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Cross Helicity Reversals in Magnetic Switchbacks

Abstract : We consider 2D joint distributions of normalized residual energy, sigma(r)(s, t), and cross helicity, sigma(c)(s, t), during one day of Parker Solar Probe's (PSP's) first encounter as a function of wavelet scale s. The broad features of the distributions are similar to previous observations made by Helios in slow solar wind, namely well-correlated and fairly Alfvenic wind, except for a population with negative cross helicity that is seen at shorter wavelet scales. We show that this population is due to the presence of magnetic switchbacks, or brief periods where the magnetic field polarity reverses. Such switchbacks have been observed before, both in Helios data and in Ulysses data in the polar solar wind. Their abundance and short timescales as seen by PSP in its first encounter is a new observation, and their precise origin is still unknown. By analyzing these MHD invariants as a function of the wavelet scale, we show that magnetohydrodynamic (MHD) waves do indeed follow the local mean magnetic field through switchbacks, with a net Elsasser flux propagating inward during the field reversal and that they, therefore, must be local kinks in the magnetic field and not due to small regions of opposite polarity on the surface of the Sun. Such observations are important to keep in mind as computing cross helicity without taking into account the effect of switchbacks may result in spurious underestimation of sigma(c) as PSP gets closer to the Sun in later orbits.
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https://hal-insu.archives-ouvertes.fr/insu-02937929
Contributor : Nathalie Pothier <>
Submitted on : Monday, September 14, 2020 - 3:30:38 PM
Last modification on : Wednesday, September 23, 2020 - 4:36:01 AM

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Michael Mcmanus, Trevor Bowen, Alfred Mallet, Christopher Chen, Benjamin Chandran, et al.. Cross Helicity Reversals in Magnetic Switchbacks. Astrophysical Journal Supplement, American Astronomical Society, 2020, 246 (2), pp.67. ⟨10.3847/1538-4365/ab6dce⟩. ⟨insu-02937929⟩

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