A New Analytical Chorus Wave Model Derived from Van Allen Probe Observations

Chorus waves play an important role in the dynamic evolution of relativistic electrons in the Earth's radiation belts. The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument onboard the Van Allen Probe satellites provides continuous measurements of chorus waves. Using more than 5 years (2012-2018) of Van Allen Probe EMFISIS data, we developed a new analytical model for upper- and lower-band chorus waves. In this work,, we describe the construction of the chorus wave database and present the method of model development. By applying polynomial fits to chorus wave root-mean-square (RMS) amplitudes, we developed regression models for lower- and upper- band chorus as a function of geomagnetic activity (Kp), L, magnetic latitude (λ ), and magnetic local time (MLT). Dependence on Kp is separated from the dependence on λ , L, and MLT as Kp-scaling law to simplify calculation of diffusion coefficients and inclusion into radiation belt codes. MLT- and magnetic latitude-dependent frequency models for upper- and lower-band chorus waves are also developed. This empirical model is valid in all MLTs, L-shell range from 3.5 to 6, magnetic latitude up to 19^o, and Kp≤ 6. The results of our study show, that the dependence on L is different for upper- and lower-band chorus, which implies different energy sources for different wave bands. When Kp is higher than 4, the RMS amplitudes of upper band chorus waves on the dayside and lower-band chorus waves in the afternoon sector decrease with Kp. At mid-latitudes, chorus waves on the dayside are stronger than those on the nightside. This analytical chorus wave model is convenient for inclusion in quasi-linear diffusion calculations of electron scattering rates and particle simulations in the inner magnetosphere.