Infrared behavior of closed superstrings in strong magnetic and gravitational fields

A large class of four-dimensional supersymmetric ground states of closed superstrings with a non-zero mass gap are constructed. For such ground states we turn on chromo-magnetic fields as well as curvature. The exact spectrum as function of the chromo-magnetic fields and curvature is derived. We examine the behavior of the spectrum, and find that there is a maximal value for the magnetic field H_{\rm max}\sim M_{\rm planck}^2. At this value all states that couple to the magnetic field become infinitely massive and decouple. We also find tachyonic instabilities for strong background fields of the order {\cal O}(\mu M_{\rm planck}) where \mu is the mass gap of the theory. Unlike the field theory case, we find that such ground states become stable again for magnetic fields of the order {\cal O}(M^2_{\rm planck}). The implications of these results are discussed.