The idea for this text emerged over several years as the authors
participated in research projects related to analysis of data from
NASA's RHESSI Small Explorer mission. The data produced over the
operational lifetime of this mission inspired many investigations
related to a specific science question: the when, where, and how of
electron acceleration during solar flares in the stressed magnetic
environment of the active Sun.
A vital key to unlocking this science problem is the ability to produce
high-quality images of hard X-rays produced by bremsstrahlung radiation
from electrons accelerated during a solar flare. The only practical way
to do this within the technological and budgetary limitations of the
RHESSI era was to opt for indirect modalities in which imaging
information is encoded as a set of two-dimensional spatial Fourier
components.
Radio astronomers had employed Fourier imaging for many years. However,
differently than for radio astronomy, X-ray images produced by RHESSI
had to be constructed from a very limited number of sparsely distributed
and very noisy Fourier components. Further, Fourier imaging is hardly
intuitive, and extensive validation of the methods was necessary to
ensure that they produced images with sufficient accuracy and fidelity
for scientific applications.
This book summarizes the results of this development of imaging
techniques specifically designed for this form of data. It covers a set
of published works that span over two decades, during which various
imaging methods were introduced, validated, and applied to observations.
Also considering that a new Fourier-based telescope, STIX, is now
entering its nominal phase on-board the ESA Solar Orbiter, it became
more and more apparent to the authors that it would be a good idea to
put together a compendium of these imaging methods and their
applications. Hence the book you are now reading.
