Sources of the Solar Wind

The solar wind originates from different regions and processes on the Sun. The fast solar wind emerges from coronal holes, which are cool, dark areas with open magnetic field lines. The slow solar wind is more variable and is thought to originate from a variety of sources, including the boundaries of coronal holes, active regions, and streamers. Understanding the sources of the solar wind is crucial for predicting space weather and its effects on Earth.

A figure illustrating the relationship between the helium abundance, solar wind turbulent fluctuations, and the solar wind's birth at the Sun. — A schematic for the Sun-solar wind connection. The coronal plasma becomes the solar wind when it achieves supersonic speeds at the sonic critical surface ( $r_{c}$ ). The helium abundance ( $A_{He}$ ) is set low in the solar atmosphere below $r_{c}$ . The Alfvénicity or cross helicity ( $∣ σ_{c} ∣$ ) reaches a maximum between $r_{c}$ and the Alfvén surface ( $r_{A}$ ), indicated by the growing S-curve. Above $r_{A}$ , $∣ σ_{c} ∣$ decays as Alfvénic fluctuations continue accelerating the solar wind (growing blue arrows). Both $A_{He}$ and $∣ σ_{c} ∣$ depend on the local magnetic field topology (orange and red lines). Together, they provide a snapshot of the processes below ( $A_{He}$ ) and above ( $∣ σ_{c} ∣$ ) the interface where the coronal plasma is accelerated into the solar wind. From Alterman & D'Amicis (2025), The Astrophysical Journal Letters.