Annual and Diurnal Variations in Fireball Rates

The frequency at which fireballs appear varies in several ways. First of all there is a distinct increase of brighter meteors within some showers (see below). Then, there is a seasonal variation: around the time of the vernal equinox (March 21st), fireball rates are about three times the rates seen around the autumnal equinox (September 23rd) in the northern hemisphere. This ratio was derived by Halliday and Griffin (1982) for meteorite falls; it was proven by Rendtel and Knöfel (1989) for visual and photographic fireballs.

The energy transformation which takes place during the atmospheric flight of a meteoroid strongly depends on its geocentric velocity. Normally, we would expect a faster meteor of given size to appear brighter than a slower particle of comparable size. In the case of a larger meteoroid producing a bright fireball or even a meteorite- like event, we also have to consider the fact that the lower the velocity is, the deeper the meteoroid can penetrate into the atmosphere before ablation ceases, making the energy transformation very effective in denser layers of the atmosphere and thus producing a brighter meteor. A meteor of the same size but of high velocity only causes such an energy transformation at a higher altitude. It is clear that the velocity at entry into the Earth’s atmosphere is lowest when a meteor comes from the antapex (where its relative velocity is equal to difference of velocities between the particle and the Earth), while it is highest when it approaches from the apex. Around the vernal equinox the antapex reaches its northernmost declination and is located nearest the zenith in the evening sky. As in the case of meteor showers, a higher elevation of the radiant means more visible meteors. That is why we can expect a greater number of fireballs in spring than in autumn for sites north of the equator. Since this is a physical model, the relation in principle holds also for the southern hemisphere. Bright fireballs should be more frequent in the southern spring, when the antapex reaches its southernmost position. The real picture may differ somewhat from this because the distribution of meteoroids is not isotropic along the Earth’s orbit.

Furthermore, some authors suggest we can expect fireball showers (e.g. Astapovich and Terentjeva, 1968), but experience shows the influence of these on regularly observed rates to be very low. In past centuries some reports of fireball showers have been noted (Dall’Olmo, 1978) but it could possibly be that such showers are only reports of bright meteors between magnitudes +1 mag and -2 mag.

The diurnal variation is also affected by the same situation. At 18 h local time (LT) the antapex reaches its maximum elevation. Fireballs possibly associated with meteorite falls are thus most likely to appear around 18 h LT and least likely at about 06 h LT. Halliday and Griffin (1982) derived a ratio of 2:1 (18 h LT to 06 h LT) from model calculations for meteorite falls at mid-latitudes. Hughes (1981) found a ratio of 3:1 from analyses of 644 meteorite falls (without restriction in latitude). Rendtel and Knöfel (1989) analyzed the frequency of visual and photographic fireballs at mid-latitudes and found a ratio of 4:1.

Annual variations of fireball frequencies to be seen from the equator must be caused by variations of the meteoroid distribution along Earth’s orbit. In case of an isotropic distribution there would be no annual variation at all. The reason for this is the geometry of the apex / antapex positions. Both rising and setting of near-ecliptic regions means that they appear or disappear almost vertically at the horizon. This is independent of the seasons. The only change is an alteration of the azimuth of the rising or setting point, but this does not cause any real difference. On the other hand, the very steep rising of the apex during the night causes a very strong diurnal variation. At the poles we find the opposite. Virtually no diurnal variation will occur at all since any celestial point circles the sky at a constant altitude. But there will be a significant annual variation. For half the year the apex / antapex are situated below and above the horizon respectively. At mid-latitudes we find an increasing effect of annual, but a decreasing effect of diurnal, variations. Superimposed on this is a gentle decrease of fireball frequency towards the poles, because the orbits of the meteoroids responsible for producing these meteors are concentrated near the ecliptic. Consequently the expected rates will be highest between the tropics where the ecliptic reaches its highest positions in the sky. As lready pointed out, this is valid only if we assume an isotropic distribution of meteoroid orbits in the plane of the ecliptic.