David Santos-Costa

Evidence for short-term variability of Jupiter's decimetric emission from VLA observations

Aims. We present the first evidence of short-term variations of Jupiter’s radiation-belt emission obtained with interferometric measurements. Over a two-month period of observational time in 2002, the Jovian synchrotron emission was observed with the Very Large Array (VLA). Methods. The images constructed at the wavelength of 6 cm demonstrate significant changes in the spatial structure of the brightness distribution. The comparisons of the two-dimensional maps with another campaign of VLA observations made in May 1997 confirm our discovery of changes in Jupiter’s synchrotron emission on a time-scale of days to weeks. Results. For a series of central meridian longitudes (CMLs), the radiation peak near 1.4 RJ was observed to shift back and forth from one side of the planet to the other between October and December 2002. The change in the location of the emission peak was found to be the result of fluctuations in the peak brightness distribution by 10% up to 40%. These fluctuations are too significant to be associated with the small variation of the geometric parameter DE (the declination of the Earth as seen from Jupiter) during the campaign of observations. We have demonstrated that the variability of the synchrotron emission peak was observed when the angular sectors covering the Jupiter SIII longitudes λIII where the field strength along the equatorial magnetic surface is maximum or minimum were monitored. Conclusions. Short-term variations of Jupiter’s synchrotron emission are expected to be reported when specific CMLs are observed and changes in the distributions of the 30–40 MeV electron population occur simultaneously. Our discussions led to the conclusion that the short-term variations of Jupiter’s Decimetric brightness distribution may be the result of submicrometre charged dust particles undergoing significant electromagnetic perturbations while interacting with the radiation-belt electrons in the Jovian ring’s innermost component.

Multifrequency analysis of the Jovian electron-belt radiation during the Cassini flyby of Jupiter

Aims. We examine Very Large Array (VLA) observations of Jupiter to present evidence of fluctuations in the emission produced by the electron belt in January 2001. Investigating the source of fluctuations will provide new opportunities to discuss the scenarios of temporal changes in Jupiter’s synchrotron radiation (i.e., the electron belt) in future data analysis and modeling work. Methods. To discuss the electron belt dynamics during the Cassini flyby of Jupiter, we compare the radio measurements from 2−3 January 2001 with VLA observations obtained on 20−21 December 1988, when viewing geometry and array configuration are comparable. All data are scaled to a standard Earth-Jupiter distance of 4.04 AU for comparison purposes. Brightness distribution maps with identical spatial resolutions and cartography of the equatorial radiation are constructed and examined at the wavelengths of ∼21 cm and ∼90 cm. Results. Rotation-averaged maps show that the emission from the equatorial zones of maximum intensity is weaker by ∼5−40%, but the brightness distribution is spatially more extended on 2−3 January 2001, resulting in a total emission at both wavelengths stronger by ∼35%. Between observation periods, the brightness distributions are observed to evolve differently during the planet rotation. Tomographic reconstructions of the equatorial radiation support our conclusion that the electron-belt population was differently distributed around the planet in December 1988 and January 2001. Conclusions. Our analysis of VLA data sets suggests that the spatial distribution of the electron belt on 2−3 January 2001 is different from that usually observed. Our knowledge of solar activity at the time of the Cassini flyby of Jupiter suggests that the emission from the electron radiation belt was responding to external influences, most likely to solar wind structures rather than to solar radio flux, on a timescale of days to a couple of weeks. Combined results from a multisource data analysis ‐ including spacecraft and radio observations ‐ are needed to confirm this relationship.

Imaging Jupiter's radiation belts down to 127 MHz with LOFAR

Context. Observing Jupiters synchrotron emission from the Earth remains today the sole method to scrutinize the distribution and dynamical behavior of the ultra energetic electrons magnetically trapped around the planet (because in-situ particle data are limited in the inner magnetosphere). Aims. We perform the first resolved and low-frequency imaging of the synchrotron emission with LOFAR at 127 MHz. The radiation comes from low energy electrons (~1-30 MeV) which map a broad region of Jupiters inner magnetosphere. Methods (see article for complete abstract) Results. The first resolved images of Jupiters radiation belts at 127-172 MHz are obtained along with total integrated flux densities. They are compared with previous observations at higher frequencies and show a larger extent of the synchrotron emission source (>=4 RJ). The asymmetry and the dynamic of east-west emission peaks are measured and the presence of a hot spot at lambda_III=230 {\deg} ± 25 {\deg}. Spectral flux density measurements are on the low side of previous (unresolved) ones, suggesting a low-frequency turnover and/or time variations of the emission spectrum. Conclusions. LOFAR is a powerful and flexible planetary imager. The observations at 127 MHz depict an extended emission up to ~4-5 planetary radii. The similarities with high frequency results reinforce the conclusion that: i) the magnetic field morphology primarily shapes the brightness distribution of the emission and ii) the radiating electrons are likely radially and latitudinally distributed inside about 2 RJ. Nonetheless, the larger extent of the brightness combined with the overall lower flux density, yields new information on Jupiters electron distribution, that may shed light on the origin and mode of transport of these particles.