  The quasi-periodic oscillations (QPOs) in black hole (BH) systems of
different scales are interpreted based on the magnetic reconnection of the
large-scale magnetic fields generated by the toroidal electric currents flowing
in the inner region of accretion disk, where the current density is assumed to
be proportional to the mass density of the accreting plasma. The magnetic
connection (MC) is taken into account in resolving the dynamic equations of the
accretion disk, in which the MC between the inner and outer disk regions, the
MC between the plunging region and the disk, and the MC between the BH horizon
and the disk are involved. It turns out that the single QPO frequency of
several BH systems of different scales can be fitted by invoking the magnetic
reconnection due to the MC between the inner and outer regions of the disk,
where the BH binaries XTE J1859+226, XTE J1650-500 and GRS 1915+105 and the
massive BHs in NGC 5408 X-1 and RE J1034+396 are included. In addition, the
X-ray spectra corresponding to the QPOs are fitted for these sources based on
the typical disk-corona model.
