ON THE INTERACTION OF ISOTROPIC PICKUP IONS WITH OBLIQUE ALFVEN WAVES

We consider the oblique Alfven wave instability for isotropic shells o f pickup ions which was introduced by Wu and Yoon. We provide some fur ther physical insight into this instability but show that the results published to date have not been correct. Our analysis indicates that t he instability disappears when the thickness of the shell in velocity space becomes comparable to the average radius of the shell. Thus the generation of these waves will not lead to a filling-in of the shell.