Prashant Baruwal

Small-amplitude fluctuations in the magnetized solar wind are typically measured by a single spacecraft. In the magnetohydrodynamic description, fluctuations are expressed in terms of the fundamental modes admitted by the system. An important question is how to resolve observed sets of fluctuations, typically plasma moments such as density, velocity, pressure, and magnetic field, into their constituent fundamental modal components. In particular, identifying these components for the same plasma parcel and their properties observed at two radially aligned locations has not yet been investigated in detail. We apply the linear mode decomposition technique recently developed by G. P. Zank et al. to a plasma parcel measured by Solar Orbiter at 0.88 au and Wind at 0.99 au during their radial alignment on 2021 November 9, identifying eight modes and examining how their properties evolve with radial distance in the inner heliosphere. Our study shows that the contribution of transverse magnetic energy to the total transverse magnetic energy corresponding to Alfvén modes decreases by ∼17%, while that of the magnetic island and fast modes increases by ∼15% and ∼2%, respectively. We find that the transverse magnetic energy decreases by 86% in forward Alfvén and 69% in backward Alfvén and increases by 9% in magnetic island modes. Similarly, the forward Alfvén transverse kinetic energy decreases by 61%, while the backward Alfvén kinetic energy decreases by 13%. This behavior reflects a possible resonant interaction between counterpropagating Alfvén modes that generate zero-frequency magnetic islands, thereby reducing energy imbalance.

The characterization of solar wind fluctuations provides insight into magnetofluid turbulence. The slow solar wind with high Alfvénicity, the Alfvénic slow solar wind (ASSW), possesses a similar bulk speed as the slow wind, but with fluctuations largely similar to those found in the fast wind. ASSWs have attracted less attention from the turbulence community compared to fast wind flows. G. P. Zank et al. developed a linear mode decomposition (LMD) technique that identifies eight modes in the magnetized solar wind plasma, namely entropy, fast and slow (+/−) magnetosonic (MS), Alfvén (+/−), and magnetic island modes. Using the LMD technique, we present a comparative study of the characteristic modes found in the ASSW, classical slow solar wind (CSSW), and fast solar wind (FSW) events identified by the WIND spacecraft. We investigate the properties of the eight magnetohydrodynamic modes and their spectral characteristic differences in the three solar wind plasma environments at 1 au. We show that in the CSSW, ASSW, and FSW intervals (i) fast MS mode density fluctuations are dominant compared to entropy and slow MS modes, (ii) magnetic island modes clearly dominate Alfvén modes for the transverse fluctuating magnetic field, and (iii) slow MS modes show no significant power spectral density contribution compared to the Alfvén and fast modes in the frequency spectra for velocity fluctuations. The ASSW exhibits characteristics intermediate to the CSSW and FSW in entropy modes, compressibility, the magnetic island-to-Alfvén mode magnetic energy ratio, and kinetic variance anisotropy and magnetic energy variance anisotropy corresponding to Alfvén modes.

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.