Forecasting solar wind parameters at 1 AU using time-dependant magnetohydrodynamic simulations

The solar wind is a continuous, magnetised outflow of plasma from the Sun's surface that shapes the heliosphere and interacts with Earth's magnetic field, driving space weather phenomena. Variability in the photospheric and solar coronal magnetic field, which continually evolves, introduces changes in the formation and propagation of the solar wind. This variability leads to the development of large-scale structures, such as High-Speed Streams (HSS) and Stream Interaction Regions (SIRs), which can trigger geoeffective events.

In this study, we present results from a 2.5D magnetohydrodynamic (MHD) simulation of the heliosphere in the equatorial plane to assess the importance of incorporating the time-dependent nature of solar conditions through boundary conditions. Such boundary conditions are imperative to capture the variable behaviour of the solar magnetic field and coronal plasma. Thus, the MHD simulation is driven using six-hourly updated photospheric magnetograms to feed the Wang-Sheeley-Arge (WSA) coronal model over a 10-day period. These evolving WSA maps serve as the inner boundary conditions at 0.1 AU for the MHD simulation. The solar wind is modelled by solving the ideal MHD equations with an adiabatic equation of state, incorporating heating through a reflection-driven turbulent heating mechanism. The resulting simulation can capture time-dependant effects in the heliosphere that are absent when performing steady-state simulations using a single WSA map. The simulation outputs are validated against spacecraft data from 1 AU.

Previous studies have demonstrated that the time-dependent evolution of WSA maps captures large-scale heliospheric features with greater fidelity. An alternative approach, utilising time-dependent coronal simulations instead of WSA maps, has been shown to reproduce evolutionary features in solar wind stream structures that steady-state simulations fail to resolve. More recently, time-dependent boundary conditions driving a hydrodynamic wind model have highlighted their importance for improved forecasting at 1 AU, particularly for longer lead times, by accounting for evolving solar wind features.

The present study builds on these efforts by developing a robust and efficient simulation tool for the community, focusing on the equatorial plane which is a main region of interest for predicting space weather. It extends the impact of boundary-driven solar wind modelling from hydrodynamic approaches to an MHD framework, while also analysing forecast lead times at 1 AU. This work aims to facilitate further research into the role of time-dependent boundary conditions in modelling space weather and coronal mass ejection (CME) propagation.