Comparing meteor number fluxes from ground-based and airplane-based video observations

We operated identical Low-Light level TV cameras to observe the Leonid 1999 meteor storm, both from a ground-based station in Southern Spain and from the ARIA airplane of the Leonid Multi-Aircraft Campaign. The ground-based c amera was pointed to a fixed position about 50 degrees from the zenith, the airborne camera was pointed through a window at 75 degrees from the zenith . During the peak of the Leonid storm, the two cameras were located between 10 degrees and 20 degrees apart in geographical longitude. The recorded me teor numbers differed by a factor 5.3 +/- 0.4, the airborne camera recordin g the higher rates. This is much more than what could be expected from the geographical separation of the cameras. The different elevation angles and altitudes of the cameras can explain this. Pointing the camera low to the h orizon results in a much larger volume in the atmosphere which is observed, resulting in higher meteor count rates. However, the meteors are on averag e much further away than when observing high above the horizon. The atmosph eric extinction reduces the brightness of the meteors, effectively reducing the count rates. For two ground-based cameras, these two effects are expec ted to compensate. Due to the high altitude of the airborne camera, the inc reasing effect of the count rate dominates. We set up an atmospheric extinc tion model taking into account Rayleigh scattering that quantitatively expl ains the number difference. Using the same model, we predict number differe nces for cameras observing from the same location, but pointed at different elevation angles. For typical observing conditions, neglecting this effect can result in differences up to a factor of 10 in the derived meteor numbe rs.