4.0 Cassini RPWS Design Aspects

To achieve the scientific objectives of the Cassini mission, the instrumentation must have certain design and performance characteristics. This section gives the rationale for the RPWS instrument design and describes the performance required to achieve the scientific objectives described in the previous section. The topics discussed include: (1) field sensors, (2) frequency ranges, (3) frequency and time resolutions, and (4) sensitivities and dynamic ranges.

4.1 Field Sensors

A basic question that arises in the design of all radio and plasma wave instruments is what types of antennas should be used, electric or magnetic, and in each case, how many field components should be detected. For measurements of radio waves at frequencies well above the local characteristic frequencies of the plasma (i.e., above the electron cyclotron frequency and the electron plasma frequency), the propagation is essentially unaffected by the local plasma. At these frequencies, it is not necessary to measure both the electric and magnetic fields, since they have a constant known ratio, E = cB. In principle, it does not matter whether the electric field or the magnetic field is measured. However, for various reasons, both electrical and mechanical, it turns out that an electric antenna can operate at much higher frequencies and with a much greater sensitivity than a magnetic antenna of comparable size and weight [Gurnett, 1998]. Therefore, for high frequency radio measurements, such as for SKR and SEDs, an electric dipole antenna is preferred. For plasma waves, which occur at lower frequencies, near or below the electron cyclotron frequency and the electron plasma frequency, the situation is more complicated. Plasma waves can be either electrostatic, with no magnetic field, or electromagnetic, with both an electric field and a magnetic field. Although an electric antenna can detect both types of waves, the only way to conclusively distinguish an electrostatic wave from an electromagnetic wave is to use both an electric antenna and a magnetic antenna. If a wave can be detected with both an electric antenna and a magnetic antenna, then it is an electromagnetic wave. If it can be detected with an electric antenna, but not with a magnetic antenna, then it is an electrostatic wave. A major shortcoming of the Voyager plasma wave instrument, which used only an electric antenna, was the inability to distinguish electrostatic waves from electromagnetic waves. Therefore, for plasma wave measurements (i.e., at frequencies below the electron cyclotron frequency and the electron plasma frequency) both electric and magnetic antennas should be used.

Next we consider the number of components to be measured. The Voyager electric field antenna consisted of two cylindrical elements mounted in a V configuration, see Scarf and Gurnett [1977] and Warwick et al. [1977]. For the plasma wave instrument the two elements were used as a dipole, and for the radio astronomy instrument they were used as two orthogonal monopoles. Although the V configuration provided a limited capability to perform polarization measurements, no capability existed for performing direction-finding measurements. Since the RPWS scientific objectives require accurate determinations of source positions, as well as polarization measurements, it is essential that the RPWS be able to perform both direction finding (i.e., wave normal, k) and polarization measurements of high frequency radio signals (i.e., f > fce and f > fpe). Since at high frequencies the electric field is always perpendicular to the wave normal (E k= 0), the wave normal direction can be determined by measuring the plane of rotation of the electric field (provided the wave is not linearly polarized). Therefore, the RPWS must include full three-axis electric field measurements. The easiest way to achieve this is to use three orthogonal electric monopoles. In the low frequency plasma wave part of the spectrum (f < fpe or f < fce), it is also important to carry out wave normal measurements. Unfortunately, when the anisotropic effects of the plasma are considered, the wave normal of an electromagnetic wave cannot be determined from electric field measurements. From Poisson's equation, E = , one can see that because the charge density, , in a plasma is in general not zero, the electric field is no longer perpendicular to the wave normal, E k 0. Fortunately, Maxwell's equation B = 0 always implies that B k = 0. Therefore, to make wave normal measurements in the low frequency plasma wave part of the spectral three-axis magnetic field measurements are required. In addition, to assure that electrostatic waves can be detected and to resolve the ambiguity in the direction of propagation (i.e., along k or -k), electric field measurements are also required, although these do not necessarily have to be three-axis measurements. In addition to electric field measurements, it is also useful to have the capability to measure electron density fluctuations in the plasma wave frequency range. This capability is easily achieved by biasing the electric antenna element with a known fixed bias current (as though it were a Langmuir probe). With this bias condition, it can be shown that the voltage variations on the element are proportional to the fractional electron density variation, ne/ne. For electrostatic waves simultaneous measurements of both the electric field and the electron density variation can provide information on the wavelength of the wave [Kelley and Mozer, 1972; Wahlund et al., 1998].

