Dependence of relativistic electron precipitation in the ionosphere on EMIC wave minimum resonant energy at the conjugate equator

We investigate relativistic electron precipitation events detected by Polar Environmental Satellites (POES) in low-Earth orbit in close conjunction with Van Allen Probe A observations of electromagnetic ion cyclotron (EMIC) waves near the geomagnetic equator. We show that the occurrence rate of >0.7 MeV electron precipitation recorded by POES during those times strongly increases, reaching statistically significant levels when the minimum electron energy for cyclotron resonance with hydrogen or helium band EMIC waves at the equator decreases below similar or equal to 1.0-2.5 MeV, as expected from the quasi-linear theory. Both hydrogen and helium band EMIC waves can be effective in precipitating MeV electrons. However, >0.7 MeV electron precipitation is more often observed (at statistically significant levels) when the minimum electron energy for cyclotron resonance with hydrogen band waves is low (E-min = 0.6-1.0 MeV), whereas it is more often observed when the minimum electron energy for cyclotron resonance with helium band waves is slightly larger (E-min = 1.0-2.5 MeV). This is indicative of the warm plasma effects for waves approaching the He+ gyrofrequency. We further show that most precipitation events had energies > 0.7-1.0 MeV, consistent with the estimated minimum energy (E-min similar to 0.6 - 2.5 MeV) of cyclotron resonance with the observed EMIC waves during the majority of these events. However, 4 out of the 12 detected precipitation events cannot be explained by electron quasi-linear scattering by the observed EMIC waves, and 12 out of 20 theoretically expected precipitation events were not detected by POES, suggesting the possibility of nonlinear effects likely present near the magnetic equator, or warm plasma effects, and/or narrowly localized bursts of EMIC waves.