  The M1 decay widths of charm and bottom mesons and their excited states are
sensitive to the relativistic aspect of the quark current operators and obtain
a significant contribution from the interaction current that is associated with
the scalar confining interaction between heavy quarks ($Q$) and light
antiquarks ($\bar q$). Consequently they provide direct evidence on the latter.
The spectra and the M1 transition widths of the $D$, $D_s$ and $B$, $B_s$
mesons and their orbital excitations are calculated here within the framework
of the Blankenbecler-Sugar equation, which allows a covariant treatment, while
retaining a formal similarity to the nonrelativistic approach. The hyperfine
interaction in the $Q\bar q$ mesons is modeled by screened gluon exchange,
which shares many features with the instanton induced interaction. The quality
of the model is tested by calculation of the spectra and M1 decay widths of
charmonium.
