Pickup ions and ion cyclotron wave amplitudes upstream of Mars: First results from the 1D hybrid simulation

Abstract

Observations upstream of Mars show strong and persistent plasma wave activity near the local proton cyclotron frequency. These waves are thought to be generated by unstable populations of newborn ions produced by ionization of the Martian hydrogen exosphere. It is generally believed that the amplitudes of the waves are related to the energy of the local pickup ion population, such that the observed spatial and temporal distribution of the wave amplitudes could be used as a diagnostic of local exospheric structure and loss rate. To understand the relationship between wave amplitudes and ion pickup rate, we carry out a parametric study using the 1D hybrid simulation (kinetic ions, fluid electrons) for varying pickup rates and pickup geometry for the Martian planetary environment. Our results indicate that, contrary to expectations, the observed local ion cyclotron wave amplitudes cannot be easily related to local ion pickup rates because the growth time of the instability is long compared to the transit time of the waves past Mars. Additionally, we find the amount of pickup ion energy lost to wave growth is not a constant over all injection rates and wave amplitudes are likely affected by propagation through regions of spatially non-uniform ion production. Because the waves in the upstream region are in a state of growth rather than saturated, the simulations suggest the pickup ion distributions are likely only partially scattered. © 2012 by the American Geophysical Union.