In strong magnetic fields the degeneracy among atomic energy levels of
the same principle quantum number in hydrogen is removed by Zeeman sp
litting. We investigated the line broadening of such isolated transiti
ons by means of stationary (i.e. d lambda/dB = 0) Balmer transitions.
At the stationary points an absorption edge occurs in the flux spectru
m. Two such absorption edges were observed with high spectral resoluti
on in the spectrum of the magnetic white dwarf Grw+70 degrees 8247. Th
e local line profiles are estimated by means of the flux gradient in t
he edge and the actual position of the edge. (Local means at that area
on the stellar sphere where the corresponding transition becomes stat
ionary.) The line absorption starts 5-6 Angstrom beyond the correspond
ing turn around points and altogether the local line widths are approx
imately 20 Angstrom. Since in non-magnetic white dwarfs above 10000K,
Stark effect is the dominating line broadening mechanism, we performed
quantum mechanical calculations referring to hydrogen including both,
(strong) magnetic fields and electric fields. According to the corres
ponding electron density, the randomly fluctuating electric field, whi
ch is caused by neighbouring free electron and ions, is typical 10(7)
Vm(-1) in non-magnetic white dwarfs. Within some restrictions (e.g. on
ly electric fields parallel to the magnetic field are yet investigated
) both, the commencement of line absorption 5-6 Angstrom beyond the st
ationary wavelength and the overall width of approximately 20 Angstrom
of the local line profile, cannot be explained by such randomly fluct
uating electric fields, as far as the physical conditions (e.g. electr
on density) of non-magnetic white dwarfs are applied. This, and the in
fluence of other line broadening mechanisms, is discussed in some deta
il.