With observations of the Van Allen Probe B, we report in situ evidence of the modification of the parallel propagating electromagnetic ion cyclotron (EMIC) waves by heated He+ ions. In the outer boundary of the plasmasphere, accompanied with the He+ ion heating, the frequency bands of H+ and He+ for EMIC waves merged into each other, leading to the disappearance of a usual stop band between the gyrofrequency of He+ ions (ΩHe+) and the H+ cutoff frequency (ωH+co) in the cold plasma. Moreover, the dispersion relation for EMIC waves theoretically calculated with the observed plasma parameters also demonstrates that EMIC waves can indeed parallel propagate across ΩHe+. Therefore, the paper provides an in situ evidence of the modification of the parallel propagation of EMIC waves by heated He+ ions.
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The Van Allen Probes data are available at the websites http://www.space.umn.edu/rbspefw-data/, http://emfisis.physics.uiowa.edu/data/index, and http://www.rbsp-ect.lanl.gov/data_pub/. This work is supported by the National Natural Science Foundation of China (41174140, 41374168, and 41521063), Program for New Century Excellent Talents in University (NCET-13-0446), and Keygrant Project of Chinese Ministry of Education (2042015kf0169).
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- EMIC waves
- He ion heating
- Van Allen Probes
- ring current ions
- stop band