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

Study of VLF/LF wave propagations above seismic areas

Abstract: We report on radio transmitter signals recorded in Europe by INFREP network which is mainly devoted to search for earthquakes electromagnetic precursors (Biagi et al., 2011). We consider in this analysis the detection of transmitter signals recorded by INFREP receivers located in different regions of Europe, i.e. Romania, Italy, Greece and Austria. The aim is the investigation of the electromagnetic environment above earthquakes regions. We selected seismic events which occurred in the year 2016 and characterized by a moment magnitude (Mw) above 5.0 and a depth of less than 50 km. A common method is applied to all events and which involves the analysis of the VLF/LF signal detection taking into consideration the following parameters: (a) the distance transmitters-receivers, (b) the signal to noise ratio during the diurnal and night observations, (c) the daily and night averaged amplitude and (d) the sunset and sunrise termination times. This leads us to specify the key factors which can be considered as criteria to distinguish and to identify earthquakes precursors. We discuss in this contribution the radio wave propagation in the D- and E-layers and their impacts on the VLF/LF amplitude signal. We show that the 'seismic anomaly' requests a more precise analysis of the 'quiet' and 'disturbed' ionospheric conditions and their corresponding spectral traces on the VLF/LF transmitter signals

Emission cone of Io-controlled Jovian decameter radiation inferred from occurrence diagram

International audienceFour zones of enhanced probability are found in the CML-Io phase diagram, where the occurrence of the Jovian decameter radio emissions is plotted versus the central meridian longitude (CML) and the orbital phase of Io. These zones are the so-called Io-controlled sources Io-A, Io-B (emitted from the northern hemisphere), and Io-C, Io-D (emitted from the south). In a recent work, we have studied the occurrence probability in a polar diagram linked to the local magnetic field, making the assumption that the magnetic field intensity gradient plays the role of an optical axis for the wave propagation. For a given Jovian magnetic field model, the four sources Io-A, Io-B, Io-C and Io-D are plotted as a function of the colatitude angle θ relative to the gradient of the magnetic field (radial coordinate) and an azimuth angle ψ linked to the direction of magnetic field vector. Our previous results revealed that the angle θ is not constant and that the Jovian decameter emission controlled by Io is radiated in a hollow cone which is not axi-symmetrical around the magnetic field gradient but flattened in the direction of the magnetic field vector. The relative directions of the magnetic field and its gradient within the radio source seem to play a crucial role in the angular distribution of the occurrence probability. Thus we analyze the effect of the choice of the magnetic field model (in particular the O6, VIP4, VIT4 and VIPAL models) on this distribution and the consequences for the emission cone. The use of elliptic coordinates in a frame linked to the local magnetic field is very relevant for such a study

Beam modelling of Io-controlled Jovian decameter radiation and localized active longitude

International audiencePrevious investigations by Galopeau et al. (J. Geophys. Res., 2004, 2007) have shown that some specific Jovian active longitudes favour the Io-controlled Jovian decameter radiation. Supposing the emission is generated by the cyclotron maser instability (CMI) near the local gyrofrequency, along an active magnetic field line carried away by the satellite Io during its revolution around Jupiter, the authors explained why the occurrence probability is larger in some specific regions of the central meridian longitude (CML)-Io phase diagram. These regions correspond to the so-called Io-A, Io-B, Io-C and Io-D sources. Galopeau et al. showed that the growth rate of the waves, derived from the CMI, is larger in those source regions. In their model, the active magnetic field line is supposed to present a constant lead angle δ relatively to Io's position and the beaming of the radiation is an axi-symmetrical hollow cone characterized by a constant half-angle ?. Long term observations allow us to define accurate contours for the source regions (Io-A, Io-B, Io-C and Io-D) in the CML-Io phase diagram. The common active longitude range derived from these observational constraints leads to justify only part of the Io-controlled radiation. It is particularly evident for the southern sources Io-C and Io-D. In the present study, we report on the possibility for the emission cone not to be axi-symmetrical but to present an elliptical section, the major directions of which would be determined by both the magnetic field vector B and the gradient of the magnetic strength ?B

Solar wind control of Jovian auroral emissions.

