We report unusual Electromagnetic Ion Cyclotron (EMIC) waves with a very narrow frequency bandwidth, closely following and approaching the proton gyrofrequency. One interesting case analysis shows that magnetosonic waves, anisotropic suprathermal proton distributions, and high frequency EMIC waves are closely related. Magnetosonic waves potentially cause the resonant heating of suprathermal protons and the temperature anisotropy of suprathermal protons (10-100 eV) likely provides free energy for the excitation of high frequency EMIC waves. The statistical analysis shows that this type of EMIC waves has a typical wave amplitude of~100 pT, left-handed polarization, and small wave normal angles. Moreover, these low frequency EMIC waves typically occur near the equator in the low-density regions from dawn to dusk. These newly observed high frequency EMIC waves provide new insights into understanding the generation of EMIC waves and the energy transfer between magnetosonic waves and EMIC waves. Plain Language Summary Electromagnetic Ion Cyclotron (EMIC) waves are commonlyobserved in the Earth's magnetosphere and play an important role in causing the loss of ring current ions and relativistic electrons due to pitch angle scattering. In this study, we report unusual high frequency EMIC waves with frequency very close to the proton gyrofrequency. An interesting case study clearly shows the correlation between magnetosonic waves, the enhancement of suprathermal protons, and high frequency EMIC waves. The protons at suprathermal energies could be heated by magnetosonic waves and the anisotropic distribution of suprathermal protons is likely responsible for the excitation of high frequency EMIC waves. The statistical analysis shows that this type of EMIC waves has a typical wave amplitude of~100 pT, left-handed polarization, and small wave normal angles. These newly observed high frequency EMIC waves provide new insights into understanding the generation of EMIC waves and the energy transfer between magnetosonic waves and EMIC waves.

Electromagnetic ion cyclotron (EMIC) waves are known to typically cause electron losses into Earth's upper atmosphere at >~1 MeV, while the minimum energy of electrons subject to efficient EMIC‐driven precipitation loss is unresolved. This letter reports electron precipitation from subrelativistic energies of ~250 keV up to ~1 MeV observed by the Focused Investigations of Relativistic Electron Burst Intensity, Range and Dynamics (FIREBIRD‐II) CubeSats, while two Polar Operational Environmental Satellites (POES) observed proton precipitation nearby. Van Allen Probe A detected EMIC waves (~0.7–2.0 nT) over the similar L shell extent of electron precipitation observed by FIREBIRD‐II, albeit with a ~1.6 magnetic local time (MLT) difference. Although plasmaspheric hiss and magnetosonic waves were also observed, quasi‐linear calculations indicate that EMIC waves were the most efficient in driving the electron precipitation. Quasi‐linear theory predicts efficient precipitation at >0.8–1 MeV (due to H‐band EMIC waves), suggesting that other mechanisms are required to explain the observed subrelativistic electron precipitation.

Electromagnetic ion cyclotron (EMIC) waves can drive precipitation of tens of keV protons and relativistic electrons, and are a potential candidate for causing radiation belt flux dropouts. In this study, we quantitatively analyze three cases of EMIC-driven precipitation, which occurred near the dusk sector observed by multiple Low-Earth-Orbiting (LEO) Polar Operational Environmental Satellites/Meteorological Operational satellite programme (POES/MetOp) satellites. During EMIC wave activity, the proton precipitation occurred from few tens of keV up to hundreds of keV, while the electron precipitation was mainly at relativistic energies. We compare observations of electron precipitation with calculations using quasi-linear theory. For all cases, we consider the effects of other magnetospheric waves observed simultaneously with EMIC waves, namely, plasmaspheric hiss and magnetosonic waves, and find that the electron precipitation at MeV energies was predominantly caused by EMIC-driven pitch angle scattering. Interestingly, each precipitation event observed by a LEO satellite extended over a limited L shell region (ΔL~0.3 on average), suggesting that the pitch angle scattering caused by EMIC waves occurs only when favorable conditions are met, likely in a localized region. Furthermore, we take advantage of the LEO constellation to explore the occurrence of precipitation at different L shells and magnetic local time sectors, simultaneously with EMIC wave observations near the equator (detected by Van Allen Probes) or at the ground (measured by magnetometers). Our analysis shows that although EMIC waves drove precipitation only in a narrow ΔL, electron precipitation was triggered at various locations as identified by POES/MetOp over a rather broad region (up to~4.4 hr MLT and~1.4 L shells) with similar patterns between satellites.

Whistler mode waves are important for precipitating energetic electrons into Earth's upper atmosphere, while the quantitative effect of each type of whistler mode wave on electron precipitation is not well understood. In this letter, we evaluate energetic electron precipitation driven by three types of whistler mode waves: plume whistler mode waves, plasmaspheric hiss, and exohiss observed outside the plasmapause. By quantitatively analyzing three conjunction events between Van Allen Probes and POES/MetOp satellites, together with quasi‐linear calculation, we found that plume whistler mode waves are most effective in pitch angle scattering loss, particularly for the electrons from tens to hundreds of keV. Our new finding provides the first direct evidence of effective pitch angle scattering driven by plume whistler mode waves and is critical for understanding energetic electron loss process in the inner magnetosphere. We suggest the effect of plume whistler mode waves be accurately incorporated into future radiation belt modeling.

Magnetospheric plasma waves play a significant role in ring current and radiation belt dynamics, leading to pitch angle scattering loss and/or stochastic acceleration of the particles. During a non‐storm time dropout event on 24 September 2013, intense electromagnetic ion cyclotron (EMIC) waves were detected by Van Allen Probe A (Radiation Belt Storm Probes‐A). We quantitatively analyze a conjunction event when Van Allen Probe A was located approximately along the same magnetic field line as MetOp‐01, which detected simultaneous precipitation of >30 keV protons and energetic electrons over an unexpectedly broad energy range (>~30 keV). Multipoint observations together with quasi‐linear theory provide direct evidence that the observed electron precipitation at higher energy (>~700 keV) is primarily driven by EMIC waves. However, the newly observed feature of the simultaneous electron precipitation extending down to ~30 keV is not supported by existing theories and raises an interesting question on whether EMIC waves can scatter such low‐energy electrons.