Hard X-Ray Imaging of Solar Flares

1 Hard X-ray Emission in Solar Flares1.1 A Brief Overview of Solar Flares1.2 Hard X-Ray Emission from Flares and its Significance1.2.1 Acceleration of Nonthermal Electrons1.2.2 Hard X-Ray Production by Accelerated Electrons: The Bremsstrahlung Process1.2.3 Relation of the Mean Source Electron Spectrum to the Accelerated Spectrum1.3 History of Solar Hard X-Ray Imaging Observations2 X-Ray Imaging Methods2.1 Medical Imaging2.2 Astrophysical X-ray Imaging Techniques2.2.1 Absorption2.2.2 Scattering2.2.3 Reflection2.2.4 Diffraction3 RHESSI and STIX3.1 RHESSI Design / Brief History of Concept Development3.2 The RHESSI imaging concept3.3 Strengths and Limitations of the RHESSI RMC Imaging Technique3.4 RHESSI Imaging Example3.5 SSW and the RHESSI GUIs3.6 STIX Design / Brief History of Concept Development3.7 The STIX imaging concept3.8 STIX software3.9 RHESSI vs. STIX - a comparison of strengths and limitations4 Image Reconstruction Methods4.1 The Essence of the Image Reconstruction Problem4.1.1 Count-based versus visibility-based imaging4.1.2 Point Spread Functions4.2 The ill-posedness of the image reconstruction problem4.3 The Regularization Concept4.4 Numerical Optimization5 Count-based Imaging Methods5.1 Back-projection5.2 CLEAN5.2.1 Two-step CLEAN Method5.3 Forward Fit5.4 Pixon5.4.1 Maximum Entropy Methods5.4.2 The Pixon Methodology5.5 Expectation Maximization6 Visibility-based Imaging Methods6.1 Visibilities6.2 Visibility-based Methods6.3 VIS_FWDFIT6.4 Bayesian Optimization6.5 MEM_NJIT and MEM_GE6.6 uv_smooth6.7 VIS_CLEAN and Multi-scale CLEAN6.8 Compressed Sensing - VIS_CS and VIS_WV6.9 Electron Flux Maps7 Application to Solar Flares7.1 Number and Nature of Hard X-Ray Sources in the 2002 February 20 Event7.2 The Physical Nature of Multiple Hard X-Ray Sources in the 2002 July 23 Event7.3 Properties of the Electron Acceleration Region7.3.1 Using the VIS_FWDFIT method to estimate the acceleration region length and density7.3.2 Using the MEM_NJIT method to revisit earlier results7.4 Empirical Determination of the Electron Energy Loss Rate7.5 Hard X-Ray Imaging and the Global Energetics of Solar Flares8 Future Possibilities8.1 STIX8.2 NuSTAR8.3 FOXSI8.4 Advanced Spaced-based Solar Observatory8.5 GRIPS8.6 Hard X-ray Polarimetry8.7 ConclusionReferencesIndex