Jovian Radio Emissions Research Articles

УСУНЕННЯ ЗАВАД У ДИНАМІЧНИХ СПЕКТРАХ ЗА НАЯВНОСТІ ПОТУЖНОГО СИГНАЛУ. Частина 2. ВУЗЬКОСМУГОВІ ЗАВАДИ УМОВНО-СТАЦІОНАРНОГО ХАРАКТЕРУ

Subject and Purpose. A new method for detecting narrow band interferences is discussed, with the use of an example record of Jovi- an Io-C decametric radio storm as obtained with the UTR-2 array on April 10, 2020. We aimed at developing an efficient and simple algorithm based on a detailed analysis of the effect the radio interference environment and the frequency response of the telescope may have on the efficiency of the interference mitigation procedure. Methods and Methodology. The ‘orthogonal detection’ method proposed for identifying linear interference patterns in dynamic spectra of powerful radio sources has been adapted for application to narrow band interferences of various spectral widths and variable brightness, which often happen to be located close to (or intersect with) the signal of interest. In order to minimize the impact of the telescope’s frequency response, a discretized frequency-scanning technique is used, which permits a steady approximation of the averaged spectrum by low order polynomials, as well as removal of the frequency trend which hinders distinguishing between the low-level interference signals and those coming from the source. Results. An efficient approach to the problem of detecting long lasting, narrowband interferences in dynamical spectra (con- ditionally stationary interferences) is proposed. The algorithm has been tested on the example of a powerful storm of Jovian radio emission. The proposed technique can be especially useful in situations where the signal of interest overlaps with the interference in the time-frequency domain. Conclusions. The new approach to the problem of interference mitigation, based on the development of combined algorithms of signal separation in the time-frequency domain, has been shown to offer an effective method of signal processing. In contrast to the previously used methods based on calculation of statistical moments for amplitudes, the new approach allows avoiding use of the same statistical indicators for both interference detection and subsequent steps of data analysis intended for building physical models and interpreting the observational data.

Effect of a Magnetospheric Compression on Jovian Radio Emissions: In Situ Case Study Using Juno Data

AbstractDuring its polar orbits around Jupiter, Juno often crosses the boundaries of the Jovian magnetosphere (namely the magnetopause and bow shock). From the boundary locations, the upstream solar wind dynamic pressure can be inferred, which in turn illustrates the state of compression or relaxation of the system. The aim of this study is to examine Jovian radio emissions during magnetospheric compressions, in order to determine the relationship between the solar wind and Jovian radio emissions. In this paper, we give a complete list of bow shock and magnetopause crossings (from June 2016 to August 2022), and the associated solar wind dynamic pressure and standoff distances inferred from Joy et al. (2002, https://doi.org/10.1029/2001JA009146). We then select two sets of magnetopause crossings with moderate to strong compression of the magnetosphere for two case studies of the response of the Jovian radio emissions. We confirm that magnetospheric compressions lead to the activation of new radio sources. Newly activated broadband kilometric emissions are observed almost simultaneously with compression of the magnetosphere, with sources covering a large range of longitudes. Decametric emission sources are seen to be activated more than one rotation later only at specific longitudes and dusk local times. Finally, the activation of narrowband kilometric radiation is not observed until the magnetosphere is in its expansion phase.

Jovian Periodicities (~10 h, ~40, 20, 15 min) at ACE, Upstream from the Earth’s Bow Shock, on 25–27 November 2003

