Jovian Decametric Radiation Research Articles

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.

An oblate beaming cone for Io‐controlled Jovian decameter emission

AbstractWe investigate the emission cone of the Io‐controlled Jovian decameter radiation from the layout of the sources in the central meridian longitude (CML)‐Io phase diagram where the occurrence of the radiation is plotted versus the central meridian longitude (CML) and the phase of the satellite Io. Four zones of enhanced probability are revealed in this diagram and named Io‐controlled sources Io‐A, Io‐B, Io‐C, and Io‐D. We propose an angular representation of the CML‐Io phase diagram in a coordinate system linked to the local magnetic field in the radio source and the gradient of the magnetic field. The angular distribution of the sources in such a diagram clearly shows that the radio emission is radiated in a hollow cone which is not axisymmetrical around the magnetic field gradient but flattened in the direction of the magnetic field vector. The use of elliptic coordinates allows us to compute the variable opening angle and the flattening of the cone in both Jovian hemispheres. Finally, it comes from our investigation that the flattening of the emission cone allows the theoretical existence of a Jovian active longitude sector much more easily than in the case of an axisymmetrical cone, and the zones of maximum theoretical amplification correspond to the areas of high occurrence probability observed in the CML‐Io phase diagram.

Quasi-similar decameter emission features appearing in the solar and jovian dynamic spectra

We investigate the dynamic spectra of the Sun and jovian decametric radiation obtained by the authors with the radio telescopes UTR-2 and URAN-2 (Kharkov, Poltava, Ukraine). We focus on the similar structures that appear on the dynamic spectra of those objects: S-bursts, drifting pairs, absorption bursts and zebra patterns. Similarity in structures allows us to assume that the plasma processes in the solar corona and in the jovian magnetosphere might have similar properties. We analyze and compare the main parameters of those structures and describe briefly some mechanisms of their generation that have already discussed in publications. We selected the mechanisms which, in our opinion, most completely and consistently explain the properties of the structures under consideration.

A simulation study of Io-related Jovian decametric radiation: Control factor of occurrence probability

[1] Io-Jupiter interaction leads to auroral emissions and Jovian decametric radiations (Io-DAM). The longitudinal distribution of the Io-DAM occurrence probability has been considered to be controlled by the footprint magnetic intensity of each hemisphere. Recent observations revealed that the brightness of the main auroral spot is mainly modulated by Io's magnetic latitude. In the present study, we propose that the Io-DAM occurrence probability is controlled by the north-south asymmetry of the footprint magnetic intensity, in addition to Io's magnetic latitude. Hall magnetohydrodynamic equations are solved in the corotating meridional plane, which includes the Jovian ionosphere of finite thickness. We assume that the ionospheric Pedersen conductance is inversely proportional to the footprint magnetic intensity; the conductance is nearly the same for both ionospheres at a longitude of 290°, while twice higher for the south than for the north at a longitude of 110°. Above the northern ionosphere, the parallel current density further than 20° downstream of the main spot is estimated to be 1.5–2.0 times larger for 290° than for 110°. This indicates that if the Io-DAM lead angle is large, the suppressed Io-DAM occurrence probability for the northern hemisphere around 110° would be caused by the north-south asymmetry of the footprint magnetic intensity. A large current density conducted into the south around 110° would be the source of the Io-D component, radiated from the southern hemisphere. The observed brightness of the Io-related auroras is discussed in the context of the intensity of the parallel current density.

