Investigating the origins of the Jovian decimetric emission's variability

Abstract

[1] The long-term variability of the Jovian radiation observed at 13-cm wavelength over 30 years is investigated using a physical model of the radiation belts. The model enables us to quantify the effect of the geometric parameter D E on the synchrotron emission. When a full Jupiter rotation is taken into account in the computations, the variability of the total radio flux with D E is only '1% of that measured. Such a dependence is not detectable. In contrast, radio fluctuations observed at a central meridian longitude (CML) and caused by D E changes can be very important, up to 10% of that measured. This strong dependence of the total radio flux on the geometric factor can result in difficulties for examining the origins of the Jovian synchrotron emission's fluctuations during periods where times of observation are not covering large CML range values. The numerical computations reveal that the variability either of the populations injected near the outer boundary of our simulations (Io's orbit) or the inward radial transport can, independently or in combined action, conduct variability in the decimetric emission. The theoretical fluctuations required in our computations for reproducing the radio measurements do not confirm any direct correlation between the long-term changes in the radio data and the radial transport driven by solar wind conditions or solar radio fluxes. Nevertheless, a linear relationship between the total radio flux and solar wind ram pressure for the period of 1970 to 2002 is established. The maximum correlation is reached when the P sw data are shifted by '2.7 years prior to the radio observations. Using this time-lag in the computations, the simulation results show that (P sw ) g can be associated with the variations of the particle injections and the radial transport. The correlation coefficient, associated with the fit between radio data and simulated radio flux densities, is 0.59 for the period of 1971-2002, 0.93 for the period of 1971-1972, and 0.83 for the periods of 1975-1989 and 1992-1995. The index g is set to 0.35 when only particle injections are fluctuating with time, to 0.50 when temporal variations are driving the radial transport, and to 0.15-0.20 when the particles injected in the inner magnetosphere and their transport are simultaneously following the variations of the solar wind ram pressure. The model suggests that the solar wind ram pressure fluctuations can be related to variations of the Jovian decimetric emission on timescales of months, particularly to the enhancements in total radio flux observed in 1987, 1988, 1990 and 1994. Our results thus support the idea that the increase in radio emission in July 1994 was due to the impact of Comet Shoemaker-Levy 9 into Jupiter, in addition to the response of the Jovian magnetosphere to steep changes in solar wind conditions. The divergence between observations and simulations indicates that mechanisms other than those discussed in the modeling must govern the radiation belts dynamics for the periods of

Multifrequency analysis of the Jovian electron-belt radiation during theCassiniflyby of Jupiter

“…The observation of changes in the emission from Jupiter's electron belt has been reported since the early 1970s (e.g., Gérard 1970; Klein et al 1972Klein et al , 1989. Correlations with solar wind on timescales of months to years (e.g., Bolton et al 1989;Galopeau & Gérard 2001;Santos-Costa et al 2008) and solar radio flux on timescales of days to a week (e.g., Miyoshi et al 1999;Tsuchiya et al 2011;Kita et al 2013) have been confirmed since the 1990s. However, the response of the electron belt to the solar wind on short timescales has never been considered until the present work.…”

“…Changes in the spatial distribution of the electron belt are supposedly driven by either internal or external parameters (e.g., Bolton et al 1989;Brecht et al 2001;de Pater and Brecht 2001;Horne et al 2008;Santos-Costa et al 2008, 2011Kita et al 2013, and references therein). Determining the exact cause(s) of changes in the radio maps in Fig.…”

“…Investigating the source of fluctuations in early January 2001 will provide new opportunities to re-examine the scenarios of temporal changes in Jupiter's synchrotron radiation (e.g., Bolton et al 1989;Santos-Costa et al 2008;Kita et al 2013, and references therein). The results of our data analysis will help constrain models of Jupiter's electron belt.…”

VLA observations at 6.2 cm of the response of Jupiter's electron belt to the July 2009 event

“…The Earth declination as seen from Jupiter, D E , varies slowly over a 11.9‐year cycle between the limit values of −3.3° and +3.3°. The effects of this parameter on Jupiter's decimetric emission had been investigated by carrying out data analysis of single dish and interferometric measurements but also by combining models of the electron radiation belt with synchrotron simulation codes or using a 3D Fourier mapping technique to construct the tridimensional structure of the synchrotron radiation [ de Pater and Klein , 1989; Klein et al , 1989; Hood , 1993; Dulk et al , 1997b, 1999a; Leblanc et al , 1997b; Dunn et al , 2003; Sicard et al , 2004; Kloosterman et al , 2005; Santos‐Costa et al , 2008, and references therein]. Data analyses and modeling works have both led to the conclusion that D E cannot induce drastic changes in the appearance of the 2‐D brightness distribution and distorts efficiently the longitudinal profile of the total flux density over periods of time lasting less than a few months.…”

“…However, until the late 1990s [ Miyoshi et al , 1999; Tsuchiya et al , 2010], short‐term variations non‐related to comet‐like impact could never be clearly related to any particular transient magnetospheric phenomena or solar activity. Intensity variations on broader time‐scales (from months to years) have been also observed since the 1970s [ Gérard , 1970, 1976; Gulkis et al , 1973; Berge , 1974; Klein , 1976; Klein et al , 1972, 1989, 2001; de Pater and Klein , 1989; Bolton , 1990; de Pater and Dunn , 2003; Galopeau and Gérard , 2001] and later linked to solar activity [ Bolton et al , 1989; de Pater , 1994; Galopeau and Gérard , 2001; Santos‐Costa et al , 2008].…”