It is known that Jovian radio and high energy electron emissions are observed near Earth. The question we address in this study is whether the quasi-periodic ~10 h and ~40/15–20 min (QP-10 h, QP-40/15–20 min) energetic particle and magnetic field periodicities observed by Ulysses during its distant encounter with Jupiter in 2003 were also detectable as far as the Earth’s orbit. Surprisingly, we found that at the end of the extreme 2003 Halloween events, during times of a highly disturbed Jovian magnetosphere, as inferred from strong bKOM radio emissions observed by Ulysses, and a magnetic connection of Earth with the Jovian magnetosphere, as suggested by simulation results of the interplanetary magnetic field (IMF), the ACE satellite observed, between at least 25–27 November 2013 at the Lagrangian Point L1 (LPL1), all the characteristic Jovian periodicities. In particular, by using high-time resolution data (1/5 min), we found, for the first time, quasi-permanent electron, and magnetic field QP-10/5 h, QP-40 min and QP-15/20 data variations at LPL1 for at least three days. These observations reasonably suggest that low energy (~50–~300 keV) Jovian electrons reached the Earth’s environment; the observations examined extend the lowest energy limit of the Jovian electron spectrum from 200 keV to ~50 keV. In addition, the ACE satellite observed an impressive series of QP-10/5 h energetic (≤0.05 MeV) ion bursts (EIBs) with strong cross-field intensity gradients at the onset/decay phase of the events and energy-dependent field aligned anisotropy suggesting ion streaming in the anti-sunward direction during their main phase. A comparison of simultaneously obtained measurements by ACE at the LPL1 and by Geotail upstream from the bow shock and in the magnetosphere suggests that the QP-10/5 h EIBs are inconsistent with the concept of a terrestrial origin. On the contrary, the observations indicate that the series of QP-10/5 h EIBs on 25–27 November 2003 was a spatial effect caused by the ~10/5 h quasi-periodic approach of a large-scale sheet to the Earth’s environment. The source of the ion population forming the QP-10/5 h sharp EIBs seems most probably Jovian ions accumulated in the interplanetary space, although a solar ion contribution is possible. Based on the above results, it is reasonable to suggest that the observed QP-10 h, QP-40 min and QP-15/20 periodicities are due to Jovian influence. Further research is needed to study the cause of the QP-10/5 h EIBs. This study presents new data which extend our view on the influence of the QP-10 h/QP-40/QP-15/20 min Jovian emissions from the outer to the inner heliosphere at 1 AU.

Jupiter radio emission probability tool

Jupiter is a source of intense radio emissions in the decametric wavelength range observable from ground (above ∼10 MHz) and from space (down to a few kHz). The strong anisotropy of the Jovian radio sources results in characteristic shapes in the temporal-spectral domain, which can be used to identified the various types of Jovian radio components. The Jupiter Probability Tool provides users with Jovian radio emission observability predictions, depending on the observers location, and the radio emission class. The application can be used for observation planning or data analysis for ground or space observations.

Method to observe Jupiter’s radio emissions at high resolution using multiple LOFAR stations: a first case study of the Io-decametric emission using the Irish IE613, French FR606, and German DE604 stations

ABSTRACT The Low Frequency Array (LOFAR) is an international radio telescope array, consisting of 38 stations in the Netherlands and 14 international stations spread over Europe. Here, we present an observation method to study the Jovian decametric radio emissions from several LOFAR stations (here Birr Castle in Ireland, Nançay in France, and Postdam in Germany), at high temporal and spectral resolution. This method is based on prediction tools, such as radio emission simulations and probability maps, and data processing. We report an observation of Io-induced decametric emission from 2021 June, and a first case study of the substructures that compose the macroscopic emissions (called millisecond bursts). The study of these bursts makes it possible to determine the electron populations at the origin of these emissions. We then present several possible future avenues for study based on these observations. The methodology and study perspectives described in this paper can be applied to new observations of Jovian radio emissions induced by Io, but also by Ganymede or Europa, or Jovian auroral radio emissions.

Analysis of Temporal and Structural Characteristics of Jovian Radio Emissions for Passive Radar Sounding of Jupiter’s Icy Moons

Recent studies have proposed that Jovian decametric radiation (DAM) can be effectively exploited for probing the subsurface of Jupiter’s icy moons by using passive radio sounding. However, these studies were based on the assumption that Jovian noise is white and stationary. Therefore, additional investigations into the temporal stability, predictability, and spectral properties of Jovian noise are required to fully assess the potential of passive radar sounding and improve the acquisitions planning. In this article, we investigate these properties of the Jovian DAM to understand their impact on radar sounding performance. This is done by analyzing the recently available radio spectra acquired by the JUNO Waves instrument. Results are also evaluated for the specific case of the Radar for Icy Moon Exploration (RIME) and the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) that have been selected for European Space Agency (ESA) and NASA missions to Ganymede and Europa. Our results show that the Jovian DAM is not perfectly white, but no severe distortions in the range response should be expected. The results on spatiotemporal occurrence show that Jupiter’s DAM has a variable probability of occurrence, which is rather sporadic for some frequency ranges. The results on RIME and REASON flybys show that the Jovian DAM occurrence probability is relatively low for selected sub-Jovian flybys at Europa. For the RIME Ganymede orbital phase, a large number of high occurrence passive acquisition opportunities are expected. The experimental results also show that a large bandwidth receiver would enhance the chance of recording Jovian noise.

