The Relationship Between EMIC Wave Properties and Proton Distributions Based on Van Allen Probes Observations

Chao Yue, Chae Woo Jun, Jacob Bortnik, Xin An, Qianli Ma, Geoffrey D. Reeves, Harlan E. Spence, Andrew J. Gerrard, Matina Gkioulidou, Donald G. Mitchell, Craig A. Kletzing

Research output: Contribution to journalArticlepeer-review

49 Scopus citations


Plasma kinetic theory predicts that sufficiently anisotropic proton distribution will excite electromagnetic ion cyclotron (EMIC) waves, which in turn relax the proton distribution to a marginally stable state creating an upper bound on the relaxed proton anisotropy. Here, using EMIC wave observations and coincident plasma measurements made by Van Allen Probes in the inner magnetosphere, we show that the proton distributions are well constrained by this instability to a marginally stable state. Near the threshold, the probability of EMIC wave occurrence is highest, having left-handed polarization and observed near the magnetic equator with relatively small wave normal angles, indicating that these waves are locally generated. In addition, EMIC waves are distributed in two magnetic local time regions with different intensity. Compared with helium band waves, hydrogen band waves behave similarly except that they are often observed in low-density regions. These results reveal several important features regarding EMIC waves excitation and propagation.

Original languageEnglish (US)
Pages (from-to)4070-4078
Number of pages9
JournalGeophysical Research Letters
Issue number8
StatePublished - Apr 28 2019

All Science Journal Classification (ASJC) codes

  • Geophysics
  • General Earth and Planetary Sciences


  • EMIC waves
  • Van Allen Probes
  • helium band
  • hydrogen band
  • plasma beta
  • proton temperature anisotropy


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