1. Chromospheric diagnostics using RHESSI visibilities Marina Battaglia & Eduard Kontar

2. Overview Introduction Observations –Previous work –Current work Comparison with theory Outlook

3. Introduction Aschwanden et al. 2002: Height of X-ray source decreases with energy Early in mission, using traditional forward fitting Aschwanden et al. 2002

4. Observations Kontar et al. 2008 –Limb event (6 th January 2005) –Using visibility forward fit –Find clear decrease of height with energy

5. 6 th January 2004 vis_fwdfit Energy

6. Horizontal size Eccentricity Height Vertical size The source size decreases as well!

7. Horizontal size Eccentricity Height Vertical size Using and find semi minor and major axes Can not only do position and size, but shape as well!

8. What physics can we learn from this? Begin with simplest case: collisional thick target (Brown 1971) Has maximum for each 

9. ab Height

10. From maximum position can infer density structure Semi-major axis (a) as measure for magnetic field convergence Height a

11. From maximum position can infer density structure: Aschwanden: power-law Kontar: exponential r 0 : radial distance of bottom of loop, h 0 density scale height, n 0 number density at height h=0 (from models, n 0 =1.16x10 17 cm -3 ) The bottom of the magnetic loop (r 0 ) is determined from the data via fitting

12. The simple thick target case is inaccurate What about electron pitch angle?  0 = 1 0.9 0.7 0.5 Constant pitch angle from injection

13. So far so good – but: The simple thick target case is inaccurate Semi minor axis, expected from collisional thick target Semi minor axis measured in 6 th January event

14. Magnetic field: –Change in column depth due to different path –Magnetic mirroring e-e- e-e- s n

15. Magnetic field: Expect effect on width of semi – minor axis How does it affect the maximum position? –To be continued

16. Outlook Is the 6 th January 2004 unique? Energy Clean mem_njit Pixon vis_fwdfit

17. southern footpoint Radial positionFWHM

18. Energy Clean mem_njit Pixon vis_fwdfit

19. southern footpoint Radial positionFWHM

20. Summary and Outlook Analysis of footpoints using vis_fwd confirms previous measurement of energy dependent source height Can now also determine the size and shape of sources Can do it with more than just one event Can infer pitch angle distributions and magnetic field convergence from the source size and geometry.