In this study, we use observations of THEMIS and Van Allen Probes to statistically study the modulations of chorus emissions by variations of background magnetic field and plasma density in the ultra low frequency range. The modulation events are identified automatically and divided into three types according to whether the chorus intensity is correlated to the variations of magnetic field only (Type B), plasma density only (Type N) or both (Type NB). For THEMIS observations, the occurrences of the types B and N are larger than type NB, while for Van Allen Probes observations most events are Type N. The chorus intensity is mostly correlated to the magnetic field strength negatively and plasma density positively. Modulation event occurrences peak at the dawn sector. The chorus intensity tends to increase when the magnitude of the magnetic field perturbation increases, but little dependence on the amplitude of plasma density perturbation is found.