  The usefulness and versatility of the PSRSALSA open-source pulsar
data-analysis project is demonstrated through an analysis of the radio pulsar
B1839-04. This study focuses on the phenomenon of bi-drifting, an effect where
the drift direction of subpulses is systematically different in different pulse
profile components. Bi-drifting is extremely rare in the pulsar population.
Various tools in PSRSALSA, including those allowing quantification of
periodicities in the subpulse modulation, their flux distribution, and
polarization properties, are exploited to obtain a comprehensive picture of the
radio properties of PSR B1839-04. In particular, the second harmonic in the
fluctuation spectra of the subpulse modulation is exploited to convincingly
demonstrate the existence of bi-drifting. Bi-drifting is confirmed with a
completely independent method allowing the average modulation cycle to be
determined. Polarization measurements were used to obtain a robust constraint
on the magnetic inclination angle of less than 35 deg. Two distinct emission
modes are discovered to be operating, with periodic subpulse modulation being
present only during the weaker mode. Despite the variability of the modulation
cycle and interruption by mode-changes, the modulation pattern responsible for
the bi-drifting is strictly phase locked over a timescale of years such that
the variability is identical in the different components. The phase locking
implies that a single physical origin is responsible for both drift directions.
Phase locking is hard to explain for many models, including those specifically
proposed in the literature to explain bi-drifting, and they are therefore shown
to be implausible. It is argued that within the framework of circulating
beamlets, bi-drifting could occur if the circulation were severely distorted,
possibly by distortions in the magnetic field.
