Conference Proceedings
2011 EN
Baptiste Cecconi
Dynamic spectra of planetary radio emissions depend on physical and geometrical conditions: emission process; energy of emitting electrons; angle between the source magnetic field and the wave direction, which varies with frequency; location of the observer... Their modeling is an intrinsically 3-dimensional problem for which a code has been developed: SERPE / ExPRES (Simulateur d’´Emissions Radio Plan´etaires et Exoplan´etaires / Exoplanetary and Planetary Radio Emissions Simulator). This tool has been successfully applied to the modeling of arc-shaped radio emissions generated by the Io-Jupiter electro-dynamic interaction, as well as of Saturn’s Kilometric Radiation dynamic spectra. It allowed us to determine the energy of the emitting electrons, to identify the important of the oblique mode in the rarefied auroral plasmas of Jupiter and Saturn and to clarify the link between the Io-Jupiter radio emissions and of the UV spot at the magnetic footprint of Io. In light of these results, we will briefly review the characteristics of radio emissions related to Jupiter’s aurorae and to satellite-Jupiter interactions. The EJSM (Europa Jupiter System Mission) mission is a unique opportunity to study the Jovian magnetosphere (from the close environment of Galilean satellites and their coupling with the Jovian magnetic field, to the auroral regions of Jupiter). It should carry the first goniopolarimetric radio receiver in Jovian orbit. We will illustrate with Cassini results the scientific enhancement of radio astronomy science brought by such capabilities that will not be available to any other mission around Jupiter. We will show how our present knowledge of Jupiter’s decameter radio emissions may allow us to optimize the scheduling of low-frequency radar observations. Finally, we will discuss the unique opportunities that would be offered by multi-spacecraft magnetospheric observations in the Jovian system.
Conference Proceedings
2011 EN
S. J. Bolton
Juno is the next mission to Jupiter. Juno’s overarching scientific goal is to understand the origin and evolution of Jupiter. As the archetype of giant planets, Jupiter holds the key to understanding the origin of our own solar system and the origin of the extra-solar planetary systems now being discovered around other stars. Juno’s investigation of Jupiter focuses on four themes: Origin, Interior Structure, Atmospheric Composition and Dynamics, and the Polar Magnetosphere. Juno’s scientific measurements include global maps of the gravity and magnetic fields, microwave radiometry of Jupiter’s deep atmosphere and a full suite of fields and particles measurements of Jupiter’s polar magnetosphere. Juno’s 32 polar orbits extensively sample Jupiter’s full range of latitudes and longitudes. High sensitivity radiometric measurements yields information on Jupiter’s deep atmosphere (down to ~1000 bars) which will be used to infer the global abundance of water, and to investigate the complex meteorology of Jupiter’s atmosphere. Determining the Jovian water abundance and whether a solid core exists within Jupiter permits discrimination between various scenarios of the formation of Jupiter. The gravity data constrain the planet’s interior rotation, core size and interior structure. The magnetic field measurements investigate how the interior dynamo works and examine the depth of generation of Jupiter’s powerful magnetic field. Fields and particles measurements as well as UV and IR polar images investigate Jupiter’s auroral physics to determine what drives Jupiter’s remarkable northern and southern lights. An overview of the mission and science objectives will be presented with an emphasis on Juno’s investigation of Jupiter’s polar magnetosphere and radio emission.
