An unusual interplanetary event: encounter with a comet?

Abstract

The interplanetary magnetic field near the ecliptic plane is characterized both by its variability in direction and by its constancy of magnitude. Exceptions to this constancy are depressions in magnitude associated with current sheets and the occasional compressions associated with shocks and stream–stream interactions. Pioneer Venus, orbiting Venus 0.72 AU from the Sun, has encountered an even rarer change in the magnitude of the magnetic field, a slow rise to a sharp maximum and then a decay, surrounding a strong current sheet. Similar structures have apparently not been reported to occur in the interplanetary medium. A survey of three years of both ISEE3 and Pioneer Venus interplanetary magnetic field data reveals what are perhaps small analogues of this feature. However, these features are an order of magnitude shorter in duration and about a factor of three smaller in amplitude. These too are rare, occurring about twice per year on average in both sets of data. An explanation that is consistent with these observations is that Venus passed through the wake of an active comet; other possible explanations seem less likely. However, no known comet was near Venus at the time of this observation. If the observed disturbance was in fact caused by a comet, the symmetry of the signature suggests that it was near perihelion. The signature in plasma and field, described here, indicates that its orbit would be prograde and inclined to the orbit of Venus. The putative comet seems to have approached Venus from above the ecliptic plane.