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We study the generation of helical magnetic fields during inflation byconsidering a model which does not suffer from strong coupling or largeback-reaction. Electromagnetic conformal invariance is broken only duringinflation by coupling the gauge-invariants $F_{\mu\nu}F^{\mu\nu}$ and$F_{\mu\nu}{\tilde{F}}^{\mu\nu}$ to a time-dependent function $I$ with a sharptransition during inflation. The magnetic power spectrum is scale-invariant upto the transition and very blue-shifted after that. The subsequent evolution ofthe helical magnetic field is subjected to magneto-hydrodynamical processes,resulting in far larger coherence lengths than those occurring after adiabaticdecay. Scale-invariant quadratic gravity is a suitable framework to test themodel, providing a natural physical interpretation. We show that fully helicalmagnetic fields are generated with values in agreement with the lower bounds onfields in the Intergalactic Medium derived from blazar observations. This modelholds even at large/intermediate energy scales of inflation, contrary to whathas been found in previous works.

Inflationary helical magnetic fields with a sawtooth coupling