  The magnetic field dependence of the electronic specific heat C(H) in the
s-wave superconductor NbSe_2 shows curvature at low fields, resembling the near
H^1/2 term in C(H) which has been reported in high-T_c superconductors and
attributed to a d-wave pairing state. In NbSe_2 we find that the low field
behavior in C(H) is described quantitatively by the expansion of vortex cores
and the field dependence of the magnetic induction B above H_c1. The associated
change in the density of quasiparticle states localized in the vortex cores
provides a simple explanation for the "low-field anomaly" in C(H) observed in
s-wave superconductors.
