The Science Definition Team is now preparing a second report and has requested an additional report to be prepared by an ad hoc science team, in another area of magnetospheric imaging, plasma sounding. This ad hoc science team, (the Plasma Sounder Team) has prepared the following report and will present its findings at the next meeting of the IMI Science Definition Team, February 22-23, 1993, in Washington, D. C. The members of the Plasma Sounder Team are given below.
The current IMI strawman payload includes imagers for observing the geocorona in the far ultra-violet (FUV), the aurora in the FUV and X-ray wavelengths, heavy ions in the plasmasphere from scattered extreme ultra-violet (EUV) radiation, and the ring current by energetic neutral atom detection. The purpose of this report is to introduce radio sounding techniques for the purpose of remotely measuring plasma density and to show how this technique naturally complements the instruments already defined for the IMI mission.
For probing the various magnetospheric plasma regions and determining their densities and distances a Radio Plasma Imager (RPI) will transmit and receive radio wave pulses at frequencies ranging from 3 kHz to 3 MHz. Using the echo delay and the direction of arrival of the echo signals, it will be possible to generate two-dimensional cross-sectional images of the plasmasphere and auroral zone during a single orbit, and to monitor the positions of critical plasma and magnetic boundaries such as the plasmapause and magnetopause on time scales of a few minutes.
Combining the RPI capabilities with those of the other IMI instruments would enable fundamental questions of magnetospheric morphology and dynamics to be addressed. For example, the interaction between the ring current and the inner magnetospheric plasma can be determined by the energetic neutral atom (ENA) and RPI instruments. Similarly, comparing the data from the ENA and geocorona imagers with the RPI electron density measurements would enable the study of the relationship between the loss and replenishment of neutral hydrogen from the geocorona and the inner plasmaspheric structure. In the auroral zone, RPI measurements would permit comparison of auroral emission patterns, and AKR source locations, with auroral plasma cavity density structures. While the IMI instruments are producing images of the plasmasphere and aurora, the location of the magnetopause can be monitored nearly continuously by the RPI. These combined observations would show global cause-and-effect relationships of fundamental importance to magnetospheric structure and dynamics.
The RPI considered for this study uses two long dipoles antennas in the satellite spin plane (500 meter tip-to-tip length) and a short dipole along the spin axis (10 meter). Pulses of a few milliwatts to a few watts will be transmitted from the long antennas, thus causing little if any interference with other instruments. Current estimates are that the antennas should weigh roughly 18 kg, instrument should weigh roughly 14 kg, require about 25 watts of power to operate and a data rate of about 5 kilobits/second with onboard processing and data compression.
Satellite sounders have proven to be quite successful since the mid 1960's. Ground based sounders using advanced digital techniques have proven that the RPI will face minimal technological risks. We believe that the RPI electron plasma density and range measurements will be critical for interpreting the data from other IMI instruments and for defining pertinent regions of the magnetosphere. The scientific benefits of the RPI to the IMI mission would be extremely high.
The main scientific objective of the Inner Magnetospheric Imager (IMI) Mission is to understand the global evolution of the structure of the magnetosphere and the relationships between key magnetospheric plasma regimes using imaging instruments. The IMI Science Definition Team is in the process of completing an initial scientific and engineering study with an emphasis on instruments capable of imaging the magnetosphere in the far and extreme ultraviolet radiation, X-ray radiation, and energetic neutral atoms. The purpose of this report is to introduce the technique of radio wave sounding by a Radio Plasma Imager (RPI) to detect remote magnetospheric plasma regimes and show how this technique is a natural complement to the instruments already defined for the IMI mission.
As technology advanced, ionospheric swept-frequency sounders were incorporated into satellites in order to obtain Ne profiles of the topside of the ionosphere, above the density maximum and thus inaccessible from ground based sounders. ISIS (International Satellites for Ionospheric Studies), one of the most successful long-lasting international space programs, produced more than 50 satellite-years of ionospheric topside-sounder data. These data were used to combine Ne profiles obtained from successive ionograms to produce orbital-plane Ne contours from the satellite altitude to the altitude of the F layer peak density [Nelms and Lockwood, 1966 and Benson, 1985 and references therein]. Topside/bottomside density comparisons and multi-spacecraft rendezvous studies have indicated that the accuracy of the ISIS sounder-derived Ne values (even at the most remote distances) is typically a few percent and no greater than 10% [Jackson, 1969b; Whitteker et al., 1976; Hoegy and Benson, 1988 and references therein]. In addition, the ISIS satellites also demonstrated that the sounders could operate in a manner compatible with other instruments on the same spacecraft. This compatibility was particularly well illustrated with ISIS 2 which, in addition to producing Ne profiles, produced the first monochromatic auroral images from space for scientific investigations [Lui and Anger, 1973; Shepherd et al., 1976].