Conference Proceedings
2011 EN
Laurent Lamy
During the flyby of the Earth by Cassini in 1999, the Radio and Plasma Wave Science (RPWS) instrument recorded one month of quasi-continuous observations of Auroral Kilometric Radiation (AKR). Analyzing the Stokes parameters of incoming radio waves, we found AKR to be 100% circular left-handed (LH) or right-handed (RHW). We analyzed separately the northern - RH - emission, from the southern - LH - one with respect to the magnetic equator. AKR power variations reveal (i) a log-normal distribution at time scales of minutes, (ii) bursts of emission at time scales of a few hours, and (iii) a clear modulation at 24 hours, with a weaker modulation at 12 hours (especially visible for LH emissions). The prominent 24 h modulation is found to modulate LH and RH AKR in phase opposition. This is interpreted as being due to visibility effects related to the precession of the terrestrial magnetic dipole, making Cassini oscillate relative to the average AKR beaming pattern from each hemisphere. We accordingly quantified the AKR beaming vs explored latitudes. On time scales shorter than a few hours, LH and RH emissions are found to be correlated. This is attributed to the actual conjugacy of the corresponding sources, simultaneously turned on by substorm occurrence. The geometrical anti-correlation (at 24 h) dominates close to Earth, while the short term correlation (substorms) dominates far from Earth, where the detection threshold makes the visibility less important than the occurrence of substorms. Finally, the 12 h modulation is detected when it is not masked by strong visibility effects, i.e. for the LH emission which is observed mostly near the magnetic equator along the path of Cassini. A 12 h modulation being also observed in some geomagnetic indices, we suggest that a physical process (e.g. semi-diurnal variable efficiency of the reconnection between interplanetary and geomagnetic fields, or magnetotail oscillations) may be responsible of the observed AKR 12 h modulation.
Conference Proceedings
2011 EN
D. A. Gurnett
The discovery of Saturn kilometric radiation (SKR) was made by the Voyager spacecraft over three decades ago. The Voyager observations showed that SKR had a well-defined clock-like amplitude modulation with a period of 10 hr 39 min 24±7 sec. Since then much has changed. In 2000 radio observations by the Ulysses spacecraft showed that the SKR modulation period varied by as much as several minutes on time scales of years. This long-term variability was subsequently confirmed by radio measurements from the Cassini spacecraft, which was put in orbit around Saturn on 1 July 2004. We now know that there are three basic types of Saturnian radio emissions: SKR at frequencies from about 50 to 1,200 kHz; narrowband (NB) emissions in two frequency ranges, near 5 kHz and near 20 kHz; and whistler-mode auroral hiss at frequencies from a few Hz to several kHz. All of these radio emissions display long-term variations in their modulation periods of up to one percent or more on time scales of years, with smaller variations on shorter time scales. For several years prior to Saturn’s recent equinox (August 2009) these radio emissions displayed two dominant periods of about 10.6 and 10.8 hours. The 10.6-hour period has been shown to be associated with SKR and auroral hiss originating from the northern auroral zone; and the 10.8-hour period has been associated with SKR and auroral hiss originating from the southern auroral zone. The narrowband emissions observed during this period have the same two periods as the SKR and auroral hiss, but do not have the corresponding north-south symmetry. As equinox approached the two periods converged and now appear to have crossed several months after equinox. A similar crossing of the two components may have occurred in the Ulysses measurements of SKR during the Saturn’s equinox fourteen years ago. Possible models for explaining these complex long term variations will be discussed.
Conference Proceedings
2011 EN
Roland Karlsson
We report on solar radio bursts observed by the RPWS experiment on board the Cassini spacecraft in the period from 1st January 2004 to 31st March 2010. In this time intervals of about six years a limited number of strong solar type III bursts, less than 300, has been recorded. This is mainly due to the solar activity which reaches its minimum in 2008–2009. In this study we consider type III solar bursts observed at frequencies lower than 1.2 MHz generated in the interplanetary medium. We analyse the solar bursts with the aim to estimate the Cassini local time (LT) occurrence rate, where the Kronian day has been divided into eight LT sectors. Our results are combined with the Cassini orbits where the LT and the distance to the planet are taken into consideration. We show that the type III burst occurrence rates depend on the solar activity, however the day side sector (midday to early afternoon) exhibits the lowest rate of occurrence.
