Global Morphology of Chorus Waves in the Outer Radiation Belt and the Effect of Geomagnetic Activity and fpe/fce
By Kaine Bunting (British Antarctic Survey)
Chorus waves are naturally occurring plasma waves often observed in the Earth’s outer radiation belt that strongly influence the behaviour of energetic electrons. These waves can both accelerate electrons to relativistic energies, which poses a threat to satellites, as well as scatter electrons into Earth's atmosphere, where they are consequently lost.
The ratio between the electron plasma frequency (fpe) and electron gyrofrequency (fce) holds information on both electron density and magnetic field strength and significantly influences the efficiency of these processes, with electron acceleration being most effective during periods of low fpe/fce.
Bunting et al. (2026) analyses a combined 24.5 years of wave data from three THEMIS satellites to investigate the effect of fpe/fce, geomagnetic activity and normalized frequency on the spatial distribution and intensity of chorus waves.
The strongest waves are generally observed on the dawn-side of the Earth during active geomagnetic conditions. Figure 1 shows global plots of the equatorial (|MLAT| < 9°) chorus wave intensity during active conditions (AE > 200nT). At intermediate relative frequencies (0.3fce < f < 0.4fce), chorus is largely independent of fpe/fce. However, at low frequencies (flhr < f < 0.1fce), strong waves are most often associated with high fpe/fce (>10) and at high frequencies (0.5fce < f < 0.7fce), chorus is strongest at low fpe/fce (<6).
Overall, this study highlights the critical role of fpe/fce on the spatial distribution and dynamic behaviour of chorus waves under varying geomagnetic conditions, as well as its influence on wave-particle interactions. During a geomagnetic storm fpe/fce outside of the plasmapause may gradually change from low to high values over the course of the recovery phase, suggesting that the role of chorus may change from efficient acceleration early in the recovery phase to little or no acceleration and even loss toward the end of the recovery phase.
See publication for details:
Bunting, K. A., Meredith, N. P., Bortnik, J., Ma, Q., Matsuura, R., & Shen, X.-C. (2026). Global morphology of chorus waves in the outer radiation belt and the effect of geomagnetic activity and fpe/fce. Journal of Geophysical Research: Space Physics, 131, e2025JA034737. https://doi.org/10.1029/2025JA034737

Figure 1 - Global maps of the average chorus wave intensity during active geomagnetic conditions (AE > 200nT) in the equatorial region (MLAT < 9°) as a function of L* and magnetic local time for, from top to bottom, increasing relative frequency, and, from left to right, increasing fpe/fce. The maps extend linearly out to L* = 10 with noon at the top and dawn to the right. The average intensities are shown in the large panels and the corresponding sampling distributions in the small panels to the bottom right of each large panel.