Expansion of High-speed Solar Wind Streams from Coronal Holes through the Inner Heliosphere

Abstract

Coronal holes (CHs) are regions of open magnetic flux that are the source of high-speed solar wind (HSSW) streams. To date, it is not clear which aspects of CHs exert the most influence on the properties of the solar wind as it expands through the Heliosphere. Here, we study the relationship between CH properties extracted from Atmospheric Imaging Assembly images using the Coronal Hole Identification via Multi-thermal Emission Recognition Algorithm and HSSW measurements from Advanced Composition Explorer at L1. For CH longitudinal widths Δ θ CH  < 67°, the peak solar wind velocity ( v max ) is found to scale as v max  ≈ 330.8 + 5.7 Δ θ CH km s −1 . For larger longitudinal widths (Δ θ CH  > 67°), v max is found to tend to a constant value (∼710 km s −1 ). Furthermore, we find that the duration of HSSW streams (Δ t ) are directly related to the longitudinal width of CHs (Δ t SW  ≈ 0.09Δ θ CH ) and that their longitudinal expansion factor is f SW  ≈ 1.2 ± 0.1. We also derive an expression for the CH flux-tube expansion factor, f FT , which varies as f SW  ≳  f FT  ≳ 0.8. These results enable us to estimate the peak speeds and durations of HSSW streams at L1 using the properties of CHs identified in the solar corona.