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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. Identification and Classification of Relativistic Electron Precipitation at Earth Using Supervised Deep Learning

   https://doi.org/10.3389/fspas.2022.858990  

   Capannolo, Luisa ; Li, Wen ; Huang, Sheng ( March 2022 , Frontiers in Astronomy and Space Sciences)

   We show an application of supervised deep learning in space sciences. We focus on the relativistic electron precipitation into Earth’s atmosphere that occurs when magnetospheric processes (wave-particle interactions or current sheet scattering, CSS) violate the first adiabatic invariant of trapped radiation belt electrons leading to electron loss. Electron precipitation is a key mechanism of radiation belt loss and can lead to several space weather effects due to its interaction with the Earth’s atmosphere. However, the detailed properties and drivers of electron precipitation are currently not fully understood yet. Here, we aim to build a deep learning model that identifies relativistic precipitation events and their associated driver (waves or CSS). We use a list of precipitation events visually categorized into wave-driven events (REPs, showing spatially isolated precipitation) and CSS-driven events (CSSs, showing an energy-dependent precipitation pattern). We elaborate the ensemble of events to obtain a dataset of randomly stacked events made of a fixed window of data points that includes the precipitation interval. We assign a label to each data point: 0 is for no-events, 1 is for REPs and 2 is for CSSs. Only the data points during the precipitation are labeled as 1 or 2. By adopting a longmore »short-term memory (LSTM) deep learning architecture, we developed a model that acceptably identifies the events and appropriately categorizes them into REPs or CSSs. The advantage of using deep learning for this task is meaningful given that classifying precipitation events by its drivers is rather time-expensive and typically must involve a human. After post-processing, this model is helpful to obtain statistically large datasets of REP and CSS events that will reveal the location and properties of the precipitation driven by these two processes at all L shells and MLT sectors as well as their relative role, thus is useful to improve radiation belt models. Additionally, the datasets of REPs and CSSs can provide a quantification of the energy input into the atmosphere due to relativistic electron precipitation, thus offering valuable information to space weather and atmospheric communities.« less
3. Superfast precipitation of energetic electrons in the radiation belts of the Earth

   https://doi.org/10.1038/s41467-022-29291-8  

   Zhang, Xiao-Jia ; Artemyev, Anton ; Angelopoulos, Vassilis ; Tsai, Ethan ; Wilkins, Colin ; Kasahara, Satoshi ; Mourenas, Didier ; Yokota, Shoichiro ; Keika, Kunihiro ; Hori, Tomoaki ; et al ( March 2022 , Nature Communications)

   Energetic electron precipitation from Earth’s outer radiation belt heats the upper atmosphere and alters its chemical properties. The precipitating flux intensity, typically modelled using inputs from high-altitude, equatorial spacecraft, dictates the radiation belt’s energy contribution to the atmosphere and the strength of space-atmosphere coupling. The classical quasi-linear theory of electron precipitation through moderately fast diffusive interactions with plasma waves predicts that precipitating electron fluxes cannot exceed fluxes of electrons trapped in the radiation belt, setting an apparent upper limit for electron precipitation. Here we show from low-altitude satellite observations, that ~100 keV electron precipitation rates often exceed this apparent upper limit. We demonstrate that such superfast precipitation is caused by nonlinear electron interactions with intense plasma waves, which have not been previously incorporated in radiation belt models. The high occurrence rate of superfast precipitation suggests that it is important for modelling both radiation belt fluxes and space-atmosphere coupling.
4. ELFIN observations of energetic electron precipitation and backscatter: implication for losses, atmospheric effects, and magnetospheric populations.

   Angelopoulos, V. ( December 2020 , Fall AGU 2020)

   We report on the behavior of precipitating and backscattered energetic electrons as function of latitude, energy and pitch-angle across a wide range of local times. ELFIN’s two spinning satellites from a 450km altitude, near-polar orbit, permit excellent resolution of pitch-angles (22.5deg) well within the loss cone, and allow clear discrimination of locally trapped and field-aligned electrons between 50keV and 5MeV (dE/E ~ 40%). We find that at times of low precipitation (fluxes <10% of trapped) both precipitating and backscattered electrons are present and their ratio is close to 1. This is likely because atmospheric scattering contributes to loss-cone filling, both up and down the field line. When precipitation is significant (flux >10% of trapped, up to an energy Epmax) it dominates the upward-to-downward flux ratio at energies as low as 0.2 times Epmax, rendering that ratio very low (<10%). However, below ~0.2Epmax, as well as above Epmax, backscattering is a significant fraction of precipitation. We discuss the possible reasons for this backscatter. We also discuss the implications of our findings for electron losses from the radiation belts, for modeling atmospheric effects of energetic electron precipitation and for populating the magnetosphere with field-aligned energetic electrons.
5. Meso-Scale Electrodynamic Coupling of the Earth Magnetosphere-Ionosphere System

   https://doi.org/10.1007/s11214-022-00940-0  

   Yu, Yiqun ; Cao, Jinbin ; Pu, Zuyin ; Jordanova, Vania K. ; Ridley, Aaron ( December 2022 , Space Science Reviews)

   Within the fully integrated magnetosphere-ionosphere system, many electrodynamic processes interact with each other. We review recent advances in understanding three major meso-scale coupling processes within the system: the transient field-aligned currents (FACs), mid-latitude plasma convection, and auroral particle precipitation. (1) Transient FACs arise due to disturbances from either dayside or nightside magnetosphere. As the interplanetary shocks suddenly compress the dayside magnetosphere, short-lived FACs are induced at high latitudes with their polarity successively changing. Magnetotail dynamics, such as substorm injections, can also disturb the current structures, leading to the formation of substorm current wedges and ring current disruption. (2) The mid-latitude plasma convection is closely associated with electric fields in the system. Recent studies have unraveled some important features and mechanisms of subauroral fast flows. (3) Charged particles, while drifting around the Earth, often experience precipitating loss down to the upper atmosphere, enhancing the auroral conductivity. Recent studies have been devoted to developing more self-consistent geospace circulation models by including a better representation of the auroral conductance. It is expected that including these new advances in geospace circulation models could promisingly strengthen their forecasting capability in space weather applications. The remaining challenges especially in the global modeling of the circulation system are alsomore »discussed.

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