Lyndsay Fletcher, University of Glasgow
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Transcript of Lyndsay Fletcher, University of Glasgow
Lyndsay Fletcher, University of Glasgow
Ramaty High Energy Solar Spectroscopic Imager
Fast Particles in Solar FlaresThe view from RHESSI (and TRACE)
MRT Newton Institute Aug 18th
Spectroscopy
Imaging: X-ray and gamma-rays
Coronal sources
Footpoint sources
Estimates of reconnection rate
Conclusions
R
Ge Detector High Resolution Spectrum
1keV bins at < 100keV
MRT Newton Institute Aug 18th
2.2 MeV line
Thermal bremsstrahlung
Non-thermal bremsstrahlung
Movie: Eduard Kontar
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Holman et al 2003.
photons
Power-law electrons
Electrons from photons: forward fitting
Photon spectrum I() is related to source-averaged electron spectrum:
e.g. F(E) modelled with Maxwellian plus two power-laws
Solve – eg as minimisation problem with smoothing
Piana et al 2003
Bars – inversion
Full line – forward fitting
Electrons from photons: numerical inversion
Photon spectrum I() is related to source-averaged electron spectrum by
Write as discretised matrix equation
The devil in the details
Are features in the source spectrum real properties of the spectrum?
Or do they arise because of simplifications made in deducing them?
Typically, neither forward-fitting nor inversion takes account of:
- non-uniform ionisation of chromosphere
- photospheric hard X-ray albedo
- electron-electron bremsstrahlung
Forward fitting (at present) further ignores the possibility of
- multi-thermal plasmas
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Effect of Albedo
(Kontar, Alexander and Brown 2004, in prep.)
Inversion with correction for reflection of photons from photosphere can smooth out some of the interesting features
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Higher energy emission from higher in the looptop– Strongly implies multi-thermal distribution
Source position as a function of energy
Figure: Amir Caspi, UCB
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Comments on the electron energy budget/spectrum
The (minimum) total energy deposited by non-thermal electrons is comparable to the peak total energy in the thermal plasma
We cannot uniquely determine the low-energy cutoff or turn-over in the power-law electron component.
Most spectra require a double power-law fit above the thermal component (but may disappear with further corrections to cross- section)
Total energy deposited by non-thermal electrons is ~ 2 .1024 J in a large (X) flare (assuming cold target, collisionally thick)
We can in most cases obtain an upper limit to the cutoff / turnover of typically 20 - 40 keV.
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Coronal density ~ 109cm-3
So need to accelerate all the electrons in 1027 cm3 every second
Electron number flux
Max number flux = 2-5 1036 electrons s-1
Holman et al 2003
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2.2 MeV centroid (i.e. protons) displaced from 50 keV centroid (i.e. electrons) by ~ 20” (~5 sigma result)
No H, EUV, X-ray enhancement at 2.2 MeV centroid location
(From Hurford et al. 2003)
July 23: electrons and ions
Protons with 10s of MeV energy undergo spallation reactions on heavy ions,
produce neutrons which are slowed down and undergo capture on H
Neutron capture line at 2.223MeV
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NB. TRACE image from ~ 45 mins later
2.2 MeV image (protons)is integrated over 15 minutes
• Electrons and protons both close to ribbons • 2) possible small
difference of position:< 15” ( ~104 km)
• e and p are accelerated in loops of similar size
October 28: electrons and protons
MRT Newton Institute Aug 18 2004
Image: courtesy Krucker & Hurford
October 28 Coronal Source
Coronal sources can be well-fitted with thermal bremsstrahlung spectra.
Temperatures up to ~ 40 MK
First appear just before or ~ simultaneously with footpoints
Often move during flare (limb events)
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Image: courtesy Krucker & Hurford
RHESSI CLEAN images at different energies: 3 Nov 2003
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Image: Astrid Veronig
Evolution of RHESSI footpoints and looptop source
Time evolution: black white
Footpoints: 70-100 keVLoop top: 20-25 keV
Image: Astrid Veronig
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Inferring coronal reconnection rate
Reconnection produces a coronal electric field – may directly accelerate particles
Outside reconnection region: E + v B = 0
Measure of E given by rate of advection of B into reconnection region
2-D configuration
xBEz
The flare is clearly a 3-D configuration.
However, we still expect high fluxes of fast particles at times of high reconnection rate
xB
Ez
xx-B
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Flux, spectrum and ‘reconnection rate’
Movement of RHESSI sourcecentroids (30-50keV) show chromospheric mappings of evolving coronal field
Rapidly reconfiguring magnetic fields should in principle provide a high energy input rate for acceleration of particles
(Fletcher & Hudson 2002)
High HXR flux/hard spectrum occur during intervals of rapid footpoint separation
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July 23, 2002
Courtesy: Säm Krucker
Good correlation between particle flux and ‘reconnection rate’ in later phase of flare, when footpoint motion is ~ regular
October 29: HXR flux and footpoint motion.
Images: Säm Krucker
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July 17 2002 flare: TRACE observations
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time
Fletcher, Pollock & Potts 2004
~130 separate tracks
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Flare footpoints on ~ simultaneous magnetogram
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UV footpoint source intensity variations
Peaks in v BLOS for individual footpoints show significant correlationin time with peaks in the UV brightness, during impulsive phase
Observations Monte-Carlo simulationsPeaks within 2s 25 5% 8 2%Peaks within 8s 45 5% 25 5%
v BLOS I1600 v BLOS I1600
Typical examples:
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v B ~103Vm-1
v B ~1.5x103Vm-1
Typical value of v BLOS ~ several 100 V m-1
Hard X-ray footpoints occur where v BLOS ~ 1 kV m-1
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Pairs of correlated footpoints
pairs of footpoints for which UV time profiles highly correlated
(lines join pairs with linear correlation coefficient > 0.8)
P1
P2
N
Potential field extrapolation (zero free energy)
P1P2N
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Conclusions
MRT Newton Institute Aug 18 2004
• RHESSI spectroscopy gives new insights into source-averaged electron distributions
• There is still more to be explored in the details: e.g. non-isothermality,
• We need full imaging spectroscopy (particularly of coronal sources) to get closer to acceleration/heating mechanism
• Understanding displacements between signatures of electrons and protons will require better understanding of the magnetic structure (as well as the acceleration mechanisms)
• There are suggestions of a good correlation between accelerated electron flux, and a measure of the instantaneous reconnection rate