Spatial quasi-periodic variations of the plasma density and magnetic field in zebra radio sources

Context. Radio bursts and their fine structures are an integral part of solar flares. Fine in particular used for diagnostics flare processes. The so-called zebras belong to the most important such structures. Aims. We analyze seven zebra events order search spatial variations plasma density magnetic field zebra-stripe sources. Methods. improved method estimating gyroharmonic numbers frequencies. compared observed frequencies with those calculated model. differences these vary thus show field. Results. In six out analyzed zebras, we found a rather high correlation coefficient (about 0.7 higher) between strictly periodic function. These explained by torsional or sausage magnetoacoustic waves loop which sources located. present wavelengths dependence on frequency estimate periods.