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Description of the objective:


  • Outward propagating stochastic fluctuations, Alfvénic modes within the fast streams (Belcher and Davis, 1971; Tu and Marsch, 1995).
    • Excitement by magnetic activity and reconnection in the chromospheric network.
    • Large-scale torsional Alfvén waves generated from magnetic restructuring related to interchange reconnection (Lynch et al., 2014).
    • Alfvén waves in the low corona caused by leakage of wave power from chromospheric oscillations (Tomczyk et al., 2007).
    • Presence or not of large-amplitude Alfvénic fluctuations with ⎟B⎜=const (Matteini et al., 2014) in the inner heliosphere, implication for their stability and their role as a source for turbulence.

  • Identification of the drivers at the "outer scale" (large scale) end of the turbulent cascade.

  • Inward propagating Alfvén waves (already active at 0.3 AU as observed by Helios):
    • Local production by velocity shears (Roberts et al., 1992).
    • Parametric decay mechanisms (Malara et al., 2000).
    • Generation of inward component by expansion (relevant for polar wind where shear effects are negligible) (Velli et al., 1989; Verdini et al., 2009).
    • Nonlinear generation by compressive MHD cascade (Marsch & Mangeney, 1987).
    • Carefully distinguish real local production of inward modes from compressive events masking a generation process (Bavassano and Bruno, 1989).

  • How does the behavior of MHD waves observed remotely in the core of coronal holes and at the CH/QS boundaries match what is observed in situ in the solar wind that emanates from these regions?

 

Remarks:


  • Note that the McIntosh paper (McIntosh et al., Nature, 475, 477) observed waves in structures seen above the limb (spicules) and in active region loops (on-disc). In coronal holes seen on the disc, it will be probably easier to observe oscillating structures in Lyman alpha (HRI) than in the other passbands. In Ly-alpha, there are dark, fibril-like structures that are seen with a better contrast with respect to the background (optical thickness), while in 304 or 174, the structures are optically thin and the contrast may be less favorable. But using HRI and SPICE will probably require off-pointing, so we should implement two different observational strategies: one with and one without off-pointing.
  • For the objective of linking with in-situ observations, the "minimal" option should be without SPICE or EUI/HRI, and no off-pointing required. EUI/FSI 174 and 304 at 30-s cadence during 1-2 hours minimum (there is currently no program fitting this cadence with FSI). Alternating both filters would probably require filter wheel motions incompatible with EMC requirements. If so, FSI 304 may be preferred. Or 1 hour of FSI 304 followed by 1 hour of FSI 174. To save telemetry, it is possible to store only 1/4 of the FOV as long as it covers the core and one boundary of the CH. The 30-s cadence is sufficient to observe waves as in McIntosh et al., Nature, 475, 477. But the spatial resolution of FSI may not be sufficient to resolve fine structures and their motions, especially for observations on the disc, which are necessary as the objective is to measure the wind emanating from the RS observations.
  • "Optimal" option: with SPICE and HRI; off-pointing and mosaic may be necessary to observe the source region of the wind that will be measured at the spacecraft within a few days from the RS observations. The observed region must cover both the core of the CH and at least one of the CH/QS boundaries.
  • HRI 174 and HRI Ly-alpha at 30-s cadence during 1-2 hours (EUI Coronal hole mode (C) can fit). Observations in coronal holes require deep exposures (at least in HRI 174), which requires minimal jitter.
  • FSI 304 at 30-s cadence.
  • SPICE rasters for measurement of the line widths (to determine the non-thermal motions, supposed to correspond to LOS-integrated effect of Alfvén waves), for at least transition region and coronal lines (optionally: also lines for abundance measurements, for which the total intensity may be enough and not the full profile). Cadence is not an issue as we measure the integrated effect in the line width (for observation of the wave motions in Doppler shifts, sit-and-stare (i.e. no slit scan) may be preferable).