The combination of Galileo/PWS and Wind/WAVES observations allows the study ofthe flux density variation of the Jovian hectometric emissions (HOM) observed from31 August 1996 until 24 October 1996. It is found that the HOM emission presents periodicfeatures; each one is called a ‘‘HOM event.’’ Such episodic emissions were concurrentlyobserved by both experiments with similar spectral characteristics. The fluctuations of theJovian hectometric emissions and the solar wind parameters are found to exhibit quasisimilarvariations when a time lag of about 153 days is taken into consideration. Also, ‘‘HOMenhancements,’’ like the ‘‘injection events’’ first reported by Mauk et al. (1997), are found tooccur at some specific longitudes. The occurrence of these magnetospheric eventsincreases at two ‘‘active longitudes,’’ i.e., 45 CML and 180 CML. The solar windseems to be at the origin of both phenomena. Solar particles go through the polarregions where they interact with the Jovian magnetic field and give rise in the auroraland equatorial regions to an increase of Jovian hectometric emissions and/or injection events

Beaming Cone of Io-Controlled Jovian Decameter Radio Emission and Existence of Localized Active Longitude

International audienceThe occurrence probability of the Jovian decameter radio emissions depends on two essential parameters: the central meridian longitude (CML) and the orbital phase of the satellite Io. Four main zones of enhanced occurrence probability emerge from the CML-Io phase diagram: the so-called Io-controlled sources Io-A, Io-B, Io-C and Io-D. We study the compatibility of the location of these sources with the existence of a specific active longitude range, anchored in Jupiter's magnetic field, and favoring the radio emissions. A theoretical model, based on the cyclotron maser instability (CMI), was proposed a few years ago in order to explain the existence of such active longitudes, assuming that the radiation was emitted at the local gyrofrequency in a hollow cone of constant angle, along a magnetic field line carried away by Io through its revolution around Jupiter. Unfortunately this model was not able to justify the dimension in longitude of all the Io-controlled sources, in particular those located in the Jovian southern hemisphere (Io-C and Io-D). We show that the azimuthal distribution of the four occurrence regions (Io-A, Io-B, Io-C and Io-D) around the gradient of the local magnetic field is not constant so that the emission cone (in each Jovian hemisphere) presents a significant flattening in the direction of the magnetic field vector. Introducing a beaming cone with an elliptical section makes the location and extension in longitude of the sources (in the CML-Io phase diagram) compatible with the existence of an active longitude. A theory of the CMI, acting in an inhomogeneous medium in which the magnetic field vector and the gradient of its modulus are not aligned, shall be required in order to justify the flattening of the emission cone

Role of the Jovian magnetic field on the occurrence probability of Io-controlled decameter emissions in a polar diagram

International audienceFour zones of enhanced probability are found in the CML-Io phase diagram, where the occurrence of the Jovian decameter radio emissions is plotted versus the central meridian longitude (CML) and the orbital phase of Io. These zones are the so-called Io-controlled sources Io-A, Io-B (emitted from the northern hemisphere), and Io-C, Io-D (emitted from the south). In a recent work, we have studied the occurrence probability in a polar diagram linked to the local magnetic field, making the assumption that the magnetic field intensity gradient plays the role of an optical axis for the wave propagation. For a given Jovian magnetic field model, the four sources Io-A, Io-B, Io-C and Io-D are plotted as a function of the colatitude angle θ relative to the gradient of the magnetic field (radial coordinate) and an azimuth angle ψ linked to the direction of magnetic field vector. Our previous results revealed that the angle θ is not constant and that the Jovian decameter emission controlled by Io is radiated in a hollow cone which is not axi-symmetrical around the magnetic field gradient but flattened in the direction of the magnetic field vector. The relative directions of the magnetic field and its gradient within the radio source seem to play a crucial role in the angular distribution of the occurrence probability. Thus we analyze the effect of the choice of the magnetic field model (in particular the O6, VIP4, VIT4 and VIPAL models) on this distribution and the consequences for the emission cone. The use of elliptic coordinates in a frame linked to the local magnetic field is very relevant for such a study

Beam modelling of Io-controlled Jovian decameter radiation and localized active longitude

International audiencePrevious investigations by Galopeau et al. (J. Geophys. Res., 2004, 2007) have shown that some specific Jovian active longitudes favour the Io-controlled Jovian decameter radiation. Supposing the emission is generated by the cyclotron maser instability (CMI) near the local gyrofrequency, along an active magnetic field line carried away by the satellite Io during its revolution around Jupiter, the authors explained why the occurrence probability is larger in some specific regions of the central meridian longitude (CML)-Io phase diagram. These regions correspond to the so-called Io-A, Io-B, Io-C and Io-D sources. Galopeau et al. showed that the growth rate of the waves, derived from the CMI, is larger in those source regions. In their model, the active magnetic field line is supposed to present a constant lead angle δ relatively to Io's position and the beaming of the radiation is an axi-symmetrical hollow cone characterized by a constant half-angle ?. Long term observations allow us to define accurate contours for the source regions (Io-A, Io-B, Io-C and Io-D) in the CML-Io phase diagram. The common active longitude range derived from these observational constraints leads to justify only part of the Io-controlled radiation. It is particularly evident for the southern sources Io-C and Io-D. In the present study, we report on the possibility for the emission cone not to be axi-symmetrical but to present an elliptical section, the major directions of which would be determined by both the magnetic field vector B and the gradient of the magnetic strength ?B