Abstract/Summary

Previous work has established an empirical relationship between densities gained from coronal rotational tomography near the ecliptic plane with solar wind outflow speeds at heliocentric distance r₀ = 8R⊙. This work aims to include solar wind acceleration, and thus velocity profiles out to 1 au. Inner boundary velocities are given as a function of normalized tomographic densities, ρN, as V₀ = (75 * ᵉ⁻[⁵.²⁺ρᴺ] + 108), and typically range from 100 to 180 km s−1. The subsequent acceleration is defined as V(r) = V₀(1+αIP[1-e⁽⁻[ʳ⁻ʳ⁰]/ʳᴴ⁾]), with αIP ranging between 1.75 and 2.7, and rH between 50 and 35 R⊙ dependent on V0. These acceleration profiles approximate the distribution of in situ measurements by Parker Solar Probe (PSP) and other measurements at 1 au. Between 2018 November and 2021 September these constraints are applied using the HUXt model and give good agreement with in situ observations at PSP, with a ∼6% improvement compared with using a simpler constant acceleration model previously considered. Given the known tomographical densities at 8 R⊙, we extrapolate density to 1 au using the model velocities and assuming mass flux conservation. Extrapolated densities agree well with OMNI measurements. Thus coronagraph-based estimates of densities define the ambient solar wind outflow speed, acceleration, and density from 8 R⊙ to at least 1 au. This sets a constraint on more advanced models, and a framework for forecasting that provides a valid alternative to the use of velocities derived from magnetic field extrapolations.