Abstract

Establishing the sources of the fast and slow solar wind is important for understanding their drivers and their subsequent interaction in interplanetary space. Although coronal holes continue to be viewed as the main source of the fast solar wind, there is recent evidence that the quiet Sun provi... Show moreEstablishing the sources of the fast and slow solar wind is important for understanding their drivers and their subsequent interaction in interplanetary space. Although coronal holes continue to be viewed as the main source of the fast solar wind, there is recent evidence that the quiet Sun provides other spatially concentrated sources. To identify the underlying physical characteristics of the outflow from coronal holes, solar disk observations from the Solar and Heliospheric Observatory (SOHO) are considered. These observations encompass photospheric line-of-sight magnetic field measurements from the Michelson Doppler Imager (MDI), Fe X 171 Å passband imaging from the Extreme-ultraviolet Imaging Telescope (EIT), and Ne VIII 770 Å spectral observations with outflows inferred from their corresponding Doppler blueshifts, at solar minimum and maximum and at different latitudes, from the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument. The sharp variations of outflows within the SUMER field of view, referred to as velocity gradients, are introduced as a new diagnostic. It is shown that, in general, coronal holes are indistinguishable from the quiet Sun, whether in their outflows or their gradients. Surprisingly, however, when enhanced unbalanced magnetic flux from active regions extends into neighboring coronal holes, both outflows and their gradients become significantly enhanced within the coronal holes and along their boundaries. The same effect is observed in the quiet Sun, albeit to a lesser extent. These findings point to the possibility that active regions can lead to enhanced plasma outflows in neighboring coronal holes. Show less