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1. The ELFIN Mission

   https://doi.org/10.1007/s11214-020-00721-7  

   Angelopoulos, V. ; Tsai, E. ; Bingley, L. ; Shaffer, C. ; Turner, D. L. ; Runov, A. ; Li, W. ; Liu, J. ; Artemyev, A. V. ; Zhang, X.-J. ; et al ( August 2020 , Space Science Reviews)

   Abstract The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or heretoforth simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (∼93 ∘ inclination), nearly circular, low-Earth (∼450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss cone bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (T orbit ∼ 90 min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50 keV to 5 MeV electrons with $\Delta $ Δ E/E < 40% and a fluxgate magnetometer (FGM) on a ∼72 cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC tomore »5 Hz Nyquist (nominally) with <0.3 nT/sqrt(Hz) noise at 1 Hz. The spinning satellites (T spin $\,\sim $ ∼ 3 s) are equipped with magnetorquers (air coils) that permit spin-up or -down and reorientation maneuvers. Using those, the spin axis is placed normal to the orbit plane (nominally), allowing full pitch-angle resolution twice per spin. An energetic particle detector for ions (EPDI) measures 250 keV – 5 MeV ions, addressing secondary science. Funded initially by CalSpace and the University Nanosat Program, ELFIN was selected for flight with joint support from NSF and NASA between 2014 and 2018 and launched by the ELaNa XVIII program on a Delta II rocket (with IceSatII as the primary). Mission operations are currently funded by NASA. Working under experienced UCLA mentors, with advice from The Aerospace Corporation and NASA personnel, more than 250 undergraduates have matured the ELFIN implementation strategy; developed the instruments, satellite, and ground systems and operate the two satellites. ELFIN’s already high potential for cutting-edge science return is compounded by concurrent equatorial Heliophysics missions (THEMIS, Arase, Van Allen Probes, MMS) and ground stations. ELFIN’s integrated data analysis approach, rapid dissemination strategies via the SPace Environment Data Analysis System (SPEDAS), and data coordination with the Heliophysics/Geospace System Observatory (H/GSO) optimize science yield, enabling the widest community benefits. Several storm-time events have already been captured and are presented herein to demonstrate ELFIN’s data analysis methods and potential. These form the basis of on-going studies to resolve the primary mission science objective. Broad energy precipitation events, precipitation bands, and microbursts, clearly seen both at dawn and dusk, extend from tens of keV to >1 MeV. This broad energy range of precipitation indicates that multiple waves are providing scattering concurrently. Many observed events show significant backscattered fluxes, which in the past were hard to resolve by equatorial spacecraft or non-pitch-angle-resolving ionospheric missions. These observations suggest that the ionosphere plays a significant role in modifying magnetospheric electron fluxes and wave-particle interactions. Routine data captures starting in February 2020 and lasting for at least another year, approximately the remainder of the mission lifetime, are expected to provide a very rich dataset to address questions even beyond the primary mission science objective.« less
2. Energetic Electron Precipitation Observed by FIREBIRD‐II Potentially Driven by EMIC Waves: Location, Extent, and Energy Range From a Multievent Analysis

   https://doi.org/10.1029/2020GL091564  

   Capannolo, L. ; Li, W. ; Spence, H. ; Johnson, A. T. ; Shumko, M. ; Sample, J. ; Klumpar, D. ( March 2021 , Geophysical Research Letters)

   We evaluate the location, extent, and energy range of electron precipitation driven by ElectroMagnetic Ion Cyclotron (EMIC) waves using coordinated multisatellite observations from near‐equatorial and Low‐Earth‐Orbit (LEO) missions. Electron precipitation was analyzed using the Focused Investigations of Relativistic Electron Burst Intensity, Range and Dynamics (FIREBIRD‐II) CubeSats, in conjunction either with typical EMIC‐driven precipitation signatures observed by Polar Orbiting Environmental Satellites (POES) or with in situ EMIC wave observations from Van Allen Probes. The multievent analysis shows that electron precipitation occurred in a broad region near dusk (16–23 MLT), mostly confined to 3.5–7.5 L‐shells. Each precipitation event occurred on localized radial scales, on average ∼0.3 L. Most importantly, FIREBIRD‐II recorded electron precipitation from ∼200 to 300 keV to the expected ∼MeV energies for most cases, suggesting that EMIC waves can efficiently scatter a wide energy range of electrons.
3. Estimating the Impacts of Radiation Belt Electrons on Atmospheric Chemistry Using FIREBIRD II and Van Allen Probes Observations

