Collective modes and screening in an electric-magnetic dual plasma

We study the linear response of an effective relativistic two-fluid medium carrying separately conserved electric and magnetic charge currents. The model is defined by the duality-symmetric Maxwell equations with electric and magnetic sources, together with Lorentz-force dynamics for two fluids with independent inertia and possible Carter-type entrainment. The magnetic component is treated as an effective charge-carrying constituent, so the analysis uses only the closed two-fluid equations. Around a homogeneous, neutral, and unmagnetized background, the transverse electromagnetic response contains two stable branches whose cutoffs are set by the electric and magnetic plasma frequencies and are exchanged by electric--magnetic duality. In the longitudinal sector, entrainment mixes the electric and magnetic density oscillations, turns their crossing into an avoided crossing, and gives the stability condition $\kappa^2<1 ,$ equivalent to positive definiteness of the two-fluid momentum matrix. Resolving the magnetic component into monopole and antimonopole species gives a neutral branch selected by magnetic charge conjugation $C_m$. In this branch the net magnetic current vanishes, so the long-range monopole field is absent, while the total magnetic density can still produce screened collective response. The resulting picture is that magnetic charge can be statically hidden but dynamically visible. A robust observable signature is the density scaling $\omega_{\rm coll}^2\sim\omega_{pm}^2\propto n^0_{(m)},$ which may survive dissipative broadening even when sharp ideal-plasma poles are not resolved. We briefly comment on possible dyonic interpretations of magnetically neutral composites, but the linear-response results do not rely on that interpretation.