Later spacecraft-borne sounders, employing digital technology, were developed in Japan for ISS-b (Ionosphere Sounding Satellite) [Maruyama and Matuura, 1984], Ohzora (also called EXOS-C) [Oya et al., 1985], and for Akebono (or EXOS-D) [Oya et al., 1990] and in the former USSR for Intercosmos 19 and Cosmos 1809 [Shuiskaya et al., 1990]. Relaxation sounders, which transmit very low power, are designed to stimulate plasma resonances in the ambient medium near the spacecraft for diagnostic purposes, have been flown on a variety of spacecraft over the last 15 years: GEOS 1 & 2 and ISEE 1 [Etcheto et al., 1983], Jikiken (EXOS B) [Oya and Ono, 1987], Viking [Perraut et al., 1990], Oedipus A [James, 1991], and Ulysses [Stone et al., 1992].
The greatest advance in sounding techniques has been employed in bottomside (ground based) ionospheric sounders over the last few decades, the Advanced Ionospheric Sounders (AIS), as described in the recent book by Hunsucker [1991]. In addition to measuring the amplitude and time delay of the returned pulse, as done by simple ground based ionosondes, these AIS instruments measure the exact frequency, phase, Doppler shift, polarization, and direction of arrival of the echo. Two sounders in this class are the Dynasonde (developed at the U.S. Department of Commerce Laboratories in Boulder) and the Digisonde (developed at the University of Massachusetts, Lowell). An important feature of these instruments is the high degree of flexibility in measurement format since operations are controlled by software. Evidence that their scientific capabilities go far beyond the Ne profiles of the standard ionosondes is indicated from results that show, for example, turbulence, winds and structures, [Wright and Hunsucker, 1983 and Buchau et al., 1988]. These latest techniques can now be implemented in space-borne instruments to make sounding of the magnetosphere possible with modest power.
fp (kHz) | r (Re) | R (Re)(1) | reff (Re) | S/N (dB)(2) | |
Plasmapause | 3 to 100 | 2-4 | +(2 to 4) | 3 to 12 | >= 0 |
Magnetopause | 20 to 50 | 4-9 | -(10 to 15) | <2.9 | >= 0 |
Plasmasphere | 30 to 1000 | 2-5 | +(1 to 4) | 3 to 30 | 20 to 30 |
Ionosphere | >1000 | 5 | +1 | 55 | 20 |
Auroral Cavity | 3 to 30 | 0.5 | +-5? | 0.5 | -30 to +10 |
Plasmoids | 1 to 10 | >10 | ~10 | >20 | <-50 |
(1) + defocusing, -focusing.
(2) S/N for L = 500 m and Va<1000 Vrms<=10 W.
Modern digital processing can add another 20 to 40 dB of processing gain to the analog S/N ratios shown in Table 1, which means that all target regions except the plasmoids will have a post-processing S/N ratio of more than 10 dB, which is adequate for their detection under quiet conditions, i.e., for [Phi] = 10-22 W/m2Hz.
The expected noise margin for echoes from the plasmasphere, for L = 500 meters, should be 20 to 30 dB, and, after digital signal processing, 40 to 70 dB. Accurate density and direction-of-arrival measurements should therefore be possible throughout this region during quiet times. Even in the presence of solar or magnetospheric noise 40 dB above the cosmic noise (as occurs during relatively rare solar noise storms) the measurements should still be possible near the ionosphere, where they are needed most. At the plasmapause, where the post-processing noise margin is expected to be 20 to 50 dB, it should be possible to measure the direction frequently and the distance and density most of the time. For the magnetopause it should be permissible to sacrifice range resolution by using longer coded pulses and a 100 Hz bandwidth (with 0.25 Re resolution) in order to increase the noise margin to 30 to 60 dB and hence permit adequate distance and direction measurements more often in the presence of stronger solar noise. Measuring low density contours of the auroral cavity, on the other hand, will require integration and must rely upon magnetic models for direction, since the noise margin with integration will probably be only -10 to +40 dB, and detecting plasmoids in the tail will probably not be possible.
