1. A Local Reacceleration Thick Target Model (LRTTM) (a modification of the Collisional Thick Target Model CTTM -Brown 1971) Brown, Turkmani, Kontar, MacKinnon and Vlahos AA submitted

2. Collisional TTM Acceleration Radiation only No accln Collisional Transport NO Acceleration

3. The Collisional Thick Target Model CTTM Brown 71, 73 etc Hudson 72 etc MERITS OF CTTM Provides a ‘cartoon’ scenario for flare impulsive phase emissions roughly fitting observations Provides a ‘cartoon’ scenario for flare impulsive phase emissions roughly fitting observations Collisional transport is easy to work with even though we know it cannot really be valid! Separates acceleration site from HXR (TT Injection) source – ie no acceleration in HXR source. Simple but v restrictive

4. PROBLEMS WITH CTTM  Inefficiency of bremss => 1. Beam density ~ coronal loop density unless loop area there >> footpoint area unless loop area there >> footpoint area 2. Very large no. Ne of e’s accelerated >> IP & radio Ne  Downward beaming => Strong albedo bumps in HXR spectra - not observed. Data => comparable upward and downward fluxes (Kontar and Brown 2006)  Does not really tally with EM(t) and T(t) data  Beam driven evaporation does not work – self choking

5. HXR Source Requirements Regardless of model, observed HXR flux fixes required value of source nonthermal EM For a large HXR event

6. For any thick target model the N 1 source electrons of life  need ‘replenished’ at a rate For the CTTM collisional case  =tcoll ~ 1/n and F 1 is independent of n If there is LOCAL REACCELERATION inside the HXR source  is increased and F 1 reduced. In other words the photon yield per electron is increased

7. ONE CANDIDATE FOR THE LOCAL REACCELERATION – ELECTRIC FIELDS IN CURRENT SHEET CASCADE OF DISTRIBUTED ENERGY RELEASE (Galsgaard…. Vlahos… Turkmani…..) MHD defines stochastic electric fields Test particle acceleration occurs in these in both the corona and then after injection to the chromosphere

8. CSC E fields electron motion Corona Chromosphere

9. A Local Reacceleration A Local Reacceleration Thick Target Model (LRTTM)

10. E(t) for 10 test electrons 1 CTTM & 9 LRTTM E(t ) t/tcollCTTM LRTTM

11. Photon emission rate for test electrons LRTTM CTTM

12. Cumulative photon emission of test electrons over lifetime in thick target CTTM LRTTM

13. SOME LRTTM vs CTTM PROPERTIES  Needs lower electron flux and number (but as much beam power) as CTTM. How much lower depends on uncertain parameter values (resistivity etc). More consistent with radio and IP values.  Electrons much less anisotropic (less albedo)  Like CTTM, predicts HXR footpoints displaying rapid structure, syhnchronism and time of flight delays BUT  Footpoint/coronal contrast higher than CTTM  MUCH higher proportion of beam power goes into chromosphere, and deeper – may help with evaporation and WLF problems

14. OVER TO RIM FOR CSC DETAILS !