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
