Collisional and viscous damping of MHD waves in partially ionized plasmas of the solar atmosphere

Magnetohydrodynamic (MHD) waves are widely considered as a possible source of heating for various parts of the outer solar atmosphere. Among the main energy dissipation mechanisms which convert the energy of damped MHD waves into thermal energy are collisional dissipation(resistivity) and viscosity. The presence of neutral atoms in the partially ionized plasmas of the solar photosphere, chromosphere and prominences enhances the efficiency of both these energy dissipation mechanisms. A comparative study of the efficiency of MHD wave damping in solar plasmas due to collisional and viscous energy dissipation mechanisms is presented here. The damping rates are taken from Braginskii 1965 and applied to the VAL C model of the quiet Sun (Vernazza et al. 1981). These estimations show which of the mechanisms are dominant in which regions. In general the correct description of MHD wave damping requires the consideration of all energy dissipation mechanisms via the inclusion of the appropriate terms in the generalized Ohm’s law, the momentum, energy and induction equations. Specific forms of the generalized Ohm’s Law and induction equation are presented that are suitable for regions of the solar atmosphere which are partially ionised

Jovian 1.2-kHz nonthermal continuum radiation

Nonthermal continuum is observed at 1.2 kHz on Voyagers 1 and 2 within the Jovian magnetosphere. It is seen in the magnetospheric cavity on both the dayside and nightside, being most intense in the magnetotail lobes when Voyagers were above the plasma sheet. In these regions the radiation was distinctly left‐hand polarized. The observations are considered within the context of other plasma waves reported to exist in the Jovian magnetosphere and of analogous emissions observed at earth. Drawing in particular from our knowledge of continuum source regions within the terrestrial magnetosphere, it is suggested that the most likely main source of the 1.2‐kHz Jovian continuum is the morning/prenoon magnetopause

Using large radio telescopes at decametre wavelengths

International audienceWith the aim of evaluating the actual possibilities of doing, from the ground, sensitive radio astronomy at decametre wavelengths (particularly below ˜50MHz), an extensive program of radio observations was carried out, in 1999-2002, by using digital spectral and waveform analysers (DSP) of new generation, connected to several of the largest, decametre radio telescopes in the world (i.e., the UTR-2 and URANs arrays in Ukraine, and the Nançay Decametre Array in France). We report and briefly discuss some new findings, dealing with decametre radiation from Jupiter and the Solar Corona: namely the discovery of new kinds of hyper fine structures in spectrograms of the active Sun, and a new characterisation of Jupiter's "millisecond" radiation, whose waveform samples, with time resolution down to 40 ns, and correlated measurements, by using far distant antennas (3000 km), have been obtained. In addition, scattering effects, caused by the terrestrial ionosphere and the interplanetary medium, could be disentangled through high time resolution and wide-band analyses of solar, planetary and strong galactic radio sources. Consequences for decametre wavelength imaging at high spatial resolution (VLBI) are outlined. Furthermore, in spite of the very unfavourable electromagnetic environment in this frequency range, a substantial increase in the quality of the observations was shown to be provided by using new generation spectrometers, based on sophisticated digital techniques. Indeed, the available, high dynamic range of such devices greatly decreases the effects of artificial and natural radio interference. We give several examples of successful signal detection in the case of much weaker radio sources than Solar System ones, down to the ˜1Jy intensity level. In summary, we conclude that searching for sensitivity improvement at the decametre wavelength is scientifically quite justified, and is now technically feasible, in particular by building giant, phased antenna arrays of much larger collecting area (as in the LOFAR project). In this task, one must be careful of some specifics of this wavelength range - somewhat unusual in "classical" radio astronomy - i.e., very high level and density of radio interference (telecommunications) and the variable terrestrial ionosphere

Evidence of jovian active longitude: 2. A parametric study.

In a previous work, we developed a model allowing a theoretical location of the Io‐controlled decameter radio sources (Io‐A, Io‐B, Io‐C, and Io‐D) in the central meridian longitude‐Io phase diagram. This model considers the cyclotron maser instability to be at the origin of most auroral planetary radio emissions. We derive the efficiency of this theoretical mechanism at the footprint of the Io flux tube during a complete revolution of the satellite around Jupiter, and we show that some longitudes in the northern and southern hemispheres favor the radio decameter emission and lead to a probability of higher occurrence. In order to make the calculation easier, we suppose that electrons are accelerated in the neighborhood of Io and follow an adiabatic motion along magnetic field lines carried by the satellite. We also assume that the source of free energy needed by the cyclotron maser instability to amplify the waves derives from a loss cone distribution function built up by electrons which have disappeared in Jupiter's ionosphere. We study the effect of several parameters on the theoretical location of the sources in the central meridian longitude‐Io phase diagram, in particular the Jovicentric declination of the Earth and the frequency of emission

Evidence of jovian active longitude: 1. Efficiency of cyclotron maser instability.

Long-term observations of Jupiter's decametric radiation have shown that a great part of emission is modulated by two dominant factors: the planetary rotation and the orbital phase of Io. The first one indicates that the occurrence probability of the radiation depends on the observer's longitude, while the second factor points to a control of part of the radio emission by Io. Within the framework of the cyclotron maser instability, which is supposed to be the mechanism at the origin of most planetary radio emissions, we estimate the efficiency of this theoretical mechanism at the footprint of the Io flux tube during a complete revolution of the satellite around Jupiter. Our study is based on several simplifying hypotheses: on one hand, we suppose that electrons are accelerated in the neighborhood of Io and follow an adiabatic motion along magnetic field lines carried by the satellite; on the other hand, we assume that a loss cone built up by electrons which have disappeared in Jupiter's ionosphere constitutes the main source of free energy needed by the cyclotron maser instability to produce the radiation. We calculate the maximum growth rate of the waves amplified by the mechanism as a function of the jovicentric longitude of Io. It emerges that some longitudes in the Northern and Southern Hemispheres favor the radio decametric emission and lead to a higher occurrence probability. Our results are compared to the occurrences observed for the sources Io-A, Io-B, Io-C, and Io-D in the usual central meridian longitude-Io phase diagram