INDIVIDUAL AND INTEGRATED PULSE PROPERTIES OF PSR B0943-CHANGING PHENOMENON(10 INVOLVED IN THE MODE)

The characteristics of the ''burst'' (B) mode and ''quiescent'' (Q) mo de pulse sequences-long known from studies at or below 103 MHz-are ide ntified at 430MHz for the first time. An 18-minute, polarimetric obser vation begins with a long B-mode sequence, which has a higher average intensity, regular drifting subpulses, and a preponderance of primary polarisation-mode radiation. An abrupt transition to a Q-mode sequence is then marked by a) weaker average intensity, but occasional very br ight individual subpulses: b) a complete cessation of drifting subpuls es, with disorganized subpulses now being emitted over a much wider lo ngitude interval, and c) near parity between the primary and secondary polarisation modes, resulting in pronounced depolarisation, both of i ndividual pulses and the average profile. Careful study, however, of p rofile changes before and after this mode change reveals slower variat ions which both anticipate the abrupt transition and respond to it. A slow attenuation of the intensity level of the dominant component is o bserved throughout the duration of the B-mode sequence, which then acc elerates with the onset of the Q-mode sequence. This slow variation ap pears to represent a ''pre-switching transition'' process; and the com bination of effects on slow and abrupt time scales are finally respons ible for the formation of the characteristic B- and Q-mode average pro files.