The effects of reflected protons on the plasma environment of the moon for parallel interplanetary magnetic fields

For the unique case of magnetic field parallel to the solar wind flow, a column of reflected protons can accumulate upstream from the Moon. We investigate observations from the ARTEMIS probes for an extended period with this geometry. During this time, P2 observes strong wave turbulence in two frequency bands above and below the ion cyclotron frequency near the Moon, not seen by P1 farther from the Moon. The lower frequency oscillations prove consistent with kinetic magnetosonic waves resonantly generated by reflected protons, and test particle calculations confirm that a significant column of reflected protons lies upstream when the waves occur. The reflected protons perturb a large volume of plasma around the Moon, extending upstream as well as into the wake. The waves observed near the Moon during this time period have many similarities to those found in the terrestrial foreshock and at comets, suggesting the potential for comparative studies. Key Points For parallel fields, reflected protons can travel well upstream from the Moon Reflected protons drive large amplitude low frequency waves in a large volume The reflected‐proton‐driven waves perturb the wake boundary