A simulation study of the current‐voltage relationship of the Io tail aurora

The Io tail aurora extends for approximately 100 degrees downstream in longitude from the Io footprint aurora. Observations indicate that the brightness of the Io tail aurora continuously decreases along the footpath while its peak altitude remains constant. According to the quasi‐steady theoretical frame, this suggests that the field‐aligned voltage is constant while the parallel current density decreases in the downstream direction. The mechanism that realizes the current‐voltage relationship of the Io tail aurora remains unresolved. In this paper, we apply a new multimagnetofluid code to the Io‐Jupiter system to clarify the origin of the current‐voltage relationship. The code solves a set of equations that includes the electron convection term in Ohm's law, which enables us to simulate the current‐driven ion acoustic instability in the fluid frame. The instability forms a transition layer at a high altitude, which accelerates the magnetospheric electrons and blocks the magnetospheric ions, leading to the formation of a density depleted region called an auroral cavity. We find that if the ionospheric proton density decreases at the same rate as the parallel current density, the timescale on which the transition layer disappears is consistent with the longitudinal extent of the tail aurora, and the potential gap is constant all along the tail. We discuss the possibility that the fringe, wideband repetitive bursts of the Io‐related Jovian decametric radiation, is excited in the auroral cavity.

Comment on “Jovian slow‐drift shadow events” by T. Koshida et al.

] Koshidaetal.[2010a]describespecificunusualfeaturesof the Jovian decametric (DAM) radiation, called “slow-driftshadow” (SDS) events. These events consist of brief (a fewtens of milliseconds long) extinctions of the slowly varyingIo-induced Jovian decametric emission (so-called«L-burst»)in the 21–23 MHz range. SDS drifts from high to low fre-quencies across the observed 21–23 MHz range in a fewhundred milliseconds.

Asymmetrical features of frequency and intensity in the Io‐related Jovian decametric radio sources: Modeling of the Io‐Jupiter system

Part of the Io‐related Jovian decametric radiation (Io‐DAM) has been thought to be excited in the auroral cavity formed on field lines downstream of Io. Since source regions of Io‐DAM called Io‐A and Io‐B are located in dusk and dawn local times despite having nearly equal magnetic longitudes, some of the observed asymmetries between Io‐A and Io‐B events are expected to be due to the difference in the local times. We developed a static Vlasov code, applied it to the Io‐Jupiter system, and investigated source structure in order to clarify the ways in which the characteristics of Io‐DAM are affected by the plasma in the Jovian ionosphere. Generally there are various solutions which satisfy the quasi‐neutrality condition for almost identical boundary conditions. With regard to the solutions with two transition layers, if the altitude of a low‐altitude transition layer (LATL) is higher, the voltage at the LATL and the ionospheric proton current density is smaller. Similarly, if the altitude of a high‐altitude transition layer (HATL) is higher, the voltage at the HATL and the magnetospheric electron current density is larger. A solution with a smaller ionospheric density for Io‐B than for Io‐A indicates lower altitude of the LATL for Io‐B, which is consistent with the observed high‐frequency limit higher for Io‐B than for Io‐A. This suggests that the high‐frequency limit is affected by the local time. It is also expected that the difference in ionospheric proton current densities would be associated with the observed asymmetry of emission intensity.

A BLIND SEARCH FOR MAGNETOSPHERIC EMISSIONS FROM PLANETARY COMPANIONS TO NEARBY SOLAR-TYPE STARS

This paper reports a blind search for magnetospheric emissions from planets around nearby stars. Young stars are likely to have much stronger stellar winds than the Sun, and because planetary magnetospheric emissions are powered by stellar winds, stronger stellar winds may enhance the radio luminosity of any orbiting planets. Using various stellar catalogs, we selected nearby stars (<~ 30 pc) with relatively young age estimates (< 3 Gyr). We constructed different samples from the stellar catalogs, finding between 100 and several hundred stars. We stacked images from the 74-MHz (4-m wavelength) VLA Low-frequency Sky Survey (VLSS), obtaining 3\sigma limits on planetary emission in the stacked images of between 10 and 33 mJy. These flux density limits correspond to average planetary luminosities less than 5--10 x 10^{23} erg/s. Using recent models for the scaling of stellar wind velocity, density, and magnetic field with stellar age, we estimate scaling factors for the strength of stellar winds, relative to the Sun, in our samples. The typical kinetic energy carried by the stellar winds in our samples is 15--50 times larger than that of the Sun, and the typical magnetic energy is 5--10 times larger. If we assume that every star is orbited by a Jupiter-like planet with a luminosity larger than that of the Jovian decametric radiation by the above factors, our limits on planetary luminosities from the stacking analysis are likely to be a factor of 10--100 above what would be required to detect the planets in a statistical sense. Similar statistical analyses with observations by future instruments, such as the Low Frequency Array (LOFAR) and the Long Wavelength Array (LWA), offer the promise of improvements by factors of 10--100.

