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

Abstract

Coronal holes (CHs) are regions of open magnetic flux which are the source of high speed solar wind (HSSW) streams. To date, it is not clear which aspects of CHs are of 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 AIA (Atmospheric Imaging Assembly) images using CHIMERA (Coronal Hole Identification via Multi-thermal Emission Recognition Algorithm) and HSSW measurements from ACE (Advanced Composition Explorer) at L1. For CH longitudinal widths $\Delta {\theta }_{CH}<$67${}^{\circ }$, the peak SW velocity ($v_{max}$) is found to scale as $v_{max}\approx 330.8+5.7\Delta {\theta }_{CH}$~km~s${}^{-1}$. For larger longitudinal widths ($\Delta {\theta }_{CH}>$67${}^{\circ }$), $v_{max}$ is found to tend to a constant value ($\sim$710~km~s${}^{-1}$). Furthermore, we find that the duration of HSSW streams ($\Delta t$) are directly related to the longitudinal width of CHs ($\Delta t_{SW}$~$\approx$~0.09$\Delta {\theta }_{CH}$) and that their longitudinal expansion factor is $f_{SW}\approx 1.2\pm 0.1$. We also derive an expression for the coronal hole flux-tube expansion factor, $f_{FT}$, which varies as $f_{SW}\gtrsim f_{FT}\gtrsim 0.8$. 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.

arXiv comment8 pages, 4 figures, Accepted for publication in The Astrophysical Journal Letters (APJL) on 25th November 2018

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• On the Dependency between the Peak Velocity of High-speed Solar Wind Streams near Earth and the Area of Their Solar Source Coronal Holes
• Influence of coronal hole morphology on the solar wind speed at Earth
• On the short term stability and tilting motion of a well-observed low-latitude solar coronal hole
• Space weather: the solar perspective -- an update to Schwenn (2006)