Transverse acceleration of oxygen ions by electromagnetic ion cyclotron resonance with broad band left‐hand polarized waves

Abstract

Central plasma sheet (CPS) ion conies are oxygen dominated with peak energies ranging from tens to hundreds of eV centered around pitch‐angles between 115 and 130 degrees. Because of the lack of correlation between the CPS conics and the observed currents and/or electron beam‐like structures, it is not likely that all of these conies are generated by interactions with electrostatic ion cyclotron waves or lower hybrid waves. Instead, we suggest that the observed intense broad band electric field fluctuations in the frequency range between zero and a hundred Hz can be responsible for the transverse energization of the ions through cyclotron resonance heating with the left‐hand polarized electromagnetic waves. This process is much more efficient for heating the oxygen ions than hydrogen ions, thus providing a plausible explanation of the oxygen dominance in CPS conies. Simple algebraic expressions are given from which estimates of conic energy and pitch angle can be easily calculated. This suggested mechanism can also provide some preheating of the oxygen ions in the boundary plasma sheet (BPS) where discrete aurorae form.