   https://doi.org/10.1029/2020JD033098  

   Duderstadt, K. A. ; Huang, C. ‐L. ; Spence, H. E. ; Smith, S. ; Blake, J. B. ; Crew, A. B. ; Johnson, A. T. ; Klumpar, D. M. ; Marsh, D. R. ; Sample, J. G. ; et al ( March 2021 , Journal of Geophysical Research: Atmospheres)

   This study considers the impact of electron precipitation from Earth's radiation belts on atmospheric composition using observations from the NASA Van Allen Probes and NSF Focused Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics (FIREBIRD II) CubeSats. Ratios of electron flux between the Van Allen Probes (in near‐equatorial orbit in the radiation belts) and FIREBIRD II (in polar low Earth orbit) during spacecraft conjunctions (2015–2017) allow an estimate of precipitation into the atmosphere. Total Radiation Belt Electron Content, calculated from Van Allen Probes RBSP‐ECT MagEIS data, identifies a sustained 10‐day electron loss event in March 2013 that serves as an initial case study. Atmospheric ionization profiles, calculated by integrating monoenergetic ionization rates across the precipitating electron flux spectrum, provide input to the NCAR Whole Atmosphere Community Climate Model in order to quantify enhancements of atmospheric HO_xand NO_xand subsequent destruction of O₃in the middle atmosphere. Results suggest that current APEEP parameterizations of radiation belt electrons used in Coupled Model Intercomparison Project may underestimate the duration of events as well as higher energy electron contributions to atmospheric ionization and modeled NO_xconcentrations in the mesosphere and upper stratosphere.
4. Characteristics of Electron Microburst Precipitation Based on High‐Resolution ELFIN Measurements

   https://doi.org/10.1029/2022JA030509  

   Zhang, Xiao‐Jia ; Angelopoulos, Vassilis ; Mourenas, Didier ; Artemyev, Anton ; Tsai, Ethan ; Wilkins, Colin ( April 2022 , Journal of Geophysical Research: Space Physics)

   We present statistical characteristics of electron microburst precipitation using high time‐resolution measurements from the low‐altitude Electron Losses and Fields InvestigatioN (ELFIN) CubeSats. The radial distribution of the equatorial projection of microbursts as a function of geomagnetic activity suggests that they are produced by resonant interaction with quasi‐parallel lower‐band chorus waves. ELFIN electron flux measurements provide the first statistical models of microburst energy spectra from 50 keV to 2 MeV. Microbursts with energies up to 150 keV have a relatively flat pitch‐angle spectrum. Estimates of scattering rates required to produce the observed flat spectra suggest that such precipitation signatures are due to near‐equatorial electron scattering by chorus wave packets with peak amplitudes of 0.4–0.9 nT, well above the threshold for nonlinear resonant interaction. More rare microbursts, exceeding 500 keV, are observed preferentially near dawn during disturbed periods. We interpret them as evidence of scattering by intense ducted chorus waves propagating from the equator up to middle latitudes with little attenuation.
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 ( July 2023 , Space Science Reviews)

   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$$\Delta L\sim 0.56$) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or equatorial observations of the EMIC waves reveal that the energy of moderate and strong precipitation at ELFIN approximately agrees with theoretical expectations for cyclotron resonant interactions in a cold plasma. Using multiple years of ELFIN data uniformly distributed in local time, we assemble a statistical database of ∼50 events of strong EMIC wave-driven precipitation. Most reside at$L\sim 5-7$$L\sim 5-7$at dusk, while a smaller subset exists at$L\sim 8-12$$L\sim 8-12$at post-midnight. The energiesmore »of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an$L$$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$$\sim 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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