Outstanding questions around the energy of EMIC wave driven electron precipitation

Abstract:

It has long been predicted through theory that Electromagnetic Ion Cyclotron Waves (EMIC) should drive significant energetic electron precipitation from the radiation belts into the lower ionosphere. Recently, ground based observations of EMIC waves and ionospheric disturbances have been combined with Low-Earth Orbit precipitation measurements made by the POES constellation to conclusively identify hundreds to thousands of examples of EMIC driven electron scattering into the atmosphere [e.g., Hendry et al., doi:10.1002/2015JA022224, 2016]. However, this large set of events appear to have significant fluxes of electrons with energies of hundreds of keV, which is generally lower than that expected by theory based on first order cyclotron resonance. Differential electron fluxes tend to peak at ~250 keV, with only ~11% peaking in the 1-4 MeV range [Hendry et al., doi:10.1002/2016GL071807, 2017]. These experimental observations appear to confirm the theory of Non-Resonant Interactions with EMIC waves with "sharp edges" [Chen et al., doi:10.1002/2016JA022813, 2016].

At the same time, there is a growing body of satellite observations suggesting that EMIC waves are efficient scatters of ~2-8 MeV ultra-relativistic electrons [e.g., Usanova et al., doi:10.1029/2013GL059024, 2014]. These observations are supported by theory [e. g., Shprits et al., doi:10.1038/ncomms12883, 2016], where cyclotron resonance is most efficient at ultra-relativistic energies, and can lead to non-reversible electron "dropouts" in the trapped fluxes populations. Multiple case studies have been presented [e.g., Shprits et al., doi:10.1002/2016GL072258, 2017; Aseev et al., doi:10.1002/2017JA024485, 2017], and there appears to be growing conviction in the community around the importance of these waves in this energy range. However, at this time there is a lack of any clear observations of the ultra-relativistic electron precipitation

In this presentation we discuss whether the existing precipitation observations are consistent (or not) with the idea that ultra-relativistic electrons are lost during EMIC wave scattering events. We also examine the nature of the signature in ground based precipitation monitors expected from the impact of ultra-relativistic electron losses into the atmosphere.