Based on 7 years' observations from Time History of Events and Macroscale Interactions during Substorms (THEMIS), we investigate the statistical distribution of electric field Pc5 ULF wave power under different geomagnetic activities and calculate the radial diffusion coefficient due to electric field, DLLE, for outer radiation belt electrons. A simple empirical expression of DLLE[]THEMIS is also derived. Subsequently, we compare DLLE[]THEMIS to previous DLL models and find similar Kp dependence with the DLLE[]CRRES model, which is also based on in situ electric field measurements. The absolute value of DLLE[]THEMIS is constantly higher than DLLE[]CRRES, probably due to the limited orbital coverage of CRRES. The differences between DLLE[]THEMIS and the commonly used DLLM[]normalB‐normalA and DLLE[]Ozeke models are significant, especially in Kp dependence and energy dependence. Possible reasons for these differences and their implications are discussed. The diffusion coefficient provided in this paper, which also has energy dependence, will be an important contributor to quantify the radial diffusion process of radiation belt electrons.

[1] Ultralow frequency (ULF) waves in the Pc4 and Pc5 bands are ubiquitous in the inner magnetosphere and have significant influence on energetic particle transport. Investigating the source and characteristics of ULF waves also helps us better understand the interaction processes between the solar wind and the magnetosphere. However, owing to the limitation in instrumentation and spatial coverage, the distribution of ULF waves in local time and L shell in the inner magnetosphere has not been completely studied. The recent Time History of Events and Macroscale Interactions During Substorms (THEMIS) mission provides unique opportunities to investigate the spatial distribution of ULF pulsations across different L shells with full local time coverage in the inner magnetosphere during solar minimum, with both electric and magnetic field measurements. Pc4 and Pc5 pulsations in the electric field observations are identified throughout 13 months of measurements, covering 24 h in local time. The pulsations are characterized as either toroidal or poloidal (including compressional) mode, depending on the polarization of the electric field. Subsequently, the pulsations' occurrence rate and wave power distributions in radial distance and local time are recorded. While the distributions of both Pc4 and Pc5 events vary greatly with radial distance and local time, Pc4 events are more frequently observed in the inner region around 5-6 R E and Pc5 events are more frequently observed in the outer region around 7-9 R E , which suggests that the field line resonance is an important source of the ULF waves. In the flank regions, the wave power is dominated by the toroidal mode, likely associated with the KelvinHelmholtz (KH) instability. In the noon sector, the Pc5 ULF wave power is dominated by the poloidal mode, likely associated with the solar wind dynamic pressure disturbance. The KH instability plays an important role, suggested by our observations, during the solar minimum when the solar wind dynamic pressure is relatively weak. We also find that the contributions to the Pc5 ULF wave power from the external sources are larger than the contributions from the internal sources. These statistical results are important in characterizing Pc4 and Pc5 waves and also important for any efforts to model the transport of energetic particles in the magnetosphere.

[1] Radial diffusion is one of the most important acceleration mechanisms for radiation belt electrons, which can be enhanced from drift-resonant interactions with large-scale fluctuations of the magnetosphere's magnetic and electric fields (Pc5 range of ULF waves). In order to physically quantify the radial diffusion coefficient, D LL , we run the global Lyon-Fedder-Mobarry (LFM) MHD simulations to obtain the mode structure and power spectrum of the ULF waves and validate the simulation results with available satellite measurements. The calculated diffusion coefficients, directly from the MHD fields over a Corotating Interaction Region (CIR) storm in March 2008, are generally higher when solar wind dynamic pressure is enhanced or AE index is high. In contrary to the conventional understanding, our results show that inside geosynchronous orbit the total diffusion coefficient from MHD fields is dominated by the contribution from electric field perturbations, rather than the magnetic field perturbations. The calculated diffusion coefficient has a physical dependence on m (or electron energy) and L, which is missing in the empirical diffusion coefficient, D LL Kp as a function of Kp index, and D LL Kp are generally greater than our calculated D LL during the storm event. Validation of the MHD ULF waves by spacecraft field data shows that for this event the LFM code reasonably well-reproduces the B z wave power observed by GOES and THEMIS satellites, while the E j power observed by THEMIS probes are generally underestimated by LFM fields, on average by about a factor of ten.