If a short antenna is used for reception (see Appendix A, equations A15 to A18), the antenna must be tuned in order to obtain a similar performance as the one discussed for the 500 m transmit/receive antenna. This is illustrated in Figure 5, where the maximum range curves are shown for reception with an untuned and a tuned 10 m antenna and an untuned 500 m antenna; a 500 m transmit antenna was again assumed.
An RPI could be based on the design of the Digisonde Portable Sounder, or DPS [Haines et al., 1989; Reinisch et al., 1992] developed at the University of Massachusetts, Lowell for ground based ionospheric observations. The large echo ranges involved allow a long pulse width of 10 msec resulting in a range resolution of 1,500 km for the sounding of the magnetopause. A 1 msec pulse width would be selected for sounding of the near satellite environment. The pulse repetition rate would be limited by the largest distances to be probed (~ 25 Re or 1 sec of travel time). The pulse repetition frequency would not be higher than 1 Hz. A 12 dB (S/N = r(16)), processing gain could be obtained by transmitting a 16 bit phase coded signal, resulting in a 160 ms transmitter pulse for magnetopause sounding. For the near satellite sounding, the 1ms pulse would be transmitted uncoded. The maximum transmission duty cycle would therefore be 16 percent. Figure 6 shows a block diagram of a possible RPI instrument.
Modern digital techniques developed in recent years, like those now used routinely in advanced ground based sounders, include simultaneous Doppler integration and pulse compression. For Doppler integration, pulses are transmitted N times at the same frequency providing N complex measurements, for each range. The N measurements are Fourier transformed to give a Doppler spectrum. This spectrally coherent integration results in a signal to noise (S/N) improvement equal to r(N). For example, with N = 128 the S/N improvement becomes r(128) = 11.3 or 21 dB. There is an additional advantage to spectral integration. Assuming that the observed spectrum is primarily the result of the satellite motion, it will give additional information on the direction of echo arrivals.
For pulse compression, a long pulse consisting of M short pulses, each of length [tau], is transmitted. The phase is constant within each short pulse, but the phase changes between short pulses. In reception, the long pulse of length M[tau] is compressed to length [tau], by cross correlating the received signal with the transmission sequence. The S/N improvement in pulse compression equals r(M). For M = 64, the S/N improvement is 8 or 18 dB. Using both Doppler integration and pulse compression as in the examples above, the total S/N improvement would be 39 dB.
The computational requirements to perform the doppler integration and pulse compression processing are minimal, and an 8 bit microprocessor could easily handle the required correlation and Fourier transform algorithms. For example, integration over 128 pulses would require approximately 2 minutes for collection of the raw data samples. At the end of this reception period, the processor would have to pulse-compress 200 range bins (100 for ordinary and 100 for extraordinary polarization) requiring 6400 operations and compute 200 Doppler spectra, requiring an additional 80,000 operations. Sixteen bit integer processing will provide sufficient (96 dB) dynamic range. Since relatively simple microprocessors perform one million instructions per second (1 MIPS), or 120 million instructions in 2 minutes, this is a trivial load.
The Doppler processing of the received signals provides a number of advantages: (1) S/N improvement by phase coherent integration on all Dopplers; (2) direction of arrival information based on observed radar velocities relative to the satellite velocity; (3) spectral characterization of local plasma resonances.
Not all ranges can be simultaneously sampled with the same resolution and the RPI will, therefore, have to operate in a number of modes with varying transmission and processing characteristics. Two examples are shown below:
Long Range (Magnetopause) Mode
Receiver Bandwidth : 100 Hz (0.25 Re resolution)
Pulse Waveform : 160 msec (16, 10 msec phase code bits,
18 dB processing gain)
Pulse Integration : 32 pulses (15 dB processing gain)
Pulse Repetition Rate : 1 Hz (about 25 Re range ambiguity)
Transmitter Power : 10 W
Frequency Range : 20 kHz to 100 kHz in 4 kHz steps
Measurement Run-Time : 11 minutes (32 sec per frequency step)
Short Range Mode
Receiver Bandwidth : 1 kHz (150 km resolution)
Pulse Waveform : 1 msec unmodulated pulse
Pulse Integration : 64 pulses (18 dB processing gain)
Pulse Repetition Rate : 100 Hz
Transmitter Power : 10 W
Frequency Range : 100 kHz to 3 MHz in 100 kHz steps
Measurement Run-Time : 20 sec
A multitude of different modes can be provided for optimal deployment.
