We propose to continue a highly-successful collaboration of solar theorists, mathematical modelers and data analysis experts, aimed at extracting the fullest information on the accelerated electron spectrum throughout the volume of a solar flare.

This knowledge is the key to understanding the processes of particle acceleration in solar flares and, by extension, in other astrophysical sites as well.

Over the past few years, under ISSI support, we have developed well-tested rigorous techniques to determine the source electron spectrum from observations of the emitted hard X-ray spectrum.

The previous work has opened up new opportunities that we want to exploit and for this reason we propose several followup activities:

• incorporate these techniques into robust, user-friendly, codes, to be made available to the community through the Solar SoftWare (SSW) tree;

• analyse a comprehensive set of solar flare events using these new techniques and tools, and thereby determine both “typical” and “extreme” properties of solar flares;

• explore the (rather subtle) effects of adding bound-free emission to the previously assumed free-free continuum;

• explore the development of a “bivariate” analysis, aimed at determining both the energy and angular distributions of the accelerated electrons solely from “one-dimensional” spectral observations.