On the mechanism of the 'Noisar' phenomenon in magnetic close binary systems

The mechanisms of 'Noisar' phenomenon in AM Herculis-type stars are discussed. In an accretion column above the surface of the magnetized degenerate star the instability of some types may be excited, such as axi-symmetrical quasi-periodical penetration of the low-density 'bulks' from the column axis to be outer parts; 'boiling' with 'bulks' moving inside or outside the column; 'tornado' with low-density region rapidly rotating around the column axis; 'switchings' of the accretion from one half of the 'polar cap' to another and vice versa. The oscillations of different plasma clots ('spaghetti') may interfer causing flux changes as well. Such 'Noisar' oscillations appear in different regions of the accretion column (at the upper shock and near the column base, respectively), so hard and soft X-ray fluxes might not have correlation in their variability. The observations are in qualitative agreement with the models. (author)