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1. EMIC wave events during the four GEM QARBM challenge intervals

   https://doi.org/10.1002/2018JA025505  

   Engebretson, M. J.; Posch, J. L.; Braun, D. J.; Li, W.; Ma, Q.; Kellerman, A. C.; Huang, C.-L.; Kanekal, S. G.; Kletzing, C. A.; Wygant, J. R.; et al (August 2018, Journal of geophysical research. Space physics)

   This paper presents observations of electromagnetic ion cyclotron (EMIC) waves from multiple data sources during the four Geospace Environment Modeling challenge events in 2013 selected by the Geospace Environment Modeling Quantitative Assessment of Radiation Belt Modeling focus group: 17 and 18 March (stormtime enhancement), 31 May to 2 June (stormtime dropout), 19 and 20 September (nonstorm enhancement), and 23–25 September (nonstorm dropout). Observations include EMIC wave data from the Van Allen Probes, Geostationary Operational Environmental Satellite, and Time History of Events and Macroscale Interactions during Substorms spacecraft in the near-equatorial magnetosphere and from several arrays of ground-based search coil magnetometers worldwide, as well as localized ring current proton precipitation data from low-altitude Polar Operational Environmental Satellite spacecraft. Each of these data sets provides only limited spatial coverage, but their combination shows consistent occurrence patterns and reveals some events that would not be identified as significant using near-equatorial spacecraft alone. Relativistic and ultrarelativistic electron flux observations, phase space density data, and pitch angle distributions based on data from the Relativistic Electron-Proton Telescope and Magnetic Electron Ion Spectrometer instruments on the Van Allen Probes during these events show two cases during which EMIC waves are likely to have played an important role in causing major flux dropouts of ultrarelativistic electrons, particularly near L* ~4.0. In three other cases, identifiable smaller and more short-lived dropouts appeared, and in five other cases, these waves evidently had little or no effect. 

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2. Statistical Characteristics of Leaked AKR Observed at South Pole Station, Antarctica

   https://doi.org/10.25546/103087  

   Labelle, J; Schwartz, N (January 2023, Trinity College Dublin)

   Fischer, G; Jackman, C M; Louis, C K; Sulaiman, A H; Zucca, P (Ed.)

   There is mounting evidence of a component of terrestrial auroral kilometric radiation (AKR) that is converted to whistler mode and radiated downward toward the planet, observable even at ground level. Three years of data from South Pole Station in 2018-2020 provide statistics of characteristics of leaked AKR at ground level. The events occur in an approximately 90--day interval around winter solstice, apparently requiring darkness in the ionosphere to be observed at ground level. They favor pre--midnight/midnight magnetic local times, which is consistent with the connection of AKR, observed in space, to auroral substorms. The frequency distribution of ground{level AKR is truncated compared to that observed in space, with primarily the higher end of the frequency range being observed, 400--600 kHz, corresponding to the low altitude range of source heights, 2500-3500 km, assuming generation at the electron cyclotron frequency. Approximately half of the events have maximum radiance exceeding 1.5×10^18 W/m2/Hz, with the strongest events exceeding 10^16 W/m2/Hz; these intensities are up to two orders of magnitude lower than those observed in the ionosphere, suggesting that most of the leaked AKR is at large wave normal angles that cannot penetrate the Earth{ionosphere boundary. 

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3. Full-wave modeling of EMIC wave packets: ducted propagation and reflected waves

   https://doi.org/10.3389/fspas.2023.1251563  

   Hanzelka, Miroslav; Li, Wen; Ma, Qianli; Qin, Murong; Shen, Xiao-Chen; Capannolo, Luisa; Gan, Longzhi (October 2023, Frontiers in Astronomy and Space Sciences)

   Electromagnetic ion cyclotron (EMIC) waves can scatter radiation belt electrons with energies of a few hundred keV and higher. To accurately predict this scattering and the resulting precipitation of these relativistic electrons on short time scales, we need detailed knowledge of the wave field’s spatio-temporal evolution, which cannot be obtained from single spacecraft measurements. Our study presents EMIC wave models obtained from two-dimensional (2D) finite-difference time-domain (FDTD) simulations in the Earth’s dipole magnetic field. We study cases of hydrogen band and helium band wave propagation, rising-tone emissions, packets with amplitude modulations, and ducted waves. We analyze the wave propagation properties in the time domain, enabling comparison within situobservations. We show that cold plasma density gradients can keep the wave vector quasiparallel, guide the wave energy efficiently, and have a profound effect on mode conversion and reflections. The wave normal angle of unducted waves increases rapidly with latitude, resulting in reflection on the ion hybrid frequency, which prohibits propagation to low altitudes. The modeled wave fields can serve as an input for test-particle analysis of scattering and precipitation of relativistic electrons and energetic ions. 

