Growth rate measurement of ULF waves in the ion foreshock

Abstract

We report the first satellite measurement of the ultralow frequency (ULF) wave growth rate in the upstream region of the Earth's bow shock. We employ the two identical ARTEMIS spacecraft orbiting the Moon to characterize crescent-shaped reflected ion beams and relatively monochromatic ULF waves. The event presented here features spacecraft separation of ∼2.5 Earth radii (0.9 ± 0.1 wavelengths) in the solar wind flow direction along a nearly radial interplanetary magnetic field. The ULF wave growth rate is estimated and found to fall within dispersion solver predictions during the initial growth time. Observed frequencies and wave numbers are also within the predicted range. Other ULF wave properties such as the phase speed, obliquity, and polarization are consistent with expectations from resonant beam instability theory and prior satellite measurements. These results will inform future missions near bow and interplanetary shocks as well as future nonlinear studies related to turbulence and dissipation in the heliosphere.