Gyrating ions and large‐amplitude monochromatic MHD waves upstream of the Earth's bow shock

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Abstract
Episodes of nearly monochromatic, low‐frequency (∼0.03 hz) hydromagnetic waves are occasionally observed upstream of the earth's bow shock. These waves have previously been associated with suprathermal ions of the “intermediate” type of distribution and have been attributed to the early stage of disruption of a field‐aligned ion beam through the electromagnetic ion beam instability. However, high time resolution (3 s) measurements of two‐dimensional ion distributions during two nearly monochromatic wave events reveal that the ion distributions associated with these waves are, in fact, “gyrating ions.” Such distributions consist of suprathermal ions with parallel and perpendicular velocities confined to a fairly narrow range of (nonzero) values. The ions are also often confined to a fairly narrow range of gyrophase angle (“gyrophase bunched”). In one of the two cases, the observed frequency of the waves agrees quite well with the Doppler shifted resonance frequency of waves in right‐hand resonance with the observed gyrating ions. In the second case, the observed frequency is lower than the predicted frequency by a factor of ∼1.5–2.