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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