  The neutron stars in low-mass X-ray binaries (LMXBs) are usually thought to
be weakly magnetized objects accreting matter from their low-mass companions in
the form of a disk. Albeit weak as compared to those in young neutron-star
systems, the neutron-star magnetospheres in LMXBs can play an important role in
determining the correlations between spectral and temporal properties. Parallel
tracks appearing in the plane of kilohertz (kHz) quasi-periodic oscillation
(QPO) frequency versus X-ray flux can be used as a tool to study the
magnetosphere-disk interaction in neutron-star LMXBs. For dynamically important
weak fields, the formation of a non-Keplerian magnetic boundary layer at the
innermost disk truncated near the surface of the neutron star is highly likely.
Such a boundary region may harbor oscillatory modes of frequencies in the kHz
range. We generate parallel tracks using the boundary region model of kHz QPOs.
We also present the direct application of our model to the reproduction of the
observed parallel tracks of individual sources such as 4U 1608--52, 4U
1636--53, and Aql X-1. We reveal how the radial width of the boundary layer
must vary in the long-term flux evolution of each source to regenerate the
parallel tracks. The run of the radial width looks similar for different
sources and can be fitted by a generic model function describing the average
steady behavior of the boundary region in the long term. The parallel tracks
then correspond to the possible quasi-steady states the source can occupy
around the average trend.
