On the connection between the magneto-elliptic and magneto-rotational
  instabilities
  It has been recently suggested that the magneto-rotational instability (MRI)
is a limiting case of the magneto-elliptic instability (MEI). This limit is
obtained for horizontal modes in the presence of rotation and an external
vertical magnetic field, when the aspect ratio of the elliptic streamlines
tends to infinite. In this paper we unveil the link between these previously
unconnected mechanisms, explaining both the MEI and the MRI as different
manifestations of the same Magneto-Elliptic-Rotational Instability (MERI). The
growth rates are found and the influence of the magnetic and rotational effects
is explained, in particular the effect of the magnetic field on the range of
negative Rossby numbers at which the horizontal instability is excited.
Furthermore, we show how the horizontal rotational MEI in the rotating shear
flow limit links to the MRI by the use of the local shearing box model,
typically used in the study of accretion discs. In such limit the growth rates
of the two instability types coincide for any power-type background angular
velocity radial profile with negative exponent corresponding to the value of
the Rossby number of the rotating shear flow. The MRI requirement for
instability is that the background angular velocity profile is a decreasing
function of the distance from the centre of the disk which corresponds to the
horizontal rotational MEI requirement of negative Rossby numbers. Finally a
physical interpretation of the horizontal instability, based on a balance
between the strain, the Lorentz force and the Coriolis force is given.
