Reply to comment by B. Cecconi on "Spectral features of SKR observed by Cassini/RPWS: Frequency bandwidth, flux density and polarization"

International audienceThe main purpose of the paper by Galopeau et al.[2007] was to classify the spectral features of the Saturniankilometric radiation (SKR) starting from three physicalobserved parameters: the frequency bandwidth, the fluxdensity, and the pol arization. We show in the presentresponse that an unsupervised application of arbitrary auto-matic criteria during the data processing (such as a signal-to-noise ratio greater than 23 dB) can totally judge a weaknatural emission as a background noise. As a consequence,such a situation may lead to consideration of only the datapresenting a degree of circular polarization close to 100%and neglect a huge part of the data. Galopeau et al. [2007]considered a phenomenological aspect and gave an estima-tion of the Stokes parameters. This approach leads to firstrecognizing spectral components (flux density and band-width) in the frequency range from 3.5 kHz to 1200 kHz,and then deriving the Stokes parameters for each compo-nent. The Cassini/RPWS instrument provides long-lastingcoverage of radio emissions at Saturn with unprecedentedinstrumental capabilities

Remote sensing of the Io torus plasma ribbon using natural radio occultation of the Jovian radio emissions

International audienceWe study the Jovian hectometric (HOM) emissions recorded by the RPWS (Radio and Plasma Wave Science) experiment onboard the Cassini spacecraft during its Jupiter flyby. We analyze the attenuation band associated with the intensity extinction of HOM radiation. This phenomenon is interpreted as a refraction effect of the Jovian hectometric emission inside the Io plasma torus. This attenuation band was regularly observed during periods of more than 5 months, from the beginning of October 2000 to the end of March 2001. We estimate for this period the variation of the electron density versus the central meridian longitude (CML). We find a clear local time dependence. Hence the electron density was not higher than 5.0 × 104 cm−3 during 2 months, when the spacecraft approached the planet on the dayside. In the late afternoon and evening sectors, the electron density increases to 1.5 × 105 cm−3 and reach a higher value at some specific occasions. Additionally, we show that ultraviolet and hectometric wavelength observations have common features related to the morphology of the Io plasma torus. The maxima of enhancements/attenuations of UV/HOM observations occur close to the longitudes of the tip of the magnetic dipole in the southern hemisphere (20° CML) and in the northern hemisphere (200° CML), respectively. This is a significant indication about the importance of the Jovian magnetic field as a physical parameter in the coupling process between Jupiter and the Io satellite

Analysis of pre-seismic ionospheric disturbances prior to 2020 2 Croatian earthquakes

Abstract: We study the sub-ionospheric VLF transmitter signals recorded by the Austrian Graz station in the year 2020. Those radio signals are known to propagate in the Earth-ionosphere waveguide between the ground and lower ionosphere. The Austrian Graz facility (geographic coordinates: 15.46◦E, 47.03◦N) can receive such sub-ionospheric transmitter signals, particularly those propagating above earthquake (EQ) regions in the southern part of Europe. We consider in this work the transmitter amplitude variations recorded a few weeks before the occurrence of two EQs in Croatia at a distance less than 200 km from Graz VLF facility. The selected EQs happened on 22 March 2020 and 29 December 2020, with magnitudes of Mw5.4 and Mw6.4, respectively, epicenters localized close to Zagreb (16.02◦E, 45.87◦N; 16.21◦E, 45.42◦N), and with focuses of depth smaller than 10 km. In our study we emphasize the anomaly fluctuations before/after the sunrise times, sunset times, and the cross-correlation of transmitter signals. We attempt to evaluate and to estimate the latitudinal and the longitudinal expansions of the ionospheric disturbances related to the seismic preparation areas

Nonaxisymmetrical component of Saturn's magnetic field derived from Cassini radio and magnetometer data

International audienceA dipole model ("birotor dipole") was proposed for Saturn's inner magnetic field: this dipole presents the particularity to have North and South poles rotating around Saturn's axis at two different angular velocities; this dipole is tilted and not centered. The spin rates and the associated rotation phases are derived from a continuous wavelet transform analysis of the intensity of the Saturnian kilometric radiation (SKR) signal received at 290 kHz between July 2004 and June 2012 by the radio and plasma wave science (RPWS) experiment on board Cassini. 57 revolutions of the spacecraft, the periapsis of which is less than 5 Saturnian radii, have been selected for this study. For each of these chosen orbits, it is possible to fit with high precision the measurements of the MAG data experiment given by the magnetometer embarked on board Cassini. A nonrotating external magnetic field completes the model. This study suggests that Saturn's inner magnetic field is neither stationary nor fully axisymmetric. These results can be used as a boundary condition for modelling and constraining the planetary dynamo and they can be a starting point for the study of Saturn's inner structure and the comparison with the interior of Jupiter. An average of the birotor dipole magnetic potential is performed on the azimuthal angle revealing an average axisymmetrical component which can be compared with the published zonal models proposed for Saturn's magnetic field. The presence of a "birotor quadrupole" is required in the present model to improve the agreement for the third order Gauss coefficient

