Search

Jovian decameter emission is known to exhibit almost total polarization. We consider the elliptical polarization to be a consequence of linear-mode coupling in the Jovian magnetosphere outside the source region. We determine conditions of emission propagation along the ray path that are necessary for self-consistent explanatation of the polarization observations and show that the ellipticity (axial ratio of the polarization ellipse) is determined by the magnetospheric plasma density ne in a small region a distance of about half the Jovian radius from the radiation source. The plasma density in the region is quite low, ne

GPS/MET, a multichannel Global Positioning System (GPS) receiver onboard the small research satellite MicroLab 1, is the first example of a research tool of great importance for ionospheric research. In the near future, other satellites with GPS/GLONASS (Global Navigation Satellite System (also GNSS)) receivers will be launched. Their main purpose is lower atmosphere research, but because of the necessity to correct for plasma influences, “ionospheric” data will be available as a side product. The occultation of GNSS signals offers the possibility to gain very good quality height profiles of electron density by means of classical inversion techniques. The profiles are averaged horizontally. This paper concentrates on the possibility to complement inversion results with electron content data gained on the ground using beacon signals of low orbiting satellites (e.g., the U.S. Navy Navigation Satellite System (NNSS)). The data combination offers several possibilities for ionospheric tomography. Several GNSS scanning satellite scenarios are assessed, and their ionospheric imaging/tomography merits are discussed. An example result for the inversion of GPS/MET data is shown. The results of simulations with model ionosphere data are used to demonstrate tomographic reconstruction techniques based on the combination of “space” and “ground” electron content. The simulation results have direct applicability to observed data.

Rheometry serves as a method for the determination of effective length vectors of short antennas by means of electrolytic tank measurements. This paper reports on the application of rheometry to the three linear monopoles mounted for the purposes of the Radio and Plasma Wave Science Experiment on the Cassini spacecraft, which will fly to planet Saturn. The voltage signals induced by incoming waves from the Saturnian radio emissions will be recorded for further evaluation. By direction-finding techniques one will trace back from the collected data to the source regions of the received radio waves and determine the wave polarization. An accurate direction finding is only possible if the effective length vectors of the antennas, which are affected by the spacecraft body, are known to a certain degree of accuracy. It is investigated how rheometry enables the determination of the effective length vectors with the help of a scale model. After a detailed discussion of the fundamentals of rheometry, the application of rheometry to the Cassini scale model is described. The results of the measurements are graphically depicted and discussed with the requirements for direction finding taken into consideration. Finally, an overview of the inflight antenna calibration is given, which will be possible by utilizing the strong Jovian radio emissions during Cassini's Jupiter flyby.

Radio occultation sensing of the terrestrial atmosphere can be performed by using signals from Global Positioning System (GPS) or Global Navigation Satellite System (GLONASS) satellites, which are received by satellites in low Earth orbit. The signal transmitted from a GPS/GLONASS satellite, when occulted, suffers propagation effects in the ionosphere and atmosphere, including carrier and group phase shifts and ray path bending. Those propagation effects give direct information on the refractivity of the medium of occultation. In order to sense the neutral atmosphere only, we have to carefully correct for the ionospheric contribution to the propagation effects whose influences are poorly known at present time. In this paper, we provide a straightforward method for model-independent correction of the ionospheric effects based on the correction of the ionospheric bending angle rather than correction of the optical path lengths, as done by previous studies. Using the presented correction, the accuracy of retrieved temperature profiles of the atmosphere is better than 1 K up to heights of some 55 km during typical ionospheric conditions. The formalism can easily be implemented in on-line analyses of phase data obtained by the radio occultation technique. This promising technique may thus in fact become an extremely valuable tool for supporting climate monitoring as well as weather and climate modeling and prediction.

For experimental observation of the Jovian decametric emission, it is important to know the low-frequency limit of the received radiation, which depends on the Jovian elevation (angular position from the local horizon) and the ionospheric electron density distribution. We develop the corresponding analysis and present results for different Jovian positions and different models of the ionospheric electron density profile. This consideration is of special interest for the periods 1993 to 1997 and also 2005 to 2009, years for low Jovian positions observed from northern hemisphere observatories where Jupiter is in the southern hemisphere in the celestial equatorial coordinate system. For several ionospheric models we determine the using range which depends on the Jovian elevations. By the use of empirical ionospheric models from Feichter and Leitinger, [1990, 1993] of the total electron content (TEC) from which an electron density maximum can be derived, we found that the solar activity highly influences the electron peak density which, in turn, acts on the low-frequency limit of the detectable Jovian decametric emission from ground-based observatories.

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.

