Explanation of dominant oblique radio emission at Jupiter and comparison to the terrestrial case

The Io-Jupiter S-bursts are series of quasi-periodic impulsive decameter radio emissions from the magnetic flux tube connecting Jupiter to its closest galilean satellite Io. This paper discusses the possibility, suggested by previous works by Hess et al., that the S-bursts are triggered by upgoing electrons accelerated (downward) by trapped Alfvén waves, that have mirrored above the Jupiter ionosphere. According to this theory, the S-bursts would correspond to wave modes that propagate at oblique angles with respect to the magnetic field. Oblique propagation is also inferred for the more slowly varying components of Io-Jupiter radio emissions. Previous works, mainly based on observations of the terrestrial AKR, whose generation process is closely related to those of S-bursts, showed that these waves are emitted on perpendicular wave modes. This discrepancy between the Jovian and Terrestrial cases has led to a controversy about the credibility of the S-bursts model by Hess et al. In the present paper, we show that indeed, the most unstable wave modes for Earth AKR, and Io-Jupiter S-bursts, as they are seen from ground based radio-telescopes, are not the same. Several causes are evaluated: observational bias, the different degree of plasma magnetization above Earth and Jupiter, the role of a cold plasma component and of plasma auroral cavities. Furthermore, we make predictions about what kind of radiation modes a probe crossing the low altitude Io-Jupiter flux tube will see.     

Variants of tilted-V events in Jupiter's decametric radio spectra

Spectra of complex Jovian S-storms can be interpreted as groups of tilted-V variants. In such an approach the basic components (wide-range S-bursts, narrow-range S-trains, emissions of type N, and shadow events) are arranged in a predictable sequence. It seems that the application of tilted-V variants offers some order for the chaos evident in many S-storm spectra.     

S-bursts in Jupiter's decametric radio spectra

Dynamic spectra of Jupiter's S-bursts are observed with sweep-frequency and multi-channel receivers operating at frequency ranges 21–30 and 20.85–23.20 MHz, respectively. Spectra obtained with time resolutions of 0.2, 0.02, and 0.004 s are compared, the frequency resolution being 50 kHz. The most normal appearance of S-bursts is in trains with a frequency range of the order of 1 MHz. Narrow-band Strains also occur. Narrow-band L-emissions in region B are often associated with S-bursts, obviously in the manner described by Flagget al. (1976). Synoptic spectral observations indicate that region B for S-bursts exhibits a drift in longitude similar to that for L-bursts. The Io phase profile for S-bursts has a maximum in the vicinity of 80° in region B and 230° in region C. S-bursts observed in 1976 have higher drift rates than those compiled by Krauscheet al. (1976). Region C bursts have simpler spectra and lower drift rates than region B bursts.     

Solar S-bursts at Frequencies of 10 – 30 MHz

Solar S-bursts observed by the radio telescope UTR-2 in the period 2001 – 2002 are studied. The bursts chosen for a detailed analysis occurred in the periods 23 – 26 May 2001, 13 – 16 and 27 – 39 July 2002 during three solar radio storms. More than 800 S-bursts were registered in these days. Properties of S-bursts are studied in the frequency band 10 – 30 MHz. All bursts were always observed against a background of other solar radio activity such as type III and IIIb bursts, type III-like bursts, drift pairs and spikes. Moreover, S-bursts were observed during days when the active region was situated near the central meridian. Characteristic durations of S-bursts were about 0.35 and 0.4 – 0.6 s for the May and July storms, respectively. For the first time, we found that the instantaneous frequency width of S-bursts increased with frequency linearly. The dependence of drift rates on frequency followed the McConnell dependence derived for higher frequencies. We propose a model of S-bursts based on the assumption that these bursts are generated due to the confluence of Langmuir waves with fast magnetosonic waves, whose phase and group velocities are equal.     

