The spectral extent of chorus in the off-equatorial magnetosphere

Magnetospheric chorus waves are a major driver of acceleration and loss in the Earth's outer electron radiation belt. The spectral extent of chorus is a key parameter in quantifying the global effect of chorus on energetic particle populations by determining the range of resonant electron energies. However, statistics of spectral properties are sparse, particularly in the off-equatorial magnetosphere. We use a database of chorus observations from the Polar spacecraft to generate statistics on the normalized chorus frequency (with the respect to the minimum field line gyrofrequency, min) as a function of magnetic local time (MLT) (0 < 24), L-shell (3 < L < 11), and magnetic latitude (||<65 degrees). We find that, on average, the chorus spectrum peaks in the range of 0.1-0.4 min, varying significantly with , R0 and MLT. The normalized chorus peak frequency is found to decrease with increasing R0, and decreases with increasing latitude below approximate to 25 degrees. When fit to a Gaussian spectral model, lower band chorus is found to have a bandwidth < 0.1 min, which is narrower than assumed in most diffusion models. Diffusion coefficients calculated using the University of California at Los Angeles (UCLA) Full Diffusion Code show that wave-particle interactions on the nightside are highly sensitive to both the peak frequency and bandwidth of chorus, yet on the dayside scattering is mostly sensitive to the peak frequency as a result of the wider latitudinal extent of the waves. We also find that fitting a Gaussian to the logarithm of the spectrum reduces fit errors by over 60%, indicating that inclusion of arbitrary spectral forms may improve the accuracy of wave models within radiation belt simulations.