The power consumption of the RPI is estimated to be 25 Watts. A telemetry rate of 5 kbits/s would allow sending 160 kbit of data for each transmitted frequency in the Long Range Mode. This is more than adequate to characterize signal amplitude, polarization, Doppler and incidence angle for 200 range bins. Data compression will be required in the Short Range Mode where the integration time is only 0.64 seconds.
As described in this study, the techniques of probing remote plasma
characteristics by radio waves have been well established for ionospheric
studies but no such U.S. instruments have been flown in the magnetosphere. The
successful Alouette-ISIS program has spawned numerous investigations leading to
much of our current understanding and producing a global view of the topside
ionosphere [Jackson, 1986]. With the new generation of ground based
digital radio signal instrumentation and processing techniques developed over
the last several years, it will be possible to apply the same radio wave
sounding techniques to measure distant plasma parameters and to construct two
dimensional images of various magnetospheric plasmas.
It has been shown that such images produced by a radio plasma imager, are
feasible and would be of fundamental importance in achieving the scientific
objectives of the IMI mission.
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In order to estimate the power needed to sound the magnetosphere, it is
necessary to compare the received echo power Pr to the noise Pnoise at the
receiver. The echo power, for a pulse radiated power Pt at a distance r from
the target, as shown in Figure A1, is
(A1)
where the first factor represents the effective collection area of an antenna
[see Jordan, 1950], the second represents the echo power flux for a
plane reflector [which is proportional to 1/(2r)2 rather than
1/r4 because the reflection is coherent], F2 is a
focusing factor for reflection from a curved surface, M is a receiver impedance
mismatch factor, S/N is the signal-to-noise ratio, and Gt and Gr are the
antenna pattern gains of the transmitting and receiving antennas (typically
0.75 for a short dipole antenna and circular echo polarization). If wave
retardation (usually not a major factor) is also considered, r should be
replaced by the so-called "virtual range" r' = ct/2, where t is the
echo delay, [Franklin and Maclean, 1969]. Since a magnetospheric sounder
should presumably be shielded from terrestrial interference by the ionosphere,
the principal noise sources should be incoming cosmic or magnetospheric radio
noise and receiver thermal noise:
(A2)
where [Phi] is the spectral power flux of the external noise, B is the
receiver bandwidth, n is the noise factor of the receiver (typically 3 for a
good design), k = 1.38 x 10-23 J/K is
Boltzmann's constant, and T is the temperature. The required radiated power
thus becomes
(A3)
For curved reflecting surfaces, the echo power becomes proportional to
(A4)
where R1 and R2 are the principal radii of curvature of the
reflecting surface and R is the equivalent spherical radius of curvature
[see Davies, 1990,
p. 213]. As a result, the focusing caused by curved surfaces can be allowed
for by substituting
(A5)
for r in Equation A3, where reff is the effective range of the equivalent
planar target.
The receiver impedance mismatch factor, for the receiver input circuit shown
in Figure A2, where Ra is the radiation resistance, Xa is the antenna
reactance, and Ri is the receiver input impedance, is
(A6)
for Xa > Ri. Since the approximate radiation resistance and
reactance, in
ohms, of a short dipole antenna of tip-to-tip length L and radius a, according
to Jordan [1950, Ch. 13], are
(A7)
Equation A6 becomes
(A8)
and second term in the square brackets in Equation A3 becomes
(A9)
where [lambda]1 is the breakpoint wavelength at which receiver noise begins
to dominate external noise for longer wavelengths, and hence where the range
capability of a sounder (see text) changes slope. For L/a = 10,000, Ri = 600
[Omega], [Phi] = 10-22 W/m2Hz (cosmic radio noise),
n = 3, T = 300 K, and L in meters, this breakpoint wavelength is
approximately:
(A10)
The required radiated power, for Gr = Gt = 0.75,
B = 1 kHz, and [Phi] = 10-22 W/m2Hz, therefore becomes
(A11)
Although this indicates that only a few milliwatts are sufficient for sounding
over distances of a few Earth radii, at the low frequencies required for
magnetospheric sounding, relatively long antennas and high transmitter voltages
are required to radiate such power because of the large reactance of antennas
which are substantially shorter than the wavelength. For the circuit in Figure
A3, where Va is the transmitter voltage, Xa is the antenna
reactance, and Ra is the radiation resistance:
(A12)
For example, although only about 10 mW is sufficient for sounding at distances
of 10 RE, according to Equation A11 (with S/N = 1), for an antenna length of L
= 0.05 l (500 m at 30 kHz), this would require antenna voltages of about one
kilovolt.
