to improve our modelling ability and frameworks. To make rapid and serious
progress not only in solar flare physics, but in modern space physics more
generally, we must have significant investment of time and resources in
models. This involves continued improvements of our existing field-aligned
models [some examples are discussed in section 5.2 of Kowalski et al., 2022],
but also to attempt to bridge the gap to multi-dimensional models. A fully 3D
radiative-MHD model capable of simulating an NLTE chromosphere to sub-metre
scales following flare energy injection is a daunting task that may not be
fully realised for many years, but certainly we must attempt to include multi-
dimensional effects. Efforts are already underway to study 2D/3D RT effects.
For example, Osborne and Fletcher [2022] are exploring the effects of 2D
radiation transfer, using 1D field-aligned RADYN models embedded within
regions of quiet-Sun, finding significant differences may be present
(including on the intensity of transition region lines, private
communication). Additionally, non-thermal electrons have now been included in
3D radiative-MHD simulations, with very low fluxes to explore nano-flare
heating [Bakke et al., 2018], and hopefully larger fluxes typical of flares
may be explored in the future. However, we must also think about how to handle
horizontal expansion of plasma in flare footpoints in the field-aligned models
(that can handle the small vertical scales required in the chromosphere during
flares).

Recently, Dr. Joel Allred and co-authors proposed an end-to-end modelling
framework of solar eruptive events in a white paper titled ‘Next-Generation
Comprehensive Data-Driven Models of Solar Eruptive Events’ 171717White papers
will appear in the Bulletin of the American Astronomical Society, but are for
now hosted by the NAS on their website: https://www.nationalacademies.org/our-
work/decadal-survey-for-solar-and-space-physics-heliophysics-2024-2033. A link
to Dr. Allred’s white paper is:
http://surveygizmoresponseuploads.s3.amazonaws.com/fileuploads/623127/6920789/140-a8d175a52b5836b620abf6d961febf97_AllredJoelC.pdf
submitted to the National Academy of Science Solar and Space Physics Decadal
Survey. In this framework, models that target different aspects of solar
eruptive events should be linked such that the output of one is either the
input to the next link in the chain, or strongly guides/constrains the next
link, ideally in a data-driven or data-constrained manner. Field-aligned
models would be an essential component. An example of such a chain could be:
(1) an MHD model of the build up and release of magnetic energy, that then
drives (2) a model of particle acceleration, the energy spectrum of which
drives, (3) a model that propagates and dissipates non-thermal particles,
producing (4) the radiative hydrodynamic response of the atmosphere, that (5),
ultimately predicts observables, including geometry of the original magnetic
field/loops (e.g. via a RADYN_Arcade type framework). The latter three steps
are of course already being done with the models discussed in this review
(e.g. RADYN+FP). Joined up modelling such as that described by Allred et al,
interrogated by high-resolution observations, is perhaps the best way to make
rapid progress whilst covering the vast range of scales, from MHD to kinetic,
present in solar flares.

## Conflict of Interest Statement

The author declares that the research was conducted in the absence of any
commercial or financial relationships that could be construed as a potential
conflict of interest.

## Author Contributions

GSK performed the literature review and wrote the manuscript.

## Funding

GSK acknowledges funding via a NASA ROSES Early Career Investigator Award
(Grant# 80NSSC21K0460), and the Heliophysics Supporting Research program
(Grant# 80NSSC21K0010).

## Acknowledgments

IRIS is a NASA small explorer mission developed and operated by LMSAL with
mission operations executed at NASA Ames Research center and major
contributions to downlink communications funded by the Norwegian Space Center
(NSC, Norway) through an ESA PRODEX contract. This manuscript benefited from
discussions held at a meeting of International Space Science Institute team:
“Interrogating Field-Aligned Solar Flare Models: Comparing, Contrasting and
Improving,” led by Dr. G. S. Kerr and Dr. V. Polito. I also thank the
following colleagues for their help, and patience, with answering questions
related to RADYN, PREFT, FLARIX, and HYDRAD: Dr. Joel Allred, Dr. Mats
Carlsson, Dr. Adam Kowalski, Dr. Vanessa Polito, Dr. Dana Longcope, Dr.
William Ashfield, Dr. John Unverferth, Dr. Stephen Bradshaw, Dr. Jeffrey Reep,
Dr. Jana Kašparová, Dr. Petr Heinzel, and Dr. Michal Varady. Finally, I thank
the referees for their diligent reading and useful comments which improved
both content and clarity of this review.

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