ISSI webinar by Marco Velli: Origins and Structure of the Heliosphere: Novel Discoveries and Insights from Parker Solar Probe and Solar Orbiter

Abstract: It has been established since the early observations by Helios (and later confirmed by Ulysses and SOHO) that the sources of fast solar wind streams at solar minimum are polar coronal holes. In contrast, slower solar wind streams have contributions from different sources. The larger than expected filling factor of slow solar wind has been attributed, in addition to flows surrounding the Heliospheric current sheet, to flows coming from coronal hole boundaries, i.e., regions with large expansion factors, or from regions where the mapping of the magnetic field from the photosphere into the heliosphere is complex, as identified for example by the squashing factor. This is known as the S-Web. The ubiquity of Alfvénic fluctuations in the inner Heliosphere, and the frequent observation of slow Alfvénic solar wind by Parker Solar Probe as well as Solar Orbiter, previously observed relatively rarely in Helios and Wind data, provide evidence for a picture of solar wind origins that incorporates both the expansion factor and S-web paradigms: both coronal holes with large expansion and S-Web regions act as slow sources of the solar wind, with the difference that highly expanding coronal holes provide Alfvénic slow streams, while the S-web wind is unlikely to exhibit strong Alfvénic correlations. Over the solar cycle the distribution of solar wind flows changes, and we will highlight how observations from Parker Solar Probe and Solar Orbiter have opened new perspectives on how the solar magnetic field is advected into the Heliosphere.