THE PARKER-SHEARING INSTABILITY IN AZIMUTHALLY MAGNETIZED DISCS

Using a linear analysis, in the framework of the shearing sheet approx imation, we have investigated the stability of a differentially rotati ng, stratified gaseous disc embedded in a purely azimuthal magnetic fi eld. We show that a realistic differential rotation is strong enough t o couple the different waves, and allows, in particular, an efficient coupling of the Parker mode with spiral density waves. Perturbations w ith a vertical wavelength much smaller than the scaleheight of the dis c are subject to the shearing instability. We describe its mechanism i n detail, and its interplay with the Parker instability when the verti cal stratification is taken into account.