The results of investigation of the cyclotron wave instability in the corona for Bernstein modes and plasma waves in a hybrid band have been used to interpret zebra-pattern phenomena. Two models of the generation region of parallel drifting bands are considered: the model of the point source localized at the apex of the magnetic trap and the model of a distributed source extended along the magnetic flux tube. In the first model it is assumed that a harmonic character of zebrapattern appears either in coalescence of excited Bernstein modes at different harmonics of the gyrofrequency or in coalescence of these modes with plasma waves excited in the hybrid band. In the latter case if the magnetic field changes in time a pulsating generation regime occurs. In the second model, the emission bands appear in the regions of double plasma resonance as a result of coalescence of longitudinal waves excited in the hybrid band. Estimations of the magnetic field and the nonequilibrium component density necessary for the zebra-pattern to generate are presented.* Here and below a roman II before the number of the formula or figure denotes a reference to the corresponding formula or figure in the paper by Zheleznyakov and Zlotnik (1975b) cited as II. A roman I refers to the paper by Zheleznyakov and Zlotnik (1975a). Solar Physics 44 (1975) 461-470. All Rights Reserved Copyright 9 1975 by D. Reidel Publishing Company, Dordrecht-Holland

Abstract.We discuss a source model for the origin of solar type IV burst fine structures (FS) using the data of an event in AR 7792 on 25 October 1994. After giving a comprehensive observational treatment of FS (Paper I), here we repeat the main observed facts to construct a simplified radio source model. It consists of two interacting loops (named LS1 and EL) with one spatial order of magnitude scale difference (turning heights 70 and 7 Mm). We consider the implications of this model for physical mechanisms of broad band pulsations (BBP) and zebra patterns (ZP). Our analysis leads to the conclusion that meter wave BBP and ZP originate from a common magnetic source structure -a large asymmetric coronal loop. It is shown that the BBP result from periodically repeated injections of fast electrons into the asymmetric magnetic trap. The excitation of plasma waves is due to the stream instability when these electrons are propagating along the loop. We demonstrate that a two percent quasi-periodic modulation of a magnetic field component in EL is sufficient for it to act as a periodic electron accelerator. The ZP is due to a plasma wave instability at the levels of double plasma resonance (DPR) in an inhomogeneous source distributed along the loop axis of LS1. The DPR frequencies appear at those height levels where the upper hybrid frequency is equal to a harmonic of the gyrofrequency. Two Appendices review theoretical details needed to understand the given ZP interpretation. The gyrofrequency as a function of height was derived from a force-free extrapolated field line that passes the coronal radio source. After knowing the loop turning height and the magnetic field strength we identified for a fixed observing time the harmonic number of each zebra stripe. The comparison of the calculated DPR levels with the observed zebra stripe peak frequencies yields a density law for the ZP source volume. It turns out that this is a barometric law with a temperature near 106 K. We demonstrate that the drift of the whole ZP to higher frequencies can be explained as a signature of magnetic field decrease and/or plasma cooling in the ZP source. The time delay between BBP and ZP was found to be due to the higher fast particle threshold of the DPR versus the beam instability. The present analysis confirms the double plasma resonance model for the ZP fine structure, and underlines the significance of force-free extrapolated photospheric fields for coronal magnetic field modelling.

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. Solar Physics 43 (1975) 431-451. All Rights Reserved Copyright 9 1975 by D. Reidel Publishing Company, Dordrecht-Holland 432 v.v. ZHELEZNYAKOV AND E. YA. ZLOTNIK BERNSTEIN MODES AND HYBRID WAVES 435

No abstract

Abstract. The fine structures (FS) of solar type IV radio bursts are of principal interest for flare plasma diagnostics in the low corona. In this paper we give an observational (Part I) and theoretical (Paper II) treatment of broad band radio pulsations (BBP) and zebra patterns (ZP) in a well observed flaring sigmoidal loop system of AR 7792 on 25 October 1994. We present comprehensive meter-decimeter radio spectral (Astrophysical Institute Potsdam, AIP) and meter wave heliographic (Nançay Radio Heliograph of Paris-Meudon Observatory, NRH) observations. Spectral and spatial properties of FS elements (one pulsation pulse, one single zebra stripe) as well as a statistical analysis for the whole fine structure event are presented. The source sites are compared with soft X-ray images of the flare, and with force-free extrapolated coronal magnetic fields. Both FS sources occur in a common diverging loop structure with a turning height of about 70 Mm. The BBP source is shown to appear (if seen along the loop axis) nearer to the injection site of the electrons than the ZP source. BBP do show high frequency drift (≈-250 MHz s −1 ). At a given frequency, the projected source speed is 1.1 × 10 5 km s −1 . For ZP, we find a good correlation between the inclination of a single zebra stripe to the heliographic observing frequency level in the dynamic spectrogram, and the speed of the simultaneously observed projected source motion at this frequency. The direction of the source motion at a given frequency is on average found to be perpendicular between BBP and ZP sources. During a time interval of 90 s the BBP source consists in its lower part (higher observing frequencies) of a widely spaced double source. Despite a source distance of 360 Mm both subsources are highly correlated and thus probably simultaneously driven. We come to the conclusion that both fine structures are emitted during repeated electron beam injection into an asymmetric magnetic trap configuration between a footpoint in the leading spot of AR 7792 and a trailing more dispersed footpoint. In Paper II we will show that the specifications derived from the data allow for selecting one out of several competing fine structure models which explains the simultaneous formation of BBP and ZP in the same loop structure.