In a configuration that can simulate hot accretion disks where both a toroidal and a poloidal component of the magnetic field are present, a new class of modes is found that minimizes the bending of the magnetic field lines and involves a novel effect related to (the square of) the rotation frequency gradient. These modes, which co-rotate with the plasma at the radius around which they are localized and depend explicitly on the radial width of localization, have the lowest threshold among the various types of mode that are possible as the parallel, to the field, component of the relevant mode number is decoupled from the magnitude of the height of the disk. The new effect and driving factor is related to the gradient of the Doppler shifted frequency omega - n(0) Omega(R), where Omega(R) is the rotation frequency, n(0) the toroidal mode number and omega the mode frequency. The contribution of a previously known driving factor, identified first by Velikhov and associated with the linear contribution of d Omega/dR to the perturbed toroidal field, remains important. These modes that depend on the plasma temperature being finite, survive the inclusion of relevant dissipative effects and can be expected to persist in collisionless regimes where the adopted fluid description is to be reconsidered. (C) 1998 Elsevier Science B.V.
