Constraints on Solar Wind Density and Velocity Based on Coronal Tomography and Parker Solar Probe Measurements

Abstract

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 r _0 = 8 R _⊙ . 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, ρ _N , as ${V}_{0}=\left(75\ast {e}^{-\left[5.2* {\rho }_{N}\right]}+108\right)$ , and typically range from 100 to 180 km s ^−1 . The subsequent acceleration is defined as $V(r)={V}_{0}\left(1+{\alpha }_{\mathrm{IP}}\left[1-{e}^{\left(-\left[r-{r}_{0}\right]/{r}_{{\rm{H}}}\right)}\right]\right)$ , with α _IP ranging between 1.75 and 2.7, and r _H between 50 and 35 R _⊙ dependent on V _0 . 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.

Keywords

• Solar wind
• Solar coronal mass ejections
• Space weather
• Solar corona
• Heliosphere