A re-analysis of the Jovian radio emission as seen by Cassini-RADAR and evidence for time variability

After more than a decade of operation at Titan and Saturn, the Cassini RADAR instrument is considered well understood and calibrated. In light of the recent Juno mission which is exploring the inner magnetosphere and the atmosphere of Jupiter, it is worthwhile to reconsider the original measurements of Cassini at Jupiter. The better instrument knowledge in combination with a better understanding of the ammonia distribution of Jupiter has allowed for revising the synchrotron flux density to 1.10 ± 0.07 Jansky, a factor of 2.5 larger than the initial estimate (Bolton et al., 2002). The forward model reduced uncertainties pertaining to the spacecraft pointing using a Markov-Chain Monte Carlo algorithm and constrained simultaneously a brightness model of Jupiter with a disk-averaged brightness temperature of 158.6 ± 2.4 K and depletion of ammonia at the poles (limb darking coefficient, p = 0.05). The flux density spectrum for the 2001 measurement campaign reveals a depletion of energetic electrons (>30 MeV) in contrast to an undisturbed electron population at lower energies. Comparing the Cassini radio maps to Very Large Array maps revealed a redistribution of energetic particles to higher latitudes, indicating enhanced pitch angle scattering for energetic particles. This kind of behavior has been observed in the terrestrial Van Allen belts and could be caused by the resonance of energetic electrons with electromagnetic ion cyclotron waves. We used a simplified analytic expression to determine the feasibility of this process at Jupiter. Although this process is not feasible under nominal conditions, a 10-fold enhancement of the cold plasma density, caused for example by extreme UV events, or volcanic eruptions on Io, could lead to rapid pitch angle scattering of electrons, and the subsequent removal of these particles by the atmosphere.

Zebra pattern in decametric radio emission of Jupiter

We report the systematic analysis of zebra-like fine spectral structures in decametric frequency range of Jovian radio emission. Observations were performed by the large ground-based radio telescope URAN-2 during three observation campaigns between, Sep., 2012, and May, 2015. In total, 51 zebra pattern (ZP) events were detected. These rare fine radio features are observed in frequency range from 12.5 to 29.7 MHz as quasi-harmonically related bands of enhanced brightness. ZPs are strongly polarized radio emission with a duration from 20 s to 290 s and flux densities ~105−106 Jy (normalized to 1 AU), that is, 1–2 orders lower than for Io-decametric radio emission (DAM). Occurrence of the events does not depend on the position of Io satellite but is strongly controlled by the Jovian central meridian longitude (CML). ZPs are mainly detected in two active sectors of Jovian CMLs: 100∘ to 160∘ for Northern sources (right-handed polarized) and 300∘ and 60∘ (via 360∘) for the Southern sources (left-handed). The frequency interval between neighboring stripes is from 0.26 to 1.5 MHz and in most cases this interval increases with frequency. We discussed the double plasma resonance with electrons or ions as a possible source of the ZPs. The performed analysis of the observations allows us to conclude that the observed ZPs are a new type of narrow band spectral structures in the Jovian DAM.

Detection of Jupiter decametric emissions controlled by Europa and Ganymede with Voyager/PRA and Cassini/RPWS

AbstractThe Jovian high‐latitude radio emissions produced by Jupiter's magnetosphere extend from a few kilohertz to 40 MHz. Part of the decametric (DAM) emissions are driven by the Galilean moon Io (Io‐DAM). As UV aurorae have been detected at the footprint of Europa and Ganymede, we expect that these moons drive Jovian radio emissions as well. To check this assumption, we used the ExPRES simulation code (Exoplanetary and Planetary Radio Emissions Simulator) to predict dynamic spectrum (time‐frequency spectograms) of the radio emissions controlled by the four Galilean moons. Then we compared the simulations to the Voyager/PRA and Cassini/RPWS radio observations of Jupiter (1979, and between 2000 and 2003, respectively). We present the first clear evidence for the existence of decametric emissions controlled by Europa and Ganymede. Their statistical analysis allows us to describe the average properties of the Europa‐DAM and Ganymede‐DAM emissions such as their spectrum, temporal variability, and occurrence as a function of moon phase and subobserver's longitude.

