The ELFIN Mission

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 (Torbit ∼ 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 to 5 Hz Nyquist (nominally) with <0.3 nT/sqrt(Hz) noise at 1 Hz. The spinning satellites (Tspin$\,\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.

Energetic Electron Precipitation Observed by FIREBIRD‐II Potentially Driven by EMIC Waves: Location, Extent, and Energy Range From a Multievent Analysis

“…Finally, the analysis of FIREBIRD & RBSP/POES conjunctions shows that we can identify the driver of electron precipitation observed by FIREBIRD with the aid of RBSP wave observations and/or POES proton observations, and simultaneously improve POES measurements of electron flux when they are limited by high noise levels or proton contamination. Therefore, this study highlights the importance of coordinated multisatellite studies and especially demonstrates the usefulness of low-cost CubeSat missions, like current (AC6, AC10, ELFIN (Angelopoulos et al, 2020), FIREBIRD-II, etc.) and future (GTOSat , REAL, etc.)…”

Electron Lifetimes and Diffusion Rates Inferred From ELFIN Measurements at Low Altitude: First Results

“…But trapped electrons measured by ELFIN are less than a fraction of degree above the bounce loss cone angle α LC (Angelopoulos et al., 2020). This mainly corresponds to quasi‐trapped electrons with pitch‐angles above the longitudinally averaged α LC but below the drift loss cone angle α DLC , defined as the maximum bounce loss cone angle reached at South Atlantic Anomaly (SAA) longitudes (Schulz & Lanzerotti, 1974; Tu et al., 2010).…”

“…It is worth emphasizing that chorus wave driven diffusion rates D αα , LC at L * = 4.5–6.5 decrease rapidly as electron energy increases above 10 keV for fixed wave power and plasma density (Aryan et al., 2020; Mourenas et al., 2014), and that a larger D αα , LC is likely to be more accurately estimated from the measured flux ratio J precip / J trapped (Reidy et al., 2021). This leads us to focus on measured fluxes of 100 ± 30 keV electrons, slightly above the minimum energy limit of ELFIN's detector (Angelopoulos et al., 2020).…”

“…The ELFIN mission consists of two identical 3‐Unit CubeSats launched on nearly circular ∼90 min low‐altitude (450 km) orbits with 93° inclination (Angelopoulos et al., 2020). Each spinning satellite (with 3 s spin period) carries an energetic particle detector for electrons (EPDE) measuring 0.05 − 5 MeV electrons in 16 channels with energy resolution Δ E / E < 40% and pitch‐angle resolution , a similar detector of 0.05 − 5 MeV ions (EPDI), as well as a fluxgate magnetometer (FGM) recording magnetic field waves from DC to 5 Hz Nyquist with nT noise.…”

“…The spin axis of each satellite is maintained perpendicular to the orbit plane, allowing a full pitch‐angle resolution of electrons twice per spin. To prevent a contamination of EPDE measurements by penetrating particles, detectors are in a vault with Ta (3 mm) plus Al (9 mm) walls, with apertures comprising phosphor‐bronze knife‐edge collimators (Angelopoulos et al., 2020). The expected signal‐to‐noise ratio is 10:1 in a realistic ion and electron environment, and coincidence logic further improves this ratio by a factor of 10 (also suppressing side‐penetrating background).…”