Modeling Solar Wind Variations over an 11-yr Cycle with Alfv\'en Wave Dissipation: a Parameter Study

TL;DR

This study compares Alfvén wave driven and polytropic models of solar wind over an 11-year cycle, demonstrating that Alfvén wave models better reproduce observed wind properties and their dependence on magnetic topology.

Contribution

It provides a systematic comparison showing the importance of Alfvén waves in modeling solar wind variations and reproducing observed bimodality and anti-correlations.

Findings

Alfvén wave driven model reproduces bimodal solar wind.

AW model matches observed anti-correlation between wind speed and source temperature.

Both models show similar trends in solar spin-down torque.

Abstract

We study the behaviour and properties of the solar wind using a 2.5D Alfv\'en wave driven wind model. We first systematically compare the results of an Alfv\'en wave (AW) driven wind model with a polytropic approach. Polytropic magnetohydrodynamic wind models are thermally driven, while Alfv\'en waves act as additional acceleration and heating mechanisms in the Alfv\'en wave driven model. We confirm that an AW-driven model is required to reproduce the observed bimodality of slow and fast solar winds. We are also able to reproduce the observed anti-correlation between the terminal wind velocity and the coronal source temperature with the AW-driven wind model. We also show that the wind properties along an eleven-year cycle differ significantly from one model to the other. The AW-driven model again shows the best agreement with observational data. Indeed, solar surface magnetic field topology plays an important role in the Alfv\'en wave driven wind model, as it enters directly into the input energy sources via the Poynting flux. On the other hand, the polytropic wind model is driven by an assumed pressure gradient; thus it is relatively less sensitive to the surface magnetic field topology. Finally, we note that the net torque spinning down the Sun exhibits the same trends in the two models, showing that the polytropic approach still captures correctly the essence of stellar winds.