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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 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).

The variations in the magnetic field of mesoscale structures in the ambient solar wind are important for an understanding of both their creation and their effect on the heliosphere and, in particular, on energetic particles. One fundamental scale size in the magnetic field on mesoscales is the correlation scale length perpendicular to the mean field. Crooker et al. (1982) measured this quantity at 1 AU and found it to have a characteristic size of 130 Mm during low magnetic field variance, and a characteristic length scale of 320 Mm during high variance. Collier, Slavin, and Lepping (2000) extended this analysis of magnetic field characteristic size scales at 1 AU and found that there is an additional scale size—the radius of curvature of the magnetic field—equal to 640 Mm.

The magnetic field changes that produce mesoscale length scales are either in the form of flux ropes (e.g., Feng et al., 2007; Moldwin et al., 1995; Moldwin et al., 2000) or flux tubes with magnetic field discontinuities at their boundaries (e.g., Bruno et al., 2001; Thieme et al., 1989). Magnetic flux ropes have a characteristic core magnetic field and magnetic rotation, and are often consistent with a force‐free equilibrium (Burlaga, 1988; H. Goldstein, 1983). Flux ropes in large‐scale ICMEs are a common, well‐known example. There is a population of flux ropes lasting tens of minutes to hours, which are not associated with ICMEs, and instead occur with the ambient solar wind. Unlike ICMEs, the mesoscale flux ropes are not observed to be expanding due to overpressure, they do not exhibit ICME temperature depletion, they occur more often during solar minimum, and they occur in conjunction with the heliospheric current sheet. These observations make it clear that they are a slow wind feature, and not a continuation of a distribution of ICMEs to smaller scales (Cartwright & Moldwin, 2008, 2010b; Crooker et al., 1996; Moldwin et al., 1995, 2000).

Though flux ropes occur often in the slow solar wind, statistical measurements indicate that they do not make up a large portion, with occurrence rates of only six per year. There is considerable uncertainty about this rate, however, as different identification criteria can lead to vastly different event lists, and compressive Alfvén waves can be mistaken for flux ropes (Cartwright & Moldwin, 2008, 2010b; Feng et al., 2007; Higginson & Lynch, 2018).

Flux tube structures take the form of a discontinuous, planar change in the magnetic field and plasma parameters at the boundaries of the mesoscale structures. These are typically described as tangential and rotational discontinuities (Hudson, 1970), and are abundant throughout the solar wind, occurring even in the slow wind away from the HCS and in the fast wind (Neugebauer et al., 1984).

Mesoscale structures ahead of energetic particle events can influence energetic particle transport. STEREO observations of energetic particles have shown that variations appear to be linked to flux tubes with diameters of 6000 Mm (von Rosenvinge et al., 2009). Furthermore, even shorter fluctuations in the particle properties suggest that these flux tubes have sizes that extend down to 500 Mm. These intensity changes are similar to particle “dropouts” (Chollet et al., 2007; Mazur et al., 2000). Dropouts have been attributed to changes in the magnetic connectivity between the particle detector and the particle accelerator at the Sun due to flux tubes (at scales of 10–100 Mm) that meander or mix through the interplanetary medium (Chollet & Giacalone, 2011; Giacalone et al., 2000). Particle dropouts and heat flux loss often indicate changes within the underlying magnetic topology occurring with HCS crossings or local kinks in magnetic flux tubes (Borovsky, 2008; Crooker et al., 1982). We note that the majority of sharp changes in energetic particle populations during ambient solar wind conditions occur with changes in plasma and magnetic fields at tangential discontinuities (Neugebauer & Giacalone, 2015).

Whether the smaller flux tubes and flux ropes are created at the Sun and advected with the solar wind, or whether they are created in transit is still an open question. For example, Cartwright and Moldwin (2008) argued that flux ropes are generated by reconnection across the HCS in transit, and indeed plenty of reconnection is known to occur locally in the solar wind (Gosling, 2012), though not in direct association with a mesoscale flux rope. Magnetic reconnection in the solar wind is discussed in Section 1.3.5. On the other hand, Crooker et al. (1996) argued the flux ropes were formed at the Sun as the solar wind is created. Measurements of the flux ropes as a function of radial distance from the Sun shows that there are fewer at greater distances, and that they are larger with distance (Cartwright & Moldwin, 2010b), indicating merging or expansion rather than local creation.

