Constraints on the Alfvénicity of Switchbacks

Abstract

Switchbacks (SBs) are localized structures in the solar wind containing deflections of the magnetic field direction relative to the background solar wind magnetic field. The amplitudes of the magnetic field deflection angles (θ) for different SBs vary from ∼40° to ∼160°–170°. Alignment of the perturbations of the magnetic field (Δ B ) and the bulk solar wind velocity (Δ V ) is observed inside SBs so that Δ V ∼ Δ B when the background magnetic field is directed toward the Sun (if the background solar wind magnetic field direction is anti-sunward then Δ V ∼ − Δ B , supporting anti-sunward propagation in the background solar wind frame). This causes spiky enhancements of the radial bulk velocity inside SBs. We have investigated the deviations of SB perturbations from Alfvénicity by evaluating the distribution of the parameter α, defined as the ratio of the parallel to Δ B component of Δ V to Δ V A = Δ B /4π n i m i inside SBs, i.e., α = V ∣∣/∣Δ V A∣ (α = ∣Δ V ∣/∣Δ V A∣ when Δ V ∼ Δ B ), which quantifies the deviation of the perturbation from an Alfvénic one. Based on Parker Solar Probe (PSP) observations, we show that α inside SBs has systematically lower values than it has in the pristine solar wind: α inside SBs observed during PSP Encounter 1 were distributed in a range from ∼0.2 to ∼0.9. The upper limit on α is constrained by the requirement that the jump in velocity across the switchback boundary be less than the local Alfvén speed. This prevents the onset of shear flow instabilities. The consequence of this limitation is that the perturbation of the proton bulk velocity in SBs with θ > π/3 cannot reach α = 1 (the Alfvénicity condition) and the highest possible α for an SB with θ = π is 0.5. These results have consequences for the interpretation of switchbacks as large amplitude Alfvén waves.