Solar flares: evaporation and simulation

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Nugget
Number:	464
1st Author:	Malcolm DRUETT
2nd Author:
Published:	December 18, 2023
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Contents 1 Introduction 2 Results 3 Conclusions 4 Acknowledgements 5 References

Introduction

Numerical models of solar flares provide interpretations of the energy transport and release processes, and importantly this includes some process that both heats and drives upflows of dense material from the lower atmosphere. These hot upflows are commonly known as "chromospheric evaporation", though the physics would be better described as "ablation". Chromospheric evaporation is critical to the interpretation of both the emission and the dynamics of the lower solar atmosphere, and also for the information encoded in bright loop-top ultraviolet and soft x-ray emission from previously evaporated material.

There is still debate regarding the agents responsible for this key physical process. Proposed mechanisms include simple heat conduction, Alfvén waves, and the impact of outflow jets from magnetic reconnection. The standard interpretation involves beams of energetic particles that originate in the solar corona and travel along magnetic field lines to deposit their energy in the lower atmosphere. The beam-electron model also explains the strong Bremsstrahlung emission that is observed in flare loop foot-points, produced by collisions between the beam particles and the ambient plasma.

Moreover, there is also a schism between the approaches used in 1D (field-aligned) and multi-dimensional (full MHD) models. The 1D models generally use combinations of thermal conduction and beams of energetic particles to drive chromospheric evaporation. Multi-dimensional models often either omit the process, or use a strong reconnection outflow originating in the corona and impacting the lower atmosphere to drive the evaporation.

Results

Two recent manuscripts (Refs. [1], [2]]) have produced the first multi-dimensional chromospheric evaporation by beams of electrons, and also laid key groundwork to help compare results from 1D and multi-dimensional models.

This was achieved by placing models of 1D field-aligned electron beams within a multi-dimensional model (Ref. [3]). Figure 1 illustrates the calculations. By adjusting the scheme of plasma resistivity in the corona so that a gentle initial phase produced pre-existing flare loops, we can avoid the reconnection outflow jet directly impacting onto the solar surface below (Ref. [2]), thus allowing the particle beams to drive the evaporation.

Figure 2 compares the multidimensional results with the 1D approach.

Conclusions

These results open up a new state-of-the-art in flare modelling as well as providing opportunities for collaboration between 1D and multi-dimensional modelling approaches. We can look to provide a best-of-both-worlds route forward that includes the critical physics, highlighted in detailed 1D models, within the multi-dimensional models of the future.

Multi-dimensional effects can also explain additional physics within flares. One example is reported in Ref. [4], namely that the leading edges of and expanding chromospheric flare ribbon are frequently associated with less evaporation and lower levels of beam electrons. In multi-dimensional models (Druett et al. 2023) this effect is manifested due to heat diffusion and the compression of field outside of the reconnected flare loops. Thus heat intrudes outward onto neighbouring field-lines, and thermal conduction fronts travel along that field-line, often reaching the chromosphere before the associated field-line has actually reconnected.

Acknowledgements

I would like to thank my co-authors on these papers: Wenzhi Ruan and Rony Keppens. I would also like to thank Alexander Pietrow for his constructive input and additionally, Hugh Hudson for his help editing this Nugget.

References

[1] "Chromospheric Evaporation by Particle Beams in Multi-Dimensional Flare Models"

[2] "Exploring self-consistent 2.5 D flare simulations with MPI-AMRVAC"

[3] "A Fully Self-consistent Model for Solar Flares"

[4] "Solar Flare Ribbon Fronts. I. Constraining Flare Energy Deposition with IRIS Spectroscopy"