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Lyndsay Fletcher, University of Glasgow

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Ramaty High Energy Solar Spectroscopic Imager. MRT Newton Institute Aug 18th. Fast Particles in Solar Flares The view from RHESSI (and TRACE). Spectroscopy Imaging: X-ray and gamma-rays Coronal sources Footpoint sources Estimates of reconnection rate Conclusions.

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slide1

Ramaty High Energy Solar Spectroscopic Imager

MRT Newton Institute Aug 18th

Fast Particles in Solar Flares

The view from RHESSI (and TRACE)

Spectroscopy

Imaging: X-ray and gamma-rays

Coronal sources

Footpoint sources

Estimates of reconnection rate

Conclusions

Lyndsay Fletcher, University of Glasgow

slide3

MRT Newton Institute Aug 18th

Ge Detector High Resolution Spectrum

1keV bins at < 100keV

Thermal bremsstrahlung

2.2 MeV line

Non-thermal bremsstrahlung

slide5

Electrons from photons: forward fitting

Photon spectrum I() is related to source-averaged electron spectrum:

photons

e.g. F(E) modelled with Maxwellian plus two power-laws

Power-law electrons

Holman et al 2003.

slide6

Electrons from photons: numerical inversion

Photon spectrum I() is related to source-averaged electron spectrum by

Write as discretised matrix equation

Solve – eg as minimisation problem with smoothing

Bars – inversion

Full line – forward fitting

Piana et al 2003

slide7

MRT Newton Institute Aug 18 2004

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
slide8

MRT Newton Institute Aug 18 2004

Effect of Albedo

Inversion with correction for reflection of photons from photosphere can smooth out some of the interesting features

(Kontar, Alexander and Brown 2004, in prep.)

slide9
Higher energy emission from higher in the looptop

Strongly implies multi-thermal distribution

MRT Newton Institute Aug 18 2004

Source position as a function of energy

Figure: Amir Caspi, UCB

slide10

MRT Newton Institute Aug 18 2004

Comments on the electron energy budget/spectrum

Total energy deposited by non-thermal electrons is ~ 2 .1024 J in a large (X) flare (assuming cold target, collisionally thick)

We cannot uniquely determine the low-energy cutoff or turn-over in the power-law electron component.

We can in most cases obtain an upper limit to the cutoff / turnover of typically 20 - 40 keV.

Most spectra require a double power-law fit above the thermal component (but may disappear with further corrections to cross- section)

The (minimum) total energy deposited by non-thermal electrons is comparable to the peak total energy in the thermal plasma

slide11

MRT Newton Institute Aug 18 2004

Electron number flux

Max number flux =

2-5 1036 electrons s-1

Holman et al 2003

Coronal density ~ 109cm-3

So need to accelerate all the electrons in 1027 cm3 every second

slide12

MRT Newton Institute Aug 18 2004

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

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)

slide14

MRT Newton Institute Aug 18 2004

October 28: electrons and protons

  • 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

Image: courtesy Krucker & Hurford

slide15

MRT Newton Institute Aug 18 2004

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)

Image: courtesy Krucker & Hurford

slide16

MRT Newton Institute Aug 18 2004

RHESSI CLEAN images at different energies: 3 Nov 2003

Image: Astrid Veronig

slide17

MRT Newton Institute Aug 18 2004

Evolution of RHESSI footpoints and looptop source

Footpoints: 70-100 keV

Loop top: 20-25 keV

Time evolution:

black  white

Image: Astrid Veronig

slide18

Ez

-

MRT Newton Institute Aug 18 2004

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 

The flare is clearly a 3-D configuration.

However, we still expect high fluxes of fast particles at times of high reconnection rate

slide19

MRT Newton Institute Aug 18 2004

Flux, spectrum and ‘reconnection rate’

Rapidly reconfiguring magnetic fields should in principle provide a high energy

input rate for acceleration of particles

Movement of RHESSI source

centroids (30-50keV) show

chromospheric mappings of

evolving coronal field

High HXR flux/hard spectrum occur during intervals of rapid footpoint separation

(Fletcher & Hudson 2002)

july 23 2002
July 23, 2002

Courtesy: Säm Krucker

slide21

MRT Newton Institute Aug 18 2004

October 29: HXR flux and footpoint motion.

Good correlation between particle flux and ‘reconnection rate’ in later phase of flare, when footpoint motion is ~ regular

Images: Säm Krucker

slide22

MRT Newton Institute Aug 18 2004

July 17 2002 flare: TRACE observations

slide24

time

MRT Newton Institute Aug 18 2004

~130 separate tracks

Fletcher, Pollock & Potts 2004

slide25

MRT Newton Institute Aug 18 2004

Flare footpoints on ~ simultaneous magnetogram

slide26

I1600

I1600

v BLOS

v BLOS

MRT Newton Institute Aug 18 2004

UV footpoint source intensity variations

Typical examples:

Peaks in v BLOS for individual footpoints show significant correlation

in time with peaks in the UV brightness, during impulsive phase

ObservationsMonte-Carlo simulations

Peaks within  2s 25  5% 8  2%

Peaks within  8s 45  5% 25  5%

slide27

MRT Newton Institute Aug 18 2004

Typical value of v BLOS ~ several 100 V m-1

v B ~103Vm-1

v B ~1.5x103Vm-1

Hard X-ray footpoints occur where v BLOS ~ 1 kV m-1

slide28

N

P1

P2

Pairs of correlated footpoints

pairs of footpoints for which UV time profiles highly correlated

(lines join pairs with linear correlation coefficient > 0.8)

slide29

MRT Newton Institute Aug 18 2004

Potential field extrapolation (zero free energy)

P1

N

P2

slide30

MRT Newton Institute Aug 18 2004

Conclusions

  • 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
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