By using two rotating noncollinear antennas or three spatially fixed noncoplanar antennas on a spacecraft, full information on the polarization and the direction of arrival of an electromagnetic wave can be obtained by measuring the voltages created by the electric field of the incident wave. The physical parameters (polarization and direction of arrival) of the incoming wave are related to the received voltages on the antenna system by the so-called direction-finding equations. Since the used antennas are generally of small directivity (electrically short monopoles or dipoles), the resulting system of equations is numerically close to singular, and generally no unique solution can be obtained for the physical parameters of the wave throughout the inversion process. However, there exists a very powerful tool for dealing with sets of equations that are singular or close to singular, known as singular-value decomposition (SVD), which precisely focuses the problem. For illustration, this paper analyzes the direction-finding equations for the Radio and Plasma Wave Science (RPWS) experiment on the Cassini spacecraft by using SVD techniques. It also compares the expected performances of RPWS with those of the Ulysses Unified Radio and Plasma Wave (URAP) experiment achieved at Jupiter for the kilometer and hectometer emissions. The RPWS experiment on Cassini, which will be launched in 1997, is supposed to observe wave phenomena between a few hundred Hertz and 16 MHz in the Saturnian magnetosphere.

Taking advantage of the direction finding capabilities of the Ulysses unified radio and plasma wave (URAP) experiment we derive the source locations and emission characteristics of the Jovian hectometric (HOM) and broadband kilometric (bKOM) emissions. Unlike previous studies we additionally determine the systematic error of the source direction due to the uncertainty of the antenna/ receiver calibration parameters. To obtain maximum accuracy when doing direction finding, we use HOM and bKOM events observed close to the Ulysses encounter with Jupiter. It is found that both emissions have their sources at the auroral zones of Jupiter, at dipole L shells between 7 and 11 for HOM in the northern hemisphere, and between 9 and 15 for the bKOM radiation observed at high southern latitudes after the Ulysses Jupiter encounter. For the analyzed HOM events the source location is usually between 40° and 130° central meridian longitude (CML) and 2200 and 0600 local time (LT), but probably there exist emissions from other longitudes and local times too. In contrast, the bKOM is emitted from a wide range of longitudes and local times as well. There is some evidence that both the kilometric and hectometric emissions are composed of a dominant component in the fast extraordinary (R-X) mode and a weaker ordinary (L-O) mode component. Due to the uncertainty in source direction determination the emission's cone half-angle (i.e. half-angle of the cone in which the emission is beamed) cannot be accurately determined. It is some 30°–90° for the HOM and about 40°–80° for the bKOM. The source location of bKOM is likely to be associated with open magnetospheric field lines whereas the HOM is located at field lines that connect the HOM radio sources with the inner Jovian plasma sheet and/or outer plasma torus. There is some evidence that the bKOM radio emissions are correlated with ultraviolet auroral activity.

Radio emissions from Jupiter provided the first evidence that this giant planet has a strong magnetic field and a large magnetosphere. Jupiter also has polar aurorae, which are similar in many respects to Earth's aurorae. The radio emissions are believed to be generated along the high-latitude magnetic field lines by the same electrons that produce the aurorae, and both the radio emission in the hectometric frequency range and the aurorae vary considerably. The origin of the variability, however, has been poorly understood. Here we report simultaneous observations using the Cassini and Galileo spacecraft of hectometric radio emissions and extreme ultraviolet auroral emissions from Jupiter. Our results show that both of these emissions are triggered by interplanetary shocks propagating outward from the Sun. When such a shock arrives at Jupiter, it seems to cause a major compression and reconfiguration of the magnetosphere, which produces strong electric fields and therefore electron acceleration along the auroral field lines, similar to the processes that occur during geomagnetic storms at the Earth.

For the Uranus smooth low frequency emission (SLF) two different (and contradictory) source locations (one polar, one equatorial) have been reported in the literature. Visibility studies show that polar source locations within a limited magnetic longitude range can be singled out, whereas a polar source distributed along the whole range of magnetic longitudes is compatible with observations. In this study we have computed intensity contours for comparison with the observed emission. Profiles obtained by a geometrical beaming model favor the equatorial source location.

By using PRA (Planetary Radio Astronomy), PWS (Plasma Wave System) and MAG (Magnetometer) data from Voyagers 1 and 2, essential features of the nightside Jovian plasma sheet are derived, and the density gradient of the corotating plasma structure in the middle Jovian magnetosphere is estimated. The PRA experiment gives information about the plasma wave polarization. The emission was found to be left hand polarized when the spacecraft was in the northern magnetic tail lobe, and right hand in the southern lobe. The PWS experiment enables the determination of a density profile of the Jovian plasma sheet from the knowledge of the low frequency cutoffs of nonthermal continuum radiation observed at 3 frequencies (f=562 Hz, f=1.0 kHz, f=1.78 kHz). The results are discussed and compared with those obtained from other investigations.

