Electromagnetic ion cyclotron waves suggesting minor ion existence in the inner magnetosphere observed by the Akebono satellite

It is well known that electromagnetic ion cyclotron (EMIC) waves exhibit characteristic frequencies on the basis of dispersion relations in multiple‐component plasma. We present a case of EMIC waves exhibiting a sudden decrease in intensity (characteristic lower cutoff) to just above half of the proton cyclotron frequency observed in the vicinity of the geomagnetic equator by the Akebono satellite along its trajectory during a magnetic storm in April 1989. It was found that the waves propagate with a large wave normal angle with respect to the geomagnetic field line and that they had a crossover frequency above the characteristic lower cutoff. Because of stormy conditions, ion constituents were expected to fluctuate, suggesting that the characteristic frequencies of EMIC waves should have been fluctuating as well. However, the characteristic frequencies of each event did not vary despite disturbances in the inner magnetosphere, represented by a sudden decrease in the Dstindex and electron density fluctuation. In addition, the waves were repeatedly observed within a half day after sudden decreases in Dst; however, they disappeared when the recovery of the Dstindex became moderate. Wave generation appears to be closely correlated to fresh energetic particle injection. We study the dispersion relations of EMIC waves under the condition of multiple ion species and suggest the existence of a few percent of alpha particles (He++) or deuterons (D+), which can explain the lower cutoff of EMIC waves in the inner magnetosphere. EMIC waves which had characteristic cutoff frequency were observed by AkebonoCutoff frequencies are stable in spite of disturbance of inner magnetosphereExistence of a few percent of He++ is suggested in the inner magnetosphere