Electrostatic Ion Cyclotron Waves Observed by CSES in the Equatorial Plasma Bubble

Broadband ELF electrostatic emissions have been observed by the China Seismo-Electromagnetic Satellite (CSES) inside the equatorial plasma bubbles (EPBs). Analyses reveal that the electric field fluctuates nearly perpendicular to the ambient magnetic field, with the transverse component gyrotropically distributed around the magnetic field line. The observed emissions, Doppler shifted and broadened, are found to be close to the local oxygen ion (O+) cyclotron frequency in the plasma reference frame. Accompanying ion heating and deceleration are consistent with the consequences of the ion cyclotron resonance. All these results indicate that the electrostatic emissions pervading a major part of the EPB are the O+ electrostatic ion cyclotron (EIC) waves. The inhomogeneous-energy-density-driven instability triggered by the non-uniform electric field and density gradient across the EPBs could be the source of the EIC waves. The CSES observation provides new evidence for the shear driven EIC wave in the equatorial ionosphere.