Occurrence features of simultaneous H + - and He + -band EMIC emissions in the outer radiation belt Song Fu a , Fengming He a , Xudong Gu a,⇑ , Binbin Ni a , Zheng Xiang a , Jiang Liu b,c a Department of Space Physics, School of Electronic Information, Wuhan University, Wuhan, Hubei, China b Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA, USA c Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA Received 5 November 2017; received in revised form 9 January 2018; accepted 28 January 2018 Available online 6 February 2018 Abstract As an important loss mechanism of radiation belt electrons, electromagnetic ion cyclotron (EMIC) waves show up as three distinct frequency bands below the hydrogen (H + ), helium (He + ), and oxygen (O + ) ion gyrofrequencies. Compared to O + -band EMIC waves, H + - and He + -band emissions generally occur more frequently and result in more efficient scattering removal of <5 MeV relativistic electrons. Therefore, knowledge about the occurrence of these two bands is important for understanding the evolution of the relativistic electron population. To evaluate the occurrence pattern and wave properties of H + - and He + -band EMIC waves when they occur con- currently, we investigate 64 events of multi-band EMIC emissions identified from high quality Van Allen Probes wave data. Our quan- titative results demonstrate a strong occurrence dependence of the multi-band EMIC emissions on magnetic local time (MLT) and L- shell to mainly concentrate on the dayside region of L = 4–6. We also find that the average magnetic field amplitude of H + -band waves is larger than that of He + -band waves only when L < 4.5 and AE * < 300 nT, and He + -band emissions are more intense under all other conditions. In contrast to 5 events that have average H + -band amplitude over 2 nT, 19 events exhibit >2 nT He + -band amplitude, indi- cating that the He + -band waves can be more easily amplified than the H + -band waves under the same circumstances. For simultaneous occurrences of the two EMIC wave bands, their frequencies vary with L-shell and geomagnetic activity: the peak wave frequency of H + - band emissions varies between 0.25 and 0.8 f cp with the average between 0.25 and 0.6 f cp , while that of He + -band emissions varies between 0.03 and 0.23 f cp with the average between 0.05 and 0.15 f cp . These newly observed occurrence features of simultaneous H + - and He + -band EMIC emissions provide improved information to quantify the overall contribution of multi-band EMIC waves to the loss processes of radiation belt electrons. Ó 2018 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: H + -band EMIC; He + -band EMIC; Simultaneous occurrence features; Outer radiation belt 1. Introduction Wave-particle interactions, which break one or more of the three adiabatic invariants, have tight connection with the acceleration and loss of radiation belt electrons (Horne, 2002; Thorne, 2010). Among the variety of wave modes in the inner magnetosphere, electromagnetic ion cyclotron (EMIC) waves are known to interact with rela- tivistic electrons (Horne and Thorne, 1998; Summers and Throne, 2003; Summers et al., 2007; Usanova et al., 2014; Ni et al., 2015, 2018; Cao et al., 2016; 2017) and ring current ions (Cornwall et al., 1970; Summers, 2005). This interaction causes pitch angle scattering and thus the loss of these particles. EMIC waves arise from both hot and cold ion populations—the hot population provides the free energy for the wave growth, while the cold dense popula- tion can facilitate the growth rate (e.g., Cornwall, 1965). https://doi.org/10.1016/j.asr.2018.01.041 0273-1177/Ó 2018 COSPAR. Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: guxudong@whu.edu.cn (X. Gu). www.elsevier.com/locate/asr Available online at www.sciencedirect.com ScienceDirect Advances in Space Research 61 (2018) 2091–2098