Comparing Plasma Anisotropy Associated with Solar Wind Discontinuities and Alfvénic Fluctuations

Abstract

Solar wind magnetic field fluctuations exhibit a complex multiscale nature, often encompassing ion-scale discontinuities and MHD-scale Alfvénic fluctuations. Both of these types of structures are thought to play a critical role in plasma heating and turbulence dissipation. Here we comparatively analyze the plasma pressure anisotropies within discontinuities and adjacent Alfvénic fluctuations, leveraging unique solar wind observations from orbit conjunctions between the ARTEMIS and WIND missions, along the same flow streamline, though about 150 Earth radii apart. Based on 11 cases of such observations, we compare direct measurements of plasma anisotropy from particle instruments with its estimates from anisotropic MHD theory using the ratios of correlated ion velocity and Alfvén speed variations Δ v i /Δ v A. We find that (1) sporadically observed discontinuities associated with bifurcated reconnection current sheets harbor significant parallel electron anisotropies of >0.2; (2) direct electron measurements in all events reveal a median anisotropy of ∼0.07 for Alfvénic fluctuations and ∼0.17 for discontinuities; (3) anisotropic MHD predicts even more disparate total anisotropies within Alfvénic fluctuations and discontinuities, with a median value of ∼0.15 for the former and ∼0.57 for the latter; (4) the differences between theory-predicted and directly measured anisotropies imply that the ion contribution to anisotropy is significant and likely dominant within both types of structures, an assertion which we partly verify using simultaneous ion measurements from WIND. Our observations confirm that such discontinuities play a uniquely important role in producing solar wind plasma heating and anisotropy.