Search for effects of comet S-L 9 fragment impacts on low radio frequency emission from Jupiter

Decametric radio observations of Jupiter were made before, during, and after the impacts of the fragments of the comet S-L 9 with the planet, from the University of Florida Radio Observatory, the Maipu Radio Astronomy Observatory of the University of Chile, and the Owens Valley Radio Observatory of the California Institute of Technology. The decametric radiation was monitored at frequencies from 16.7 to 32 MHz. The minimum detectable flux densities were on the order of 30 kJy, except for that of the large 26.3 MHz array in Florida, which was about 1 kJy. There was no significant enhancement or suppression of the decametric L-burst or S-burst emission with respect to normal activity patterns that might be attributed to the fragment entries. However, a burst of left-hand elliptically polarized radiation having a considerably longer duration than an L-burst was observed almost simultaneously with the impact of the large fragment Q2, and another with right-hand elliptical polarization was observed simultaneously with Q1. We consider the possibility that these two bursts were emitted just above the local electron cyclotron frequencies from the southern and northern ends, respectively, of magnetic flux tubes that had been excited in some way by the proximity of fragments Q2 and Q1.In addition to the monitoring of the decametric radiation, a search was conducted for possible comet-enhanced Jovian synchrotron radiation at 45 MHz using a large dipole antenna array at the observatory in Chile. This frequency is above the cutoff of the decametric radiation, but is considerably below the lowest frequency at which the synchrotron emission has previously been detected. The minimum detectable flux density with the 45 MHz antenna was about 5 Jy. No synchrotron emission at all was found before, during, or after the entry of the comet fragments.     

Short-lived Jovian decametric events observed with an acousto-optical radio spectrograph

S-bursts and other short-lived events in Jupiter's decametric radio spectra are studied with an acousto-optical radio spectrograph (AORS) and a charge-coupled device (CCD) readout. The lifetimes of regular S-bursts are of the order of tens of milliseconds. Lifetimes of S-bursts that occur in quasi-periodic trains are mostly below 20 ms. Short-lived bursts that do not seem to drift in frequency are also observed. Such bursts (nondrift bursts) may occur in S-trains and at the vertex of tilted-V patterns. The vertex bursts have lifetimes comparable to those of the S-train bursts. Preliminary measurements of the risetimes of bursts indicate that a significant proportion of the vertex bursts exhibit risetimes of less than 2 ms.     

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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Locations of Jovian decametric radiation sources

The location of the Jovian decametric radiation main source is determined to be the south magnetic pole while the location of the early source is found to be near the north magnetic pole, with an equal contribution from a region near the south magnetic pole. The results are based on calculations of the region observable from the Earth (ROE) for Jovian decametric radio waves that are emitted in the direction ± 10° centered on the direction perpendicular to the Jovian magnetic field and based on a Pioneer 11 model of the field at the level of the topside region of the Jovian ionosphere. Ground-based observations of the occurrence frequency of the decametric radiation as a function of Jovian longitude, which indicate a remarkable asymmetry between the early and main sources, agree with the calculated ROE area that varies as a function of CML observed from the Earth. The observations support a recent theory for the origin of the decametric radiation which is based on a wave-mode conversion from plasma waves into electromagnetic waves.     

A cometary ionosphere model for Io

The ionosphere of Jupiter's satellite Io, discovered by the Pioneer 10 radio-occultation experiment, cannot easily be understood in terms of a model of a gravitationally bound, Earth-like ionosphere. Io's gravitational field is so weak that a gravitationally bound ionosphere would probably be blown away by the ram force of the Jovian magnetospheric wind — i.e., the plasma corotating in the Jovian magnetosphere. We propose here a model in which the material for Io's atmosphere and ionosphere is drawn from the ionosphere of Jupiter through a Birkeland current system that is driven by the potential induced across Io by the Jovian corotation electric field. We argue that the ionization near Io is caused by a comet-like interaction between the corotating plasma and Io's atmosphere. The initial interaction employs the critical velocity phenomenon proposed many years ago by Alfvén. Further ionization is produced by the impact of Jovian trapped energetic electrons, and the ionization thus created is swept out ahead of Io in its orbit. Thus, we suggest that what has been reported as a day-night ionospheric asymmetry is in fact an upstream-downstream asymmetry caused by the Jovian magnetospheric wind.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30th May, 1978.     

