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					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
                Collisional TTM
Acceleration


 Collisional
 Transport
 NO
 Acceleration
                       Radiation
                       only
                       No accln
 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
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
          PROBLEMS WITH CTTM
   Inefficiency of bremss =>

1. Beam density ~ coronal loop density
   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
   HXR Source Requirements
Regardless of model, observed HXR flux fixes
 required value of source nonthermal EM




 For a large HXR event
For any thick target model the N1 source
electrons of life t need ‘replenished’ at a rate


                nvQ    brem   dt  nvQbremt                                                   nvQ     brem   dt  nvQbremt


  For the CTTM collisional case t =tcoll ~
             t ( E  )                                                                        t ( E  )




                                                           nvQBremssdt  nvQBremsst
  1/n and F 1 is independent of n                      t ( E  )




  If there is LOCAL REACCELERATION
  inside the HXR source t is increased
  and F1 reduced. In other words the
                 nvQbremsdt  nvQbremst
            t ( E  )



  photon yield per electron is increased
                                nvQbremsdt  nvQbremst
    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
CSC E fields


           Corona


                     Chromosphere




                    electron motion
   A Local Reacceleration
Thick Target Model (LRTTM)
       E(t) for 10 test electrons
         1 CTTM & 9 LRTTM


E(t)
                    LRTTM




CTTM               t/tcoll
  Photon emission rate for test electrons




                      LRTTM




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


               LRTTM



           CTTM
        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
OVER TO RIM FOR
  CSC DETAILS !

				
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posted:4/22/2013
language:English
pages:14