1. Soft-Hard Soft Spectral Behaviour From Dynamic Reconnection Test Particle Calculations C. A. Burge 1, P. Petkaki 2, A.L.MacKinnon 3 1. Department of Physics & Astronomy, University of Glasgow, Glasgow, G12 8QQ. 2. British Antarctic Survey, Cambridge, CB3 0ET. 3. DACE, University of Glasgow, Glasgow,G3 6NH. Email: c.burge@astro.gla.ac.uk, a.mackinnon@educ.gla.ac.uk, ppe@bas.ac.ukc.burge@astro.gla.ac.uka.mackinnon@educ.gla.ac.uk

2. SHS Spectral Pattern SHS = hardening of the X- ray spectrum of a flare as the HXR flux increases. The spectrum softens again as the flux decreases. Ubiquitous in solar flare HXR spectra-a clue to the electron accelerator? Hudson & Fárník (2002)‏

3. What Causes SHS? Theories include: Electric field induced by electrons causing reduction in number of low- energy electrons, and causing hardening of photon spectrum.(Zharkova & Gordovskyy 2006)‏ Changes in escape length of fast electrons could produce SHS behaviour. (Benz 1976, Grigis & Benz 2006)‏ Variation in proportion of thermal & nonthermal photons.(Aschwanden 2004)‏ Increase in efficiency of acceleration mechanism.(Aschwanden 2004)‏

4. X Type Neutral Point Linear waves traveling through a region including a magnetic null point become dissipative phenomena around the null (e.g. Craig and McClymont, 1991; Hassam, 1992; MacLaughlin and Hood, 2004). Near the null a large, localised electric field develops. Particles can become decoupled from the magnetic field and be freely accelerated in a suitably collisionless plasma. This linear reconnection is intrinsically time- dependent, with frequency depending on magnetic field strength, system scale and resistivity. Craig & McClymont (1991)‏

5. X Type Neutral Point B x = y ; B y = x 50 000 electrons with initial conditions chosen randomly from a spatially uniform, Maxwellian distribution with T= 5 x 10 6 K. We normalise speeds to c & times to the electron gyroperiod at the boundary. Final energy distributions were generated after t=5360 (1s for typical coronal conditions).

6. Particle Trajectories

7. SHS Spectral Pattern: Thick Target

8. SHS Spectral Pattern: Thin Target

9. Spectral Index Variation: Thin Target

10. SHS Spectral Pattern

12. Numbers of accelerated electrons and maximum energies attained (hence HXR spectral hardness) maximise for an intermediate range of ω. Continuously varying ω will produce SHS behaviour. This model is simple and idealised, but is interesting as several acceleration regions may exist in a flare. Conclusions