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

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

Emission cone of radio waves generated by cyclotron maser instability in nonaxisymmetrical inhomogeneous medium

International audienceFour zones of enhanced probability appear in the CML-Io phase diagram, where the occurrence of the Jovian radio emissions at decameter wavelength is plotted versus the central meridian longitude (CML) and the orbital phase of Io. These zones are the so-called Io-controlled sources Io-A, Io-B (emitted from Jupiter's northern hemisphere), and Io-C, Io-D (emitted from the south). We have plotted the occurrence probability in a polar diagram linked to the local magnetic field, making the assumption that the magnetic field intensity gradient ∇B plays the role of an optical axis for the wave propagation, and introducing an azimuth angle measured relatively to the direction of the magnetic field vector B. The results of our study allow us to conclude that the Io-controlled decameter Jovian radiation is emitted in a hollow cone flattened in a particular direction. The existence of such an emission cone leads us to understand the location of the Io-controlled sources (Io-A, Io-B, Io-C, and Io-D) in the CML-Io phase diagram and to interpret their dependence on the longitude as the manifestation of a Jovian active longitude sector, where the emission mechanism is the most efficient. We study the origin of the flattening of the emission cone in the framework of a radio emission produced by the cyclotron maser instability in an inhomogeneous medium where the local magnetic field B and the gradient of its modulus ∇B are not parallel, i.e., in a geometry without axial symmetry. We consider that the radiation propagates in the source region in the X-mode near its cutoff frequency

LOFAR-CM: A low frequency array station in the Carpathian Mountains

International audienc

LOFAR-CM: A low frequency array station in the Carpathian Mountains

International audienc

LOFAR-CM: A low frequency array station in the Carpathian Mountains

International audienc