H.-P. Ladreiter

Cassini model rheometry

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 Cassinis Jupiter flyby.

Analysis of electromagnetic wave direction finding performed by spaceborne antennas using singular-value decomposition techniques

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.

Emission characteristics and source location of the smooth Neptunian kilometric radiation

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 uncertainty of the Uranian radio source location due to the nonuniqueness of the planetary magnetic field model

Since the Voyager 2 encounter in early 1986, several investigators have attempted to localize the source regions of the smooth high-frequency radio emission which was observed by the planetary radio astronomy experiment at the nightside of Uranus. The various studies (most of them are based on the offset tilted dipole (OTD) model of the Uranian magnetic field) yielded significantly different source locations around the southern magnetic pole of Uranus. This may be a consequence of the individual a priori assumptions of the source model. However, the simplicity of the OTD model (Ness et al., 1986) also cannot adequately represent the complexity of the magnetic field at the radio source locations near the planet. The aim of this study is twofold. (1) We reanalyze the various source locations given in the literature (most of them are based on the OTD model) in the frame of the Q3 magnetic field model (Connerney et al., 1987). Our analysis moves some of the previously determined source locations from open toward closed field lines; however, the uncertainty due to the nonuniqueness of the Q3 model remains too large to exclude the possibility that open field lines are the source of smooth Uranian kilometric radiation. (2) We calculate the uncertainty of the radio source locations imposed by the nonuniqueness of the Q3 and OTD magnetic field models. We construct solutions by using generalized inversion techniques (Connerney, 1981) to obtain estimates of those magnetic field parameters (spherical harmonic coefficients up to degree and order 6) that are constrained by the magnetometer observations. The nonuniqueness of the resulting magnetic field models translates into an uncertainty about the radio source locations of some 20° in Uranocentric coordinates at altitudes of about 1.5 Uranian radii (RU). The present results are important for radio source locations at all the outer planets whose magnetic field geometries are represented by nonunique magnetic field models.

Uranus smooth low frequency emission

Abstract 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.