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


2.2 MeV line

Thermal bremsstrahlung

Non-thermal bremsstrahlung

Movie: Eduard Kontar


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

MRT Newton Institute Aug 18 2004

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


Higher energy emission from higher in the looptop– Strongly implies multi-thermal distribution

Source position as a function of energy

Figure: Amir Caspi, UCB


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.


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


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


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


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)


Image: courtesy Krucker & Hurford

RHESSI CLEAN images at different energies: 3 Nov 2003


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


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


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


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


July 17 2002 flare: TRACE observations



time

Fletcher, Pollock & Potts 2004

~130 separate tracks


Flare footpoints on ~ simultaneous magnetogram


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:


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


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


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

Date post:	02-Feb-2016
Category:	Documents
Upload:	jensen
View:	48 times
Download:	0 times

Lyndsay Fletcher, University of Glasgow

Documents