  We calculate the dynamical structure factor S(q, {\omega}) of a weakly
interacting helical edge state in the presence of a magnetic field B. The
latter opens a gap of width 2B in the single-particle spectrum, which becomes
strongly nonlinear near the Dirac point. For chemical potentials |{\mu}| > B,
the system then behaves as a nonlinear helical Luttinger liquid, and a
mobile-impurity analysis reveals interaction-dependent power-law singularities
in S(q,{\omega}). For |{\mu}| < B, the low-energy excitations are gapped, and
we determine S(q,{\omega}) by using an analogy to exciton physics.