Figure 1.12 Occurrence distribution of flux tube sizes mapped to the solar surface is plotted as the black curve. Also plotted are distributions of solar granules and supergranules, and supergranule sizes obtained with high‐resolution measurements are indicated with horizontal bars.

(Source: Taken with permission from Borovsky, 2008. © 2008, John Wiley and Sons.)

One idea is that the solar wind is filled with coherent “fossil structure” flux tubes from the solar corona that advect with the solar wind (Bruno et al., 2001; Marsch & Tu, 1993; Thieme et al., 1989). Borovsky (2008) and Collier et al. (2000) both used geometric arguments (e.g., solar rotation rate, radial expansion of the solar wind) to relate the mesoscale flux tube structure observed at 1 AU to granules. A comparison of the occurrence distribution of flux tube sizes between in situ and observed solar features is shown in Figure 1.12. Neugebauer and Giacalone (2015) argued that tangential discontinuities were preexisting flux tube boundaries formed at the Sun based on the corresponding plasma parameters, and were not consistent with in‐transit turbulence. However, it is unclear if a direct connection with granules in the photosphere is possible, and simulations show that the boundaries of flux tubes created from granulation would not survive to 1 AU (Cranmer et al., 2013).

Some of the mesoscale structures, such as the characteristic magnetic field correlation lengths, may be related to turbulence. In general, it is known that the spectra of fluctuations in the solar wind have an inertial range at smaller scales, and at larger scales, follow a 1/f form that is a condition of low‐frequency Alfvénic turbulence. The origin of the 1/f turbulent fluctuations is still debated but may itself originate from the corona (e.g., Matthaeus & Goldstein, 1986; Nicol et al., 2009) and footpoint stirring in the solar photosphere, with the inertial range being the transit turbulent decay. When compositional changes are associated with discontinuities and flux ropes, that is uncontroversial evidence that those structures were formed at the Sun (Borovsky, 2012; Borovsky & Denton, 2016; Kepko et al., 2016; Neugebauer, 2012; Viall et al., 2009). Structures without compositional change are ambiguous and could be formed either at the Sun or in transit (Owens et al., 2011).

We should point out the impact of mesoscale structures on global heliospheric structures. The variable coronal outflows imaged by coronagraphs and discussed in Section 1.2.2 indeed have repercussions on the structure of the solar wind. The standard picture of a CIR (Burlaga & Barouch, 1976; Lee, 2000; Pizzo, 1982) considers them as recurrent, uniform, and stable structures. In situ measurements made during the maximum phase of cycle 23 have revealed that interaction regions do not necessarily recur, a result of coronal hole reconfigurations and of the single‐point nature of in situ measurements (L. Jian et al., 2006). Heliospheric imaging further reveals that individual CIRs or SIRs are by no means smooth compression regions distributed along a spiral but exhibit strong variability along the interaction region in response to several dynamic processes occurring at the Sun (Rouillard, Davies, et al., 2010). First, coronal holes can rapidly form and disappear during a solar rotation period, which leads to the appearance and disappearance of interaction regions in the interplanetary medium. Second, the density variations induced by the release of the small and large‐scale streamer events discussed in the previous paragraph modifies the global structure of CIRs. In this regard, heliospheric imagery has provided new insights into the global structure of CIRs. It has been shown that in the inner heliosphere, CIRs are made up of compressed density structures (Rouillard, Lavraud, et al., 2010). The presence of strong pressure variations along the CIR surface suggests that shock formation will also be nonuniform with heliospheric location and time. Strong pressure enhancements would develop due to the compression by high‐speed streams of small‐scale transients. The shape of the CIR shock may therefore become a highly irregular surface beyond 1 AU. This could have implications for the formation of MIRs by the interaction of CIRs toward the outer regions of the heliosphere.