = C E -d + 1/2 Kd(E)

1/L aW

}Benz 1977

(

Approximate further, eliminate WL and get for observed HXR flux:

FHXR C(1/2 + 1/2[1 +(+3/2)]1/2)2

[( - 1)(+3/2)]2

log FHXR

Brown & Loran, 1985

2. RHESSI –Phoenix Observations

0

10

20

30

40

50

60

70

80

Type III

Pulsations

Narrowband spikes

Diffuse cont.

Type IV

Type I

Hf broadband(gyro-synchrotron)

befo

re

rise pe

ak deca

y

afte

r

radio emission in 201 X-ray flares >C5.0

Meter-Decimeter Radio Patterns

of X-ray selected flares

A Standard 129

B Just IIIm 8

C Afterglows 20 D No Radio 34 E Type I 10

Standard

M1.1Standard

25 – 50 keV

50 – 100 keV

irreg.pulsation

Standard

reverseddrift IIIm

25 – 50 keV

50 – 100 keV

M1.1

Standardirregularpulsation

decimetricnarrowband spikes

50 – 100 keV

25 – 50 keV

M1.1

C7.7Standard

IIIdmirreg.pulsation

hf continuum

6 – 12 keV

12 – 25 keV

25 – 50 keV

Just IIIm

C7.9Just IIIm

6 – 12 keV

12 – 25 keV

25 – 50 keV

Just IIIm

6 – 12 keV

12 – 25 keV

25 – 50 keV

C7.9

Type IV and DCIM "Afterglows"

type IV

gyro-synchrotron

Phoenix-2Radio spectrum

GOESClassX17

gyro-synchrotron

drifting structure

decimetric pulsations

Phoenix-2Radio spectrum decimetric

pulsations

decimetric patch

Type IV

DCIM

M2.3

12 – 25 keV

6 – 12 keV

3 – 6 keV

Afterglows

narrowband spikes

IIIm andhf continuum

Afterglows

3 – 6 keV

6 – 12 keV

12 – 25 keV

M2.3

regular dmpulsation

patch

100 – 300 keV regular dmpulsations

Afterglows

6 – 12 keV

12 – 25 keV

25 – 50 keV

50 – 100 keV

M5.0

No Radio

radio-quiet flare

GOES class M1.0

6 – 12 keV

12 – 25 keV25 – 50 keV

50 – 100 keV

no-radio flares

Flares C5.0 – C9.9 22 %

Flares > M1.0 12 %

All flares > C5.0 17 %

Two possible interpretations:

1. Small flares have less radio emission (sensitivity effect)

2. Large flare have more associated processes

("large flare syndrom", suggesting indirect connection)

2

1

A

B

C

Standard: reconnection at 1 and 2Just IIIm: reconnection at 2Type IV: reconnection at 2 after 1Noise storm: reconnection at 2Radio-quiet:: reconnection at 1

2

1

A

Standard: reconnection at 1 and 2Just IIIm: reconnection at 2Type IV: reconnection at 2 after 1Noise storm: reconnection at 2Radio-quiet:: reconnection at 1

Summary on HXR - Radio Correlations

• Hard X-ray and radio emissions of flares are relatively independent.

• 17% of >C5.0 flares have no coherent radio emissions (22% if type I excluded).

• Many type IIIm have no hard X-ray emission.

• Correlation is often poor, suggesting multiple acceleration sites for "standard flare pattern" and "afterglows".

• Multiple reconnection may also interprete "big flare syndrom".

Conclusions1. Where are electrons accelerated?

- often in more than one site (independent signatures)

- most IIIm (and SEDs) have only very weak hard X-ray

emission (possibly high-coronal flares).

2. How are they accelerated? - Violent acceleration processes are excluded. - If acceleration signature, why not close X-ray correlation? - Radio type IV and DCIM indicate processes long after flare

3. If loop-top, why this large number? - loop-top may be secondary acceleration site

Observational Constraints on Flare Particle Acceleration

1. Absence of radio emission in 17% of flares does not support violent acceleration processes, such as single shocks or single DC fields.

2. Consistent with heating processes (bulk energization).

3. RHESSI observations show that flares start with soft non-thermal spectrum. In the beginning it is difficult to distinguish from a thermal spectrum (γ ≈ 8).

4. The spectrum of non-thermal electrons gets harder with flux of non- thermal electrons both in time during one flare, as well as with peak flare flux (Battaglia et al. 2005).

5. The evidence supports stochastic bulk energization to hot thermal distribution and, if driven enough with power-law wings.

IIIm

irregular dmpulsation

narrowbandspikes

reversed driftIIIdm

Standard

6 – 12 keV

12 – 25 keV

25 - 50 keV

C9.0

6 – 12 keV

II

IIIdm

HF cont.

Standard

12 – 25 keV

25 – 50 keV

X1.6

IIIdmIIIdm

OVSA

Standard Irregular pulsationC9.7

Standard

6 – 12 keV

12 – 25 keV

C6.5

irreg.pulsation

Christe & Krucker

StandardRHESSI

Just IIIm

6 – 12 keV

12 – 25 keV

C8.0

Type I

12 – 25 keV

6 – 12 keV

C7.3

Type I