schematic is a view from solar north, and the right-hand schematic illustrates the 3D interaction between flows when the stream interface is inclined relative to the north-south direction."/>
Figure 1.10 Two schematics illustrating CIRs where the fast solar wind catches up and compresses the slow solar wind. The left‐hand schematic is a view from solar north, and the right‐hand schematic illustrates the 3D interaction between flows when the stream interface is inclined relative to the north‐south direction.
(Source: Figure taken from Owens & Forsyth, 2013. © 2013, Springer Nature.)
The interaction of the fast and slow solar winds, particularly in SIRs and CIRs, reduces the range of velocities of the solar wind. The slow solar wind has typical slow speeds between 300 km/s and 450 km/s. Between 0.3 and 0.4 AU, where interaction regions have not yet formed to accelerate the slowest plasma, the slow solar wind exhibits speeds less than 300 km/s nearly 10% of the time. This very slow solar wind can have speeds as low as 200 km/s inside 0.7 AU, a speed never measured at 1 AU. Wind speeds less than 300 km/s are very seldom measured in situ at 1 AU but have been extensively observed in white‐light images (Rouillard, Davies, et al., 2010). This very slow wind typically has lower temperatures and higher densities than the regular slow solar wind. The properties and the source of this VSSW as well as its solar cycle variability were analyzed by Sanchez‐Diaz et al. (2016).
1.3.4. Mesoscale Structures
There is an abundance of structures in the solar wind that are above the kinetic scales, but well below the global scales of the heliosphere. These so‐called mesoscale structures abound in the solar wind that fills the inner heliosphere and their in situ measurements provide insights on the formation mechanisms of the solar wind.
As we saw in section 1.2.2, evidence that at least some in situ mesoscale density structures originate within the solar corona, as the solar wind forms, is found in the remote imaging of the corona. The images recorded by SoHO and STEREO have provided a tracking of density fluctuations continuously at mesoscales (several 100 Mm) from the Sun to the interplanetary medium, where it is measured in situ. As we describe below, composition, electron strahl, velocity, magnetic field, plasma temperature, and plasma density measured in situ have also been used to begin to piece together a picture where time dynamics, such as reconnection, and spatial structure at the Sun combine to create mesoscale structure in the solar wind. In essence, the solar wind measured in situ is far from homogeneous and is most likely formed that way.
The solar wind measured near the HCS is one region where myriad mesoscale structures are found. Crooker et al. (1996, 1993) showed that the highly structured solar wind measurements associated with the HCS at 1 AU were not simply due to multiple crossings of a single wavy current sheet, but rather the sampling of a HCS comprising intertwined flux ropes. They suggested that some were formed in the solar corona as the result of transient activity. In a statistical survey of mesoscale flux ropes found in the solar wind, Cartwright and Moldwin (2010a) found them to by highly concentrated near the heliospheric current sheet. As previously discussed, STEREO has tracked structures of sufficient density variations from the Sun to the spacecraft making in situ measurements (Rouillard, Davies, et al., 2010; Rouillard, Lavraud, et al., 2010; Rouillard et al., 2011). This unequivocally confirms the association of many of the mesoscale structures as magnetic flux ropes measured at 1 AU at the HCS and “blobs” released from helmet streamers. We now discuss mesoscale structures measured in situ that have not yet been associated with specific coronal features because they could not be tracked from the Sun continuously in remote imaging. For these, only likely associations have been made with coronal structures observed separately near the Sun by using numerical simulations of the solar corona and wind.
As already shown in Figure 1.5, the slow solar wind and the HCS are generally associated with the helmet streamer structure in the solar corona (McComas et al., 1998). Gosling et al. (1981) showed helium abundance variations associated with the crossing of the HCS, confirming that variations associated with the HCS are of solar origin. Kilpua et al. (2009) identified in STEREO in situ data 17 different transient structures at the HCS, which they linked to time dynamics in helmet streamers, 7 of which had counter‐streaming electrons, indicating that the structures were still connected at both ends back to the Sun. Kepko et al. (2016) identified a cyclic train of mesoscale structures around the HCS. They exhibited cyclic compositional changes, confirming a solar source. One of the structures was a flux rope with counter‐streaming electrons, followed by a strahl dropout; the compositional changes indicate that magnetic reconnection in the corona created these structures.
Mesoscale structures are not restricted to the HCS. The slow solar wind includes the HCS, but can be observed as far as 30° away from the HCS (Burlaga et al., 1982), and is more generally associated with the boundary between field lines that are open to the heliosphere (coronal holes) and those that are close in the corona (streamers). Numerical modeling shows that open‐closed boundaries can be a complex web of separatrices (the so‐called S‐web; Antiochos et al., 2011) where reconnection is likely to occur. During solar maximum, these separatrices can form a complex web, mapping to many locations away from the HCS in the heliosphere (Crooker et al., 2012; Crooker et al., 2014). Mesoscale structures in the slow solar wind outside of the HCS are observed in density as seen in Figure 1.11 taken from Viall et al. (2008), but thye are also observed in magnetic field (Borovsky, 2008), and composition (Viall et al., 2009).
Figure 1.11 Solar wind number density data for 15 January 1997. Bottom x‐axis is in radial‐length scale steps, top x‐axis shows the corresponding UT. Tick marks indicate a clear 400 Mm periodicity.
(Source: Taken from Viall et al., 2008. © 2008, John Wiley and Sons.)
It is thought that interchange reconnection could be a source of mesoscale structures perhaps forming at these modeled separatrices (Higginson et al., 2017). One signature expected when interchange reconnection occurs is that the electron strahl—which always flows away from the Sun—is observed to be in the opposite sense expected from the magnetic field direction (Crooker et al., 1996; Crooker et al., 2004; S. Kahler & Lin, 1994; S. W. Kahler et al., 1996), indicating that the magnetic field is locally folded back on itself. Owens et al. (2013) shows these inverted strahl signatures in the slow, dense solar wind at 1 AU associated both with helmet streamers, and with pseudostreamers, also associated with separatrices. Stansby and Horbury (2018) and Di Matteo et al. (2019) argue that signatures of interchange reconnection away from the HCS can be identified in Helios data inside of 1 AU. They identified mesoscale structures using density and showed concurrent temperature signatures, which are retained close to the Sun, strongly suggesting a solar source.
Mesoscale structures also occur in the fast wind. One prominent example is that of microstreams (Neugebauer, 2012; Neugebauer et al., 1995; Neugebauer et al., 1997). Microstreams are observed in the fast, polar solar wind with velocity fluctuations of ±35 km/s, last 6 hr or longer, have higher kinetic temperatures, higher proton flux, and slightly FIP enhanced compared to the rest of the fast solar wind. They are associated with large angle magnetic discontinuities and compositional changes that are consistent with a solar origin. Neugebauer (2012) showed that X‐ray jets and the reconnection that causes them are the most likely sources of microstreams, though they could also be related to polar
