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The high-energy Sun - probing the origins of particle acceleration on our nearest star

Matthews, S. A and Reid, H. A. S. and Baker, D. and Bloomfield, D. S. and Browning, P. K. and Calcines, A. and Del Zanna, G. and Erdelyi, R. and Fletcher, L. and Hannah, I. G. and Jeffrey, N. and Klein, L. and Krucker, S. and Kontar, E. and Long, D. M. and MacKinnon, A. and Mann, G. and Mathioudakis, M. and Milligan, R. and Nakariakov, V. M. and Pesce-Rollins, M. and Shih, A. Y. and Smith, D. and Veronig, A. and Vilmer, N. (2022) 'The high-energy Sun - probing the origins of particle acceleration on our nearest star.', Experimental astronomy., 54 . pp. 335-360.


As a frequent and energetic particle accelerator, our Sun provides us with an excellent astrophysical laboratory for understanding the fundamental process of particle acceleration. The exploitation of radiative diagnostics from electrons has shown that acceleration operates on sub-second time scales in a complex magnetic environment, where direct electric fields, wave turbulence, and shock waves all must contribute, although precise details are severely lacking. Ions were assumed to be accelerated in a similar manner to electrons, but γ-ray imaging confirmed that emission sources are spatially separated from X-ray sources, suggesting distinctly different acceleration mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic understanding of accelerated particle spectra and the total energy budgets are therefore poorly constrained. Additionally, the recent detection of relativistic ion signatures lasting many hours, without an electron counterpart, is an enigma. We propose a single platform to directly measure the physical conditions present in the energy release sites and the environment in which the particles propagate and deposit their energy. To address this fundamental issue, we set out a suite of dedicated instruments that will probe both electrons and ions simultaneously to observe; high (seconds) temporal resolution photon spectra (4 keV – 150 MeV) with simultaneous imaging (1 keV – 30 MeV), polarization measurements (5–1000 keV) and high spatial and temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray) regimes. These instruments will observe the broad range of radiative signatures produced in the solar atmosphere by accelerated particles.

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Publisher statement:Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit
Date accepted:23 September 2021
Date deposited:05 January 2022
Date of first online publication:09 November 2021
Date first made open access:05 January 2022

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