The satellites of Jupiter as a source of very energetic magnetospheric particles

The Jovian satellites and ring are continuously bombarded by high-energy galacic cosmic rays and magnetospheric ions. Nuclear interactions will create very energetic neutrons and pions. The decay of some of these unstable particles within the Jovian magnetosphere wil result in trapped protons and ultrarelativistic electrons and positrons. Although this source is weak compared to those that yield lower-energy magnetospheric particles, it is expected to generate the most energetic Jovian particles. These processes are briefly described.     

Modelling the proto-Jovian atmospheres by a simple analytical algorithm

This paper models the proto-Jovian atmospheres by a simple analytical algorithm, which does not require the use of a computer. This method, known as the MDV method, was used to model the present structures of the Jovian planets in Doorish (1992). The protoplanetary stages of the Jovian planets, known as stage 1 of development, can be described by the Stellar Interior Equations (SIE). These equations which basically dictate the structure of a star, can also be used to model the Jovian planets (Bodenheimer, 1986). We here include tabulated results.     

Mass-injection rate from Io into the Io plasma torus

Theoretical arguments have been presented to the effect that both plasma and energy are supplied to the Jovian magnetosphere primarily from internal sources. If we assume that Io is the source of plasma for the Jovian magnetosphere and that outward flow of plasma from the torus is the means of drawing from the kinetic energy of rotation of Jupiter to drive magnetospheric phenomena, we can obtain a new, independent estimate of the rate of mass injection from Io into the Io plasma torus. We explicitly assume the solar wind supplies neither plasma nor energy to the Jovian magnetosphere in significant amounts. The power expended by the Jovian magnetosphere is supplied by torus plasma falling outward through the corotational-centrifugal-potential field. A lower limit to the rate of mass injection into the torus, which on the average must equal the rate of mass loss from the torus, is therefore derivable if we adopt a value for the power expended to drive the various magnetospheric phenomena. This method yields an injection rate of at least 10³ kg/sec, a value in agreement with the results obtained by two other independent methods of estimating mass injection rate. If this injection rate from Io and extraction of energy from Jupiter's kinetic energy of rotation has been maintained over geologic time, then approximately 0.1% of Io's mass (principally in the form of sulfur and oxygen) has been lost to the Jovian magnetosphere, and Jupiter's spin rate has been reduced by less than 0.1%.     

Jupiter's outer atmosphere

The current state of the theory of Jupiter's outer atmosphere is briefly reviewed. The similarities and dissimilarities between the terrestrial and Jovian upper atmospheres are discussed, including the interaction of the solar wind with the planetary magnetic fields. Estimates of Jovian parameters are given, including magnetosphere and auroral zone sizes, ionospheric conductivity, energy inputs, and solar wind parameters at Jupiter. The influence of the large centrifugal force on the cold plasma distribution is considered. The Jovian Van Alien belt is attributed to solar wind particles diffused in towards the planet by dynamo electric fields from ionospheric neutral winds and consequences of this theory are given.     

Dust in jupiter''s magnetosphere: Effect on magnetospheric electrons and ions

The interaction of the Jovian energetic radiation belt electrons, and the Jovian plasma, with an ambient dust population is examined. Firstly the distribution of dust, ejected from Io, in the inner magnetosphere is calculated. Using the mass loss in submicron particles of ~13g/sec, which is required to model the intensity and shape of the Jovian ring in the model of Morfill etal. (1980b), it is possible to quantitatively calculate losses of magnetospheric ions and electrons due to direct collisions with charged dust particles as well as multiple Coulomb scattering with resultant losses in the Jovian atmosphere. It is shown that the magnitude and radial dependence of the losses are sufficient to explain the electron measurements, although the possibility that some other process may be more effective cannot be ruled out. The same dust population has, on the other hand, no significant effect on the plasma, which should therefore be transported essentially loss free, except within the Jovian ring, if there are no other processes involved. Comparison with the data shows that loss free transport outside the ring does indeed satisfy the measurement constraints.     

