1996A&A...308..465Z

We investigate the formation of a plasma envelope in the magnetosphere of a hot white dwarf by cyclotron radiation pressure. The radiation pressure distribution (both in the continuum and in the cyclotron line) is derived for an optically thin, dipolar magnetosphere of an isolated, non-rotating star emitting blackbody radiation. For an isothermal, fully ionized, pure hydrogen plasma the hydrostatic solution consists of a closed plasma shell accumulated in a potential well near the equilibrium surface, where radiation pressure cancels gravity, and an equatorial disk inside this surface. The presence of a finite optical depth leads to a temporal variation of the plasma envelope and of the observed radiation. We apply these results to the strongly magnetized white dwarf GD229, which is a candidate for such a radiation-driven envelope or "diskon". The deep unidentified 2000-3000. in its UV spectrum is explained as the result of cyclotron scattering in optically thick gyroresonant layers around the star. We predict a temporal and spectral variability of this feature with a characteristic time of ≥1hr due to non-stationary plasma motions in the envelope.