Spectral analysis of the I24, I27, I31, and I32 Io flybys. The local cyclotron frequencies for ${\text{SO}}_3{}^{+}$, ${\text{SO}}_2{}^{+}$, SO⁺, and S⁺ ions are overlaid in white lines (dash-dot, solid, dashed, and dotted). The vertical lines show the location of the geometrical wake (white dashed), and closest approach (white dotted) and the location where Galileo moves from upstream to downstream of Io (X = 0, magenta dashed).