MHD simulations on the large-scale propagation of high-speed solar wind streams

TL;DR

This study uses 3D MHD simulations to analyze how high-speed solar wind streams evolve from the Sun to 1 AU, revealing complex interactions and measurement biases affecting space weather predictions.

Contribution

It demonstrates that plasma parcel tracking is unreliable with common diagnostics and highlights the importance of velocity-based metrics and 3D effects in solar wind modeling.

Findings

High-speed streams are not parcel-preserving structures.

Velocity-based diagnostics better trace plasma evolution.

3D interactions cause deceleration, heating, and magnetic flux redistribution.

Abstract

We investigate the propagation of high-speed solar wind streams from their origin near the Sun to 1 AU using three-dimensional magnetohydrodynamic simulations. By tracking both global stream structure and individual plasma parcels, we assess how local in-situ measurements relate to the underlying plasma evolution. We find that high-speed streams are not parcel-preserving structures: commonly used diagnostics such as peak velocity, density, or temperature do not trace fixed plasma elements, and feature-based radial trends can therefore misrepresent the true evolution. Instead, velocity-based relationships provide a more robust framework for linking plasma parcels across heliocentric distances. Stream evolution is dominated by interaction regions, where compression leads to deceleration of fast wind, acceleration of slow wind, and significant heating. A boundary layer forms close to the Sun and can dominate narrow streams, biasing in-situ measurements toward lower apparent velocities. We show that three-dimensional transport, in particular latitudinal flows, redistributes mass and magnetic flux and reduces center-to-flank contrasts. While radial magnetic flux is conserved, the total field strength is not in spherical sampling geometries due to non-radial components. Finally, observed stream properties and geoeffectivity depend strongly on sampling location, stream geometry, and latitudinal magnetic deflection, introducing systematic variability and asymmetries in geomagnetic response.