Discovery of Two Consecutive Profile Change Events in the Millisecond Pulsar PSR J0437-4715

The closest and brightest millisecond pulsar (MSP), PSR J0437-4715, is a high-priority pulsar for precision timing experiments, including the Parkes Pulsar Timing Array (PPTA) project, where it is used for both pulsar timing and for polarization calibration. Using approximately seven years of observations obtained with the Ultra-Wideband Low-Frequency receiver on Murriyang, CSIRO's Parkes radio telescope, we investigate the long-term spectro-polarimetric variability of PSR J0437-4715. We report the discovery of two consecutive discrete profile change events exhibiting the same phase-localised morphological variations, the first such behaviour reported in any MSP. Both events affect three phase-localised regions of the pulse profile that exhibit coupled and anti-correlated temporal evolution. We also identified, and corrected for, systematic profile variability induced by polarization calibration errors arising from long-term drift of the polarimetric properties of the receiver. The profile change exhibits frequency-dependent evolution in total intensity, together with variations in linear polarization, position angle and fractional linear polarization. These results confirm a magnetospheric origin for the events rather than propagation effects caused by the interstellar medium, and provide potential new constraints on MSP emission models.