Lyndsay Fletcher, University of Glasgow Ramaty High Energy Solar Spectroscopic Imager Fast Particles in Solar Flares The 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

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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 MRT Newton Institute Aug 18th

Holman et al 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

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 MRT Newton Institute Aug

Higher energy emission from higher in the looptop –Strongly implies multi-thermal distribution Source position as a function of energy Figure: Amir Caspi, UCB MRT Newton Institute Aug

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 ~ 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 keV. MRT Newton Institute Aug

Coronal density ~ 10 9 cm -3 So need to accelerate all the electrons in cm 3 every second Electron number flux Max number flux = electrons s -1 Holman et al 2003 MRT Newton Institute Aug

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 MRT Newton Institute Aug

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” ( ~10 4 km) e and p are accelerated in loops of similar size October 28: electrons and protons MRT Newton Institute Aug 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) MRT Newton Institute Aug Image: courtesy Krucker & Hurford

RHESSI CLEAN images at different energies: 3 Nov 2003 MRT Newton Institute Aug Image: Astrid Veronig

Evolution of RHESSI footpoints and looptop source Time evolution: black  white Footpoints: keV Loop top: keV Image: Astrid Veronig MRT Newton Institute Aug

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 EzEz - MRT Newton Institute Aug

Flux, spectrum and ‘reconnection rate’ Movement of RHESSI source centroids (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 MRT Newton Institute Aug

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 MRT Newton Institute Aug

July flare: TRACE observations MRT Newton Institute Aug

time Fletcher, Pollock & Potts 2004 ~130 separate tracks MRT Newton Institute Aug

Flare footpoints on ~ simultaneous magnetogram MRT Newton Institute Aug

UV footpoint source intensity variations Peaks in v B LOS for individual footpoints show significant correlation in time with peaks in the UV brightness, during impulsive phase Observations Monte-Carlo simulations Peaks within  2s 25  5% 8  2% Peaks within  8s45  5% 25  5% v B LOS I 1600 v B LOS I 1600 Typical examples: MRT Newton Institute Aug

v B ~10 3 Vm -1 v B ~1.5x10 3 Vm -1 Typical value of v B LOS ~ several 100 V m -1 Hard X-ray footpoints occur where v B LOS ~ 1 kV m -1 MRT Newton Institute Aug

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) P1 P2 N MRT Newton Institute Aug

Conclusions MRT Newton Institute Aug 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