A beam-maser instability: Direct amplification of radiation

The cyclotron maser instability has been applied in the past to laboratory microwave generation devices, and radiation from planetary magnetospheres, such as Earth’s kilometric radiation and Jovian decametric radiation. Recently, the same concept was put forth to explain solar coronal radio bursts. In particular, it was suggested that the ordinary (O) mode propagating in antiparallel direction with respect to the electron beam may be important. For this reason, this article revisits the maser instability theory with a particular emphasis on understanding the key difference between forward- and backward-propagating O-mode masers. It is found that the O-mode maser instability is excited over a significant range of wave phase angle which corresponds to the backward propagation. Of the various physical parameters the average beam speed and the ratio of plasma-to-gyro-frequencies, ωp/Ω, play important roles. Past discussions have emphasized the role of ωp/Ω. The present analysis also includes the effects of average beam speed on the instability. It is shown that there is a critical beam speed for the instability onset, beyond which the growth rate is quasi-independent of the beam speed. Such a finding has not been reported in the literature before.

Search for decametric occultations of Io flux tube by Ganymede

The satellite Ganymede sometimes occults the sources of the Jovian decameter radiation (DAM) associated with Io magnetic field line. The basic parameters of Ganymede occultations are calculated for 1990–2010. One of these events is found to coincide with a Io–A radio storm, which has been recorded in Nancay Observatory on 17 April 1994. In spite of the difficulty to identify the satellite shadow on sporadic DAM, the ratio of frequency emitted to calculated gyromagnetic frequency of electrons in the source is tentatively estimated as $ f / f_{{\rm c}} \geq 1.11 \pm 0.02$. Formally, this limit contradicts the present generation theories where $f_{{\rm c}}$ in the DAM source is much closer to 1. Hence, improvements to the magnetic model (VIP4) or of the distortion of the Io flux tube are needed. Two possible shadows of the satellite are tentatively identified on the DAM frequency-time spectrogram. Multiple occultations are indeed possible in the Alfven wave model of Io-DAM interaction, and the lead angle of the emitting field line is not well known. That is why the tentative location of the radio source is made for both variants.

Propagation of nonlinear slow waves produced by pressure pulses along the Io flux tube

A pressure enhancement in the vicinity of Io can be created in the course of the torus plasma flow around Io due to mass loading or it can be produced by volcanic outbursts on Io. For a given magnetic flux tube crossed by Io, a pressure pulse generates two slow magnetosonic waves propagating along the tube to the southern and northern ionosphere of Jupiter. These slow waves evolve rather quickly into shocks due to a steepening mechanism with accelerated plasma flow behind the shock front. This plasma flow streaming along the Io flux tube generates a field aligned potential difference, which can reach values of 1 kV for sufficiently strong pressure pulses. Therefore, this slow mode scenario seems to contribute to the Io controlled aurora as well as to the Io controlled Jovian decameter radiation (DAM) together with the generally accepted Alfvén wings model.

Analyses Of Jovian Decametric Radiation -Bursts Interacting With N -Bursts

Dynamic spectra of S-bursts of Jovian decametric radiations are obtained by using a high time resolution radio spectrograph which has a time resolutionof 2 msec and the bandwidth of 2 MHz.Within occurrence of 65 S-burst events observed in the period from 1983 to 1999, 26 events have been identified as the S-N burst events, which are characterized by the interaction between the S-burst emissions and the Narrow band emissions. In the dynamic spectra of the S-N burst, the trend of emissions with negative and slower frequency drift named as “Trailing Edge Emission” are often observed shortly after the appearance of the S-burst.Detailed analyses of these phenomena revealed that the Trailing Edge Emission is not a manifestation of S-burst with slower drift rate but a variation ofN-burst. The results suggested that S-burst and the associated Trailing Edge Emission are formed simultaneously started from a common region with different drift rates. It has been further suggested that the appearance of the S-burstsis not controlled by the geometrical effect between the source region and theobserver, but directly reflects the generation of the source region widelydistributed in an altitude range from a few thousands km to 30,000 km, alongthe Io flux tube.

