  In this work, $\pi$, $\sigma^+$ and $\sigma^-$ transitions between magnetic
sublevels of the $D_1$ line of all alkali atoms are considered analytically.
General block Hamiltonian matrices in presence of a magnetic field for the
ground and excited states are built in order to describe all the transitions.
Eigenvalues and eigenkets describing ground and excited levels are calculated,
"modified" and unperturbed transfer coefficients as a function of the nuclear
spin $I$, the magnetic quantum number $m$ and the magnetic field magnitude $B$
are defined. Transition cancellations are observed only for some $\pi$
transitions of each isotope. The main result is that we obtain one single
formula which expresses the magnetic field values cancelling these transitions.
These values also correspond to the case when some of other transitions
intensity have their maximum. In addition, we examine the derivative of $\pi$
transition "modified" transfer coefficients in order to find the magnetic field
values which correspond to the intensities maximum. The accuracy of the
magnetic field $B$ values is only limited by the uncertainty of the involved
physical quantities.
