The self-similar magnetic cloud model of Osherovich, Farrugia and Burlaga (1,2) is based on the exact class of MHD solutions for a magnetic flux rope with polytropic index &ggr; below unity. The problem in this model is reduced to a second order dynamic equation for the nonlinear oscillator. The corresponding effective potential for the case&ggr;>1has only an oscillating mode and the case for&ggr;<1may have two modes: oscillatory and expansion. This model suggests three classes of magnetic clouds with different evolutionary patterns. For the first class (those which cannot overcome the threshold), the profile of the magnetic cloud is rather flat at 1 AU, and the velocity of expansion is small or even shows signs of contraction. The second class (those which have energy sufficient to overcome the threshold) is well described by a free expanding flux tube. The third class has a potential without a well suggesting expansion for all energies. The non-Maxwellian electron distribution function in a cloud explains the origin of the&ggr;<1thermodynamics (3,4). A few polytropes observed in the same cloud suggest a number of magnetic tubes (4,5). To classify complex clouds, we put forward a multi-tube model based on MHD bound-state solutions (6). This model presents clouds as multiple helices embedded in a cylindrical flux rope. ©1999 American Institute of Physics.