Magnetic Field Kinks and Folds in the Solar Wind

Abstract

Parker Solar Probe (PSP) observations during its first encounter at 35.7 R have shown the presence of magnetic field lines which are strongly perturbed to the point that they produce local inversions of the radial magnetic field, known as switchbacks. Their counterparts in the solar wind velocity field are local enhancements in the radial speed, or jets, displaying (in all components) the velocity-magnetic field correlation typical of large amplitude Alfvén waves propagating away from the Sun. Switchbacks and radial jets have previously been observed over a wide range of heliocentric distances by Helios, WIND and Ulysses, although they were prevalent in significantly faster streams than seen at PSP. Here we study via numerical MHD simulations the evolution of such large amplitude Alfvénic fluctuations by including, in agreement with observations, both a radial magnetic field inversion and an initially constant total magnetic pressure. Despite the extremely large excursion of magnetic and velocity fields, switchbacks are seen to persist for up to hundreds of Alfvén crossing times before eventually decaying due to the parametric decay instability. Our results suggest that such switchback/jet configurations might indeed originate in the lower corona and survive out to PSP distances, provided the background solar wind is sufficiently calm, in the sense of not being pervaded by strong density fluctuations or other gradients, such as stream or magnetic field shears, that might destabilize or destroy them over shorter timescales.

The evolution of inverted magnetic fields through the inner heliosphere

“…Simple scaling arguments (assuming an inversion which convects outward at the background solar wind speed, while straightening out at the local Alfvén speed) indicate that non-eruptive loops should not be able to form inversions which survive out to 1 AU (Owens et al 2020). However, as above, the more detailed MHD modelling by Tenerani et al (2020) suggests that plasma effects may result in inversions surviving further into the heliosphere. Inversions which have been attributed to loop opening have been observed in streamer belt solar wind with PSP near perihelion, where the inversions are most likely to be intact (Rouillard et al 2020).…”

“…Inversions of this type are less commonly observed at 1 AU, suggesting that they may straighten out, become damped, or otherwise merge into the background solar wind before reaching such distances. Modelling of these inversions by Tenerani et al (2020) has indeed indicated that an inverted magnetic structure, travelling outwards at the Alfvén speed, could reach distances accessible to PSP, but will tend to be removed from the field at greater heliocentric distances as a result of developing density and magnetic fluctuations.…”

Switchbacks in the Near-Sun Magnetic Field: Long Memory and Impact on the Turbulence Cascade

“…The study by Borovsky [12] suggests that switchbacks do not evolve after 0.3 AU, as if they had stopped interacting with the surrounding turbulent wavefield. Several studies suggest that switchbacks observed closer to the Sun by Parker Solar Probe do interact with the ambient magnetic field: Krasnoselskikh et al [33] argue that wave activity at the boundary of switchbacks may contribute to mixing and eventually to their weakening while Tenerani and et al [56], using simulations, conjecture that they originate in the lower corona and survive out to Parker Solar Probe distances. All these results support the idea that the concentration of switchbacks is higher at the perihelion of Parker Solar Probe than farther out.…”

Parker Solar Probe Observations of Proton Beams Simultaneous with Ion-scale Waves

“…4(a). These magnetic field reversals, also known as "switchbacks", have been observed to be pervasive in PSP's initial orbits, but their origin and physical nature remains a mystery Bale et al 2019;Tenerani et al 2020;de Wit et al 2020;Horbury et al 2020). Directly before this 30 minute-duration switchback, Fig.…”