Investigation of Turbulent Fluctuations in the Fast and Alfvénic Slow Solar Wind at 1 au

Abstract We study the turbulent modes present in the Alfvénic slow and fast solar wind. This analysis focuses on two solar wind intervals: 2022 January 26–29 and February 6–9, and it is based on wind observations at 1 au. Our study examined Alfvénic slow with β < 1 and fast solar wind with β < 1 (∼O(1)), employing a superposed epoch analysis (SEA) to investigate the different turbulent modes and their power spectral densities. We find that the density fluctuations in Alfvénic slow and fast solar wind are primarily fast magnetosonic (MS) modes, although they differ in the dominant propagation direction of fast MS modes—forward propagating in the Alfvénic slow and backward propagating in the fast solar wind. In the Alfvénic slow regime, the SEA shows that the incompressible velocity fluctuations dominate the compressible contributions from fast and slow MS modes. The relative power of velocity fluctuations is largest in backward Alfvén propagating modes, ∼28.8% of the total velocity fluctuations energy. The wavenumber anisotropy of the fast MS modes associated with density fluctuations is k ⊥ ≫ k ∥ . In the fast solar wind, compressible velocity fluctuations are dominated by fast MS mode, with the velocity fluctuation energy of 43.6% in backward and 42.0% in forward components, surpassing the contributions from the incompressible Alfvénic fluctuations. We find that magnetic field fluctuations are predominantly associated with magnetic island modes, contributing 53.9% of the total magnetic fluctuations energy in the Alfvénic slow and 35.4% in the fast solar wind.