  We report distinctive core profiles in the strongest optical helium line, He
I 5876, from high-resolution high-sensitivity observations of spectral type DB
white dwarfs. By analyzing a sample of 40 stars from Keck/HIRES and VLT/UVES,
we find the core appearance to be related to the degree of hydrogen and heavy
element content in the atmosphere. New Ca K-line measurements or upper limits
are reported for about half the sample stars. He I 5876 emission cores with a
self-reversed central component are present for those stars with relatively low
hydrogen abundance, as well as relatively low atmospheric heavy element
pollution. This self-reversed structure disappears for stars with higher
degrees of pollution and/or hydrogen abundance, giving way to a single
absorption core. From our model atmospheres, we show that the self-reversed
emission cores can be explained by temperature inversions in the upper
atmosphere. We propose that the transition to a single absorption core is due
to the additional opacity from hydrogen and heavy elements that inhibits the
temperature inversions. Our current models do not exactly match the effective
temperature range of the phenomenon nor the amplitude of the self-reversed
structure, which is possibly a result of missing physics such as 3D treatment,
convective overshoot, and/or non-LTE effects. The He I 5876 line structure may
prove to be a useful new diagnostic for calibrating temperature profiles in DB
atmosphere models.