Multi-instrument study of the Jovian radio emissions triggered by solar wind shocks and inferred magnetospheric subcorotation rates

The influence of solar wind conditions on the Jovian auroral radio emissions has long been debated, mostly because it has always been difficult to get accurate solar wind and radio observations at the same time. We present here a study of Jupiter׳s radio emissions compared to solar wind conditions using radio (RPWS) and magnetic (MAG) data from the Cassini spacecraft from October to December 2000, just before its flyby of Jupiter. The spacecraft was then in the solar wind and could record both the radio emissions coming from the Jovian magnetosphere and the solar wind magnetic field (IMF). With these data, we found a good correspondence between the arrival of interplanetary shocks at Jupiter and the occurrence of radio storms. Our results confirm those from the previous studies showing that fast forward shocks (FFS) trigger mostly dusk emissions, whereas fast reverse shocks (FRS) trigger both dawn and dusk emissions. FFS-triggered emissions are found to occur 10–30h after the shock arrival when the IMF is weak (below 2nT), and quasi-immediately after shock arrival when the IMF is strong (above 2nT). FRS-triggered emissions are found to occur quasi-immediately even when the IMF is weak. We show and discuss in depth the characteristic morphologies of the radio emissions related to each type of shock and their implications.We also used simultaneous radio observations from the ground-based Nançay decameter array and from the Galileo radio instrument (PWS). From the comparison of these measurements with Cassini׳s, we deduce the regions where the radio storms occur, as well as the radio source subcorotation rates. We show that FFS-triggered emissions onset happens in a sector of local time centered around 15:00 LT, and that all the shock-triggered radio sources sub-corotate with a subcorotation rate of ~50% when the IMF is below 2nT and of ~80% when it is above 2nT. These rates could correspond to the extended and compressed states of the Jovian magnetosphere.

Formation of zebra pattern in low-frequency Jovian radio emission

We investigate the formation of zebra-like fine spectral structures (consisting of several parallel bands in the dynamic spectrum) in the Jovian broadband kilometric radiation; such radio bursts were observed by Cassini in 2000/2001. We assume that the emission is generated due to a plasma mechanism in the Io plasma torus. We have shown that the double plasma resonance effect (that was proposed earlier as a formation mechanism of the solar zebra patterns) is able to produce the observed spectral structures. The observed frequency drifts are caused, most likely, by the dynamics of the electron acceleration site. The required conditions in the emission source are discussed.

Natural radio emission of Jupiter as interferences for radar investigations of the icy satellites of Jupiter

Radar instruments are part of the core payload of the two Europa Jupiter System Mission (EJSM) spacecraft: NASA-led Jupiter Europa Orbiter (JEO) and ESA-led Jupiter Ganymede Orbiter (JGO). At this point of the project, several frequency bands are under study for radar, which ranges between 5 and 50MHz. Part of this frequency range overlaps with that of the natural jovian radio emissions, which are very intense in the decametric range, below 40MHz. Radio observations above 40MHz are free of interferences, whereas below this threshold, careful observation strategies have to be investigated. We present a review of spectral intensity, variability and sources of these radio emissions. As the radio emissions are strongly beamed, it is possible to model the visibility of the radio emissions, as seen from the vicinity of Europa or Ganymede. We have investigated Io-related radio emissions as well as radio emissions related to the auroral oval. We also review the radiation belts synchrotron emission characteristics. We present radio sources visibility products (dynamic spectra and radio source location maps, on still frames or movies), which can be used for operation planning. This study clearly shows that a deep understanding of the natural radio emissions at Jupiter is necessary to prepare the future EJSM radar instrumentation. We show that this radio noise has to be taken into account very early in the observation planning and strategies for both JGO and JEO. We also point out possible synergies with RPW (Radio and Plasma Waves) instrumentations.

