Ballooning Modes in Thin Accretion Disks: Limits for their Excitation

Abstract

The conditions that limit the possible excitation of ideal MHD axisymmetric ballooning modes in thin accretion disks are discussed. As shown earlier by Coppi and Coppi (2001), these modes are well-localized in the vertical direction but have characteristic oscillatory and non-localized profiles in the radial direction. A necessary condition for their excitation is that the magnetic energy be considerably lower than the thermal energy. Even when this is satisfied, there remains the problem of identifying the possible physical factors which can make the considered modes radially localized. The general solution of the normal mode equation describing the modes is given, showing that it is characterized by a discrete spectrum of eigensolutions. The growth rates are reduced and have a different scaling relative to those of the "long-cylinder" modes, commonly known as the Magneto Rotational Instability, that have been previously studied.