A transmitter for magnetospheric sounding thus needs to supply high voltages
at low frequencies, in addition to relatively high currents at higher
frequencies where the antenna reactance is less (e.g., 12 ma or more at 300
kHz, where the antenna becomes a resonant half-wave dipole). It is suggested
that the transmitter might thus be of high power, high impedance design (e.g.,
400 watts at 2500 ohms), but limited to a suitable maximum power on the order
of 10 watts at the higher frequencies, using active feedback circuits.
From Equations A7, A11, and A12,
(A13)
where again [lambda] is the wavelength, L is the tip-to-tip antenna length,
Va is the antenna voltage, Gr = Gt = 0.75,
a = 10-4L is the antenna
radius, [Phi] = 10-22 W/m2Hz is the cosmic noise
spectral power flux, B = 1 kHz is the receiver bandwidth, and [lambda]1 is
given by Equation A10. For a given S/N ratio, this represents the range
capability of a magnetospheric sounder at low frequencies, and it should be
noted that rmax is proportional to Va and L2,
decreasing as
1/[lambda]2 for wavelengths shorter than [lambda]1, and as
1/[lambda]3 for longer wavelengths [where, according to Equation
A10, the breakpoint frequency corresponding to [lambda]1 is
f1 = c/[lambda]1 = 1.2 x 107/
L2 kHz.m2
(e.g., 48 kHz for L = 500 m) and at that frequency
rmax = 120 Va/[lambda]1 REm/volt =
Vaf1/2500 RE/volt.kHz
(e.g., 19.2 RE for and Va = 1 kV)].
At frequencies where the output is power limited, the maximum range is given
directly by Equation A11:
(A14)
where it has been assumed that [lambda] < [lambda]1.
Special Considerations for a Short Receiving Antenna
When the receiving antenna is different from the transmitting antenna,
the L in Equation A10 is the length of the former, whereas that in Equations
A12 and A13 is the length of the latter. Consequently, for
Lr << Lt:
(A15)
where Lt is the transmitting antenna length and Lr is
the receiving antenna
length, both in meters. On the other hand, if the receiving antenna is tuned
by switching in suitable series inductors in order to cancel the Xa in Figure
A2, Equation A6 becomes
(A16)
and Equation A9 thus becomes
(A17)
where Ri = 50 [Omega] in this case (since it is now advantageous to
use a low receiver impedance), the other quantities being the same. This
gives
(A18)
For Lr = 10 meters (tip-to-tip), therefore, a tuned receiving
antenna would give roughly half the range of an untuned antenna of length equal
to the transmitting antenna, (i.e., a 6 dB weaker echo signal), whereas
without tuning, the factor becomes
Lr2/40[lambda] = 2.5/[lambda] = 2.5 x
10-5
(-92 dB) at 3 kHz ([lambda] = 100 km). It is therefore critical to
tune the receiving antenna if a short antenna is used, and with such tuning, a
length of about 10 meters should be adequate.
Although the ohmic resistance of the antenna is presumably not a significant
limiting factor for untuned antennas at low frequencies, since the antenna
currents which determine radiation and reception are controlled primarily by
the antenna reactance, this could become significant for reception on a short
tuned dipole, hence dictating that short antennas for reception must be made of
high-conductivity material.
Listed below are potential questions that might be asked about magnetospheric
sounding, and answers.
1. Won't sounding cause electromagnetic interference (EMI) to other
instruments and spacecraft systems?
