Spots structure and stratification of helium and silicon in the
  atmosphere of He-weak star HD 21699
  The magnetic star HD 21699 possesses a unique magnetic field structure where
the magnetic dipole is displaced from the centre by 0.4 +/- 0.1 of the stellar
radius (perpendicularly to the magnetic axis), as a result, the magnetic poles
are situated close to one another on the stellar surface with an angular
separation of 55$^o$ and not 180$^o$ as seen in the case of a centred dipole.
Respectively, the two magnetic poles form a large "magnetic spot".
High-resolution spectra were obtained allowing He I and Si II abundance
variations to be studied as a function of rotational phase. The results show
that the helium abundance is concentrated in one hemisphere of the star, near
the magnetic poles and it is comparatively weaker in another hemisphere, where
magnetic field lines are horizontal with respect to the stellar surface. At the
same time, the silicon abundance is greatest between longitudes of 180 -
320$^o$, the same place where the helium abundance is the weakest. These
abundance variations (with rotational phase) support predictions made by the
theory of atomic diffusion in the presence of a magnetic field. Simultaneously,
these result support the possibility of the formation of unusual structures in
stellar magnetic fields. Analysis of vertical stratification of the silicon and
helium abundances shows that the boundaries of an abundance jump (in the two
step model) are similar for each element; $\tau_{5000}$ = 0.8-1.2 for helium
and 0.5-1.3 for silicon. The elemental abundances in the layers of effective
formation of selected absorption lines for various phases are also correlated
with the excitation energies of low transition levels: abundances are enhanced
for higher excitation energy and higher optical depth within the applied model
atmosphere.