Conference Proceedings
2011 EN
V. Krupar
Solar Orbiter is an M-class mission in the ESA Science Programme Cosmic Vision 2015 – 2025 having an orbit with perihelion as low as 0.28 AU. The Radio Plasma Waves (RPW) Analyzer on board will provide new insights into the microscale phenomenon, the propagation modes of the radio waves and the localization of their source regions. The three electric antennas (each 5 meters long) are designed to be mounted on booms in a perpendicular plane to the spacecraft-Sun axis. Effective antenna lengths and directions are different from the physical ones due to their coupling with the spacecraft body. These parameters have been investigated considering various antenna placements on the spacecraft body [Rucker et al., this issue]. Results indicate that all effective antenna directions will be slightly tilted towards the Sun. This paper discusses a possible accuracy of the Direction Finding (DF) with respect to this tilt angle and uncertainties of the effective antenna parameters.
Conference Proceedings
2011 EN
M. Maksimović
One of the science goals of the Solar Orbiter mission is to study the connectivitybetween the solar corona and the inner Heliosphere as close as from 0.3 AU. Withthis respect the study of Solar radio emissions produced energetic electrons eitherflare or shock accelerated will be of prime importance. I will first review some recentfindings obtained with the help of Ulysses, WIND and Stereo observations. I willthen present the expected capabilities of the Solar Orbiter instrumentation relevantto the discussed topic.
Conference Proceedings
2011 EN
R. J. MacDowall
Locating low frequency radio observatories on the lunar surface has a number of advantages, including fixed locations for the antennas and no terrestrial inteference on the far side of the moon. Here, we describe the Radio Observatory on the Lunar Surface for Solar studies (ROLSS), a concept for a near-side, low frequency, interferometric radio imaging array designed to study particle acceleration in the corona and inner heliosphere. ROLSS would be deployed during an early lunar sortie or by a robotic rover as part of an unmanned landing. The prime science mission is to image intense type II and type III solar radio bursts with the aim of determining the sites at and mechanisms by which the radiating particles are accelerated. Secondary science goals include constraining the density of the lunar ionosphere by searching for a low radio frequency cutoff of the solar radio emissions and detecting the low energy electron population in astrophysical sources. Furthermore, ROLSS serves as a pathfinder for larger, far-side lunar radio arrays, designed for faint sources.
Conference Proceedings
2011 EN
Patrick H. M. Galopeau
Recent observations performed by the radio and plasma wave science (RPWS) experiment on board the Cassini spacecraft have revealed the presence of two distinct and variable spin modulation periods (10.6 hours and 10.8 hours) in Saturn’s radio emissions emanating from the northern and southern hemispheres respectively. The main time modulation of planetary radio emissions has always been attributed to the effect on the inner magnetosphere of the internal magnetic field which rigidly rotates with the deep interior of the planet. The magnetospheric plasma is supposed to be frozen in this magnetic field so that a north/south asymmetry in the radio modulation period should never be observed. However Saturn’s magnetic field is very particular since its dipolar moment is nearly aligned with the rotation axis of the planet. Such an alignment could bring out some phenomena in the internal structure which are masked in the case of other magnetized planets the magnetic dipole of which is significantly tilted. The existence of two separated and slowly varying periods in the saturnian magnetic field could be the signature of a dynamo the dynamics of which is governed by a Rikitake system.
Conference Proceedings
2011 EN
Nicolás André
Earth-orbiting satellites have routinely traversed the source regions of auroral kilometric radiation. This radio emission is generated via the cyclotron maser instability very close to the electron cyclotron frequency. While Cassini’s orbit has crossed auroral field lines, the radial distance at auroral latitudes is typically too high for the analogous Saturn kilometric radiation source. However, on Oct. 17, 2008, the Radio and Plasma Wave Science instrument detected the kilometric radiation at and just below the electron cyclotron frequency. At this time the spacecraft was at a distance of 5 Saturn radii, at 0.9 hours local time, and on L-shells in the range of 25 to above 30. Here the magnetic field suggests the corresponding current was directed upward, away from the planet. Low energy electron observations by the Cassini Plasma Spectrometer instrument suggest that growth of the SKR is likely due to an unstable shell-like distribution.