A redefinition of Jupiter's rotation period

We measured the rotation period of Jupiter's inner magnetosphere with precision previously unattainable, using 35 years of observations of the Jovian decametric radiation at the University of Florida Radio Observatory at frequencies between 18 and 22.2 MHz. The new rotation period is the weighted mean of 13 independent 24‐year average determinations. Each of these was found by measuring the drift of the histogram of occurrence probability versus System III (1965) central meridian longitude over an interval of approximately 24 years. The measured drift was used to correct the System III (1965) period to obtain the new value. Our weighted mean is 9 hours 55 min 29.6854 s, with a standard deviation of the weighted mean (σ) of 0.0035 s. This new rotation period is 7.4σ shorter than that of the System III (1965), indicating that the latter is in need of revision. Our measurements indicate an upper limit of about 4 ms/year on any possible Jovian rotation period drift.

Polarization Characteristics of Io-Related Jovian Decametric Radiations

Polarization measurements of Io-related Jovian decametric radiations (DAM's) were made in a frequency range from 20.5 to 25.5 MHz at the Zao observatory of Tohoku University, Japan. In the analyses of axial ratios of polarization ellipses, the following two factors were particularly investigated which were expected to deform the original axial ratios of Io-related DAM's; i.e., i) interferences caused by reflected wave components from ground, and ii) mutual cross-talks between one free-space mode wave to another free-space mode wave in anisotropic magnetized plasma along the ray paths. Totally 20 Io-related radio storms and 2 non-Io-A radio storms were selected for the analyses. Our major results are as follows: i) Most of Io-related DAM events indicate highly right-handed (RH) elliptical polarization and the mean value of the axial ratios for RH events is -0.31 ± 0.09 (standard deviation). ii) Io-A and 10-13 related DAM events indicate the significant difference in their axial ratios; i.e., -0.35 ± 0.08 and -0.26 ± 0.06 for RH Io-A and RH Io-B related DAM events, respectively. iii) Io-related DAM events indicate slightly more linear polarization than non-Io-A DAM events.

Extremely Large Enhancements of the Jovian Decameter Radio Bursts Caused by the Magnetosphere-Plasmasphere Passages of Shoemaker-Levy 9 Comet Fragments-Evolution of Jovian Decametric Radiation Feature into the State of Intense Decametric Pulsar

Observation results of the Jovian decameter wave emissions in the period of the passages of fragments of Shoemaker-Levy 9 comet through the Jovian magnetosphere indicated extremely large enhancements of Jovian decameter wave radiations (JDR), being caused by dust-plasma clouds moving across the Jovian magnetic fields. The total emission power of the decameter wave radiations was increased to the level of approximately 10 times larger than the regular maximum level of the emissions of the Jovian decameter radiation in a corresponding frequency range. The Jovian decameter wave radiations have continuously been emitted for six months, long after the passage of the fragments of Shoemaker-Levy 9 comet; the evidence suggests that the Jovian plasma environment has been completely changed by the remnants of dust clouds that were left behind the Shoemaker-Levy 9 comet fragments.

Faraday rotation in Jupiter's decametric radio emission used for remote sensing of the terrestrial ionosphere and the emission's source region at Jupiter

