Nonlinear dynamics driving the conversion of gravitational and electromagnetic waves in cylindrically symmetric spacetime

Using the "composite harmonic mapping method," we construct exact solutions for cylindrically symmetric gravitational and electromagnetic waves within the Einstein-Maxwell system, focusing on the conversion dynamics between these types of waves. In this approach, we employs two types of geodesic surfaces in H2C: (a) the complex line and (b) the totally real Lagrangian plane, applied to two different vacuum seed solutions: (i) a vacuum solution previously utilized in our studies and (ii) the solitonic vacuum solution constructed previously by Economou and Tsoubelis. We study three scenarios: case (a) with seeds (i) and (ii), and case (b) with seed (ii). In all cases (a) and (b), solutions demonstrate notable mode conversions near the symmetric axis. In case (a) with seed (i) or seed (ii), we show that any change in the occupancy of the gravitational or electromagnetic mode relative to the C-energy near the axis always reverts to its initial state once the wave moves away from the axis. Particularly in case (b) with seed (ii), nontrivial conversions occur even when the wave moves away from the axis. In this case, the amplification factors of electromagnetic modes range from an upper limit of approximately 2.4 to a lower limit of about 0.4, when comparing the contributions of electromagnetic mode to C-energy at past and future null infinities.