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4. A Case Study of Pc1 Waves Observed at the Polar Cap Associated with Proton Precipitation at Subauroral Latitudes

   https://doi.org/10.3390/atmos15020219  

   D’Angelo, Giulia; Francia, Patrizia; De_Lauretis, Marcello; Parmentier, Alexandra; Raita, Tero; Piersanti, Mirko (February 2024, Atmosphere)

   The importance of ElectroMagnetic Ion Cyclotron (EMIC) ultra-low-frequency (ULF) waves (and their Pc1 counterparts) is connected to their critical role in triggering energetic particle precipitation from the magnetosphere to the conjugated ionosphere via pitch angle scattering. In addition, as a prominent element of the ULF zoo, EMIC/Pc1 waves can be considered a perfect tool for the remote diagnosis of the topologies and dynamic properties of near-Earth plasmas. Based on the availability of a comprehensive set of instruments, operating on the ground and in the top-side ionosphere, the present case study provides an interesting example of the evolution of EMIC propagation to both ionospheric hemispheres up to the polar cap. Specifically, we report observations of Pc1 waves detected on 30 March 2021 under low Kp, low Sym-H, and moderate AE conditions. The proposed investigation shows that high-latitude ground magnetometers in both hemispheres and the first China Seismo-Electromagnetic Satellite (CSES-01) at a Low Earth Orbit (LEO) detected in-synch Pc1 waves. In strict correspondence to this, energetic proton precipitation was observed at LEO with a simultaneous appearance of an isolated proton aurora at subauroral latitudes. This supports the idea of EMIC wave-induced proton precipitation contributing to energy transfer from the magnetosphere to the ionosphere. 

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5. Energetic Electron Precipitation Driven by Electromagnetic Ion Cyclotron Waves from ELFIN’s Low Altitude Perspective

   https://doi.org/10.1007/s11214-023-00984-w  

   Angelopoulos, V.; Zhang, X.-J.; Artemyev, A. V.; Mourenas, D.; Tsai, E.; Wilkins, C.; Runov, A.; Liu, J.; Turner, D. L.; Li, W.; et al (August 2023, Space Science Reviews)

   Abstract We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data collected by the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at >0.5 MeV which are abrupt (bursty) (lasting ∼17 s, or $$\Delta L\sim 0.56$$ Δ L ∼ 0.56 ) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or equatorial observations of the EMIC waves reveal that the energy of moderate and strong precipitation at ELFIN approximately agrees with theoretical expectations for cyclotron resonant interactions in a cold plasma. Using multiple years of ELFIN data uniformly distributed in local time, we assemble a statistical database of ∼50 events of strong EMIC wave-driven precipitation. Most reside at $$L\sim 5-7$$ L ∼ 5 − 7 at dusk, while a smaller subset exists at $$L\sim 8-12$$ L ∼ 8 − 12 at post-midnight. The energies of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an $$L$$ L -shell dependence in good agreement with theoretical estimates based on prior statistical observations of EMIC wave power spectra. The precipitation ratio’s spectral shape for the most intense events has an exponential falloff away from the peak (i.e., on either side of $$\sim 1.45$$ ∼ 1.45 MeV). It too agrees well with quasi-linear diffusion theory based on prior statistics of wave spectra. It should be noted though that this diffusive treatment likely includes effects from nonlinear resonant interactions (especially at high energies) and nonresonant effects from sharp wave packet edges (at low energies). Sub-MeV electron precipitation observed concurrently with strong EMIC wave-driven >1 MeV precipitation has a spectral shape that is consistent with efficient pitch-angle scattering down to ∼ 200-300 keV by much less intense higher frequency EMIC waves at dusk (where such waves are most frequent). At ∼100 keV, whistler-mode chorus may be implicated in concurrent precipitation. These results confirm the critical role of EMIC waves in driving relativistic electron losses. Nonlinear effects may abound and require further investigation. 

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