Although clearly a valid concern, the power levels of a magnetospheric sounder
are comparable to those of a cellular or portable telephone, at frequencies (3
kHz - 3 MHz) which are easily eliminated by filtering. For well designed
equipment, therefore, EMI caused by the sounder should not be a problem. Only
the in situ plasma and particle instruments on ISIS spacecraft detected EMI
from the sounder pulses and this interference could have been eliminate or
avoided by only sampling in the passive interval between pulses (L. H. Brace,
private communication, 1993).
2. Why does sounding require such long antennas?
Because of its high capacitive reactance, transmitting with a short antenna is
quite inefficient, requiring large voltages to radiate significant power, and
as a result, antenna lengths of 500 meters or more are needed to avoid
unreasonably high antenna voltages.
3. How will the long antennas affect the other instruments?
Although a solvable problem, antennas may restrict the field of view for
certain instruments. Also, because of thermal flexing as the solar heating
varies, long antennas can slightly affect the rotation rate of a spacecraft.
The change in spacecraft rotation rate due to long antennas was found to pose
little or no problems for the auroral imagers on ISIS-2.
4. Is manmade interference a problem?
At the frequencies involved, a magnetospheric sounder is nearly completely
shielded from man-made interference by the ionosphere, and it also operates
above the altitudes that whistler-mode transmissions can reach. At frequencies
below 1 MHz, a RPI should therefore be virtually immune to interference
from terrestrial sources, and this is borne out by wave observations showing no
such signals in the pertinent regime [private communication, R. R.
Anderson]. At frequencies from 1 to 3 MHz, ground-based broadcast
emissions do occasionally leak though the ionosphere. These transmitter
frequencies are relatively narrow-band and can be avoided in real time by the
instrument.
5.What limitations are imposed by natural interference?
The principal sources of natural interference are cosmic radio noise, solar
bursts, magnetospheric continuum, and AKR, all but the first being highly
variable and the last two consisting of narrowband discrete components. For
tip-to-tip antenna lengths of 500 meters or more, the noise margin should be
adequate to overcome interference by cosmic noise, magnetospheric continuum,
and all but the strongest (and hence infrequent) solar radio bursts. AKR, on
the other hand, can be eliminated, as broadcast stations are, by clear-channel
detection and automatically setting the transmitter frequency to
interference-free channels. The effect of magnetospheric continuum and solar
radio bursts is generally loss of directional information, and of AKR, a
localized loss of spatial resolution.
6. How do you know the direction of the echo?
As discussed in this study the primary method for the
determination of an echo's direction is by measuring the amplitude and phase of
the echo signals received on the three antennas. It is important to note that
there are several other independent methods which will can also be used to
confirm the echo direction determinations derived from the phase information.
These methods are:
(1) Since echoes are generated perpendicular to the density gradient, and the
plasma density is presumably distributed uniformly along the magnetic field
throughout most of the magnetosphere, except near the ionosphere and in certain
other special situations, one can generally assume that echoes originate
perpendicular to the magnetic field.
(2) Since Doppler integration will yield primarily the Doppler shift caused by
the known velocity of a satellite, one can thus apply the techniques of
synthetic-aperture radar.
(3) Finally, once the magnetospheric and plasmaspheric density profiles have
been constructed from the above methods of echo determination, these can be
verified through ray tracing techniques [see for example: Ondoh et al.,
1978; Green and Fung, 1993; Green et al., 1993] which should
reproduce the plasmagrams and therefore verify the echo directions.
Green, J. L., and S. F. Fung, Radio sounding of the magnetosphere from a
lunar-based VLF array, to be published in Adv. Space Res., 1993.
Green, J. L., S. F. Fung, J. R. Thieman, L. Aist-Sagara, R. M. Candey, Probing
the magnetosphere by radio sounding techniques, submitted to J. Geophys.
Res., 1993.
Ondoh, T., Y. Nakamura, and T. Koseki, Feasibility of plasmapause sounding from
a geostationary satellite, Space Sci. Instrum., 4, 57-71
(1978).
7. How can one distinguish the expected echoes from those of ducting,
scattering, or other such phenomena?
Experience with Alouette and ISIS spacecraft has shown that such echoes
invariably produce easily recognized signatures in ionograms, and in fact, has
clearly demonstrated that such features can often be used to advantage in
conjunction with the usual echoes to determine specific aspects and certain
details of the density distribution not otherwise accessible.
