1. RHESSI Studies of Solar Flare Hard X-Ray Polarization Mark L. McConnell 1, David M. Smith 2, A. Gordon Emslie 4, Martin Fivian 3, Gordon J. Hurford 3, Robert P. Lin 3, and James M. Ryan 1 1 Space Science Center, University of New Hampshire, Durham, NH 2 Dept. Of Physics, U.C. Santa Cruz, Santa Cruz, CA 3 Space Sciences Laboratory, U.C. Berkeley, Berkeley, CA 4 Physics Department, University of Alabama, Huntsville, AL

2. Polarization in Solar Flares The hard X-ray continuum is dominated by electron bremsstrahlung emission. Measurements of hard X-ray polarization can shed light on the geometry of the acceleration process. Models predict polarization levels as high as 20 or 30%. Model parameters include : 1)pitch angle distribution 2)B-field geometry 3)viewing angle 4)atm density profile

3. The Polarization Signature For a fixed Compton scatter angle (  ), the azimuthal distribution of scattered photons contains the polarization signature. The amplitude of the modulation defines the level of polarization. The scattering angle corresponding to the minimum of the distribution defines the plane of polarization. Asymmetry Ratio

4. RHESSI as a Polarimeter (20 – 100 keV) A small (3 cm diam by 3.5 cm high) cylinder of Be serves as a Compton scattering element. The Ge detectors measure the distribution of the scattered radiation. The rotation of the spacecraft rotation provides an effective method for fine sampling of the scatter distribution.

5. RHESSI as a Polarimeter (20 – 100 keV) Complications : Varying background as a function of spacecraft spin angle. Scattering of solar flux from parts of the spacecraft other than the Be block. Scattering of solar flux off atmosphere (spin-dependent).

6. The Polarization Signal - Simulated Results 40 keV 80 keV Top row shows results for narrow incident beam (no spacecraft scattering). Bottom row shows results for wide incident beam (with spacecraft scattering). Note the significant degradation of signal at 80 keV.

7. Polarimeter Mode – Effective Area The effective area is defined for both a narrow beam and a broad beam. The broad beam simulation incorporates the effects of scattering of solar flux into the rear Ge segments, which leads to an increase in effective area at higher energies.

8. Polarimeter Mode – Modulation Factor The modulation factor is a measure of the quality of the polarization signal. Scattering of incident solar flux reduces the quality of the polarization signal. (The scattered flux is not modulated.)

9. Polarimeter Mode – Figure of Merit The figure-of-merit is a measure of the intrinsic capability to measure polarization. Here, it is defined as the product of (effective area) 1/2 and the modulation factor. As defined here, it does not incorporate the effects of detector background.

10. Nature of the RHESSI Data Rear Segment Data (20 – 40 keV) X4.8 Flare - 23 July 2002 00:26 – 00:42 UT

11. An Initial Approach to RHESSI Analysis Three pairs of detectors with similar background : detectors 8/9, detectors 3/5 and detectors 4/6. The data from detectors 3-6 can be used as background estimate for the polarimeter mode detectors 8/9. Limitations : Does not use detector #1 Assumes symmetric geometry No modeling of Earth albedo

12. X4.8 Flare of 23-July-2002 Analysis Interval : 00:27 – 00:43 UT

13. An Initial Approach to RHESSI Analysis Two Component Analysis 1. Systematic Component: Single sinusoid component. 2. Polarization Signal Double sinusoid component.

14. An Initial Approach to RHESSI Analysis

15. X4.8 Flare of 23-July-2002 “Background”-subtracted data

16. X4.8 Flare of 23-July-2002 Residual Polarization Signal

17. X4.8 Flare of 23-July-2002 Residual Polarization Signal

18. X1.5 Flare of 21-April-2002 Analysis Interval : 1:12 – 1:18 UT

19. X1.5 Flare of 21-April-2002 Residual Polarization Signal

20. X1.5 Flare of 21-April-2002 Residual Polarization Signal

21. X3.9 Flare of 03-November-2003 Analysis Interval : 9:48 – 9:53 UT

22. X3.9 Flare of 03-November-2003 Residual Polarization Signal

23. X3.9 Flare of 03-November-2003 Residual Polarization Signal

24. X2.5 Flare of 10-November-2004 Analysis Interval : 2:06 – 2:15 UT

25. X2.5 Flare of 10-November-2004 Residual Polarization Signal

26. X2.5 Flare of 10-November-2004 Residual Polarization Signal

27. Selected Flares 11-Nov-2004 N8W62 23-Jul-2002 S13E72 03-Nov-2003 N08W77 21-Apr-2002 S14W84

28. X4.8 Flare of 23-July-2002 20 - 40 keV Polarization Flare location : S13E72  ≈ 20%  ≈72° ± 5°

29. Polarization Measurements with Coronas-F  Uses similar concept for polarization measurements (Be scattering into CsI).  Looked at data from three events: 1)28-Oct-2003 2)29-Oct-2003 3)04-Nov-2003  Polarization detected only for 29-Oct-2003 40-60 keV polarization >75%

30. Summary  Addition of a Be scattering block provides HESSI with significant polarimetric capability.  Polarization sensitivity predicted to be less than a few percent for some X-class flares.  First analysis includes four events : 1) 23-July-2002 2) 21-April-2002 3) 03-November-2003 4) 10-November-2004  Only the July 23rd event shows evidence of polarization (at the 20% level in the 20-40 keV energy range).  Polarization angle not consistent with beamed emission at footpoint. Perhaps beamed emission with looptop source?!? Or pancake distribution at footpoint?!?