Divergent light effects in ice crystal halos created by fireballs: a case study of a 22° halo with a 20° radius

Continuously evolving video techniques have allowed the detection of ice crystal halos created by bright fireballs at nighttime for nearly 20 years. However, only a handful of observations of these highly transient phenomena are known. Compared to the sun or the moon, the light source is in these cases located much closer to the halo-producing crystals, allowing for measurable effects of this divergent illumination to manifest. Here, I report on a case of a halo created by a fireball close to the zenith, with the signature of divergent illumination being a reduction of the angular radius of the circular halo. The determined amount of shrinking of about 2° is consistent with estimations of the heights of the cirrus cloud and the fireball.