Science

Here is the list of publications related to HESPE

It is now well accepted that of the different forms of solar activity, Coronal Mass Ejections (CMEs) control space weather and have the greatest effect on the Earth, both through their impact on the Earth’s magnetosphere and through the high energy particles that they accelerate as they propagate. But it is clear that solar flares and CMEs are intimately related. They generally occur together  and the CME acceleration phase is often coincident with the impulsive phase of the associated flare. The total energy dissipated in the largest flares is comparable to the total energy of the associated CMEs, known now very reliably from the increase in the total solar activity.

It is also well established that the total energy in flare-accelerated electrons as derived from X-ray observations is a large fraction of the total flare energy – on the order of 10% or more. The total energy in accelerated ions (from gamma-ray measurements) is comparable to the total energy in accelerated electrons. However, at the present time, many core aspects of the physical processes leading to the production of high-energy solar flare radiation still remain unclear. Principal among these are (i) the acceleration of the high-energy electrons and ions which produce the X- and gamma-ray radiation, i.e. , more specifically, how so many of these particles are accelerated so quickly to such high energies; and, (ii) the transport of these particles through the solar atmosphere and into interplanetary space. The dominant theoretical model for the production of flare X-rays, the so-called flare ‘standard model’, proposes a three-part process of particle acceleration (in the solar corona), transport, and radiation (primarily in the chromosphere), in which the electron acceleration and radiation regions are spatially separated, with an electron ‘beam’ transporting energy between the two. As observations, particularly in X-rays, have improved, this model has been challenged and alternatives proposed. Theoretical developments have proceeded in two main ways:

  • Firstly there is the calculation of the detailed physics of the processes leading to the observed radiation (the ‘forward’ problem). The basic problem here is to rapidly generate a substantial non-thermal tail of electrons from a (Maxwellian) background plasma and then follow its spatial, spectral and angular evolution. For many years, hard X-ray observations have been interpreted almost exclusively in the framework of collisional transport, but more recently many tools of plasma physics theory are being brought to bear, including test-particle, kinetic and particle-in-cell calculations, MHD waves and turbulence. Crucially for our understanding of electrons in the collisionless corona, the collective modes of the plasma excited by the beam, and their feedback on the beam are now being modelled, which promises to have significant ramifications for our interpretation of electron spectra, particularly the so-called ‘low energy cutoff’. Phenomenological models which address the generation of thermal and non-thermal radiation by a single electron population, rather than as two substantially separate processes, are also in development.
  • Secondly, there is the theory of how to use the data with minimal assumptions to infer the characteristics of the particles and processes producing the radiation (the ‘inverse’ problem). A typical example is collisional bremsstrahlung, whereby X-rays provide a clean, optically thin, diagnostic for electrons. Significant progress in the interpretation of this radiation has been made using inverse theory to specify the parameters of the radiating electron spectrum. Subtle spectral features which can only be extracted using the inverse approach have allowed us to constrain the angular distribution of the radiating electrons, as well as their spatial and spectral distributions. Within the framework of the standard flare model, we are also able to infer the properties of the accelerated electron distribution, thus constraining acceleration models.

HESPE is working at important advances in scientific aspects involving: (i) advances in the theory of solar flare particle acceleration and transport (due to the availability of large databases of data analysed with the most powerful processing techniques); (ii) advances in solar physics as a whole (due to the greatly improved accessibility of the best X-ray data, from present and future missions, to the whole solar and heliospheric community, for easy integration into their ongoing analyses); (iii) better understanding of the sources of high energy particles escaping and transport from the Sun to the Earth, which is essential component of space weather.

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