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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.

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.

Numerous experimental data indicate that type III solar radio bursts are generated by the streams of fast electrons in the corona. The process of the electron acceleration in the flare region is, in general, of the character of a short time local explosion. As a result, a spatially limited stream with inhomogeneous front and back is formed in the corona. The present paper shows that such a spatial structure of fast electrons radically changes the dynamics of the stream instability development. In particular, for example, despite strong quasilinear relaxation, in this case the electron stream can generate plasma waves in the corona for a long time due to faster electrons escaping out of the front of the stream. The extension of the stream in the outer corona where the collisions are negligible is of similar character. The maximum of the energy density in the packet of excited plasma waves travels in the corona with constant mean velocity which is defined by the fast-electron energy at the moment of their formation in the flare region. Therefore, in spite of the considerable influence of quasilinear effects on the stream motion, the velocity of type III sources found in terms of the drift velocity remains unchanged. This creates the illusion of stream stabilization, The energy dissipation of plasma waves for the low frequency type IiI bursts is fully determined by Landau damping in the tail of the stream. Because of this, the temperature estimates of the outer corona from time profiles of type IIi bursts are incorrect. The theoretical curves of the time variation of radiation at different frequencies agree well with the experimental data in the hectometer wave range under the assumption that the electromagnetic wave generation takes place at the second harmonic of the plasma frequency. In this case it is necessary to decrease the electron density in the solar corona at distances of about 5 to 30 R o 4 times as compared with the previous densities previously derived from type III data.

Planets that are embedded in the changing magnetic fields of their host stars can experience significant induction heating in their interiors caused by the planet's orbital motion. For induction heating to be substantial, the planetary orbit has to be inclined with respect to the stellar rotation and dipole axes. Using WX UMa, for which the rotation and magnetic axes are aligned, as an example, we show that for close-in planets on inclined orbits, induction heating can be stronger than the tidal heating occurring inside Jupiter's satellite Io; namely, it can generate a surface heat flux exceeding 2 W m −2 . An internal heating source of such magnitude can lead to extreme volcanic activity on the planet's surface, possibly also to internal local magma oceans, and to the formation of a plasma torus around the star aligned with the planetary orbit. A strongly volcanically active planet would eject into space mostly SO 2 , which would then dissociate into oxygen and sulphur atoms. Young planets would also eject CO 2 . Oxygen would therefore be the major component of the torus. If the Oi column density of the torus exceeds ≈10 12 cm −2 , the torus could be revealed by detecting absorption signatures at the position of the strong far-ultraviolet Oi triplet at about 1304Å. We estimate that this condition is satisfied if the Oi atoms in the torus escape the system at a velocity smaller than 1-10 km s −1 . These estimates are valid also for a tidally heated planet.