Model of the Jovian magnetic field topology constrained by the Io auroral emissions

[1] The determination of the internal magnetic field of Jupiter has been the object of many studies and publications. These models have been computed from the Pioneer, Voyager, and Ulysses measurements. Some models also use the position of the Io footprints as a constraint: the magnetic field lines mapping to the footprints must have their origins along Io's orbit. The use of this latter constraint to determine the internal magnetic field models greatly improved the modeling of the auroral emissions, in particular the radio ones, which strongly depends on the magnetic field geometry. This constraint is, however, not sufficient for allowing a completely accurate modeling. The fact that the footprint field line should map to a longitude close to Io's was not used, so that the azimuthal component of the magnetic field could not be precisely constrained. Moreover, a recent study showed the presence of a magnetic anomaly in the northern hemisphere, which has never been included in any spherical harmonic decomposition of the internal magnetic field. We compute a decomposition of the Jovian internal magnetic field into spherical harmonics, which allows for a more accurate mapping of the magnetic field lines crossing Io, Europa, and Ganymede orbits to the satellite footprints observed in UV. This model, named VIPAL, is mostly constrained by the Io footprint positions, including the longitudinal constraint, and normalized by the Voyager and Pioneer magnetic field measurements. We show that the surface magnetic fields predicted by our model are more consistent with the observed frequencies of the Jovian radio emissions than those predicted by previous models.

Evidence of Jovian active longitude: 3. Observational constraints

The occurrence probability of the Io‐controlled Jovian decameter radio emissions depends on the central meridian longitude and the orbital phase of the satellite Io. Investigations by Galopeau et al. (2004, 2007) have shown that some specific Jovian “active” longitudes favor the radiation. The authors proposed a model which involves the cyclotron maser instability (CMI) as the mechanism at the origin of the Jovian radio emissions produced near the local gyrofrequency, along an active magnetic field line carried away by Io through its revolution around Jupiter. Those studies brought out the existence of an active longitude anchored in Jupiter's magnetic field and directly related to the efficiency of the CMI. In the present analysis, we model the four occurrence regions associated to Io‐controlled sources. This approach is in a reverse way of the parametric method developed by Galopeau et al. (2007). The properties of the propagation and polarization of the radio wave, derived from the CMI, are the key ingredients for the study of the beaming cone. It is shown that the lead angle of the active magnetic field line relative to Io has a significant effect on both the selection of the propagation conditions and the limit between the right‐ and left‐hand polarization states. The modeled and observed occurrence regions are found to be similar for a lead angle of about 20°. However, it seems that the behaviors of the southern and northern sources are not alike despite a common generation mechanism.

Effect of electric potential structures on Jovian S‐burst morphology

Jupiter's radio emissions are dominated in intensity by decametric radio emissions due to the Io‐Jupiter interaction. A significant part of these emissions consists of short radio bursts (so‐called S‐bursts) drifting in time and frequency. Previous analyses suggest that these emissions are cyclotron‐maser emissions in the flux tube connecting Io or Io's wake to Jupiter. We present simulations of these electrons under the assumption of acceleration by Alfvén waves in the Io flux tube. Near Jupiter, a loss cone and a ring distribution appear in the magnetically mirrored electron population, which can then amplify extraordinary (X) mode radio waves. The X‐mode growth rate is computed, which allows us to build theoretical dynamic spectra of the resulting Jovian radio emissions. Additional potential structures are assumed in the Jovian auroral region. We reconstruct their impact on the morphology of the emission. They match some of the time‐frequency patterns observed with Jovian S‐bursts. This provides the first evidence of bipolar electrostatic structures in the Jovian auroral region.

Modulation structures in the dynamic spectra of Jovian radio emission obtained with high time-frequency resolution

Aims. The wide-band dynamic spectra of Jovian decameter emission obtained over the last decade with high-frequency and high time resolution equipment on the largest decameter band antenna array, the Ukrainian T-shape Radio telescope (UTR-2), are presented. Methods. We analyzed the data obtained with the Digital SpectroPolarimiter (DSP) and WaveForm Reciever (WFR) installed at UTR-2. The combination of the large antenna and high performance equipment gives the best sensitivity and widest band of analysis, dynamic range, time and frequency resolutions. The wavelet transform method and the Fourier technique was used for further data processing. Results. The main characteristics of already known and newly detected modulation events were investigated and specified. The new receiving-recording facilities, methodology and program of observations are described in detail.

