  We develop the connection between intergalactic helical magnetic fields and
parity odd signatures in the diffuse gamma ray sky. We find that the location
and the amplitude of a peak in a parity odd correlator, $Q(R)$, can be used to
infer the normal and helical power spectra of the intergalactic magnetic field.
When applied to Fermi-LAT data, the amplitude of the observed peak in $Q(R)$
gives $\sim 10^{-14}~{\rm G}$ intergalactic magnetic field strength, which is
consistent with an earlier independent estimate that only used the peak
location (Tashiro et al. 2014). We discuss features in the observed $Q(R)$ that
further support the intergalactic magnetic field hypothesis and make
predictions for future tests.