8. Will a magnetospheric sounder be capable of triggering
AKR?
It is a matter of considerable interest whether auroral kilometric radiation
can be stimulated by natural or artificial radiation. Calvert et al.
observed AKR and suggested that the auroral radio emissions were triggered
by solar Type II and Type III radio bursts [Calvert, 1981;
Calvert, 1985; Farrell et al., 1986]. Although the emission
mechanism may still be a cyclotron maser effect such as that proposed by Wu and
Lee [1979], a sounder wave with intensity above some threshold may cause the
formation of a particle distribution suitable for exciting the cyclotron maser
instability, thus triggering the emission of AKR. [C. S. Wu, personal
communication, 1993]. If the RPI sounder signal does trigger AKR, the
resultant return signal should be distinguishable from a more normal return
pulse signal (which should preserve the encoded pulse pattern). By
transmitting at different power levels, we should be able to determine the
threshold level at which such stimulated emission occurs. By varying the
sounder wave frequency, we may also be able to determine the magnetic field
strength (and thus the altitude) at which the emission was generated. Thus
although the sounder's principal science objective is not the study of AKR, we
may nevertheless end up learning a great deal about its properties from the
instrument.
Calvert, W., The stimulation of auroral kilometric radiation by type III solar
radio bursts, Geophys. Res. Lett., 8, 1091-1094, 1981.
Calvert, W., Auroral kilometric radiation triggered by type II solar radio
bursts, Geophys. Res. Lett., 12, 377-380, 1985.
Farrell, W. M., W. Calvert, and D. A. Gurnett, AKR signal increases caused by
triggering, Geophys. Res. Lett., 13, 370-372, 1986.
Wu, C. S., and L. C. Lee, A theory of the terrestrial kilometric radiation,
Astrophys. J., 230, 621-626, 1979.
6.0 SUMMARY
The importance of global imaging to studies of magnetospheric structure and
dynamics has clearly been demonstrated by the use of the DE-1 auroral images in
magnetospheric and auroral substorm studies. Using computer analysis, images
of the magnetosphere can also be constructed from measurements of energetic
neutral atoms and UV and X-ray radiation. These remote imaging techniques have
clearly opened new dimensions in magnetospheric research and are expected to
have significant impact on space physics research in the years to come
[Scarf, 1988]. The Inner Magnetospheric Imager Mission will provide the
first opportunity to simultaneously obtain images of the various parts of the
magnetosphere in different wavelengths, providing a coordinated set of global
views or images of the magnetosphere.ACKNOWLEDGMENTS
We would like to gratefully acknowledge Drs. T. Armstrong and D. Gallagher for
allowing the plasma sounder team to present their ideas on an exciting new
instrument approach for possible inclusion on the IMI mission. In addition, we
gratefully acknowledge discussions and comments from Drs. R. R. Anderson, J.
Fainberg, D. M. Haines, D. A. Gurnett, R. G. Stone, and C. S. Wu as well as
from many members of the IMI Science Definition Team. We thank Robert Kilgore
and Robert Candey for their excellent graphic support and Dr. Kolya Tsyganenko
for providing us with background information on changes in the inner
magnetosphere due to external currents. We especially thank Margie Layfield
for her expert typing and editorial suggestions. REFERENCES
APPENDIX A: Range Capability of a Magnetospheric Sounder
Figure A1. The radiation cone subtended by the sounder antenna at a distance r
from a coherent reflector, effectively originating from an image at 2r and producing
an echo power proportional to 1/(2r)2.
Figure A2. Circuit diagram for reception, where V1 is the induced voltage
causing antenna current Ia = V1/(Xa2 +
Ri2)1/2.
Figure A3. Circuit diagram for transmission, the transmitter voltage Va
causing an antenna current of Ia = Va/(Xa2 +
Ra2)1/2.
APPENDIX B: Key Questions and Answers
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Dr. D. R. Williams, dwilliam@nssdc.gsfc.nasa.gov, (301) 286-1258
NSSDC, Mail Code 633, NASA/Goddard Space Flight Center, Greenbelt, MD 20771
NASA Approval: J. L. Green, green@nssdca.gsfc.nasa.gov
Last Revised: 13 April 1998, DRW