  The leading soft photon theorem implies that four-dimensional scattering
amplitudes are controlled by a two-dimensional (2D) $U(1)$ Kac-Moody symmetry
that acts on the celestial sphere at null infinity ($\mathcal{I}$). This
celestial $U(1)$ current is realized by components of the electromagnetic
vector potential on the boundaries of $\mathcal{I}$. Here, we develop a
parallel story for Low's subleading soft photon theorem. It gives rise to a
second celestial current, which is realized by vector potential components that
are subleading in the large radius expansion about the boundaries of
$\mathcal{I}$. The subleading soft photon theorem is reexpressed as a celestial
Ward identity for this second current, which involves novel shifts by one unit
in the conformal dimension of charged operators.
