Abstract. An intense radio flare on the dMe star AD Leo, observed with the Effelsberg radio telescope and spectrally resolved in a band of 480 MHz centred at 4.85 GHz is analysed. A lower limit of the brightness temperature of the totally right handed polarized emission is estimated as13 K considered to be more probable), which requires a coherent radio emission process. In the interpretation we favour fundamental plasma radiation by mildly relativistic electrons trapped in a hot and dense coronal loop above electron cyclotron maser emission. This leads to densities and magnetic field strengths in the radio source of n ∼ 2 × 10 11 cm −3 and B ∼ 800 G. Quasi-periodic pulsations during the decay phase of the event suggest a loop radius of r ∼ 7 × 10 8 cm. A filamentary corona is implied in which the dense radio source is embedded in hot thin plasma with temperature T ≥ 2 × 10 7 K and density next ≤ 10 −2 n. Runaway acceleration by sub-Dreicer electric fields in a magnetic loop is found to supply a sufficient number of energetic electrons.

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We offer a coherent plasma-emission mechanism for intense microwave radiation of a flare in the impulsi,Je phase. The radiation arises because of a loss-cone instability of plasma waves near the upper hybrid frequency in the energy release region (flare kernel). Under typical conditions of a flare kernel, conversion of plasma waves into electromagnetic ones causes the radio emission to dominate the second harmonic of the plasma frequency. The radiation is polarized in the extraordinary sense. The Bernstein mode loss-cone instability leads to a zebra-pattern in the microwave radiation. MHD-oscillations of the flare. kernel give rise to pulsations of the microwave and hard X-ray emission. The plasma mechanism offers new possibilities of flare plasma diagnostics.

Coronal loops, which trace closed magnetic field lines, are the primary structural elements of the solar atmosphere. Complex dynamics of solar coronal magnetic loops, together with action of possible subphotospheric dynamo mechanisms, turn the majority of the coronal loops into current-carrying structures. In that connection none of the loops can be considered as isolated from the surroundings. The current-carrying loops moving relative to each other interact via the magnetic field and currents. One of the ways to take into account this interaction consists in application of the equivalent electric circuit models of coronal loops. According to these models, each loop is considered as an equivalent electric LCR-circuit with variable inductive coefficients L, capacitance C, and resistance R, which depend on shape, scale, position of the loop with respect to neighbouring loops, as well as on the plasma parameters in the magnetic tube. Such an approach enables to describe the process of electric current dynamics in the groups of coronal loops, as well as the related dynamical, energy release and radiation processes.In the present paper we describe the major principles of LCR-circuit models of coronal magnetic loops, and show their application for interpretation of the observed oscillatory phenomena in the loops and in the related radiation.

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