Linear-Mode Decomposition as a Tool for Understanding MHD Fluctuations in the Solar Wind

Small amplitude solar wind fluctuations are measured by spacecraft in both supersonic and sub-Alfvenic regions. Fluctuations are typically expressed in terms of the fundamental magnetohydrodynamics (MHD) modes admitted by the system. An important question is how to resolve an observed set of fluctuations, such as the density, velocity, pressure and magnetic field fluctuations, into their constituent MHD modal components. Despite its importance for waves and turbulence in the solar wind, this problem has not yet been fully resolved. Here, we present a new method that identifies between wave modes and advected structures such as magnetic islands or entropy modes and computes the phase information associated with the eligible MHD modes. Our mode-decomposition method identifies the admissible modes in an MHD plasma from a set of plasma and magnetic field fluctuations measured by a single spacecraft at a specific frequency and an inferred wave number. We present data from several solar wind intervals in the supersonic and sub-Alfvenic solar wind to illustrate the identification of both propagating (wave) and non-propagating (structures) modes. This allows us to identify and characterize the separate MHD modes in an observed plasma parcel and to derive wave number spectra of entropic density, fast and slow magnetosonic, Alfvenic, and magnetic island fluctuations for the first time. This form of analysis allows us to identify the fundamental building blocks of turbulence in the magnetized solar wind.