  We argue that X-ray flaring variability observed in the transient X-ray
pulsar A 0535+26 is due to low-mode magnetospheric instability. This
instability develops at the onset of accretion, in the thin boundary layer
between the accretion disk and neutron star magnetosphere. As a result, the
matter collected in the boundary layer can rapidly fall onto the NS surface
close to the magnetic poles, but not exactly along the field lines by which the
stationary accretion proceeds. This explains the shift in cyclotron line energy
measured using RXTE data in a pre-outburst spike, with respect to the line
energy observed during the main outburst. Furthermore, the instability can
account for the difference in pulse profiles, and their energy evolution that
is different in the pre-outburst flare and main outburst.
