Instrument Information
IDENTIFIER urn:nasa:pds:context:instrument:gp.lrd::1.1
NAME LIGHTNING AND RADIO EMISSION DETECTOR
TYPE PARTICLE DETECTOR
DESCRIPTION Instrument Overview =================== The Lightning and Radio Emission Detector (LRD) instrument will be carried by the Galileo Probe into Jupiter's atmosphere. The LRD will verify the existence of lightning in the atmosphere and will determine the details of many of its basic characteristics. The instrument, operated in its magnetospheric mode at distances of about 5, 4, 3, and 2 planetary radii from Jupiter's center, will also measure the radio frequency (RF) noise spectrum in Jupiter's magnetosphere. The LRD instrument is composed of a ferrite-core radio frequency antenna (~100 Hz to ~100 kHz) and two photodiodes mounted behind individual fisheye lenses. The output of the RF antenna is analyzed both separately and in coincidence with the optical signals from the photodiodes. The RF antenna provides data both in the frequency domain (with three narrow-band channels, primarily for deducing the physical properties of distant lightning) and in the time domain with a priority scheme (primarily for determining from individual RF waveforms the physical properties of closeby-lightning). The LRD instrument has been designed to take into account large uncertainties in the nature of possible Jovian lightning. For example, since Jupiter has no well-defined surface close to the cloud system, there will be no cloud-to-ground discharges, which are the best understood type of lightning on Earth. Lightning in general, and cloud discharges specifically, are very complex physical phenomena and can generate a large variety of RF pulse types and trains: unipolar pulses, bipolar pulses, asymmetric pulses, groups and bursts of pulses. The LRD instrument is designed as a compact and versatile instrument which allows a characterization of these signals with maximum sensitivity and maximum dynamic range. During the design phase, prototype instruments have been intensively tested with Earth lightning during several measuring campaigns. The final instrument characteristics have been set with acceptable margins for the unknown conditions to Jupiter. Modelling of the propagation of RF signals in the frequency range of the LRD instrument in Jupiter's atmosphere shows that direct propagation of signals will occur to distances of order 10^4 km (Rinnert et al., 1979). Hence, it is likely that Jovian atmospheric discharges with the energy of a typical cloud-to-ground discharge on Earth (order 10^8 J) will be detected at 10^4 km or more distance within the atmosphere with the LRD instrument. As noted below, the LRD instrument also includes a 'superbolt' channel, in order to count extremely large events. Hence, in light of all the above, the flown lightning detector instrument must be designed to be as sensitive as possible, limited only by spacecraft noise. The instrument must also cover as large a dynamic range as possible. Principal Investigator ====================== The Principal Investigator for the LRD instrument was Louis Lanzerotti. Scientific Objectives ===================== Radio frequency measurements are made in a reduced mode of operation at altitudes of ~5, 4, 3, 2 planetary radii from the center of Jupiter. These data are stored in the Probe memory and then read out during the atmospheric descent phase of the mission. During the atmospheric descent, the full complement of LRD data are acquired until the loss of the Probe signal by the over-flying Orbiter and/or the demise of the Probe due to atmospheric pressure and heat. The RF data obtained in the magnetosphere will be analyzed also jointly with the Probe Energetic Particle Instrument (EPI) data to gain understanding of magnetospheric particle dynamics. In the magnetosphere, statistics on the characteristics of individual waveforms measured during a sampling interval will be accumulated at the four different altitudes. In addition, noise levels at three different spectral frequencies (3, 5, 90 kHz) will be determined during the measurement intervals. In the atmosphere mode, in addition to statistics on the waveforms and the spectral noise levels at the three narrow-banded frequencies, individual waveforms will be detected, saved, and transmitted to Earth. Such waveforms will provide powerful additional diagnostic capabilities for Jovian RF signals. The LRD instrument, as noted above, has been designed to be as sensitive as possible, limited only by the spacecraft noise, and to be as versatile as possible, limited only by the imposed limitations on power, bit rate, and reliability considerations. It is within these constraints that the scientific objectives will attempt to be achieved. Extensive measurements with Earth lightning have been made and these will be continued in order to gain the maximum understanding of the operational characteristics of the instrument, and therefore the maximum science from the Probe descent through Jupiter's atmosphere. In both the magnetosphere and atmosphere modes the component of the Jovian magnetic field perpendicular to the Probe spin axis will be determined. These data will be used for analyses of EPI data and for determining the spatial distribution of the sources of some of the detected lightning signals. Further, these data will give engineering data on the Probe spin rate. Calibration =========== Because of severe constraints as to weight and power for Probe subsystems, the LRD instrument is very compact. Further, extensive on-board compression of the data is necessary because of the limited available data rate. All sensors and instrument characteristics, of course, have been extensively tested and calibrated. For example, radiation tests were carried out on the sensor electronics and pressure tests were made of the vented electronics box. These latter tests caused a stiffening piece to be added to the microprocessor chip. The calibrations could be verified over long periods because of the delays of the launch of the Galileo spacecraft. A further verification of the instrument parameters is provided by the on-board implemented test generator (ITG). Operational Considerations ========================== Magnetosphere mode. The LRD instrument will operate in the pre-entry phase at distances from the planet's center of about 5, 4, 3, and 2 Rj. The instrument is switched on by the Probe timer at these locations. In this 'magnetosphere mode' the EPI is also in operation. As the Probe is still encapsulated within the heat shield, the MS is less sensitive and the optical sensors are covered. The outputs of the LRD instrument are as noted in the previous section, but without the waveform snapshots. The magnetosphere mode data set at each of the four locations consists of a 64 byte data frame with statistics and the 3 kHz spectral channel subdivided into parallel and perpendicular (to the magnetic field) channels. The data are stored in the Probe memory for transmission during the atmospheric descent phase of the mission. Atmosphere mode. When the LRD instrument is switched on at descent the instrument begins with a test cycle (IFT) and the first data set contains the test pulse data. After that, the instrument runs continuously until the end of the mission and outputs a complete data set every four major frame periods, 256 s. These data sets contain spectral data (the 15 kHz channel being sectored), waveform statistics data, a 1 ms time interval with a selected waveform, optical data and miscellaneous data such as magnetic field component, spin period, and engineering data. The number of complete data sets achieved during descent depends upon the length of time that the Probe survives and/or the length of time that the Probe relay signal is successfully acquired by the over-flying Orbiter. For example, if the total atmosphere data time is ~48 min, then 10 data sets would be sent back (the 11th would be acquired but there would be no time for transmittal). The 10 data sets would contain one test data set and 9 science data sets.
MODEL IDENTIFIER
NAIF INSTRUMENT IDENTIFIER
SERIAL NUMBER not applicable
REFERENCES Lanzerotti, L.J., R.E. Gold, K.A. Anderson, T.P. Armstrong, R.P. Lin, S.M. Krimigis, M. Pick, E.C. Roelof, E.T. Sarris, G.M. Simnett, and W.E. Frain, Heliosphere instrument for spectra, composition and anisotropy at low energies, Astron. Astrophys. Suppl. Ser. 92, 349-363, 1992.