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Abstract The proton radiation belt contains high fluxes of adiabatically trapped protons varying in energy from ∼one to hundreds of megaelectron volts (MeV). At large radial distances, magnetospheric field lines become stretched on the nightside of Earth and exhibit a small radius of curvatureR_Cnear the equator. This leads protons to undergo field line curvature (FLC) scattering, whereby changes to the first adiabatic invariant accumulate as field strength becomes nonuniform across a gyroorbit. The outer boundary of the proton belt at a given energy corresponds to the range of magneticLshell over which this transition to nonadiabatic motion takes place, and is sensitive to the occurrence of geomagnetic storms. In this work, we first find expressions for nightside equatorialR_Cand field strengthB_eas functions of Dst andL* to fit the TS04 field model. We then apply the Tu et al. (2014,https://doi.org/10.1002/2014ja019864) condition for nonadiabatic onset to solve the outer boundaryL*, and refine our expression forR_Cto achieve agreement with Van Allen Probes observations of 1–50 MeV proton flux over the 2014–2018 era. Finally, we implement this nonadiabatic onset condition into the British Antarctic Survey proton belt model (BAS‐PRO) to solve the temporal evolution of proton fluxes atL ≤ 4. Compared with observations, BAS‐PRO reproduces storm losses due to FLC scattering, but there is a discrepancy in mid‐2017 that suggests a ∼5 MeV proton source not accounted for. Our work sheds light on outer zone proton belt variability at 1–10 MeV and demonstrates a useful tool for real‐time forecasting. 

Abstract During magnetospheric storms, radiation belt electrons are produced and then removed by collisions with the lower atmosphere on varying timescales. An efficient loss process is microbursts, strong, transient precipitation of electrons over a wide energy range, from tens of keV to sub‐relativistic and relativistic energies (100s keV and above). However, the detailed generation mechanism of microbursts, especially over sub‐relativistic and relativistic energies, remains unknown. Here, we show that these energetic electron microbursts may be caused by ducted whistler‐mode lower‐band chorus waves. Using observations of equatorial chorus waves nearby low‐altitude precipitation as well as data‐driven simulations, we demonstrate that the observed microbursts are the result of resonant interaction of electrons with ducted chorus waves rather than nonducted ones. Revealing the physical mechanism behind the microbursts advances our understanding of radiation belt dynamics and its impact on the lower atmosphere and space weather. 

Abstract We extend our database of whistler mode chorus, based on data from seven satellites, by including ∼3 years of data from Radiation Belt Storm Probes (RBSP)‐A and RBSP‐B and an additional ∼6 years of data from Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐A, THEMIS‐D, and THEMIS‐E. The new database allows us to probe the near‐equatorial region in detail, revealing new features. In the equatorial source region, |λ_m|<6°, strong wave power is most extensive in the 0.1–0.4f_cebands in the region 21–11 magnetic local time (MLT) from the plasmapause out toL^∗ = 8 and beyond, especially near dawn. At higher frequencies, in the 0.4–0.6f_cefrequency bands, strong wave power is more tightly confined, typically being restricted to the postmidnight sector in the region 4<L^∗<6. The global distribution of strong chorus wave power changes dramatically with increasing magnetic latitude, with strong chorus waves in the region 12<|λ_m|<18° predominantly observed at frequencies below 0.3f_cein the prenoon sector, in the region 5<L^∗<8. 

Abstract In this study, we use the observations of electromagnetic waves by Detection of Electromagnetic Emissions Transmitted from Earthquake Regions satellite to investigate propagation characteristics of low‐altitude ionospheric hiss. In an event study, intense hiss wave power is concentrated over a narrow frequency band with a central frequency that decreases as latitude decreases, which coincides to the variation of local proton cyclotron frequencyf_CH. The wave propagates obliquely to the background magnetic field and equatorward from high latitude region. We use about ∼6 years of observations to statistically study the dependence of ionospheric hiss wave power on location, local time, geomagnetic activity, and season. The results demonstrate that the ionospheric hiss power is stronger on the dayside than nightside, under higher geomagnetic activity conditions, in local summer than local winter. The wave power is confined near the region where the localf_CHis equal to the wave frequency. A ray tracing simulation is performed to account for the dependence of wave power on frequency and latitude.