Space Physics Seminar fall-2016

Growth rate measurement of ULF waves in the ion foreshock

Waves generated by accelerated particles are important throughout our heliosphere. These particles often gain their energy at shocks via Fermi acceleration. At the Earth's bow shock, this mechanism accelerates ion beams back into the solar wind; the beams can then generate ultra low frequency (ULF) waves via an ion-ion right hand resonant instability. These waves influence the shock structure and particle acceleration, lead to coherent structures in the magnetosheath, and are a possible source of the ULF waves that play a key role in magnetospheric dynamics.

The present study represents the first satellite measurement of the ULF wave growth rate in the upstream region. Using the flux gate magnetometer and electrostatic analyzer instruments aboard the two ARTEMIS spacecraft at ~60 Re from Earth, we characterize crescent-shaped ion beams and relatively monochromatic ULF waves. The selected event features spacecraft separation in the solar wind flow direction along a nearly radial Interplanetary Magnetic Field. We estimate the ULF wave growth rate and find it to match dispersion solver predictions during the initial growth time. Observed frequencies and wavenumbers are also within the predicted range. Other ULF wave properties such as the phase speed and obliquity are consistent with expectations from prior satellite measurements. Multiple frequency peaks observed in ARTEMIS data and additional events characterized by diffuse ion beams are currently under investigation.