V. V. Zheleznyakov

Cyclotron wave instability in the corona and origin of solar radio emission with fine structure: I. Bernstein modes and plasma waves in a hybrid band

The longitudinal waves (Bernstein modes and plasma waves near the hybrid frequency) in a mixture of equilibrium coronal plasma and a small group of energetic electrons are investigated. The energetic electrons have a nonequilibrium momentum distribution inherent in trapped particles. The frequency dependence of the cyclotron instability increments is studied. Attention is paid to a significant role of the relativistic effects for the cyclotron instability of longitudinal waves. For sufficiently large velocity of nonequilibrium electrons the increments are shown to increase when the hybrid frequency coincides with one of the gyrofrequency harmonics (double plasma resonance). The results obtained are used in Parts II and III to explain ‘tadpoles’ and ‘zebra’-pattern in solar radio bursts.

Cyclotron wave instability in the corona and origin of solar radio emission with fine structure: III. Origin of zebra-pattern

An interpretation is suggested of the fine structure of tadpole solar radio emission based upon the theory of Bernstein mode cyclotron instability. It is shown that the frequency spectrum of an individual tadpole is similar to the frequency behaviour of the Bernstein mode increment if the velocity of electrons trapped by the magnetic field amounts nearly to 109 cm s-1. The absorption ‘body’ and emission ‘tail’ are associated with the instability due to the kinematic Doppler effect and the ‘eye’ with the instability due to the velocity dependence of the electron mass. The observed radio emission is the result of nonlinear conversion (coalescence) of Bernstein modes at different frequencies into electromagnetic waves. The appearance of separate tadpoles (interrupted character of emission in time) is attributed to the pulse injection of nonequilibrium electrons into the trap. Estimations are presented for the expected dimensions of emission region and the hot electron density necessary for Bernstein mode excitation.

On the third harmonic in solar radio bursts

Possible mechanisms of the third harmonic generation in solar radio bursts are investigated. It is shown that the most essential is the coupling of plasma waves in a source with electromagnetic radiation at the second harmonic. This phenomenon may be responsible for the occurrence of the third harmonic in the type V event on 25 October 1972 recorded by Benz, as well as in the U-burst on 3 September 1957 reported by Haddock and Takakura.

Gyro-resonance absorption of plasma waves in the corona and the fine structure of solar radio bursts

We consider the important part played by plasma wave absorption at the higher harmonics of the electron gyrofrequency (under conditions of the so-called double plasma resonance) in the solar corona. An explanation is proposed for the fine structure of Type III bursts on the basis of this effect.

Thermal cyclotron radio emission of neutral current sheets in the solar corona

Cyclotron radiation (its frequency spectrum and polarization) of thermal electrons in a neutral current sheet is considered. It is shown that cyclotron radiation is able to escape only from a thin edge of the sheet where the magnetic field is practically homogeneous. Due to this, the frequency spectrum has the form of comparatively narrow lines with integer ratio of frequencies. This fact enables one to recognize neutral current sheets in the solar corona by their radio emission.

Detection of a cyclotron line in the radio spectrum of a solar active region and its interpretation

Observations of the active region AR 7962 obtained at 2–32 cm on the RATAN-600 radio telescope on May 10–12, 1996, are presented. The high-resolution measurements detected a narrow feature near 8.5 cm against the background of the smooth spectrum of the local source associated with sunspots. This narrow-band emission is identified with a bright, pointlike, high-frequency source at 1.7 cm recorded on maps made using the Nobeyama radio telescope. The characteristics of the observed line (lifetime 3 days, brightness temperature of the order of several million Kelvin, relative width of about 10%) suggest that it can be explained as thermal cyclotron radiation at the third harmonic of the electron gyrofrequency from a compact source containing a dense, hot plasma; the corresponding higher frequency emission could be due to thermal Bremsstrahlung. Analysis of the RATAN-600 and Nobeyama data can be used to probe the magnetic field, kinetic temperature, and electron density in the radiation source in the corona.

Thermal Cyclotron Radiation from Solar Active Regions

Various frequency spectra with the fine structure resulting from the thermal cyclotron radio emission from solar active regions are discussed. The conditions in sources (distribution of magnetic field and kinetic temperature over the height) are put forward which provide the frequency spectrum as a set of cyclotron lines and high frequency cut-offs. For each kind of distribution the frequency spectrum and polarization are of peculiar character. This permits one to find the conditions in the source through the properties of the observed microwave solar radio emission. To obtain reliable data on the fine structure and judge about conditions in the sources it is necessary to study microwave solar radio emission using the swept-frequency or multi-channel receivers combined with high directional antennae.