A statistical analysis of the global distribution of plasmaspheric hiss based on Van Allen Probes wave observations

Plasmaspheric hiss plays an important role in driving the precipitation loss of radiation belt electrons via pitch angle scattering, which is also known as the major cause of the formation of the slot region between the inner and outer radiation belt. Therefore, it is of scientific importance to acquire a complete picture of the global distribution of plasmaspheric hiss. Using the thirty-three month high-quality wave data of the Van Allen Probes from September 2012 to May 2015, which provide excellent coverage in the entire inner magnetosphere, we investigate in detail the characteristics of the global distribution of plasmaspheric hiss bin-averaged wave amplitude and occurrence rate with respect to the geomagnetic activity level, L-shell, geomagnetic latitude, and magnetic local time. It is demonstrated that the bin-averaged hiss amplitude strongly depends on the level of geomagnetic activity and exhibits a pronounced day-night asymmetry. Dayside hiss shows a tendency intensifying with the disturbed geomagnetic condition, which is primarily confined to L=2.5-4.0. In contrast, the average hiss amplitude on the nightside tends to decrease. It should also be noted that plasmaspheric hiss at different amplitude levels varies distinctly with geomagnetic condition. As the geomagnetic disturbance increases, the occurrence rate of hiss wave at a smaller amplitude level (i.e., 5-30 pT) increases on the nightside but decreases on the dayside, while the occurrence pattern of higher amplitude ( 30 pT) hiss wave is opposite. For high amplitude hiss wave, the occurrence rate increases on the dayside during intense geomagnetic activities while decreases on the nightside. This is probably because during active times, suprathermal electron fluxes are larger on the nightside, which causes stronger Landau damping of whistler mode waves and thus limits the ability of chorus waves to propagate into the plasmasphere and evolve into plasmaspheric hiss. In addition, plasmaspheric hiss waves with the amplitudes ranging from 5 to 30 pT have the highest occurrence probability both around the geomagnetic equator and at higher latitudes. Our statistical results can provide a reasonable and accurate cognition complementary to the current knowledge of the global features of plasmaspheric hiss, especially in the inner magnetosphere of L=2-6, thereby offering essential input parameters of hiss wave distribution for future simulations of the dynamic spatiotemporal variations of radiation belt electrons at different energies and pitch angles under the influence of diverse solar wind and magentospheric circumstances. Therefore, we suggest that these new properties of hiss wave should be incorporated into the future modeling of radiation belt electron dynamics.