  Aims. Lightning mapper sensors on board weather satellites can be
successfully used to observe fireballs. These sensors use a very narrow
spectral band at 777nm, which is only a small fraction of the total fireball
radiation. In this spectral band, the oxygen O I-1 triplet is dominant for fast
meteors and the Planck continuum can prevail in slow meteors. It is possible to
estimate the meteor brightness in the visible spectral range from this
narrowband radiation, but it is vital to first study the dependence of this
radiation on the meteor velocity. Methods. We used observations from the
well-established European Fireball Network with newly developed digital
spectral cameras that allowed us to study the oxygen triplet in meteor spectra
and its relation to the meteor velocity and altitude. In addition, we studied
strong magnesium and sodium lines. Results. We developed a method for
calibration of fireball observation reported by Geostationary Lightning Mapper
(GLM) sensors on board the Geostationary Operational Environmental Satellite
(GOES) weather satellites. We confirm that in slow meteors, the radiation of
the Planck continuum dominates, but for faster meteors, a correction on
velocity is needed. We observe that the altitude where the oxygen line was
recorded can also affect the radiation at 777 nm. In addition, determining
whether or not the meteor showed a bright flare could also lead to a similar
effect. Thus, the meteor brightness estimate may be impacted by these
characteristics. We derived simple corrections on the altitude and on the
meteor brightness that helped to improve the overall precision of the magnitude
estimate of our sample. This allowed us to estimate the magnitude of meteors
observed by GLM with an accuracy of ~ 1 in magnitude. The Na/Mg line intensity
ratio was found to be constant for velocities above 25 km/s and increasing
toward lower velocities.