Birotor dipole for Saturn's inner magnetic field from Cassini observations

International audienceThe radio and plasma wave science (RPWS) experiment on board the Cassini spacecraft, orbiting around Saturn since July 2004, revealed the presence of two distinct and variable rotation periods in the Saturnian kilometric radiation (SKR). These two periods were attributed to the northern and southern hemispheres respectively. We suppose that the periodic time modulations present in the SKR are mainly due to the rotation of Saturn's inner magnetic field. The existence of a double period implies that the inner field is not only limited to a simple rotation dipole but displays more complex structures having the same time periodicities than the radio emission. In order to build a model of this complex magnetic field, it is absolutely necessary to know the accurate phases of rotation linked with the two periods. The radio observations from the RPWS experiment allow a continuous and accurate follow-up of these rotation phases, since the SKR emission is permanently observable and produced very close to the planetary surface. A continuous wavelet transform analysis of the intensity of the SKR signal received at 290 kHz between July 2004 and June 2012 was performed in order to calculate in the same time the different periodicities and phases. A dipole model was proposed for Saturn's inner magnetic field: this dipole presents the particularity to have North and South poles rotating around Saturn's axis at two different angular velocities; this dipole is tilted and not centered. 57 Cassini's revolutions, the periapsis of which is less than 5 Saturnian radii, have been selected for this study. For each of these chosen orbits, it is possible to fit with high precision the measurements of the MAG data experiment given by the magnetometers embarked on board Cassini. A nonrotating external magnetic field completes the model. This study suggests that Saturn's inner magnetic field is neither stationary nor fully axisymmetric. These results can be used as a boundary condition for modelling and constraining the planetary dynamo and they can be a starting point for the study of Saturn's inner structure and the comparison with the interior of Jupiter

Spin modulation periods in Saturn's radio emission as a possible signature of inner dynamo

Two distinct and variable rotation periods in Saturn's radio emissions were revealed from observations performed by the radio and plasma wave science (RPWS) experiment on board the Cassini spacecraft. These two periods, at 10.6 hours and 10.8 hours, correspond to SKR produced in 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. As a consequence, the magnetospheric plasma, which is supposed to be frozen in the magnetic field, should rotate (or sub-rotate) with a unique spin period and such a north/south asymmetry in the radio period should never be observed. However, contrary to the other magnetized planets, Saturn presents a very particular magnetic field since its dipolar moment is nearly aligned with the rotation axis of the planet. This alignment could bring out some phenomena developing in the internal structure which are probably masked in the case of the planets the magnetic dipole of which is significantly tilted. We propose to interpret the existence of the two separated and slowly varying periods in the saturnian magnetic field as the signature of the inner dynamo

Is a Rikitake Dynamo in Saturn’s Interior at the Origin of the Variability of the Radio Rotation Periods?

ISBN: 978-3-7001-7125-6International audienceRecent 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

New Magnetic Field Model for Saturn From Cassini Radio and Magnetometers Observations: The Birotor Dipole