4.2. Frequency Ranges

To decide on the frequency ranges required for the electric and magnetic field measurements, we must consider the characteristic frequencies of the radio and plasma wave phenomena to be studied in the vicinity of Saturn. The frequency ranges and spectrums of all radio and plasma wave phenomena known or predicted to occur in the vicinity of Saturn are summarized in Figures 4.2.1, 4.2.2 and 4.2.3. Figure 4.2.1 shows a model of the electron plasma frequency, fpe, and electron cyclotron frequency, fce, as a function of radial distance near the equatorial plane on the dayside of Saturn; and Figures 4.2.2 and 4.2.3 show representative electric and magnetic field spectrums at a radial distance of 10 RS . First, we consider the upper limit of the frequency range. Two high frequency limits must be considered, one for the electric field and the other for the magnetic field. The high frequency limit for the electric field is determined by the highest radio emission frequency of interest, and the high frequency limit for the magnetic field is determined by the highest plasma wave frequency of interest. As can be seen in Figure 4.2.1, lightning from Saturn's atmosphere (SEDs) has the highest frequencies.

Figure 4.2.1

Figure 4.2.2

Figure 4.2.3

From the Voyager observations it is known that the SED spectrum extends up to at least 40 MHz. Since the shape of the high frequency part of the SED spectrum is already known [Zarka and Pedersen, 1983], we do not plan to make electric field measurements as high as 40 MHz. Our primary objectives relative to SEDs are to monitor the long-term occurrence of lightning and to study variations in the low frequency cutoff imposed by Saturn's ionosphere. For these purposes it is only necessary to measure the SED spectrum to frequencies slightly greater than the maximum ionospheric cutoff frequency. Since the maximum plasma frequency of the dayside ionosphere is typically about 5 MHz, we have selected 16 MHz as the upper frequency limit for the electric field measurements. Since the ionospheric cutoff frequency varies as fcutoff = fpe/cos , where is the angle of incidence, this upper frequency limit allows the detection of lightning over a large range of incidence angles, approximately 0 ≤ ≤ 70°.

For the upper frequency limit of the magnetic field measurements we must consider the highest plasma wave frequency that is likely to be encountered in Saturn's magnetosphere. Asdiscussed earlier, magnetic field measurements have two main purposes: (1) to distinguish electrostatic waves from electromagnetic waves, and (2) to determine the wave normal direction of electromagnetic waves. During the baseline tour, the periapsis radial distance is expected to be in the range from about 4 to 6 RS, and the apoapsis is in the range from about 20 to 130 RS. As can be seen from Figure 4.2.1, the whistler mode, which is the highest frequency electromagnetic plasma wave mode that is likely to be of interest, is generally in the range from 3 to 10 kHz, with a maximum of about 12 kHz. Based on this magnetic field model we have selected 12 kHz as the upper limit for the magnetic field measurements. This allows magnetic field measurements with a simple tri-axial search coil magnetometer, which can be easily designed to respond to frequencies up to 12 kHz. In selecting this upper limit we realize that it will not be possible to detect the magnetic field of certain plasma wave modes that occur at frequencies above 12 kHz. For example, during the Titan flybys UHR emissions are expected to extend up to frequencies as high as several hundred kHz, and during Saturn Orbit Insertion (SOI), which is at about 1.3 RS, electrostatic (n + ½)fc electron cyclotron waves could extend up to frequencies as high as several hundred kHz. However, in all of these cases we feel confident that it will be easy to identify these electrostatic modes. For example, at Titan the UHR emissions will be at frequencies of ten to several hundred kHz, well above the electron cyclotron frequency, which is the highest frequency for the whistler mode. The situation is more complicated near SOI, where whistler-mode emissions could occur at frequencies as high as 500 kHz. To provide magnetic field measurements at such high frequencies would require the use of a loop antenna [Gurnett, 1998]. Although such antennas have been flown in the past, they are large and very difficult to accommodate on the spacecraft, so a decision was made that high frequency magnetic field measurements were not justified to achieve this limited objective, given the additional resources that would be required.

Next, we consider the low frequency limit of the plasma wave spectrum. Since scientifically interesting plasma wave phenomena exist down to essentially zero frequency, the low frequency cutoff of the RPWS electric and magnetic field sensors is determined almost entirely by technical considerations. For example, conventional static field magnetometers tend to have better sensitivities than search coil magnetometers at frequencies below about 1 Hz. Since a tri-axial static field magnetometer (MAG) is included on Cassini [Southwood et al., this issue], there is no reason to extend the frequency range of the RPWS magnetic antenna below about 1 Hz. Therefore, we have adopted 1 Hz as the low frequency cutoff of the RPWS magnetic field measurements. For the electric field antennas the situation is more complicated. The frequency response of the electronics for the electric field measurements could easily be extended below 1 Hz. However, because of sheath effects around the spacecraft body, such low frequency, quasi-static, electric field measurements require very long antennas, typically with lengths at least ten times the maximum dimension of the spacecraft body. Since such long antennas (~100 meters, tip-to-tip) could not be accommodated on Cassini due to spacecraft dynamics considerations, we have arbitrarily defined the low frequency cutoff of the RPWS electric field measurements to be the same as for the magnetic field measurements (i.e., 1 Hz).

