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Angular response functions are derived for four electron channels and six proton channels of the SEM-2 MEPED particle telescopes on the POES and MetOp satellites from Geant4 simulations previously used to derive the energy response. They are combined with model electron distributions in energy and pitch angle to show that the vertical 0? telescope, intended to measure precipitating electrons, instead usually measures trapped or quasi-trapped electrons, except during times of enhanced pitch angle diffusion. A simplified dynamical model of the radiation belt electron distribution near the loss cone, as a function of longitude, energy, and pitch angle, that accounts for pitch angle diffusion, azimuthal drift, and atmospheric backscatter is fit to sample MEPED electron data at L=4 during times of differing diffusion rates. It is then used to compute precipitating electron flux, as function of energy and longitude, that is lower than would be estimated by assuming that the 0? telescope always measures precipitating electrons. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Plasma waves in the Earth's magnetosphere can alter the trajectory of particles traveling along geomagnetic field lines. Specifically, electromagnetic ion cyclotron (EMIC) waves can interact with both electrons and protons and cause them to fall into the upper atmosphere of Earth. From past studies and theories, it is known that EMIC waves drive precipitation of ultrarelativistic (>~MeV) electrons and tens to hundreds of keV protons. Such electron precipitation can lead to atmospheric changes and potentially aid ozone depletion. In this work, we show a direct observation of electron precipitation from ~250 keV up to ~1 MeV, potentially driven by EMIC waves using multipoint measurements primarily from a CubeSat mission (FIREBIRD‐II) and Van Allen Probes. Quasi‐linear calculations indicate that EMIC waves are efficient in driving the electron precipitation at >0.8–1 MeV, but other mechanisms are needed to explain the observed electron precipitation down to ~250 keV. Our study also highlights the capabilities of FIREBIRD‐II studying not only microbursts, but also other precipitation patterns (e.g., driven by EMIC waves). Key Points: Strong EMIC waves were observed by Van Allen Probe A in association with phase space density dips over the similar L shell extentFIREBIRD‐II observed electron precipitation from ~250 keV up to ~1 MeV over the similar L shell extent of EMIC waves but at a later MLTQuasi‐linear theory predicts efficient precipitation at > 0.8–1 MeV but requires other mechanisms to explain the subrelativistic one [ABSTRACT FROM AUTHOR]

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. Key Points: We show three cases of proton and relativistic electron precipitation observed simultaneously with EMIC wavesEMIC‐driven precipitation was observed by POES/MetOp satellites at different locations over a broad L‐MLT regionEach precipitation event extended over ΔL ~ 0.3 on average, showing that wave‐driven pitch angle scattering is localized [ABSTRACT FROM AUTHOR]