International audienceSince the insertion of Cassini in the Saturnian system in July 2004, the radio and plasma wave science (RPWS) experiment on board the spacecraft revealed the presence of two distinct and variable rotation periods in the Saturnian kilometric radiation (SKR) which were attributed to the northern and southern hemispheres respectively. The present study is based on the hypothesis that the periodic time modulations present in the SKR are mainly due to the rotation of Saturn's inner magnetic field. The existence of a double period implies that the inner field is not only limited to a simple rotation dipole but displays more complex structures having the same time periodicities than the radio emission. In order to build a model of this complex magnetic field, it is absolutely necessary to know the accurate phases of rotation linked with the two periods. The radio observations from the RPWS experiment allow a continuous and accurate follow-up of these rotation phases, since the SKR emission is permanently observable and produced very close to the planetary surface. A continuous wavelet transform analysis of the intensity of the SKR signal received at 290 kHz between July 2004 and June 2012 was performed in order to calculate in the same time the different periodicities and phases. The rotation phases associated to the main two periods allow us to define a North and South longitude system essential for such a study. In this context, a dipole model ("birotor dipole") was proposed for Saturn's inner magnetic field: this dipole presents the particularity to have North and South poles rotating around Saturn's axis at two different angular velocities; this dipole is tilted and not centered. 57 Cassini's revolutions, the periapsis of which is less than 5 Saturnian radii, have been selected for this study. For each of these chosen orbits, it is possible to fit with high precision the measurements of the MAG data experiment given by the magnetometers embarked on board Cassini. A nonrotating external magnetic field completes the model. This study suggests that Saturn's inner magnetic field is neither stationary nor fully axisymmetric. These results can be used as a boundary condition for modelling and constraining the planetary dynamo and they can be a starting point for the study of Saturn's inner structure and the comparison with the interior of Jupiter

Average axisymmetrical component of the birotor dipole proposed for Saturn's magnetic field

International audienceThe observations from the radio and plasma wave science (RPWS) experiment on board Cassini revealed the presence of two distinct and variable rotation periods in the Saturnian kilometric radiation (SKR) which were attributed to the northern and southern hemispheres respectively. These data also allowed a continuous and accurate follow-up of the rotation phases, linked with the two periods, since the SKR emission is permanently observable and produced very close to the planetary surface. A continuous wavelet transform analysis of the intensity of the SKR signal received at 290 kHz between July 2004 and June 2012 was performed in order to calculate in the same time the different periodicities and phases. The rotation phases associated to the main two periods allow us to define a North and South longitude system essential for such a study. In this context, a dipole model ("birotor dipole") was proposed for Saturn's inner magnetic field: this dipole presents the particularity to have North and South poles rotating around Saturn's axis at two different angular velocities; this dipole is tilted and not centered. 57 Cassini's revolutions, the periapsis of which is less than 5 Saturnian radii, have been selected for this study. For each of these chosen orbits, it is possible to fit with high precision the measurements of the MAG data experiment given by the magnetometers embarked on board Cassini. A nonrotating external magnetic field completes the model. This study suggests that Saturn's inner magnetic field is neither stationary nor fully axisymmetric. These results can be used as a boundary condition for modelling and constraining the planetary dynamo and they can be a starting point for the study of Saturn's inner structure and the comparison with the interior of Jupiter. An average of the birotor dipole magnetic potential is performed on the azimuthal angle revealing an average axisymmetrical component which can be compared with the published zonal models proposed for Saturn's magnetic field. The presence of a "birotor quadrupole" is required in the present model to improve the agreement for the third order Gauss coefficient

Complementarity of radio and magnetic observations by CASSINI in the making of a magnetic field model for Saturn

International audienceTwo distinct and variable rotation periods in Saturn’s radio emissions were revealed by observations performed by the radio and plasma wave science (RPWS) experiment on board the Cassini spacecraft. These two periods, first measured at 10.6 hours and 10.8 hours, were quickly attributed to SKR produced in the northern and southern hemispheres respectively. Later observations showed that these two periods varied and became equal after the time of Saturn’s equinox. Most of magnetospheric phenomena in Saturn’s environment are affected by the planet spin despite the apparent steep axisymmetry of the internal magnetic field. The existence of a double period makes the study of the planetary magnetic field much more complicated and the building of a field model, based on the direct measurements of the MAG experiment from the magnetometers embarked on board Cassini, turns out to be uncertain. The first reason is the difficulty for defining a longitude system linked to the variable period, because the internal magnetic field measurements from MAG are not continuous. The second reason is the existence itself of two distinct periods which could imply the existence of a double rotation magnetic structure generated by Saturn’s dynamo. However, the radio observations from the RPWS experiment allow a continuous and accurate follow-up of the rotation phase of the variable two periods, since the SKR emission is permanently observable and produced very close to the planetary surface. A wavelet transform analysis of the intensity of the SKR signal received at 290 kHz was performed in order to calculate the rotation phase of each Saturnian hemisphere. A dipole model was proposed for Saturn’s inner magnetic field: this dipole presents the particularity to rotate around Saturn’s axis at two different angular velocities; it is tilted and not centered. Then it is possible to fit the MAG data for each Cassini’s revolution around the planet the periapsis of which is less than 5 Saturnian radii. This study suggests that Saturn’s inner magnetic field is neither stationary nor fully axisymmetric