  We study the massless limit in synchrotron radiation and one-photon pair
creation in magnetic field. In this limit Schwinger critical field
$H_0=m^2c^3/(e\hbar)$ tends to zero, so two characteristic quantum parameters
$\eta=H/H_0,\;\chi=\eta E/mc^2$ are infinite, and the standard approximations
used in analytical calculations fail. Applying Schwinger's proper time methods
we derive simple expressions for synchrotron radiation spectra emitted by
massless charges of spins $s=0,\,1/2$ and the pair creation probability
distribution in the quasiclassical (high Landau levels) regime exhibiting
simple scaling properties and possessing universal spectral shapes.

High Energy Physics - Theory;High Energy Astrophysical Phenomena;High Energy Physics - Phenomenology