Expected heliospheric attributes of Jovian pickup ions from the extended neutral gas disk

The MIMI CHEMS Instrument on the Cassini Orbiter detected Jovian pickup ions almost an AU upstream of Jupiter during the 2001 flyby. The clue to their planetary origin is the presence of singly ionized sulfur ions in quantities exceeding those expected from the interstellar gas entering the heliosphere (Nature 415 (2002) 994). Earlier modeling of the extended Jovian neutral gas disk suggested how the combination of the orbiting, localized Jovian source and interplanetary ionization processes should combine to produce a distinctive reservoir for heliospheric pickup ion production, different from its interstellar gas counterpart. Here the expected characteristics of pickup ions from the Jovian source are considered using a simplified model. The results provide an idea of the signatures in physical and phase space that reflect both the initial velocities and directionalities of the parent neutral population. Long-term measurements can easily test for these attributes given sufficient spatial and ion energy coverage.     

Spectrophotometry of Jupiter

Spectrophotometric maps of Jupiter were made between 24 and 27(UT) November 1974 on the McMath Solar Telescope at Kitt Peak National Observatory. We report a comparison between observed scaled reflectivities of the Jovian North Tropical Zone and the North Equatorial Belt at System II longitudes between 195 and 205 degrees. The belt/zone reflectivity ratios between 430 and 740nm are related to the optical transmission curves of the organic and/or sulfur polymers synthesized by Khare and Sagan in a simulated Jovian atmosphere.     

Low order effect of crand input in the Jovian atmosphere

Modeling of the Jovian atmosphere shows that cosmic ray induced albedo neutron decay is inadequate to account for Pioneer 10 and 11 projected electron levels on Jupiter. High energy solar protons must also be excluded as an important neutron decay source. Analysis of neutron flux data near the top of the Jovian atmosphere can lead to the determination of He/H₂ and³He/⁴He ratios for the Jovian atmosphere.     

Effect of parallel electric fields on whistler mode waves in an anisotropic Jovian magnetospheric plasma

The effect of parallel electrostatic field on the amplification of whistler mode waves in an anisotropic bi-Maxwellian weakly ionized plasma for Jovian magnetospheric conditions has been carried out. The growth rate for different Jovian magnetospheric plasma parameters forL = 5.6R _j has been computed with the help of general dispersion relation for the whistler mode electromagnetic wave of a drifted bi-Maxwellian distribution function. It is observed that the growth or damping of whistler mode waves in Jovian magnetosphere is possible when the wave vector is parallel or antiparallel to the static magnetic field and the effect of this field is more pronounced at low frequency wave spectrum.     

Observations at the inner edge of the Jovian current sheet: evidence for a dynamic magnetosphere

It is widely recognized that Io, the innermost of the Galilean satellites, releases matter into the rapidly-rotating Jovian magnetosphere at rates that may be as high as a ton per second. Following ionization, this iogenic, heavy-ion plasma dominates the dynamics of the Jovian magnetosphere. On average this plasma must be lost at a rate that balances its generation but we do not know whether this process is steady or intermittent. Measurements by the Galileo magnetometer suggest that this process is unsteady. By estimating the magnetic and particle stresses from these observations, we further can derive a mass density profile that is consistent with earlier measurements of the current sheet density and that is consistent with estimates of the radial transport of mass in the middle Jovian magnetosphere.     

An analytical model of the Comet's collision with Jupiter

An important feature observed in the wake of the Jupiter-comet clash was the appearance of the ring structure axisymmetrically positioned around the center of the impact. The persistent expansion of the dark rings and its speed indicated an outward propagating gravity wave (Benka, 1995). We employ an analytical model of constant density, uniform finite depth and inviscid fluid layer to investigate the wave motion produced by the impact of Comet Shoemaker-Levy 9 on the Jovian atmosphere. The relevant thermal effects are neglected and an explosion resulting from the collision is then described by an initial impulsive pressure at the surface of the Jovian atmosphere. Under the assumption that all the kinetic energy of a comet fragment is completely converted into the energy of wave motions in the Jovian atmosphere, an analytical formula describing the relationship between the resulting wave motion in the atmosphere and the parameters of a comet fragment (the radius, density and speed) is derived. Results from the present simple analytical model give a qualitative agreement with observations regarding the distance and speed of the waves.