Proposal of a Novel Physical Parameter Characterizing Solar Wind Speed in a Wave-Driven Model

TL;DR

This study investigates how the shape of magnetic flux tubes affects solar wind speed using numerical simulations, proposing a new physical parameter that improves upon existing empirical models for space weather prediction.

Contribution

It introduces a novel physical parameter based on flux-tube shape profile that enhances solar wind speed modeling beyond traditional asymptotic expansion factors.

Findings

Solar wind speed varies significantly with flux-tube expansion height.

Radial flux-tube shape profile correlates better with wind speed than expansion factor.

A new physical parameter capturing flux-tube shape improves empirical modeling.

Abstract

Empirical solar wind speed models play an important role in enabling space weather forecasting with low computational cost. Among these, one model called WS model is based on the asymptotic expansion factor. However, it is known that it fails in the case of pseudostreamers. In this study, as a first step toward constructing a solar wind speed empirical model based on physical parameters, we investigated the effect of the radial profile of flux-tube shape on the solar wind speed using one-dimensional numerical simulations. In the simulations, ad hoc Alfv\'en waves are injected from the photosphere at $r=R_\odot$ as the energy source, and the MHD equations are solved out to the interplanetary space at $r=70R_\odot$ to reproduce solar wind acceleration. As a result, even when the coronal base magnetic field and the asymptotic expansion factor are fixed, the final solar wind speed varies by approximately 300 km s$^{-1}$ depending on changes in the expansion height or non-monotonic expansion. Additionally, across all simulations performed, a better correlation is found with the quantities that reflect the information about the radial profile of flux-tube shape than the asymptotic expansion factor. Our results suggest that, as a physical characteristic parameter of the solar wind speed, an operation that can account for the expansion factor throughout the corona is necessary.