V. V. Zaitsev

Dependence of the Io-related decametric radio emission of Jupiter on the central meridian longitude and Io's "active" longitudes

Context. The statistical analysis of the Io-related decametric radio emission of Jupiter shows that this emission depends precisely on the central meridian longitude. This dependence is the result of the existence of Io’s “active” longitudes, i.e. particular regions of Io’s orbit, which are fixed with respect to the Jovian magnetic field and at which Io-related emission occurs more often. Aims. The paper considers the mechanism of the formation of Io’s “active” longitudes. Methods. The formation of Io’s “active” longitudes is caused by two factors: first, the change of the efficiency of particle acceleration in Io’s ionosphere, depending on Io’s longitude, and second, the degree of broadening of the angular spectrum of accelerated electrons during their passing through the plasma torus. Results. It is shown that the mechanism considered explains rather well why Io-related decametric bursts begin to appear much more often in longitudes of the range 120 ◦ < λIo < 300 ◦ (λIo is the longitude in the frame III), and why one predominantly observes the emission from the sources located in the northern Jovian hemisphere.

Acceleration of electrons in Titan's ionosphere

[1]xa0A consideration of the acceleration mechanism which supplies the fast electrons to the source of Saturnian kilometric radiation (SKR) and an interpretation of the recently reported observational indications of the influence of Titan on the SKR are presented. The proposed mechanism operates by the effect of the different magnetization of the electrons and ions in Titans ionosphere which in the course of Titans motion through the Saturnian magnetic field causes the creation of a charge-separation electric field. This field has a component parallel to the magnetic field and accelerates part of the ionospheric electrons (called “runaway electrons”). The performed estimates show that the mechanism accelerates the runaway electrons up to an energy of ∼5 keV. The power of the acceleration mechanism is sufficient for SKR generation and also for the ultraviolet luminescence of Titans atmosphere. The weakening of the SKR when Titan passes on the dayside of Saturn is due to a decrease of the magnetic field strength near the dayside magnetopause, when the Moon escapes the Saturnian magnetosphere, as well as due to the break in the magnetic connection between the electron acceleration region on Titan and the SKR sources. The latter prevents the penetration of the accelerated electrons into the radiation generation region. When Titan is on the nightside of Saturn, it enters into shell L∼14, which is stretched owing to the ring current. In this case, the electrons that accelerated in the ionosphere of Titan can reach the nightside SKR sources and activate them and therefore being the reason for the Titan influence on the SKR.

Origin of ultraviolet emission source in the Jovian ionosphere at the feet of the Io flux tube

A model for an explanation of far ultraviolet (FUV) emissions observed from the Io flux tube footprints at the Jovian ionosphere is presented. An acceleration of electrons and ions of the ionospheric plasma up to sufficient energies for an effective excitation of H2 Lyman and Werner bands is proposed. The neutral particles within the Jovian ionosphere are excited due to their collisions with the charged particles relatively moving to the neutrals in the ionospheric plasma. The motion is induced by the Ampere force which arises due to the interaction between the electric current flowing within the Io magnetic flux tube and its own specific magnetic field. It is established that only a sufficiently strong field-aligned current is capable of providing the required velocity of the relative motion. Our estimates show that in principle the proposed mechanism can provide the level of FUV emissions observed from the Io flux tube footprints.