With the advent of the Parker Solar Probe (PSP) we are able to directly measure properties of the solar wind closer to the Sun than ever before (Fox et al. 2016). One of the early highlight discoveries of PSP is the omnipresence of strong local deflections of the magnetic field in the solar wind, mostly referred to as switchbacks (Kasper et al. 2019; Horbury et al. 2020). These structures, also called folds, jets, or spikes, are not necessarily full reversals of the local magnetic field, unlike some names suggest. In fact, the measured distribution of deflections resembles a power law, with most of the deflections at small angles relative to the Parker spiral (Dudok de Wit et al. 2020). There is evidence that switchbacks are localized kinks in the magnetic field and not polarity reversals or closed loops (McManus et al. 2020; Whittlesey et al. 2020). However, switchbacks are not a new finding. They have already been observed for several decades, by, e.g., Helios (e.g., Horbury et al. 2018), out to 1.3 au by Ulysses (e.g., Balogh et al. 1999; Neugebauer & Goldstein 2013). The novelty in the PSP observations is their sharpness and omnipresence (among other features; see Dudok de Wit et al. 2020), indicating that switchbacks are a more frequent feature closer to the Sun. This observation lends the possibility that switchbacks may originate lower in the solar atmosphere. The formation mechanism(s) of switchbacks, and whether they represent large-amplitude Alfvén waves or structures advected by the solar wind, is as of yet not known. Several explanations about their origin have been put forth. Among these theories, interchange reconnection is the most focused upon (Yamauchi et al. 2004; Fisk 2005; Fisk & Kasper 2020). In this scenario, switchbacks are generated in the solar corona, and form at the reconnection sites between open and closed magnetic fluxes. In some studies, it is argued that the interchange reconnection results in magnetic flux ropes, which are ejected by the reconnection outflow, and are advected by the wind (Drake et al. 2020). In other studies, reconnection is thought to generate either Alfvénic (He et al. 2020) or fast magnetosonic (Zank et al. 2020) wave pulses. Alternatively, as the distribution of deflections appears to be featureless and monotone in switchbacks (Dudok de Wit et al. 2020), they may not be a distinct feature but a manifestation of the ensuing turbulent dynamics in the solar wind (Squire et al. 2020). Thus, it is still not clear whether switchbacks originate in the lower solar atmosphere, or represent dynamic features of solar wind turbulence. Additionally, it is not clear whether they are wavelike perturbations, propagating at the Alfvén or some other characteristic speed, or structures that are advected by the wind, such as flux ropes. However, some observations offer good constraints on the nature of switchbacks. Given that switchbacks are characterized by a strong Alfvénic correlation of their velocity and magnetic field perturbations, a nearly constant magnetic pressure, and velocity enhancements along the propagation direction, an interpretation in terms of propagating nonlinear Alfvén waves is a plausible scenario (Matteini et al. 2014). Alternatively, their localization in the perpendicular direction and the kink-like geometry may indicate their kink fast magnetoacoustic nature (e.g., Van Doorsselaere et al. 2008). An additional option is the association of the switchbacks with kink solitons, which keep their shape because of the balance between nonlinear and dispersive effects (see, e.g., Ruderman et al. 2010), i.e., a stationary nonlinear kink wave pulse. Other observations, such as a sharp rise in ion temperature at the boundaries of switchbacks are more compatible with an origin by reconnection (Farrell et al. 2020; Mozer et al. 2020).
Citation: Yamauchi et al. 2004
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