Ultrarelativistic Electrons in Jupiter's Inner Magnetosphere: First Observation of Angular Distributions in the 2.5 to 6 R J Region

Jupiter has been recognized as a probable location of magnetically trapped ultrarelativistic electrons since its discovery as a source of polarized decimeter radio waves. A series of spacecraft (Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, Cassini, and New Horizons) have flown by and orbited Jupiter with the goal of observing the trapped radiation that generates the radio waves. Follow-on studies have attempted to reconcile intensity patterns of Jovian radio emission with direct measurements of ultrarelativistic electrons that radiate via a synchrotron process. These studies have been limited by the absence of observations of measurement of the angular distributions of ultrarelativistic electrons. Since the strongest synchrotron emission from an electron occurs near its mirror point, the distribution of synchrotron emission is expected to follow the distribution of the mirror points. From the description of magnetic trapping, it further follows that observing the angular distribution at low magnetic latitudes allows one to infer the distribution of mirror points along the magnetic field. Observations made with the Energetic Particle Detector aboard the Galileo orbiter during its penultimate Jovian orbit provided excellent angular distribution observations on the magnetic field lines containing the electrons which account for most of the Jovian decimeter radiation. Because ultrarelativistic electrons are especially difficult to observe directly with low-mass flight instruments, inferring angular distributions for such particles required extensive modeling of instrument responses that matched the observations. We report these results here and suggest their implications for past and future studies of astrophysical synchrotron emission.

Generation of the jovian radio decametric arcs from the Io Flux Tube

Jupiter's radio emissions are dominated in intensity by decametric radio emissions due to the Io‐Jupiter interaction. Previous analyses suggest that these emissions are cyclotron‐maser emissions in the flux tubes connecting Io or Io's wake to Jupiter. The electrons that trigger the emission are thought to be accelerated toward Jupiter. We present simulations of this hot electron population under the assumption of impulsive acceleration, consistent with the accelerated electron populations seen in the Io's wake by Galileo during its flyby in 1995. After the impulsive acceleration phase, the electrons are supposed to have an adiabatic motion along the magnetic field lines. Near Jupiter, a loss cone and a shell appear in the magnetically mirrored electron population. Such features are able to amplify extraordinary (X) mode radio waves. We compute the X‐mode growth rate, which allows us to build theoretical dynamic spectra of the resulting Jovian radio emissions. The study of the growth rates obtained by simulation and their comparison with ground‐based radio observations provide strong constraints on the acceleration process and energy sources at the origin of the Io‐controlled decameter emissions.

Jovian S burst generation by Alfvén waves

Jupiter's radio emissions are dominated in intensity by decametric radio emissions due to the Io‐Jupiter interaction. Previous analyses suggest that these emissions are cyclotron‐maser emissions in the flux tubes connecting Io or Io's wake to Jupiter. Electrons responsible for the emission are thought to be accelerated from Io to Jupiter. We present simulations of this hot electron population under the assumption of acceleration by Alfvén waves in the Io flux tube. Outside of limited acceleration regions where a parallel electric field associated with Alfvén waves exists, the electrons are supposed to have an adiabatic motion along the magnetic field lines. Near Jupiter a loss cone appears in the magnetically mirrored electron population, which is able to amplify extraordinary (X) mode radio waves. The X‐mode growth rate is computed, which allows us to build theoretical dynamic spectra of the resulting Jovian radio emissions, whose characteristics match those observed for Jovian S bursts.

Satellite control of Jovian 2–6 MHz radio emission using Voyager data

Jupiter's satellite Io no longer has the only known effect in controlling the low‐frequency Jovian radio emissions. Menietti et al. (1998, 2001) used a long and contiguous set of data in the range of 2.0–5.6 MHz from the Galileo spacecraft and found correlations with the orbital phase of Ganymede and Callisto. Recently, Higgins et al. (2006) used all of the Galileo orbiter data and found that Europa's phase shows a minor but statistically significant effect on the lower range of decametric emissions. In this paper, results are found that confirm previous work for all four Galilean satellites by combining Voyager 1 and Voyager 2 flyby data in the frequency range of 2.1–5.8 MHz. The data were analyzed using the unique occurrence probability data compilation technique that includes 10‐hour averaging and satellite bias removal. A statistical significance Z‐test was computed to compare sample populations for each enhanced region of occurrence against the background. We find significance values greater than the 95% confidence level (Z > 2σ) for Io, Europa, Ganymede, and Callisto. Least squares fits to histograms of relative occurrence probability also show similar results. The cause of these correlations is believed to be Alfvénic interactions between the individual satellite and Jupiter.