  We present a detailed single pulse study of PSR B1055-52 based on
observations at the Parkes radio telescope. The radio emission is found to have
a complex modulation dominated by a periodicity of ~20 times its rotational
period P (0.197s), whose phase and strength depends on pulse longitude. This
periodicity exhibits a phase-locked delay of about 2.5P between the main pulse
(MP) and interpulse (IP), presumed to be the opposite poles of the pulsar. This
delay corresponds to a light travel distance of many times the light cylinder
radius. More complex modulations are found within the MP on timescales down to
about 9P, and both these and the principal modulation vary strongly across the
(at least) 7 components which the MP and IP exhibit. The nature of the single
pulse emission, which ranges from smooth and longitudinally extended to
`spiky', is also component-dependent. Despite these disparities, the total
pulse intensity distributions at the MP and IP are virtually identical in
shape, suggesting a common emission mechanism. In an attempt to account for the
complex modulations we examine a number of physical models, both intrinsic
(which presuppose the pulsar to be an isolated neutron star) and extrinsic
(appealing to the presence of circumstellar material to modulate the emission).
Significant objections can be made to each model, so this pulsar's behaviour
patterns remain a crucial challenge to theorists.
