Intermittent strange nonchaotic attractors (SNAs) appear typically in quasiperiodically forced period-doubling systems. As a representative model, we consider the quasiperiodically forced logistic map and investigate the mechanism for the intermittent route to SNAs using rational approximations to the quasiperiodic forcing. It is thus found that a smooth torus is transformed into an intermittent SNA via a phase-dependent saddle-node bifurcation when it collides with a new type of "ring-shaped" unstable set. Besides this intermittent transition, other transitions such as the interior, boundary, and band-merging crises may also occur through collision with the ring-shaped unstable sets. Hence the ring-shaped unstable sets play a central role for such dynamical transitions. Furthermore, these kinds of dynamical transitions seem to be "universal," in the sense that they occur typically in a large class of quasiperiodically forced period-doubling systems. Recently much attention has been paid to the study of quasiperiodically forced systems because of the generic appearance of strange nonchaotic attractors (SNAs) [1]. SNAs were first described by Grebogi et al. [2] and since then have been extensively investigated both numerically [3,4,5,6,7,8,9,10,11,12,13,14,15,16] and experimentally [17]. SNAs exhibit some properties of regular as well as chaoic attractors. Like regular attractors, their dynamics is nonchaotic; like typical chaotic attractors, they exhibit fractal phase space structure. Furthermore, SNAs are related to Anderson localization in the Schrödinger equation with a spatially quasiperiodic potential [18], and they may have a practical application in secure communication [19]. Therefore, dynamical transitions in quasiperiodically forced systems have become a topic of considerable current interest. However, their mechanisms are still much less understood than those in unforced or periodically forced systems.Here we are interested in the dynamical transition to SNAs accompanied by intermittent behavior, as reported in [12]. As a parameter passes a threshold value, a smooth torus is abruptly transformed into an intermittent SNA. Near the transition point, the intermittent dynamics on the SNA was characterized in terms of the average interburst time and the Lyapunov exponent. This route to an intermittent SNA is quite general and has been observed in a number of quasiperiodically forced period-doubling maps and flows (e.g., see [13,14]). It has been suggested [15] that the observed intermittent behavior results through interaction with an unstable orbit. However, in the previous work, such an unstable orbit was not located, and thus the bifurcation mechanism for the intermittent transition still remains unclear.In this paper, we study the underlying mechanism of the intermittency in the quasiperiodically forced logistic map M [5] which is a representative model for the quasiperiodically forced period-doubling systems:M :where x ∈ [0, 1], θ ∈ S 1 , a is the nonlinearity parameter of the logistic map, and ω and ...