APRENDIZAJE DE LOS NIÑOS DEL PREESCOLAR

The current consensus regarding the formation of suprathermal solar wind electrons is that they likely originate in the solar corona (e.g. Vi˜nas et al., 2000; Stverak et al., 2008; Che and Goldstein, 2014). However, there are a number of different mechanisms that can be invoked to explain the generation of election populations and their existence within the solar wind. Hence, a firm agreement on their origin has yet to be reached.

With regard to strahl, it is generally accepted that this population is formed by electrons with sufficient energy to escape the electrostatic potential of the Sun (Pierrard et al., 2001). Once beyond the potential well, the electron collision fre- quency is reduced to the extent that both electron energy and magnetic moment are conserved (Hammond et al., 1996). Magnetic field strength decreases with distance from the Sun. Hence, as the electrons travel outwards, they experience strong adia-

2.2. Solar Wind Electrons 89 batic focussing, resulting in the formation of a highly field-aligned electron popula- tion (Owens and Forsyth, 2013). In fact, theoretical investigation has demonstrated that, for typical coronal hole conditions, this effect is strong enough to produce a clear strahl signature in the electron distribution function within as little as 10 solar radii (Smith et al., 2012).

With regard to suprathermal electrons in general, it has been found that the suprathermal tails of electron velocity distribution functions can be explained via the velocity filtration model. In this model a pre-existing population of suprather- mals in the low corona undergoes velocity filtration in gravitational and electrostatic fields resulting in temperature that increases with height through the solar atmo- sphere without invoking any local heating source (Scudder, 1992). The velocity filtration model predicts the evolution of the electron velocity distribution function at higher altitudes in the solar wind to form core, halo and strahl like populations, similar to those observed within the solar wind (Vi˜nas et al., 2000). However, this model does not explain how the suprathermals are generated or how they are main- tained within the solar wind.

Proposed models for the formation and evolution of electrons within the corona thus frequently invoke a combination of expansion effects, Coulomb collisions and/or wave-particle interactions to explain the formation of a thermal core and a beam-like suprathermal tail (e.g., Pierrard et al., 2001; Vocks et al., 2008; Smith et al., 2012; Landi et al., 2012). The presence of the halo population is then often explained by scattering of suprathermal electrons via further wave-particle interac- tions within the solar wind or by global reflection in the heliosphere (e.g. Saito and Gary, 2007; Pagel et al., 2007; Smith et al., 2012; Landi et al., 2012; Pavan et al., 2013). However, it has also been shown that it is possible for the halo population to form in the inner corona before subsequent adiabatic focussing results in the for- mation of a strahl beam (Che and Goldstein, 2014). This process was demonstrated using solar nanoflares as a source for electron acceleration, producing energetic electron beams, which in turn trigger instabilities that generate kinetic Alfv´en and whistler wave turbulence and produce a halo population via scattering (Che and Goldstein, 2014). Moreover, it was found that Coloumb collisions were insufficient to thermalise the distribution before the plasma was advected into the solar wind

2.2. Solar Wind Electrons 90 and thus that the halo feature may be preserved, as long as some form of scatter- ing is present at larger radial distances to counter the effect of adiabatic focussing. It should be noted that all electron evolution models use specific conditions which can be very different. For example the strong magnetic strength gradient presumed for coronal hole wind (as in Smith et al. (2012)) is very unlike the approximations appropriate for the very slow solar wind observed near sector boundaries, where the mean radial magnetic field is near zero (as in Che and Goldstein (2014)).

The solar wind is a weakly collisional plasma. At 1 AU the mean free path is comparable with the typical length scales of the system (Stverak et al., 2008) and electrons should experience negligible Coulomb collisions (e.g. Vocks et al., 2005; Stverak et al., 2008). The magnetic field strength also continues to decrease with distance from the Sun. It thus follows that, in the absence of other influences, the strahl beam should continue to narrow with heliocentric distance, becoming highly collimated within ∼ 0.5 AU (Owens et al., 2008), with a beam width < 1◦ by ∼ 1 AU (Anderson et al., 2012). Nevertheless, strahl beams observed at 1 AU have pitch angle widths that are often significantly larger than predicted to be due purely to expansion effects and are frequently > 20◦ (e.g., Owens et al., 2008; Anderson et al., 2012). Hence, adiabatic focussing cannot be the sole effect experienced by the suprathermal electrons in the solar wind.

The presence of this broader strahl and the presence of halo electrons at all pitch-angles, implies that suprathermal electrons must be subject to some form of scattering process, or processes, as they travel outwards from the Sun. The negli- gible collisions experienced by suprathermal electrons suggests that these mecha- nisms must be some form of wave-particle interaction. This implication is supported by observations of average strahl pitch angle width that continues to increase with heliospheric radial distance beyond 1 AU (Hammond et al., 1996), see Figure 2.14. In addition, it has also been observed that the fractional density of strahl electrons relative to total electron density decreases with heliospheric radial distance, while that of the halo electrons increases (e.g., Maksimovic et al., 2005; Stverak et al., 2009, see also Figure 2.13). This finding implies that not only are strahl electrons indeed subject to some form of in-transit scattering process but also that, eventually, they are likely scattered to form part of the quasi-isotropic halo population. How-

2.2. Solar Wind Electrons 91

(a) (b)

Figure 2.13: Radial evolution of the relative densities of thermal and suprathermal electron populations for (a) slow and (b) fast solar wind observations (Stverak et al., 2009). In both plots, the symbols represents the mean value for the core (squares), halo (diamonds), strahl (circles) and the sum of strahl and halo (stars). Stverak et al. (2009) notes that the sum of suprathermal populations appears to be more variable in the fast solar wind.

ever, it should also be noted that theoretical investigations have demonstrated that there are conditions under which the opposite may be true and strahl is generated by halo electrons. For example, it has been shown that a broad strahl-like feature could be produced by pitch-angle scattering in the solar wind if the halo population has a large enough drift relative to the core (Seough et al., 2015).

Thus, the origins of the both the suprathermal field-aligned strahl and the quasi- isotropic halo remain unclear. In order to better understand the coronal origins of these suprathermal electrons, it is necessary to determine what processes affect the solar wind electrons in-transit.