  Recent optical/UV data of Geminga (Mignani et al. 1998) have strengthen the
significance of the emission feature present over the thermal continuum best
fitting the EUV/soft X-ray data. Here we present a phenomenological model
interpreting the data in terms of an ion cyclotron feature, arising from a thin
outer layer of hot plasma covering Geminga's polar caps. The width of the
feature favours for Geminga the strongly oblique rotator configuration, as
already deduced from the soft X-ray data (Halpern & Ruderman 1993). Our
interpretation implies a magnetic field of 3 - 5 10d11 G, consistent with the
value deduced from the dynamical parameters of the pulsar. This would represent
the first case of an independent measurement of the surface magnetic field of
an isolated neutron star. The expected time modulation of Geminga's feature is
also discussed.
