Studying Magnetospheric Collisionless Beam Instabilities and ULF Waves in the Laboratory

Collisionless ion-ion beam instabilities parallel to the background interplanetary magnetic field are an important feature of planetary bow shocks, driving ultra-low frequency (ULF) waves that partially determine the ion velocity distribution in the parallel foreshock. However, the growth and saturation phases of these instabilities are difficult to study in situ. A recent series of parameter-scaled experiments at the University of California, Los Angeles aims to supplement spacecraft data by observing the growth of one such instability, the Right-Hand Resonant Instability (or RHI), in the laboratory. The instability is driven by a super-Alfvénic “beam” of laser-produced plasma expanding parallel to a background magnetic field over 80 ion inertial lengths through the large, magnetized ambient plasma of the Large Plasma Device (LAPD). Dimensionless parameters are comparable to the terrestrial magnetosphere. Measurements of the magnetic field with an array of 3-axis magnetic flux “bdot” probes show waves with dimensionless spectral properties similar to ULF waves observed in the terrestrial foreshock, consistent with excitation of the RHI. Measurements are also compared to hybrid simulations of parallel shock formation.