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 plumes (Neugebauer et al., 1997; Poletto, 2015). The distinction is difficult, because jets and plumes are themselves related (Raouafi et al., 2016). Simulations support the plume–microstream connection (Velli et al., 2011), and the jet–microstream/Alfvén wave connection (Karpen et al., 2017).
Horbury et al. (2018) found even smaller structures in Helios data at 0.3 AU, lasting tens of seconds to minutes, and reaching up to 1000 km/s. They are Alfvénic in nature, exhibiting large magnetic field deflections. These structures may form during jets from the chromosphere and/or low corona. Borovsky (2016) showed hours‐long structures in the fast solar wind with large variations in number density, temperature, magnetic field strength, composition, electron strahl, and proton specific entropy, and also argue these mesoscale structures map to features in the solar corona. In contrast to the dynamic sources described above, Borovsky (2016) argues that these mesoscale structures are the result of relatively time stationary coronal flux tubes.
Pressure balances structures where the magnetic pressure balances the thermal pressure (Burlaga & Ogilvie, 1970) are also prevalent in the fast solar wind (Bavassano et al., 2004; Reisenfeld et al., 1999; Thieme et al., 1990). Unlike microstreams, McComas et al. (1996) showed that PBSs were not distinguishable from the rest of the fast solar wind, and may not be relics of transient coronal structure.
Mesoscale structures in the solar wind are an important part of the solar terrestrial connection, because they can drive magnetospheric dynamics. Often, mesoscale structures are cyclic, identified as discrete frequencies in plasma density (Di Matteo & Villante, 2017; Sanchez‐Diaz et al., 2017; Viall et al., 2008) and dynamic pressure (Kepko & Spence, 2003; Kepko et al., 2002) Sometimes the structures exhibit periodicities in all plasma components (Stephenson & Walker, 2002). They are observed to directly drive global oscillations of the magnetosphere at the exact same frequencies (Kepko & Spence, 2003; Kepko et al., 2002; Viall et al., 2009; Villante et al., 2013), even by ground‐based magnetometer on Earth (Villante et al., 2016) in radar oscillations in the high latitude ionosphere (Fenrich & Waters, 2008), polar UV imaging data (Liou et al., 2008), and even the equatorial ionosphere (Dyrud et al., 2008). MHD simulations have confirmed that cyclic solar wind dynamic pressure structures directly drive magnetospheric oscillations, and locations of field line resonance will even amplify the waves (Claudepierre et al., 2010; Hartinger et al., 2014).