Streamer Blow-out CMEs and Particle Pressure

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 anywhere close 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. Therefore SEPs are probably present in the low corona at times. This Nugget discusses one potential consequence of SEPs near the Sun.

Solar radiation belts

By analogy with the Earth's radiation belts, energetic particles can be trapped in the Sun-girdling streamer belt of closed magnetic fields (Ref. [1]), satisfying all three of the adiabatic invariants that allow the Van Allen belts to exist (see also Nugget No. 37). At present it is only speculation about the sources solar radiation-belt particles, but some are surely present as cosmic-ray secondaries in high-energy interactions.

Shock-wave acceleration of SEPs gives us another likely source of particles for solar radiation belts. Modeling (e.g., Ref. [2]) clearly points to efficient shock acceleration within streamer volumes. Figure 1 illustrates one geometry (shock origin external to the streamer), but flare/CME sources may occur within the streamer domain as well, in the form of the "spine-and-fan" magnetic geometry of "anemone" active regions.

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.

Particle pressure and "streamer blow-out" events

If global shock waves crossing streamers can leave trapped high-energy particles within closed-field regions, what will happen? The particles either precipitate or augment the plasma pressure, depending on pitch angle. Any increase of pressure will alter the quasi-equilibrium of the streamer, altering the structure of the separatrices defining the streamer cusp. An SEP event can produce a huge flux of high-energy particles, arguably enough to match the ambient streamer magnetic field and thus doubling the total pressure. For example, 10³³ protons at 100 MeV contain only 10³⁰ erg, but exert the pressure of an 0.1 Gauss field in a volume R_Sun³. In such a case one would probably see a "streamer blow-out" CME; Figure 2 shows how such an event, also called a "bugle", appears in synoptic coronagraph data.

Figure 2: Synoptic coronagraph data for one month in 1997, illustrating the time developments of streamers. Time runs from right to left as the Sun rotates; the streamer disappearing in the indicated blow-out first grows in brightness for a few days, and then suddenly disappears.

This mechanism seems plausible; an impulsive flare accelerates SEPs, which pressurizes the accessible field volume. This drives expansion and creates or augments a CME; the energy added to expand the field subsequently reappears in the normal CSHKP process.

Conclusions

We picked the "streamer breakout" CME class because its name is so evocative, though it's quite possible that particle pressure is not the definitive mechanism in this case. But the important message here is that non-thermal particle pressure can have a powerful effect on magnetic structures in the solar corona in general. This mechanism must be functioning, since we observe SEPs and can infer their presence the corona. MHD simulations in general cannot paint a complete picture of coronal dynamics without recognizing this elementary consideration. The rather poor older-style graphics in this Nugget reflect the fact that we could have come to this conclusion decades ago! The dramatic discovery soft X-ray detection of dimming at the very bottom of the corona (e.g., Ref. [3]) should have sent this clear message already.

References

[1] "Coronal Radiation Belts"

[2] "Observations of coronal structure during sunspot maximum"

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