Phase spiral unwinding. Top left: density contrast in the $\sqrt{J_z}\cos {\theta }_z-\sqrt{J_z}\sin {\theta }_z$ space. Top middle: same data mapped into the $\sqrt{J_z}-{\theta }_z$ space. The cyan dots indicate the phase of the k = 1 Fourier mode. Top right: Filtered $\sqrt{J_z}-{\theta }_z$ map. The red dots indicate the phase of the k = 1 Fourier mode for the filtered map. Bottom left: phase of the k = 1 Fourier mode mapped into the Ω_z–θ_z space, where Ω_z is the vertical frequency. The blue line shows the best-fit linear model. Bottom right: $\sqrt{J_z}-{\theta }_z$ map reconstructed from the fitting result.