  Novel x-ray detection technology enabled the first profile measurements of
the birth and growth dynamics of runaway electrons (REs) at the edge of
tokamaks during quiescent RE studies at the Madison Symmetric Torus. The
formation of an off-axis RE seed with linear growth rates has been resolved for
low energies, a hollow streaming parameter and large electric fields
($E_{\parallel}/E_{D}$) in agreement with theory and simulations. Secondary
exponential growth rates have also been spatially resolved for the first time
and are consistent with a convective transport of the order of the Ware pinch
and energies up to $10^3\times T_{e,0}$. Numerical simulations are shown to
reproduce the experimental observations including the off-axis runaway electron
generation, radial transport and exponential growth at the core, as well as
suppression due to $m=3$ resonant magnetic perturbations.
