Global Morphology of Chorus waves in the Outer Radiation Belt and the Effect of Geomagnetic Activity and fpe/fce

Whistler-mode chorus waves are electromagnetic emissions in the Earths’ magnetosphere that play a central role in the dynamics of the outer radiation belt, contributing to both acceleration and loss of relativistic electrons. The efficiency of these processes strongly depend on the wave intensity and the ratio of the electron plasma frequency to the electron gyrofrequency (f_pe/f_ce). Using approximately 24.5 years of THEMIS wave observations, we examine how chorus wave intensity and spatial distribution vary with relative frequency, geomagnetic activity and f_pe/f_ce. The strongest chorus waves are observed during periods of high geomagnetic activity (AE > 200nT). At low relative frequencies (f_lhr < f < 0.1f_ce), equatorial chorus is strongest during periods of high f_pe/f_ce, primarily within 5 < L* < 8 and 22-12 MLT. In contrast, at high relative frequencies (0.5f_ce < f < 0.7f_ce), equatorial chorus is strongest when f_pe/f_ce is low, between 4 < L* < 6 and 21-09 MLT. At intermediate relative frequencies (0.3f_ce < f < 0.4f_ce), chorus intensities are observed in the same region and are largely independent of f_pe/f_ce. We find that off-equatorial chorus emissions are also largely independent of f_pe/f_ce. Results show that the locations of peak chorus intensity are controlled by the availability of resonant source electrons and reduced Landau damping. Our findings identify regions where chorus-driven acceleration of electrons to relativistic energies is expected to be the most significant.