Primordial Magnetic Field Generated in Natural Infation
  We study the simple gauge invariant model ${f^2}FF$ as a way to generate
primordial magnetic fields (PMF) in Natural Inflation (NI). We compute both
magnetic and electric spectra generated by the ${f^2}FF$ model in NI for
different values of model parameters and find that both de Sitter and power law
expansion lead to the same results at sufficiently large number of e-foldings.
We also find that the necessary scale invariance property of the PMF cannot be
obtained in NI in first order of slow roll limits under the constraint of
inflationary potential, $V\left( 0 \right) \simeq 0$. Furthermore, if this
constraint is relaxed to achieve scale invariance, then the model suffers from
the backreaction problem for the co-moving wave number, $k \lesssim 8.0\times
10^{-7} \rm{Mpc^{-1}}$ and Hubble parameter, $H_i \gtrsim 1.25\times 10^{-3}
\rm{M_{\rm{Pl}}}$. The former can be considered as a lower bound of $k$ and the
later as an upper bound of $H_i$ for a model which is free from the
backreaction problem. Further, we show that there is a narrow range of the
height of the potential $\Lambda $ around ${\Lambda _{\min }} \approx
0.00874{M_{{\rm{Pl}}}}$ and of $k$ around ${k_{\min }} \sim
0.0173{\rm{Mp}}{{\rm{c}}^{ - 1}}$, at which the energy of the electric field
can fall below the energy of the magnetic field. The range of $k$ lies within
some observable scales. However, the relatively short range of $k$ presents a
challenge to the viability of this model.
