We study the generation of electromagnetic fields during inflation when the conformal invariance of Maxwell ’ s action is broken by the kinetic coupling f ^ { 2 } ( \phi ) F _ { \mu \nu } F ^ { \mu \nu } of the electromagnetic field to the inflaton field \phi .
We consider the case where the coupling function f ( \phi ) decreases in time during inflation and , as a result , the electric component of the energy density dominates over the magnetic one .
The system of equations which governs the joint evolution of the scale factor , inflaton field , and electric energy density is derived .
The backreaction occurs when the electric energy density becomes as large as the product of the slow-roll parameter \epsilon and inflaton energy density , \rho _ { E } \sim \epsilon \rho _ { inf } .
It affects the inflaton field evolution and leads to the scale-invariant electric power spectrum and the magnetic one which is blue with the spectral index n _ { B } = 2 for any decreasing coupling function .
This gives an upper limit on the present-day value of observed magnetic fields below 10 ^ { -22 } { G } .
It is worth emphasizing that since the effective electric charge of particles e _ { eff } = e / f is suppressed by the coupling function , the Schwinger effect becomes important only at the late stages of inflation when the inflaton field is close to the minimum of its potential .
The Schwinger effect abruptly decreases the value of the electric field , helping to finish the inflation stage and enter the stage of preheating .
It effectively produces the charged particles , implementing the Schwinger reheating scenario even before the fast oscillations of the inflaton .
The numerical analysis is carried out in the Starobinsky model of inflation for the powerlike f \propto a ^ { \alpha } and Ratra-type f = \exp ( \beta \phi / M _ { p } ) coupling functions .