Pickup ion driven turbulence in the polar heliosphere: A stochastic growthmodel

A new characterization of interplanetary magnetohydrodynamic (MHD) turbulen ce over the polar regions of the Sun is presented. The pickup of interstell ar ions in the supersonic solar wind is thought to generate significant lev els of magnetic held fluctuations, which can then scatter the ions toward i sotropy in the solar wind frame. Wave generation by pickup ions in the oute r polar heliosphere is observed infrequently. Furthermore, pickup ion proto ns and helium distributions are found to exhibit pronounced anisotropies in quasi-radial magnetic field regions of the solar wind. A stochastic growth model is developed here for the growth of MHD waves driven by pickup ion i nstabilities in the polar solar wind. This is closely related to stochastic growth theories developed in other contexts. By considering temporal and s patial variations in the local interplanetary magnetic field (IMF) we compu te the mean wave growth rate and variance of pickup ion excited MHD waves. It is found that the mean wave growth rate can be very small (and even nega tive), but the variance can be large in the polar solar wind, indicating th at localized regions experience significant wave growth in a stochastic fas hion. This suggests that pickup ion driven turbulence in the outer polar he liosphere has a bursty or intermittent character, occurring in clumps, whic h makes the detection of individual. wave growth events rare. The stochasti c growth model is shown to be self-consistent, predicting very bursty wave growth by numerous long-lived beamlike fluctuations in the pickup ions duri ng characteristic wave growth times, and justifying qualitatively the persi stence of beamlike anisotropies in the observed pickup ion distribution. Su ch a picture presents an entirely different characterization of low-frequen cy MHD turbulence driven by the pickup of interstellar atoms in the polar r egions of the solar wind from that of homogeneous MHD turbulence.