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

Chorus waves are naturally occurring plasma waves often observed in the terrestrial radiation belts that strongly influence the behavior of energetic electrons. These waves can both accelerate electrons to relativistic energies as well as scatter electrons into Earth's atmosphere, where they are consequently lost. A key factor which influences the efficiency of these processes is the ratio of two frequencies (the plasma frequency and the electron gyrofrequency). We analyze a combined 24.5 years of wave measurements from the THEMIS spacecraft to show how chorus wave intensity and spatial location vary with relative frequency, geomagnetic activity and this frequency ratio (fpe/fce). The strongest waves are generally observed on the dawn side of the Earth during active geomagnetic conditions. At intermediate relative frequencies, strong waves can occur across a wide range of fpe/fce values. In contrast, at low and high frequencies, strong waves are most often associated with high and low fpe/fce values respectively. We show that the location of the strongest waves can be largely explained in terms of the source electrons being in the required energy range for resonance and the absence of Landau damping and highlight regions where electron acceleration to relativistic energies is likely to be most significant.