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.

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.

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.