Observations of Fireballs

A bright fireball is an impressive event. Even well-trained observers may be fascinated so much by the appearance that they forget what data to record; the impression dominates and data can remain uncertain. Of course, it is impossible to do fireball-observing exercises as the objects are totally unpredictable in appearance and are also relatively uncommon. In the case of bright meteors one tends to note more information than usual so it is useful to have a standard way of recording fireball data, and to know which data are important to note.

Immediately after the fireball’s appearance you should look at your watch or clock to determine the time, preferably to an accuracy of one minute. Later, this clock or watch should be compared with an official time signal. In some regions special time signal transmitters exist (e.g. in Western Europe there is the DCF-77 transmitter on 77.5 kHz) while as an alternative, time signal controlled clocks and wrist-watches are now decreasing in price. Also the satellites of the Global Positioning System (GPS) give the exact time worldwide on a GPS-receiver.

Next the apparent path determined by the first and last sightings of the trail should be immediately recorded, before the exact data begin to fade in your memory. If you store your observational data on tape, or if you witness a fireball when not carrying out a meteor watch, you should note at least 3 prominent points of its trail.

Only then you should note additional observations such as fragmentation, color, apparent velocity, flares and any sounds. Finally, note the apparent magnitude, as this information can usually be remembered best. The certainty of the magnitude is likely to be lower in the case of very bright fireballs, those which are brighter than the quarter moon (about magnitude -9). You may use comparisons where these are helpful, such as “brighter than the full moon”, or intervals (e.g. between magnitude -11 and magnitude -14). If possible, you should note any magnitude variations (the light curve) too.

Meteor sounds deserve special mention. Although they are noted only on rare occasions, they are important. The “regular” sounds are audible only after a few minutes have elapsed since the optical observation, as sound travels through air far slower than light. While recording the data described above you should be ready to listen out for these sounds. Meteor noises can reach the surface from any point of the trajectory lower than about 60 km and may consist of bangs or swishing sounds, or possibly other noises. However, there may also be noises heard synchronously with the meteor’s optical appearance. These are certainly not hallucinations! Such anomalous sounds appear to propagate via very low frequency (VLF) radio-waves, and seem to be generated especially in the upper part of the trajectory. These waves of course propagate with the velocity of light and if there are dielectric media near the surface, such as massive objects or atmospheric electric activity, they can then be converted into sound waves (ReVelle, 1975; Annett, 1980; Keay, 1980; Kn√∂fel, 1991; Keay, 1993); Keay and Ceplecha, 1994).

It is essential to identify and exclude other possible sources for supposed meteor noises, for example motors, airplanes, other technical sources, or animals. According to ReVelle (1975), meteor sounds may usually only be expected if a fireball is brighter than magnitude -8 (visual).

Photographic fireball networks provide useful information, but even here visual data are also necessary. Only a few photographic stations operate a pair of guided / unguided cameras or have an electronic time registration, and exact timings are vital for further calculations. Furthermore, a photograph gives no information about the real color, persistent train or sounds, and fragmentation data are difficult to derive from a still picture.

The collision of the meteoroid with atmospheric particles causes ablation, and thereby ionization, of both atmospheric and meteoroid constituents. While most of these excited states exist for about 10^-8 seconds only, there are also some atomic states which may persist for times exceeding several seconds (metastable levels). The emissions from such levels may be visible for variable amounts of time, and are designated as persistent trains (see Baggaley, 1980). They appear bright in the night sky after the meteor itself has disappeared. A large meteoroid entering the atmosphere leaves a substantial amount of material in its wake which is distributed along its trajectory. This may form the so-called smoke train, which does not emit light. There are two possibilities for seeing smoke trains. Firstly, in the case of a daylight event a smoke trail along the meteor’s bright path may be noted. Secondly, such trains could become visible in twilight when the brightness of the sky has already decreased. The altitude at which the trains appear may still be lit by the Sun even in late twilight. Consequently, these trains will appear to be bright. Both types of train occur along the luminous trajectory of the meteor at altitudes above 20 km. At these levels strong winds normally exist which will cause the form of the trains to vary within a relatively short time frame. More details can be found in the IMO Photographic Handbook (Rendtel, 1993).

Please report all fireball observations via the online Fireball Report Form.