August 11-15, 2014


High transparency of photosphere plasma for electromagnetic waves polarized across strong magnetic field on white dwarfs

Sergey Koryagin (Institute of Applied Physics, Russian Acad. of Science)

We showed that, in the photosphere of a white dwarf with strong magnetic field, the collisional absorption significantly decreases at the frequencies below the electron cyclotron frequency for the electromagnetic waves linearly polarized across the magnetic field lines (for the so-called extraordinary waves in a magnetized plasma). It is well known that, in an isotropic plasma without magnetic field, the collisional absorption increases with increase of the wavelength. We found out that, under the specific conditions in the magnetic white dwarf photosphere, on the contrary, the absorption decreases with increase of the wavelength for the definite polarization. As a result, the extraordinary waves can come out from the deeper and hotter photosphere layers than the ordinary waves and, in this way, can determine the linear polarization of the observed infrared and optical radiation at the frequencies below the electron cyclotron frequency.

The uncovered transparency of the cool magnetized plasma stems from the fact that an electron makes many cyclotron revolutions when it passes the region near a nucleus during a Coulomb collision (for the typical photosphere temperature 10^4 K, the magnetic field should exceed 10^7 G). So, the electron motion across the magnetic field reduces to the slow electric drift in the Coulomb electric field of a nucleus and the external magnetic field. The ``freezing'' of the electron transverse motion during a collision causes the significant decrease both of the bremsstrahlung power and the collisional absorption for the waves polarized across the magnetic field (against the corresponding values in the isotropic unmagnetized plasma). We analytically obtained the frequency dependence of the collisional absorption coefficient for the extraordinary waves which demonstrates the wide minimum at the corresponding frequency below the electron cyclotron frequency.

Mode of presentation: oral