Io-DAM arcs modeled by ExPRES for the December 13, 2020. The polarization of the arcs (a) and the respective values of the beaming cone aperture angle θ (b) are shown in t-f planes. The variation in the observer’s (at the Earth) ϕ_Io is shown in red in plot a, and the variation in ϕ_Io (PhIo) is indicated also in red in plot b. The dashed black line indicates the moment when ϕ_Io is 180°. For ϕ_Io < 180°, the emissions are labeled as Io-B or Io-D; and for ϕ_Io ≥ 180°, the emissions are labeled as Io-A or Io-C.