Particle Pressure and CMEs

Nugget
Number:	529
1st Author:	Hugh HUDSON
2nd Author:	Ed CLIVER
Published:	June 22, 2026
Next Nugget:	TBD
Previous Nugget:	White-Light and Lyman-alpha Emissions in Solar Flares: Timing, Timescale, Energy, and Scaling

Introduction

We know about solar energetic particles (SEPS) from many observations from instruments on space probes, and also from ground-based observations such as those of neutron monitors. SEP events occur only intermittently, often following powerful flares, and when they do occur the particle fluxes far exceed those of the cosmic rays, even out to GeV energies. We cannot make in situ observations closer than about 10 solar radii to the Sun, and the propagation of these "solar cosmic rays" throughout the heliosphere remains somewhat mysterious. But we can tell that the highest-energy particles come from acceleration mechanisms deriving energy from coronal magnetic fields. Most of the magnetic energy is in the low corona, so accelerated particles must be present in the low corona at times. This Nugget discusses the pressure exerted by high-energy particles low in the corona, which can rival the magnetic pressure.

The physics and the numbers

The pressure exerted by high-energy particles basically reflects the Lorentz force, and so it is anisotropic. This distinguishes it from the gas pressure exerted by the thermal core of the plasma, and its effect is to expand the magnetic field in the perpendicular direction. From SEP event observations, we can roughly quantify its significance relative to the magnetic pressure. A major CME may have a total energy of 10³² erg, and we could conservatively take 1% as the particle fraction of the total energy (Ref. [1]). This is then 10³⁰ erg in particle energy. In the volume of a shell at 1 R_☉, 0.1 R_☉ thick and 1 sr in diameter, the particle energy density would correspond to a magnetic field of 1 G, and is very likely the dominant pressure component in a closed field structure. Note that particle pressure applied to a closed magnetic field would be outward, in opposition to the inward force of magnetic tension.

CME dynamics

Flare-associated CMEs can originate impulsively, in the low corona (Ref. [2]), in which case there is a dramatic and very sudden dimming. Other CMEs may arise from streamer expansion or filament eruption, as shown in Figure 1. The right panel, originally published by Paul Wild, strongly suggests that a global shock wave could penetrate closed fields, as in a coronal helmet streamer and accelerate particles there. This has been borne out by modeling (e.g., Ref. [3]).

Figure 1: Sketches showing global waves emitted from the low corona: left, showing the flank of a wave moving laterally, but somehow not entering closed-field regions; and right, remedying that omission.

Conclusion

Non-thermal particle pressure typically has not been considered in modeling CME initiation or propagation in the low corona. With or without shock waves, the mechanism seems plausible: an electrodynamic process accelerates particles, which pressurize the accessible field volume. This drives expansion and creates or augments a CME and its interplanetary aftermath; some of the energy added to expand the field can then return in the normal CSHKP process. The important message here is that non-thermal particle pressure can have a powerful effect on magnetic structures in the solar corona in general. MHD simulations intrinsically cannot paint a complete picture of coronal dynamics because they do not include this basic physics.

References

[1] "How Efficient are Coronal Mass Ejections at Accelerating Solar Energetic Particles?"

[2] "EIT and LASCO Observations of the Initiation of a Coronal Mass Ejection"

[3] "The Acceleration of High-energy Protons at Coronal Shocks: The Effect of Large-scale Streamer-like Magnetic Field Structures"