Evolution of Fluctuations during a Radial Alignment of the Solar Orbiter and Wind Spacecraft

Abstract 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.