A 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 Titan’s ionosphere which in the course of Titan’s 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 Titan’s 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.
Comment on "Low-energy electron production by relativistic runway electron avalanches in air" by J. R. Dwyer and L. P. Babich
We have recently introduced an irregularity index λ for daily sunspot numbers ISSN, derived from the well-known Lyapunov exponent, that attempts to reflect irregularities in the chaotic process of solar activity. Like the Lyapunov exponent, the irregularity index is computed from the data for different embedding dimensions m (2-32). When m = 2, λ maxima match ISSN maxima of the Schwabe cycle, whereas when m = 3, λ maxima occur at ISSN minima. The patterns of λ as a function of time remain similar from m = 4 to 16: the dynamics of λ change between 1915 and 1935, separating two regimes, one from 1850 to 1915 and the other from 1935 to 2005, in which λ retains a similar structure. A sharp peak occurs at the time of the ISSN minimum between cycles 23 and 24, possibly a precursor of unusual cycle 24 and maybe a new regime change. λ is significantly smaller during the ascending and descending phases of solar cycles. Differences in values of the irregularity index observed for different cycles reflect differences in correlations in sunspot series at a scale much less than the 4-yr sliding window used in computing them; the lifetime of sunspots provides a source of correlation at that time scale. The burst of short-term irregularity evidenced by the strong l-peak at the minimum of cycle 23-24 would reflect a decrease in correlation at the time scale of several days rather than a change in the shape of the cycle.
This study investigates the relationship between the equatorial atmospheric angular momentum oscillation in the nonrotating frame and the quasi-diurnal lunar tidal potential. Between 2 and 30 days, the corresponding equatorial component, called Celestial Atmospheric Angular Momentum (CEAM), is mostly constituted of prograde circular motions, especially of a harmonic at 13.66 days, a sidelobe at 13.63 days, and of a weekly broadband variation. A simple equilibrium tide model explains the 13.66 day pressure term as a result of the O1 lunar tide. The powerful episodic fluctuations between 5 and 8 days possibly reflect an atmospheric normal mode excited by the tidal waves Q1 (6.86 days) and σ1 (7.095 days). The lunar tidal influence on the spectral band from 2 to 30 days is confirmed by two specific features, not occurring for seasonal band dominated by the solar thermal effect. First, Northern and Southern Hemispheres contribute equally and synchronously to the CEAM wind term. Second, the pressure and wind terms are proportional, which follows from angular momentum budget considerations where the topographic and friction torques on the solid Earth are much smaller than the one resulting from the equatorial bulge. Such a configuration is expected for the case of tidally induced circulation, where the surface pressure variation is tesseral and cannot contribute to the topographic torque, and tidal winds blow only at high altitudes. The likely effects of the lunar-driven atmospheric circulation on Earth's nutation are estimated and discussed in light of the present-day capabilities of space geodetic techniques.
 Two very bright ultraviolet (UV) radiation sources (equatorial spots) which are located on the limb of Io near its equator have been detected in a series of observations with the Hubble space telescope. In this paper, we propose the mechanism that provides the sufficient energy of the equatorial spots to explain their high brightness in the UV wavelength range. According to the proposed model, this UV radiation is generated due to electrons which are formed as a result of additional ionization of the atmosphere in the front part of the satellite. These secondary electrons in crossed electric and magnetic fields are shifted downstream into Io’s flanks. The optical depth of the source increases on the flanks of Io’s atmosphere (from the vantage point of the observer), and we therefore observe the brightest UV radiation in this region, the value of which is in good agreement with the measured values. Also, a reasonable explanation is given for the main observed properties of the UV equatorial spots, such as (1) a correlation between the brightness of the emission and the magnetic longitude of Io and as a result, Io’s distance from the plasma torus centrifugal equator; (2) a correlation between the equatorial spot location and the planetary magnetic field orientation; and (3) the excess of the brightness of anti-Jovian UV source over the brightness of sub-Jovian source.
We develop a theory of formation of a fine structure in the dynamic spectra of the Jovian decametric radio emission. Main attention is paid to the formation of narrowband (NB) emission and quasiperiodic trains of short (S) bursts. Our model is based on the effects of occurrence of the amplitude‐frequency modulation and extension of the frequency spectrum of a signal during propagation of radiation in a medium with timevaried parameters. It is shown that nonstationary disturbances of the planetary magnetic field and strong frequency dispersion of the plasma at frequencies close to the cutoff frequency of the extraordinary wave in the Jovian ionosphere play a crucial role in the formation of NB emission and quasiperiodic trains of S bursts. As a result of the numerical experiments, it was concluded that the amplitude‐frequency characteristics of an initially continuous signal can drastically vary as a functions of the form of the magnetic field disturbance in the Jovian ionosphere. Structures similar to those observed in the real experiments, ranging from NB emission and quasiperiodic trains of S bursts to more complex structures, arise in the dynamic spectrum. Time variation in the conditions of generation and propagation of decametric radiation in the Jovian ionosphere is reflected in the dynamic spectrum as a time variation in the fine structure of the radiation. For example, a structure of the NB emission type is replaced by a quasiperiodic train of S bursts and vice versa.