In situ observations of EMIC waves in O⁺ band by the Van Allen Probe A

Through polarization and spectra analysis of the magnetic field observed by the Van Allen Probe A, we present two typical cases of O⁺ band electromagnetic ion cyclotron (EMIC) waves in the outer plasmasphere or plasma trough. Although such O⁺ band EMIC waves are rarely observed, 18 different events of O⁺ band EMIC waves (16 events in the outer plasmasphere and two events in the plasma trough) are found from September 2012 to August 2014 with observations of the Van Allen Probe A. We find that the preferred region for the occurrence of O⁺ band EMIC waves is in L = 2-5 and magnetic local time = 03-13, 19-20, which is in accordance with the occurrence region of O⁺ ion torus. Therefore, our result suggests that the O⁺ ion torus in the outer plasmasphere during geomagnetic activities should play an important role in the generation of EMIC waves in O⁺ band.