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The dispersion relation for electromagnetic ion cyclotron (EMIC) waves is analyzed for a multi-ion cold plasma. Several cases accounting for the relative contribution of the nitrogen (N+) and the oxygen (O+) ions and wave normal angle (theta) are presented. It is found that the presence of N+ significantly changes the dispersion properties of EMIC waves, leading to a new frequency band with additional cutoff, crossover, and resonance frequencies just above the oxygen cyclotron frequency (OO+). The method for estimation of ions concentration from cutoff frequencies based on observation is also revisited, and N+ concentration is determined. The minimum resonant energy of N+ band is also calculated. This new N+ band is relevant to reduce the discrepancy in mode detection near Omega(O+) in the observed wave spectrum in order to quantify the transport and energization of N+, in addition to O+, for their relative contribution to the loss and/or scattering mechanisms.

Plain Language Summary EMIC waves are important due to their role in the dynamics of the magnetosphere. The theoretical prediction of a new N+ band in EMIC waves has been proposed, which has totally different dispersion properties than O+ band and can lead to motivation for finding the possible solution to refine data of current missions with such mass resolution to distinct N+ from O+. The observational evidence of low-frequency N+ band EMIC waves of this theoretical finding may become possible after the mass resolution. In this letter, we demonstrate that the relative contribution of N+ to O+ introduces a new N+ band and significantly affects the dispersion properties of EMIC waves.