The observations of the planetary radio astronomy (PRA) experiment aboard Voyager 2 reveal the existence of smooth and bursty radio emission. Both recur in a rather regular pattern with a 16.1-hour period (the Neptunian spin period). We describe the phenomenology of the smooth component in terms of frequency, polarization, and occurrence in magnetic longitude and latitude. The existence of both right-handed and left-handed polarized emissions is consistent with two sources (one in each hemisphere) which radiate independently in the TUX mode. Because Voyager passed Neptune at less than 5000 km from the surface at high northern magnetic latitudes, the radio sources were occulted by the planet near the encounter. We have taken advantage of this occultation to locate the northern hemisphere sources by calculating the radio horizon (based on the offset tilted dipole (OTD2) model) for two spacecraft positions close to the encounter. We find that the northern source is located at high magnetic latitudes δm > 40°. By using a geometrical beaming model which assumes emission in a hollow cone pattern we fit the observed PRA intensity profile. The best fit is obtained for a radio source at L=6, thus confirming δm > 40°. The longitudinal extent of the source is at least 180°, from −90° to +90° magnetic longitude. To locate the southern sources we use the large excursions of Voyager 2 to high southern magnetic latitudes. The source is found to be located also at high magnetic latitudes but is possibly more limited in longitude than the northern source. We estimate the uncertainty in source location due to the very limited knowledge of the magnetic field near the Neptunian surface, and we show that the current OTD2 model is satisfactory for the source locations for the lowest observed frequencies; however, an angular uncertainty of about 20° remains for sources in the northern hemisphere. The observed pattern of the smooth emission is strongly frequency dependent, which is in agreement with the azimuthal asymmetries of the magnetic field as predicted by the OTD2 model.

The Ulysses unified radio and plasma wave (URAP) experiment is well suited to observe the Jovian hectometric radio emission (HOM) and so complement the observations made by the low-frequency band receiver on Voyager. When the spacecraft encounters Jupiter, in February 1992, observations will be possible from considerably higher magnetic latitudes, in both hemispheres, than those accessible to Voyager. It should also be possible to determine the source location, as URAP can measure the direction of the incoming waves and all four Stokes parameters. Based upon the HOM beaming and source model of Ladreiter and Leblanc (1990a,b), the following predictions can be made: (1) During the inbound trajectory, Ulysses will, at first, be close to the Jovian equatorial plane when equal amounts of right-handed (RH) and left-handed (LH) circularly polarized HOM should be observed from the northern hemisphere and the southern hemisphere, respectively. During the three days immediately before closest approach, the spacecraft will move to a higher northerly magnetic latitude where it will be outside the main HOM emission beam so that a relatively weak RH polarized signal should be observed. (2) At closest approach, Ulysses will be very close to the low-frequency HOM sources and will cross the HOM shadow zone, which extends out to about 20 Jovian radii (RJ) close to the magnetic equatorial plane. (3) During the outbound trajectory, the spacecraft will be at a fairly high southerly magnetic latitude and outside the main HOM emission beam, so that a relatively weak LH polarized signal should be observed.

Ray tracing using realistic magnetic field and plasma models has been performed to locate the Jovian hectometric radiation (HOM) sources. To consider only a few possible source locations, we have assumed that the radiation mechanism is the cyclotron maser instability (CMI), and taken into account the characteristics of the HOM including the recent results by Ladreiter and Leblanc (1989) (hereafter paper 1). The emission is assumed to escape from the planet in the extraordinary wave mode within a thin beam of hollow cones whose apexes are distributed uniformly in longitude at the northern and southern hemispheres at altitudes where the gyrofrequency fc is less but nearly equal to the observed wave frequency. The rays are traced from possible sources located at f/fc = 1.05 and labeled by the dipole shell parameter L. For each source, we have determined the emergence wave normal angle θcrit above which the rays no longer travel within the HOM emission beam nearby the magnetic equator (paper 1). We show that the HOM sources cannot be located on field lines within the Io flux tube. However, wave refraction due to the Io torus plays a crucial role for the propagation and beaming of the waves toward the magnetic equator. It is found that HOM sources located at field lines 15 ≤ L ≤ 30 agree best with the observations when assuming that the cone half angle of emission is about 70°–90° with respect to the local magnetic field. Thus, the source is associated with the tail field aurora where the magnetic field lines connect the polar region to the Jovian magnetic tail and is extended in altitude from 2 to 7 Jovian radii (RJ). Our results explain in detail the radio phenomenology of the hectometric radiation, in particular the strong latitudinal beaming, the lowest observed frequencies (≈ 40 kHz), the solar wind control, the absence of HOM emission when Voyager 1 was within the Io torus, and the absence of Io control on the HOM. The results are discussed in the context of the other Jovian radio sources, namely the sources of the non-Io decametric and kilometric emissions.

By using planetary radio astronomy (PRA), plasma wave system (PWS), and Magnetometer (MAG) data from Voyager 1 and 2 (V1 and V2), essential features of the nightside Jovian plasma sheet are derived, and the density gradient of the corotating plasma structure in the middle Jovian magnetosphere is calculated. The PRA experiment gives information about the plasma wave polarization. To determine the density profile of the plasma sheet, the authors have derived the hinge point position of the plasma disc from MAG data and used the low-frequency cutoffs observed at three frequencies (562 Hz, 1 kHz, and 1.78 kHz) from the PWS experiment. They show that the hinge point position varies with the solar wind ram pressure, and the plasma disc thickness decreases with distance up to about 60 R{sub J}. The average thickness for an isodensity contour corresponding to 1 kHz is 3.29 R{sub J} for V1 and 3.16 R{sub J} for V2.