Evolution of large-amplitude Alfvén waves and generation of switchbacks in the expanding solar wind

Abstract

Motivated by recent Parker Solar Probe (PSP) observations of "switchbacks" (abrupt, large-amplitude reversals in the radial magnetic field, which exhibit Alfvénic correlations) we examine the dynamics of large-amplitude Alfvén waves in the expanding solar wind. We develop an analytic model which makes several predictions: switchbacks should preferentially occur in regions where the solar wind plasma has undergone a greater expansion, the switchback fraction at radii comparable to PSP should be an increasing function of radius, and switchbacks should have their gradients preferentially perpendicular to the mean magnetic field direction. The expansion of the plasma generates small compressive components as part of the wave's nonlinear evolution: these are maximized when the normalized fluctuation amplitude is comparable to $\sin \theta$, where $\theta$ is the angle between the propagation direction and the mean magnetic field. These compressive components steepen the primary Alfvénic waveform, keeping the solution in a state of nearly constant magnetic field strength as its normalized amplitude $\delta B/B$ grows due to expansion. The small fluctuations in the magnetic-field-strength are minimized at a particular $\theta$-dependent value of $\beta$, usually of order unity, and the density and magnetic-field-strength fluctuations can be correlated or anticorrelated depending on $\beta$ and $\theta$. Example solutions of our dynamical equation are presented; some do indeed form magnetic-field reversals. Our predictions appear to match some previously unexplained phenomena in observations and numerical simulations, providing evidence that the observed switchbacks result from the nonlinear evolution of the initially small-amplitude Alfvén waves already known to be present at the coronal base.

20 pages, 1 table, 9 figures. Submitted to ApJ

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Cited by in corpus (8)

• Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum
• Magnetic Reconnection as the Driver of the Solar Wind
• The Structure and Origin of Switchbacks: Parker Solar Probe Observations
• The preferential orientation of magnetic switchbacks, implications for solar magnetic flux transport
• On the properties of Alfvénic switchbacks in the expanding solar wind: the influence of the Parker spiral
• Density And Velocity Fluctuations of Alpha Particles in Magnetic Switchbacks
• Switchback Deflections Beyond the Early Parker Solar Probe Encounters
• Features of magnetic field switchbacks in relation to the local-field geometry of large-amplitude Alfvénic oscillations: \emph{Wind} and \emph{PSP} observations