Emission properties and bidrifting subpulses in pulsar PSR J0815+0939

Context. We investigate the properties of the emission region in PSR J0815+0939 as revealed by the bidrifting subpulses. Aims. We identify the emission properties as implied by different states of emission, including the charge density and the subpulse number on the carousel, in association with the different subpulse drift properties in bidrifting. Methods. The investigation is based on the rotating carousel, incorporating the model for obliquely rotating pulsar magnetospheres of multiple emission states. For the subpulse drift velocity given by the electric drift, the latter changes as the emission state changes, resulting in the observed different drift-bands. Results. Our results show that observable emission of this pulsar comes from two rotating carousels. The carousel at higher height is occupied by two different emission states. We find that the subpulse drift direction is different for emission coming either from different emission states on the same carousel or from similar emission states, but located on different carousels. We consider that establishing the subpulse number on a carousel may involve a mechanism that is not a function of the age or the obliquity angle of the pulsar. We discuss that bidrifting may be a common phenomenon, but its detection requires particular emission characteristics of the pulsar. We relate different subpulse drift modes in bidrifting with drifting subpulses of time-dependent drift rates, and speculate that they may be two manifestations of the same underlying mechanism