A peculiar fine structure in the dynamic spectrum of the solar radio emission discovered by the Radio Telescope UTR-2 spectrograph (Kharkiv, Ukraine) in the frequency band 20-30 MHz is discussed. The structure is observed against the background of a broadband type IV radio burst and consists of the parallel drifting narrow bands of enhanced (versus the background) emission and absorption. The observed structure differs from the widely known zebra pattern at the meter and decimeter wavelengths by the opposite directions of the frequency drift within the limits of a single stripe at a given time. It is shown that the observed peculiarities can be understood in the framework of the plasma mechanism of the radiation origin by virtue of the double plasma resonance effect in a nonuniform coronal magnetic trap. The source model providing the peculiar frequency drift of the zebra stripes is proposed.
Strong and weak aspects of different theories of fine structure on solar radio
emission dynamic spectra observed as several or numerous quasi-equidistant bands of
enhanced and reduced radiation (zebra pattern) are discussed. Most of the works which
propose zebra pattern interpretation are based on the plasma mechanism of radio emission
generation, which consists of excitation of plasma (electrostatic) waves and their
subsequent transformation into electromagnetic emission. Plasma waves arise due to kinetic
or hydrodynamic instability at the upper hybrid frequencies at the levels of double
plasma resonance in a distributed source. Some works are devoted to considering whistlers
as the main reason for stripes in emission and absorption occurring in the dynamic spectra.
An alternative theory of zebra pattern origin suggests that of a compact source with
trapped plasma waves is present in the corona. Another interpretation is based on special
effects that may occur when radio waves propagate through some periodic structure in
All suggested mechanisms are analyzed with relation to their capability to give the
best fit for the observed fine structure features in the framework of the source model with
reasonable physical parameters. It is shown that the theory based on the effect of double
plasma resonance in a nonhomogeneous coronal loop is the best-developed theory for the
origin of zebra pattern at the meter-decimeter wavelengths at the present time.