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

Michele Piana is full professor of Numerical Analysis at the Dipartimento di Matematica of the Università di Genova, research associate at CNR - SPIN Genova and Principal Investigator of the Methods for Image and Data Analysis (MIDA) Group. He was Visiting Scholar at the Department of Mathematical Sciences at the University of Delaware, researcher at the Istituto Nazionale di Fisica della Materia (INFM), Associate Professor of Computer Science at the Dipartimento di Informatica of the Università di Verona. He spent visiting periods at the Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology; Department of Physics and Astronomy, University of Glasgow; NASA Goddard Space Flight Center; and Institute for Data Science, Fachhochschule Nordwestschweiz. He has authored and co-authored more than 100 papers in refereed journals on solar physics, medical imaging, computational neuroscience, and applied mathematics. He has been a member of the Data Analysis Team of the NASA RHESSI mission and is currently co-Investigator for STIX on the Solar Orbiter and HXI on ASO-S. He has been Unit Coordinator for the FLARECAST project in Horizon 2020 and Coordinator of the HESPE project in FP7. He has been Deputy Rector for International Affairs and Deputy Rector for Research and Technological Transfer at the Università di Genova. He is currently Scientific Director of the Life Science Computational Laboratory, jointly established by the Università di Genova and the Ospedale Policlinico San Martino IRCCS Genova.



Brian Dennis has been actively involved in designing, building, and operating X-ray and gamma-ray spectrometers and imaging spectrometers during his over 50-year career as an astrophysicist at NASA's Goddard Space Flight Center. These include spectrometers on the 5th and 8th Orbiting Solar Observatories in the 1960s and `70s and on the Solar Maximum Mission in the 1980's, and the Ramaty High Energy Solar