Expanding haloes in cometary comae

Abstract

Observations indicate that cometary activity can be rather violent. Flare-ups, sunward-pointing jets and periodic formation of concentric haloes expanding outwards can also be persistent features in some comets. For example, a halo structure was observed in the coma of Comet Halley during its last visit¹. It is argued here that halo formation may be related to the propagation of shock pairs driven through the cometary atmosphere as a result of spin-modulation of the cometary outgassing rate. Even though the non-stationary process could be less dramatic than the example considered here, its effect on the chemistry and expansion of the neutral gas as well as dynamics of the cometary plasma must be significant.