Radio sounding in space: magnetosphere and topside ionosphere

Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 87–98

www.elsevier.nl/locate/jastp

Radio sounding in space: magnetosphere and topside ionosphere B.W. Reinischa; ∗ , D.M. Hainesa , R.F. Bensonb , J.L. Greenb , G.S. Salesa , W.W.L. Taylorc a Center

for Atmospheric Research, University of Massachusetts Lowell, 600 Suolk Street, Lowell, MA 01854, USA b NASA Goddard Space Flight Center, Code 630, Greenbelt, MD 20771, USA c Raytheon ITTS=GSFC, Code 630, Greenbelt, MD 20771, USA Received 12 November 1999; accepted 21 January 2000

Abstract Modern sounding techniques have been developed for the space-borne exploration of Earth’s magnetosphere and topside ionosphere. Two new satellite instruments will use the advanced techniques of the ground-based Digisondes. The Radio Plasma Imager (RPI), a low-frequency sounder with 500-m dipole antennas designed to sweep from 3 kHz to 3 MHz, will be part of NASA’s IMAGE mission to be launched in February 2000 into an elliptical orbit with an altitude at apogee of 7Re . While in the magnetospheric cavity, RPI will receive echoes from the magnetopause and the plasmasphere and will measure the direct response of the magnetosphere’s con guration to changes in the solar wind. With three orthogonal dipole antennas (two 500-m tip-to-tip antennas in the spin plane used for transmission and reception, one 20-m antenna along the spin axis for reception only) the arrival angle of returning echoes can be determined with high accuracy. The other instrument is the TOPside Automated Sounder (TOPAS), which was originally conceived for the Ukrainian WARNING mission with a launch date in 2001. Using one antenna for transmission and three orthogonal 10-m antennas for reception, TOPAS will be able to determine the arrival angle of ionospheric echoes and their wave polarization. It will then be possible to automatically scale the topside ionograms and calculate the electron density pro les in real time. Operating as a high-frequency radar, TOPAS c 2001 Elsevier will for the rst time measure topside plasma velocities by tracking the motions of plasma irregularities. Science Ltd. All rights reserved. Keywords: Radio sounding; Magnetosphere; Topside ionosphere

1. Introduction Radio sounding is a well-established technique that was rst deployed in the 1920s for ionospheric sounding from the ground (Breit and Tuve, 1926). In recent years, advanced digital sounders have been developed for ground-based observations that provide detailed information about the structure and dynamics of the bottomside ionosphere (Reinisch, 1996). These modern sounders measure more than just the time of ight and amplitude of the echoes, they also ∗

Corresponding author. Tel.: +1-978-934-4903; fax: +1-978459-7915. E-mail address: bodo [email protected] (B.W. Reinisch).

determine the arrival angle, wave polarization and Doppler frequency. Radio sounding relies on total re ection of radio waves from plasma structures that have a plasma frequency fN equal to the radio frequency f. It is, therefore, not possible to receive echoes re ected from the topside ionosphere or the magnetosphere on the ground since the F2 layer prevents all transmitted waves with frequencies f ¡ fo F2 from propagating beyond the height of the F2 layer peak. Topside ionospheric sounders, rst described by Franklin and Maclean (1969), recorded the amplitudes and echo delay times of ionospheric echoes as a function of frequency in the same way as it was done by the ground-based sounders of the time. Data from the highly successful Alouette=ISIS topside sounders (Jackson et al., 1980; Jackson, 1986) were

c 2001 Elsevier Science Ltd. All rights reserved. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 0 ) 0 0 1 3 3 - 4

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Fig. 1. (a) RPI transmits omidirectionally and can receive echoes from the magnetopause, plasmapause and the cusp. Two orthogonal 500-m dipoles are used for transmission. (b) Vertical and oblique echoes received by a topside sounder. The Doppler shifts of the echo signals are proportional to the dierence between the satellite velocity vs and the velocity vl of the irregularities.

the basis for nearly 1000 scienti c publications (Benson et al., 1998). The typical frequency range of the analog sounders was 0.1–20 MHz (Pulinets, 1989) corresponding to the plasma frequencies in the ionosphere. For magnetospheric sounding, the required frequency range is 3 kHz to 3 MHz, corresponding to √electron densities Ne ≈ 105 – 1011 m−3 , since fN =Hz ≈ 9 Ne =m−3 . The cartoons in Fig. 1 illustrate the geometries for magnetospheric and topside sounding. As discussed below, transmission will be nearly omnidirectional and echoes can be returned from many directions. In the magnetospheric case, the possible arrival angles cover the full sphere, and for topside sounding the lower halfsphere.

