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The fast wind, which does not exhibit strong first ionisation potential (FIP) enhancements, could come directly from the photosphere, from small cool coronal loops and open magnetic funnels (Tu et al., 2005; Schwadron and McComas, 2003) at the base of coronal holes or spicules, which also exhibit small FIP enhancements. Remote observations have revealed many cases of macrospicules undergoing reconnection and erupting within coronal holes [(Yamauchi et al., 2005]). Do they contribute to the fast solar wind streams? Polar plumes have also long been suspected to be a significant source of fast solar wind [(Deforest et al., 1997]), as well as polar regions within plumes ("interplume lanes") (Giordano et al., 2000). Micro-streams of plasma originating in the coronal holes may be related to polar plumes [(Neugebauer et al., 1995]), though evidence for this is controversial [(e.g., McComas et al., 1996]).  However, the relation could be difficult to observe with Solar Orbiter since large amplitude Alfvénic fluctuations generate micro-streams signals in the fast stream (Matteini et al., 2013).


In order to address this objective, we need to observe if the fast wind comes from the different above sources. This objective can be split into two different observational strategies:

  • A pure coronal characterization by pointing to the center of a coronal hole and its boundaries, mostly focused on remote sensing observations (closest possible at high latitude).
  • An attempt to observe fast wind in situ and link it to its origin back to the coronal hole. In order to be connected, we have to choose a coronal hole at the west limb, and observe it from a location close to the Sun (at least one perihelion window would be good for the high-resolution observations of plume-interplume FIP variation) and preferably during high-latitude windows. Another possibility would be to observe during a window before perihelion in order to obtain the overall context and then observe during the perihelion for connectivity. Polar coronal holes could also be observed with METIS from the equatorial plane (no need of high-latitude windows, since it can observe the sky above the hole). In this case, however, there is less chance of doing linkage science. Low-latitude coronal holes should also be observed for intermediate speed outflow. Even if this has a lower priority, it should not be neglected since this is the typical solar wind observed at Earth.
  • The minimum of the solar activity cycle is preferable and/or the declining phase for the polar coronal holes. Low-latitude coronal holes can be observed at any phase of the cycle, but the probability to be at the right longitude at perihelion is low.

The needed observations include:

  •  EUI/HRI in Coronal Hole mode with 1 min of cadence, during 1-2 hours and 12 hours at lower cadence.
  • PHI at high resolution at 1 min cadence.
  • SPICE: FIP and velocity maps. A combination of composition, dynamics and a 30’’-wide movie. 3.2 hours for SPICE, 3 times per day.
  • In situ instruments: normal mode for the connection and burst modes for more details. MAG Burst mode is required for ion cyclotron wave identification in order to distinguish from different acceleration and heating mechanisms as well as small scale changes. Recent Helios data analysis shows velocity changes with jet-like features probably well below the 40s cadence. We would, therefore, need 3D distributions at 1s scales in order to determine the properties inside and outside these features.

Other remarks:

  • Additional observations from near-Earth assets are desirable but not required.
  • EMC Quiet is required for linkage science. Noisy periods can be tolerated during the RS observations, but EMC quiet is required approximately 12 hours later (depending on which distance Solar Orbiter is).
  • EPD, STIX are not required for this objective.


The possible remote sensing targets should include wide regions of well extended coronal holes, as well as smaller regions for focusing on the following:

  • Small cool coronal loops. 
  • Open magnetic funnels at the base of coronal holes (Tu et al., 2005; Schwadron and McComas, 2003).
  • Spicules with short lifetimes, fast motions, and hot plasma components. Macrospicules reconnection and eruption within coronal holes (Yamauchi et al., 2005).
  • Polar plumes (Deforest et al., 1997).
  • Polar regions within plumes (“interplume planes”) (Giordano et al. 2000).
  • Diverging polar regions of the extended corona.
  • Coronal hole boundaries.

The SOOP that addresses this objective is L_SMALL_HRES_HCAD_Fast_-Wind, which also addresses objective