Surface Density Gradient Research Articles

The excitation of density waves at the Lindblad and corotation resonances by an external potential

We calculate the linear response of a differentially rotating two-dimensional gas disk to a rigidly rotating external potential. The main assumptions are that the sound speed is much smaller than the orbital velocity and that the external potential varies on the scale of the disk radius. We investigate disks both with and without self-gravity. The external potential exerts torques on the disk only at the Lindblad and corotation resonances. The torque is positive at the outer Lindblad resonance and negative at the inner Lindblad resonance; at corotation the torque has the sign of the radial gradient of vorticity per unit surface density. The torques are of the same order of magnitude at both types of resonance and are independent of the sound speed in the disk. The external potential also excites density waves in the vicinity of the Lindblad and corotation resonances. The long trailing wave is excited at a Lindblad resonance. It transports away from the resonance all of the angular momentum which is deposited there by the external torque. Short trailing waves are excited at the corotation resonance. The amplitudes of the excited waves are the same on both sides of the resonance and are small unless the disk is almost gravitationally unstable. No net angular momentum is transported away from the corotation region by the waves. Thus the angular momentum deposited there by the external torque accumulates in the gas. We briefly discuss the behavior of particle disks and prove that the external torques on particle disks are identical to those on gas disks.

The critical effect of the surface density gradient on surface plasmon dispersion

An analytical expression is given for the linear dispersion coefficient of surface plasmons on metals and semiconductors. The influence of the surface density profile in the selvage region is included by a model which yields a critical width.

Effects of a Density Gradient on the Magneto-Kerr Effect at the Boundary of a Collisional Plasma

Results are presented of calculations showing the effects of collision frequency and surface density gradient on the polarization of microwaves reflected from a plasma surface. Experimental observations are compared with these results and estimates made of collision frequency and density gradient.