MULTIFREQUENCY VLA OBSERVATIONS OF THE CRAB-NEBULA AT DIFFERENT EPOCHS - EXPANSION AND RADIO SPECTRA OF FILAMENTS

High-resolution radio maps of the filamentary structure of the Crab Nebula made using VIA data at 327, 1400 and 4800 MHz at epochs 1989.1, 1983.9 and 1984.5 are presented. The filaments in the outer regions show large proper motions consistent with the expansion of the nebula, although some anomalous motions or structural changes are seen in the north-western boundary. The radio spectrum of the filaments steepens away from the centre, with [a] approximately 0.3 within 100 arcsec of the centre and [a] approximately 0.4 in the outer regions. The polarization at 6 cm suggests that the magnetic field is along the filaments, especially in the loops and arches, having a possible origin in the toroidal magnetic field of the nebula. The spectrum suggests that, while the diffuse nebular emission is from pulsar-generated electrons, the filaments in the outer regions contain electrons generated by both pulsar and shock acceleration.