EPIC Simulations of Bright Companions to Neptune's Great Dark Spots

We present a three-dimensional model of the bright companion clouds associated with Neptune's Great Dark Spots (GDSs). Our results support the hypothesis that the bright companions of the southern-hemisphere GDS discovered in 1989 and of the northern-hemisphere GDS discovered in 1994 are methane clouds that form at or just below the tropopause and that they are caused by lifting in a manner analogous to the formation of orographic clouds. We vary the vertical position of the GDS and find that the companion cloud is a robust feature except when the anticyclone itself does not survive. When a GDS is started with its top in the stratosphere it drifts much too rapidly toward the equator and quickly disperses. On the other hand, if its top is well below the tropopause there is a tendency for the companion clouds to be too large. Hence the top of a GDS is probably at the tropopause. Along an open streamline that threads a bright companion, the typical pressure and temperature drops are about 3 mb and 1 K, respectively, corresponding to a lift of about half a kilometer or 4% of the pressure scale height, and the relative horizontal wind speed is about 45 m s^-1 eastward through the cloud.