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Hard X-Ray Solar Flare Polarimetry with RHESSI

Hard X-Ray Solar Flare Polarimetry with RHESSI. M.L. McConnell 1 , D.M. Smith 2 , A.G. Emslie 3 , R.P. Lin 2 , and J.M. Ryan 1 1 Space Science Center, University of New Hampshire, Durham, NH 2 Space Sciences Laboratory, U.C. Berkeley, Berkeley, CA

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Hard X-Ray Solar Flare Polarimetry with RHESSI

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  1. Hard X-Ray Solar Flare Polarimetry with RHESSI M.L. McConnell1, D.M. Smith2, A.G. Emslie3, R.P. Lin2, and J.M. Ryan1 1Space Science Center, University of New Hampshire, Durham, NH 2Space Sciences Laboratory, U.C. Berkeley, Berkeley, CA 3Physics Department, University of Alabama, Huntsville, AL

  2. Polarization of Bremsstrahlung Radiation Photons tend to be emitted perpendicular to electron’s plane of motion. The polarization vector tends to be parallel to the direction of acceleration. Degree of linear polarization can reach 80%.

  3. 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. • Model parameters include : • pitch angle distribution • geometry of the B-field • viewing angle Models predict polarization levels as high as 20 or 30%.

  4. Single vertical flux tube Integrated over a loop Langer and Petrosian (1977) Leach and Petrosian (1983) Predictions for Solar Flare Polarization

  5. q is the Compton Scatter Angle h is the Azimuthal Scatter Angle Basic Principles of Compton Polarimetry (10 keV – 30 MeV) Polarimetry relies on the fact that… photons tend to Compton scatter at right angles to the incident polarization vector incident photon scattered photon

  6. Asymmetry Ratio The Polarization Signature For a fixed Compton scatter angle (q), 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.

  7. Modulation Factor Modulation Factor for a 100% polarized beam represents a figure-of-merit for the polarimeter : A B

  8. Minimum Detectable Polarization (MDP) S = source counting rate B = background counting rate T = observation time = modulation factor for 100% polarization Sensitivity can be improved by : Increasing S (efficiency or geometric area) Decreasing B Increasing T Increasing Q (optimizing geometry)

  9. RHESSI as a Polarimeter (20 – 100 keV) In the case of RHESSI, the scattering medium is a small (3 cm diameter by 3.5 cm high) cylinder of Be placed within the array of Ge detectors. 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, as well as a means of minimizing systematic effects.

  10. Segmented Ge detectors The segmented nature of the Ge detectors means that low energy photons can reach the rear Ge segments only by scattering off other material.

  11. Monte Carlo Simulations We have used a modified version of GEANT3 to carry out Monte Carlo simulations of the polarimetric capabilities of RHESSI. A valid polarimeter event is one which produces a measurable energy deposit in the rear segment of Ge detectors 1, 8, or 9. Detector 2 is not included because it is not currently operating as a segmented detector. Detectors 3, 4, 5, 6 and 7 do not contribute significantly to the polarization signal and are therefore also excluded from the analysis. We have simulated a very narrow beam (just covering the front surface area of the Be) to study the intrinsic polarimetry parameters of RHESSI. A broader beam (covering the full front surface of the telescope tube) has been used to study the effects of scattering into the rear Ge segments.

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

  13. Polarimeter Mode – Effective Area The effective area is defined for both a narrow beam directed towards the front surface of the Be scattering block and a broad beam directed across the full front of the HESSI telescope tube. 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 .

  14. Polarimeter Mode – Modulation Factor The modulation factor is a measure of the quality of the polarization signal. Here we can see that scattering of incident solar flux reduces the quality of the polarization signal. (The scattered flux is not modulated.)

  15. 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.

  16. Ambient (Non-Flare) Background The measured detector background at lower energies is somewhat lower than was expected based on pre-flight estimates using data from the Ge detector on Wind/TGRS. Data are shown here for the three detectors that are used in polarization studies.

  17. HESSI Sensitivity to Solar Flare Polarization Minimum Detectable Polarization (MDP) For M-class flares, sensitivity levels of 20-40% may still be achievable in the lowest energy bands.

  18. Candidate Flare Events There have been two X-class flares since the launch of RHESSI. Both are currently under study, but no results have yet been attained. Shown below are the count rate time histories covering each event. 21-Apr-2002 : Class X1.5 : 00:59-02:51 UT time intervalof flare 20-May-2002 : Class X2.1 : 15:08 - 15:38 UT time intervalof flare

  19. Summary • Addition of a Be scattering block provides HESSI with significant polarimetric capability. • Despite detrimental impact of the scattering of incident solar flux by the spacecraft, HESSI retains significant polarization sensitivity. • Polarization sensitivity can be less than a few percent for some X-class flares. • Work in progress: • Estimate of polarization sensitivities for specific flares. • Study of the impact of Earth-scattered flux. • Development of analysis tools. • Investigation of polarization sensitivity for non-solar sources (gamma-ray bursts and the Crab pulsar).

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