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Flocculation, switchbacks, and loss of Alfvenicity: Indicators of shear-driven turbulence in the young solar wind?

Abstract : Since the first preliminary announcements of Parker Solar Probe results [1], there has been increased discussion of "switchbacks" and speed enhancements such as those observed in Helios data [2]. A familiar explanation relies on outward propagation of large amplitude remnants of magnetic reconnection at lower altitudes. Such a mechanism is plausible and difficult to rule out. However, another possibility exists, namely that the onset of strong shear-driven turbulence, beginning just outside the Alfvén critical region, may induce the switchbacks through large-scale perturbation of the flow. This scenario is consistent with a suite of observable effects already apparent in imaging [3] and in situ datasets [2]. DeForest et al. interpreted the transition from elongated striae to relatively isotropic flocculae as a signature of the onset of shear-driven turbulent activity some 20-80 Rs from the photosphere, where the magnetic field ceases to be a dominant constraint on transverse motions; this interpretation has received support from turbulence-driven global simulations of the solar wind [4]. The presence of velocity shears is also strongly suggested by coronal imaging at lower altitudes [5]. Somewhere above the conventional Alfvén point such shears can begin supplying turbulence energy [6] while also destroying Alfvénicity by injection of kinetic energy but not cross helicity [7]. If indeed the flocculation signifies large fluctuations or even turnover associated with vortices, then specific features of shear driven turbulence may be anticipated in imaging data from the upcoming PUNCH mission and in ongoing analysis of in situ Parker Solar Probe observations. Details of these signatures will be given here. Research supported in part by grant RTA5980003 from the Thailand Research Fund, by NASA under NNX17AB79G, 80NSSC18K1210, 80NSSC18K1648, and by the PSP ISOIS project as subcontract under NNN06AA01C.

[1] S. Bale, invited talk, 2018 Fall AGU Meeting

[2] T. Horbury, L. Matteini & D. Stansby, MNRAS 478, 1980 (2018)

[3] C. DeForest et al., Astrophys. J. 828, 66 (2016)

[4] R. Chhiber et al., Astophys. J. Lett. 856, L39 (2018)

[5] C. DeForest et al., Astrophys. J. 862, 18 (2018)

[6] G. Zank et al., JGR 101, 17093 (1996); B. Breech et al., JGR 113, A08105 (2008)

[7] D. A. Roberts et al., JGR 97, 17115 (1992); see also Fig. 3 of D. A. Roberts, Astrophys. J. 711, 1044 (2010)

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Submitted on : Wednesday, February 9, 2022 - 7:26:32 AM
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W. H. Matthaeus, D. J. Ruffolo, C. Deforest, T. Parashar, M. L. Goldstein, et al.. Flocculation, switchbacks, and loss of Alfvenicity: Indicators of shear-driven turbulence in the young solar wind?. American Geophysical Union, 2019, San Francisco, United States. 15 pp. ⟨insu-03562461⟩



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