A Statistical Study of Parametric Decay Instability of Alfvén Waves in the Inner-Heliosphere

The low-beta parametric decay instability of magnetohydrodynamic Alfvén waves, coupling large amplitude forward propagating Alfvén modes with backwards propagating Alfvén modes and forward propagating compressions, is an attractive mechanism in explaining observations of an Alfvénic turbulent cascade and the slow-mode nature of compressions in the solar wind. Recent theoretical work has investigated the parametric decay instability of Alfvén waves in the solar wind in the presence of weak turbulence and temperature anisotropies (e.g. Tenerani et al. 2017, and Chandran 2018). Additionally, observational results have demonstrated that the distribution of fluctuation amplitudes at 1 AU is bounded by high growth rates of the parametric decay instability (Bowen et al. 2018). The increasing parametric instability growth rates with fluctuation amplitude and decreasing beta suggest that the decay process may be dynamically relevant in the inner heliosphere. In this work, we perform a statistical study of Helios observations at 0.3 AU to determine growth rates and timescales associated with parametric instability in the solar wind. We additionally analyze power spectral densities of the magnetic field to determine wavenumber scaling of parametric growth rates in the solar wind and investigate the connection between parametric decay and observational signatures of turbulent magnetic spectra.