  The nondipole under-the-barrier dynamics of the electron during strong-field
tunneling ionization is investigated, examining the role of the Coulomb field
of the atomic core. The common analysis in the strong field approximation is
consequently generalised to include the leading light-front non-dipole Coulomb
corrections and demonstrates the counter-intuitive impact of the sub-barrier
Coulomb field. Despite its attractive nature, the sub-barrier Coulomb field
increases the photoelectron nondipole momentum shift along the laser
propagation direction, involving a strong dependence on the laser field. The
scaling of the effect with respect to the principal quantum number and angular
momentum of the bound state is found. We demonstrate that the signature of
Coulomb induced sub-barrier effects can be identified in the asymptotic
photoelectron momentum distribution via a comparative study of the
field-dependent longitudinal momentum shift for different atomic species with
state-of-the-art experimental techniques of mid-infrared lasers.
