F(E , Ω ,r )=??? 1) What is the energy distribution, F(E) ?

1. DEDUCING ELECTRON PROPERTIES FROM HARDX-RAY OBSERVATIONSEduard KontarUniversity of Glasgow (aka Chapter 7)

2. F(E,Ω,r)=??? 1) What is the energy distribution, F(E)? 2) What is the angular distribution, F(E,Ω)? 3) What is spatial distribution, F(E,r)? From X-rays to electrons… Emission cross-sections Unknown electron distribution Observed X-rays

3. The major questions discussed: • X-RAY EMISSION PROCESSES AND ENERGETIC ELECTRONS • PRIMARY AND COMPTON BACK-SCATTERED X-RAYS • THE ELECTRON ENERGY SPECTRUM • ELECTRON ANGULAR DISTRIBUTION • ELECTRON SPATIAL DISTRIBUTION

4. X-rays emission processes For spatially integrated spectrum: Thin-target case: For the electron spectrum F(E)~E-δ, a) Electron-ion bremsstrahlung (free-free emission) Dominant process for energies ~10 – 400 keV the photon spectrum is I(ε)~ ε-δ-1 In the simplest form Kramers’ approximation: b) Electron-electron bremsstrahlung (free-free emission) Dominant process for energies above 400 keV the photon spectrum is I(ε)~ ε-δ c) Recombination emission (free-bound emission) Could be importantprocess for energies up to 20 keV the photon spectrum is shifted by ionisation potential and I(ε)~ ε-δ-2 (The process requires high temperatures and detailed ionisation calculations)

5. Electron-ion and electron-electron bremsstrahlung:

6. Direct and reflected HXRs: Flaring region • Downward emitted X-ray photons can be either • Compton scattered X-rays into observer direction • or • Photoelectrically absorbed Id Iu Observer Observed Flux = Direct + Scattered

7. Primary and reflected X-rays Observed flux Primary Reflected flux Reflected flux can be ~40-45 % of the total flux even for an isotropic source!

8. Albedo and extended sources mechanisms: => Detection of halo “halo" component

9. Spatially integrated spectrum: forward fitting

10. Spatially integrated spectrum: model-independent approach

11. Low-energy cut-offs, dips:

12. Temperature distribution of thermal plasma Differential emission measure

13. Electron angular distribution (mean spectrum): Anisotropy of HXR: Statistics of RHESSI flares with Emitting electron distribution is close to isotropic (beamed models can be ruled out)

14. HXR polarization measurements: Current polarization measurements are questionable, but are essential for a number of science questions.

15. Electron spatial distribution:

16. Spatial distributions of energetic electrons:

17. Spatial distributions of energetic electrons: RHESSI measurements of the moments (flux, centroid, size) • X-ray source motions => reconnection rate, motion of loop top sources • X-ray positions versus energy => density structure, • X-ray source sizes => magnetic structure in footpoints, the size of acceleration region

18. Wish list: Thermal plasma evolution in flares: What is the relation between thermal-nonthermal components of flare emission? Differential emission measure of hot plasma, spectroscopy below ~10 keV What is the distribution of non-thermal electrons below 10-20 keV? Better dynamic range of images: What is HXR emission from the loop legs, above the loop top region ? Relation between the HXR sources? Imaging polarimetry: What is the polarization (spatial distribution of polarization) of HXR emission in solar flares? direct/scattered HXRs, directivity, angular distribution of energetic electrons Higher temporal resolution: Beyond the standard flare model ? What is the role of collective effects in solar flare electron transport?