Magnetic nulls in the outer magnetosphere of Jupiter: Detections by Pioneer and Voyager spacecraft

First reported during the Ulysses flyby of Jupiter in February 1992, null events are sporadic dropouts in the magnetic field strength detected in the outer Jovian magnetosphere, with the magnitude of the field strength decreasing to values that can be less than 0.2 nT. The events occur in regions where the ambient field is primarily southward, that is, aligned with the local planetary dipole field direction and are distinct from the field depressions encountered in the center of the magnetodisk in the middle magnetosphere. They also appear distinct from the field structures encountered at the magnetopause. Here we report a survey of the occurrence of such events in the magnetic field data recorded by the earlier Pioneer and Voyager spacecraft passages through the Jovian system. All of the previous spacecraft to fly through the Jovian magnetosphere recorded field null events on their inbound passes where the field magnitude dropped below 2 nT. There is little or no evidence of any field reversal in the center of events. Only Pioneer 11, which was the only spacecraft to exit on the dayside of the magnetosphere, recorded events on its outbound pass.