Description of the objective:
Study of the anti-correlation between the speed of the solar wind and the expansion rate of the magnetic field. Wang and Sheeley (1990), Arge and Pizzo (2000), and others observe an anti-correlation between solar wind speed and the so-called expansion factor of a flux tube near the Sun calculated with a Potential-Field-Source-Surface (PFSS) model. Near the edges of coronal holes, the flux-tube expansion factor is larger and generates a slower solar wind (Antonucci et al., 2006). The physical connection between flux-tube expansion and solar wind speed is unclear. Larger flux-tube expansion increases the downward electron and proton heat flux (e.g., Cranmer et al., 2007) and creates an energy sink that slows the solar wind (Schwadron and McComas, 2003). However, detailed solar models (e.g., Lie-Svendsen et al., 2002) have difficulty accounting for the solar wind properties under these circumstances. If slow solar wind has a coronal hole origin, we expect a smooth transition across the coronal hole boundary. Boundary layers outside the coronal hole (Schwadron et al., 2005) need to be correlated with their remotely observed source structures. Furthermore, an “anomalous” slow solar wind is observed to be originating from the boundaries of coronal holes with the same level of Alfvénic fluctuations as in the nearby fast wind, but of considerable smaller amplitude (Antonucci et al., 2005; D’Amicis et al., 2015).
- This objective can be addressed with two combined observing strategies:
- At first, several (2-3) days of disc centre pointing for overall coronal hole configuration during a perihelion window that would scan over many latitudes. PHI and EUI/FSI will give us a ~3D view of the coronal hole edges and METIS can observe close to the Sun, while SPICE will do composition mapping scanning.
- At a second phase, the coronal hole boundary will be mapped with SPICE mosaics during 1 day (with METIS being off). The SPICE raster area should be optimized to make sure that both open and closed field boundary is captured. The choice of the lines has to be optimized depending on the type of target.
- An alternative strategy would be to observe with SoloHI and METIS during a high-latitude window, before trying to do the linkage science at the following perihelion window.
- The connectivity being of main importance for the solar wind origin objectives, we will need modeling in order to choose the right pointing.
- Observations during two successive windows (high-latitude – especially for high-latitude coronal holes - and the following perihelion) are preferred.
- It is easier to perform these observations during the minimum or declining phase of solar activity, when we have a strong magnetic dipole. Even though it will not be easy to choose where to point during the maximum, it is worth planning it as well at least once during the maximum.
- Earth context observations are important for better constraining the models and improving pointing decisions.
- Parker Solar Probe joint observations at a radial alignment would be beneficial, especially if PSP happens to be at its perihelion.
The SOOP that addresses this objective is L_BOTH_HRES_LCAD_CH-Boundary-Expansion, which was specifically created for this objective and does not currently cover any other sub-objectives. All other slow wind origin objectives are addressed by the similar SOOP L_SMALL_HRES_HCAD_Slow-Wind-Connection.
The figure shows some of the observed properties of fast wind from coronal holes and slow wind that may emanate from coronal hole edges, from loops beyond the coronal or the helmet streamer. The cut-out shows the complex magnetic structure modelled at the base of a fast wind flux tube that rapidly expands out of the chromosphere and into the corona. Understanding the dynamics of the solar magnetic atmosphere, and its signatures in the measured solar wind holds the key to understanding the sources of all solar wind (adapted from Tu et al., 2005 and Schwadron and McComas, 2003).
(Wang and Sheeley, 1990)
(Arge and Pizzo, 2000)
Schwadron et al. 2005: Coronal Hole Boundary Layer model
Slow wind general considerations (D'Amicis et al., 2015):
2 types of slow wind:
1) "standard": from coronal streamers or active regions, non-Alfvénic fluctuations
2) "anomalous": from the boundary of coronal holes, same level of Alfvénic fluctuations as in the nearby fast wind, but of considerably smaller amplitude.
The parameter that better discriminates between fast and slow wind source regions is the O7+/O6+ ratio:
High values are found within the slow wind from coronal streamers while low values are found in the fast wind coming from coronal holes (as already found by Geiss et al. 1995).
In this plot we clearly observe that higher values of this ratio characterize event 2 while events 1 and 3 have similar characteristics (also considering the larger fluctuation ampli- tudes in velocity, for example). Moreover, although during events 2 and 3 the speed value is roughly constant, the amount of O7+/O6+ is different. For this reason we identify event 2 as “slow wind 1st type” while event 3 as “slow wind 2nd type.”
The different values of the O7+/O6+ ratio is an indication that we are observing plasma coming from different source regions. Mapping back onto a synoptic map.
Our finding supports results by Antonucci et al. (2005) who, using UVCS observations, found evidence for the existence of two kinds of slow solar wind typically originating from different source regions, coronal streamers, and coronal holes’ boundaries, respectively.
[ ... ]
In conclusion, we can unambiguously identify the slow wind of 1st type as the “classical” slow wind coming from coronal streamers while the 2nd type is the “anomalous” slow wind coming from the boundaries between closed and open magnetic field regions, which have characteristics more similar to fast wind than classical slow wind.
Wang, Y.-M. and Sheeley, N.R., 1990, Solar wind speed and coronal flux-tube expansion, ApJ, 355, 726.
Arge, C.N., and Pizzo, V.J., 2000, Improvement in the prediction of solar wind conditions using near-real time solar magnetic field updates, JGR, 105, A5.
Schwadron N.A et al., 2005, Solar wind from the coronal hole boundaries, JGR, 110, A04104.]
D'Amicis R. and Bruno R., 2015, On the origin of highly Alfvénic slow solar wind, ApJ, 805, 84.