  We propose a novel effect that accounts for the photon emission from a
quark-gluon plasma in the presence of a weak external magnetic field. Although
the weak magnetic photon emission from quark-gluon plasma only leads to a small
correction to the photon production rate, the induced photon spectrum can be
highly azimuthally anisotropic, as a consequence of the coupled effect of the
magnetic field and the longitudinal dynamics in the background medium. With
respect to a realistic medium evolution containing a tilted fireball
configuration, the direct photon elliptic flow from experiments is reproduced.
In comparison to the experimental data of direct photon elliptic flow, in
heavy-ion collisions the magnitude of the magnetic field before 1 fm/c can be
extracted. For the top energy of RHIC collisions, right after the
pre-equilibrium evolution, $|eB|$ is found no larger than a few percent of the
pion mass square.