Faraday rotation in the Jovian decametric radiation (DAM) is a well-known phenomenon when performing observations from ground-based telescopes which are sensitive to linear polarization. It is due to the different phase velocity of the ordinary and extra-ordinary wave modes that results in a rotation of the major axis of the elliptically polarized Jovian decametric emission when propagating through a magneto-active plasma. The amount of rotation is frequency dependent and therefore the Faraday rotation can be well recognized on the dynamic spectra by intensity minima (when the major axis of the polarization ellipse is perpendicular to the antenna elements) and maxima (when the major axis is parallel to the antenna elements) as a function of frequency. The study of Faraday rotation allows remote sensing of the terrestrial ionosphere and/or the source regions of the decametric emission in the Jovian magnetosphere. Investigating Jovian decametric observations containing Faraday rotation made at the Kharkov radio-telescope (Ukraine), it is found that the polarization angle at Jupiter (orientation of the major axis of the polarization ellipse at the radio source) cannot unam-biguously be determined with these data. Results on the polarization angle of Jovian DAM already given in the literature are still contradictory. Also it is not yet clear whether or not the total amount of Faraday rotation observed is due to the terrestrial ionosphere or if there is additionally a considerable contribution from the Jovian magnetosphere. Using modeled Faraday rotation patterns derived by electromagnetic ray-tracing through a realistic terrestrial ionospheric model enable most of the conflicting results to be explained in the literature by systematic errors due to over-simplification made in the respective assumptions. It is concluded that the amount of Faraday rotation observed in Jovian DAM spectra can be explained by the terrestrial ionosphere. At least in the case of low ionospheric plasma densities, the quadratic formula Ω = C 0 + C 1 f 2 + C 2 f 4 is sufficient to describe the number of Faraday fringes Ω as a function of frequency. Also, the polarization angle is confirmed to be parallel to B at the Jovian source position.

Absence of magnetic field constraints on the source region of Jovian decametric radiation

We have re‐examined the Voyager 1 and 2 planetary radio astronomy radio emission data in light of new magnetic field models. A previous study based on earlier models [Genova and Aubier, 1985] indicated that an apparent offset (∼70° measured at the orbit of Io) exists between the instantaneous Io flux tube and the source region of Jovian decametric radiation (DAM). Our new survey indicates that no such offset is necessary based on more recent magnetic field models. In addition, examples of DAM that are probably due to 2nd harmonic gyroemission were observed, as well as emission that may be due to mechanisms other than the cyclotron maser instability.

Infrared signature of Jupiter's Io interaction is detected

One of the most intriguing problems in the study of planetary magnetospheres is the electrodynamic interaction between Io, a satellite of Jupiter, and the planet's magnetosphere. The first indication of this unique relationship was found in remote observations of radio emission from Jupiter's magnetosphere, first detected in 1955. Observations of Jovian decameter radiation (DAM) at Earth had established a correlation between the probability of observation of DAM and the position of Io in its orbit about Jupiter. Io's influence on the generation of Jovian DAM was attributed to the workings of an electric generator of enormous scale [Goldreich and Lynden‐Bell, 1969]. Now, images of Jupiter's disc reveal a localized infrared emission feature associated with this interaction (Figure 1).

Faraday Effect in the Jovian Decametric Radiation Observed with a New Radiospectrograph at Wakkanai.

Dynamic spectrum observations of the Jovian decametric radiation (JDR) have been carried out since 1993 at the Wakkanai Radio Observatory using a new radiospectrograph system, which covers with a frequency range from 20 to 40 MHz. The Faraday effect in JDR spectra is analyzed for two kinds of JDR events; emissions from lo-related (lo-A) source and those from non lo-related source. A significant excess of the Faraday rotation to the terrestrial contribution is disclosed by the comparison with satellite beacon measurements taken simultaneously at Wakkanai. This result suggests that the Faraday effect by the Jovian plasma cannot be neglected for both Io-A and non Io-A emissions.

Elliptically polarized Jovian decametric radiation: An investigation of the electron cyclotron maser mechanism

A model for the elliptical polarization of Jovian decametric radiation is presented, based on the electron cyclotron mechanism. The aim is to determine whether the observed elliptical polarization is consistent with the radiation being generated by mildly relativistic electrons streaming along converging magnetic field lines. The growth rate for electron cyclotron maser emission is considered, assuming a drifting DGH distribution function for the streaming electrons. Constraints on the allowable parameters of this distribution function are made by the observed polarization, timescale, bandwidth, and angular range of the radiation.