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.
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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.
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