4.3. Frequency and Time Resolutions

It is well known that the frequency and temporal structure of Saturnian radio emissions and plasma waves vary over an extremely large range [Scarf et al.,1984; Zarka, 1998]. Some types of waves, such as continuum radiation and whistler-mode hiss, have smooth continuous spectrums that can be resolved with very modest frequency and time resolution. On the other hand, certain other types of waves, such as lightning-generated whistlers, whistler-mode chorus, and cyclotron maser radiation, have extremely complicated frequency-time structures. These structures often extend down to frequency and time resolutions on the order of f t ~ 1. Furthermore, in some cases it is necessary to stress high frequency resolution (i.e., small f), such as in the analysis of plasma resonances, whereas in other cases it is necessary to stress high-
time resolution (i.e., small t), such as in the analysis of lightning and dust impacts. Resolving these conflicting demands is one of the main challenges that must be faced in designing a radio and plasma wave investigation.

What little is known about the fine structure of radio emissions and plasma waves at Saturn comes almost entirely from the Voyager measurements. The Voyager plasma wave instrument had a 16-channel spectrum analyzer spanning the frequency range from 10 Hz to 56 kHz, and a wideband waveform receiver that covered the frequency range from 50 Hz to 10 kHz. The time resolution of the 16-channel measurements was 4 seconds. During selected intervals, the wideband waveform provided high-resolution 48-second "snapshots" of the electric field waveform with a sample rate of 28,800 samples/sec. However, the number of wideband frames that could be transmitted was severely limited by data rate considerations. The Voyager planetary radio astronomy instrument, which made measurements from 20 kHz to 40 MHz, had somewhat better frequency resolution ( f/f ~ 1 to 5 percent), but relatively poor time resolution (~6 seconds/sweep). Although the radio astronomy instrument also included a high-rate mode that allowed rapid (millisecond) sampling of a selected channel, again the amount of high-rate data collected was severely limited.

Since it is highly likely that radio emissions and plasma waves in Saturn's magnetosphere have fine-scale structures comparable to those observed in the Earth's magnetosphere, it is important that the RPWS instrument be designed with sufficient frequency and time resolution to resolve these structures. In particular, the RPWS should provide a substantial improvement relative to the Voyager radio and plasma wave instruments. Our basic approach to achieving this goal is to make measurements on two widely different frequency and time scales: nearly continuous low-rate spectral measurements with a frequency resolution on the order of a few percent and a time resolution on the order of a few seconds, and short-duration high-rate wideband waveform measurements with frequency and time resolutions approaching the limit, f t ~ 1. To provide flexibility, the frequency and time resolution of the low-rate measurements must be controlled by a reprogrammable microprocessor, so that they can be changed to accommodate unexpected results. The high-rate wideband waveform measurements will allow us to produce high resolution frequency-time spectrograms. Since the entire waveform is transmitted to the ground, these measurements have the advantage that the frequency and time resolution of the spectral processing can be adjusted during the ground processing to provide the optimum resolution for the phenomena being investigated, the only limit being that f t 1. To provide high-resolution measurements of high frequency radio emissions, the waveform receiver must also have a frequency conversion mode of operation that can provide waveform measurements in selected frequency bands at high frequencies. This mode of operation will allow us to determine whether SKR has fine structure comparable to terrestrial AKR. Such high-resolution measurements of SKR will provide fundamental constraints on the mechanism by which these radio emissions are generated.

4.4. Sensitivities and Dynamic Ranges

The RPWS instrument must have sufficient sensitivity to detect the weakest signals of interest in the vicinity of Saturn, and still have adequate dynamic range to respond to the strongest signals without saturating. Figures 4.2.2 and 4.2.3 show the range of electric and magnetic field strengths that must be measured for various phenomena in the vicinity of Saturn. For some phenomena, such as the SKR and SED events, the intensities vary considerably. In these cases, the spectrums were selected from periods of relatively high intensity at a radial distance of 10 RS. As can be seen, the intensities vary over a wide range. For electric fields the spectral densities that must be measured range from a minimum of about 10-18 V2m-2Hz-1 at 106 Hz, to a maximum of about 10-6 V2m-2Hz-1 at 1 Hz, a total range of 120 dB. For the magnetic field the spectral densities that must be measured range from a minimum of about 10-7 nT2Hz-1 at 103 Hz to a maximum of about 101 nT2Hz-1 at 1 Hz, a total range of 80 dB. Since it is very difficult to achieve a total dynamic range of 120 dB, special techniques must be used to accommodate this very large range of signal strengths. For example, since the intensities tend to increase toward lower frequencies, the gain near the front end of the receiving system must be decreased at low frequencies in order to avoid saturation when strong low frequency signals are present. Also, since digital waveforms typically cannot accommodate such large dynamic ranges, automatic gain control systems must be used for all waveform measurements. These and a variety of other techniques must be used in order to assure that the instrument can perform reliable measurements over the large range of field strengths illustrated in Figures 4.2.2 and 4.2.3.