To correctly interpret the echoes in terms of plasma density and echo location it is, therefore, necessary to determine the arrival angles of the echoes together with the distance to the re ection point and the wave polarization. Section 4 describes how the polarization ellipse and the angle of arrival are determined from in-phase and quadrature SAMPLES of the signals from three orthogonal antennas. Ionospheric sounders on the ground usually use interferometric techniques with three or more spaced antennas to determine the arrival angle, and crossed dipoles to nd the polarization (Reinisch, 1996). A single-point measurement of these quantities with three orthogonal antennas on the ground is dicult to carry out because of the ground effects, but it can in principle be done (Afraimowich et al., 1999; Marie et al., 1999). Both the interferometric and the single point technique assume that only one signal of frequency f arrives at a given time at the receive antennas. This condition is normally not satis ed since echoes from dierent directions will superimpose. For pulsed signals, only echoes from targets with a virtual distance between R0 and R0 + c PW=2 will superimpose where c is the free-space speed of light and PW the pulse width. The virtual distance, or range, is de ned as R0 = 0:5c te where te is the measured echo travel time. Pulsing the transmit signal reduces the number of time-coincident echoes, but there can still be many echoes dependent on the structure of the plasma which would void the direction nding techniques referenced above. Bibl and Reinisch (1978) overcame this diculty by rst Fourier analyzing the echo signals, thus separating any time-coincident echoes by making use of the direction-dependent Doppler shifts. It is very unlikely that echoes from dierent directions have the same propagation delay between te and te + PW=2 and the same Doppler shift d = K · (v − vS )=, where K = (2=)n is the wave vector, v is the target velocity, vS is the spacecraft velocity,  is the free-space wavelength, and n is the wave normal. This echo source identi cation technique, using interferometric Doppler imaging (IDI), had been pioneered for ionospheric radio sounding from the ground (where vS = 0) by Bibl and Reinisch (1978), but is equally applicable in space where the single-point direction nding technique replaces the interferometry. Modern sounding techniques will now be applied to the remote sensing of plasma structures above the peak of the F2 layer. The radio plasma imager (RPI) will be ying on NASA’s magnetospheric IMAGER satellite to be launched in February 2000, and the TOPAS is being developed for a satellite orbiting at ∼ 1000 km altitude. Both instruments use in-phase and quadrature sampling of signals received on three orthogonal antennas and advanced signal processing techniques including pulse compression and Fourier integration. The instruments and the dierent waveforms used are brie y described in Sections 2 and 3. The calculations of the polarization ellipse, the wave mode, and the arrival angle, which are the same for both instruments, are discussed in Section 4.

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2. The radio plasma imager on the IMAGE mission NASA’s IMAGE mission is scheduled for launch from Vandenberg AFB, CA, in March 2000. The 14.5-h orbit will be highly elliptical with 7Re altitude at apogee and 1000 km at perigee. The mission’s scienti c objective is to determine the dynamic characteristics of the magnetosphere in response to changes in the solar wind. The changing characteristics of the magnetospheric plasma structures, from the magnetopause to the auroral ionosphere, can be captured by remote sensing from a single satellite using imaging techniques. IMAGE, which will be NASA’s rst Medium-class Explorer (MIDEX) mission, will carry three dierent types of sensors: neutral atom, UV and the radio plasma imagers. Burch of Southwest Research Institute in San Antonio, TX, is the principal investigator of the IMAGE mission (Burch, 2000). 2.1. The RPI instrument The RPI functions as a radar, transmitting radio waves simultaneously into all directions and measuring the arrival angle and delay time of all echoes (Reinisch et al., 2000; Green et al., 1998). It will use three orthogonal thin-wire dipole antennas for reception, two of which are 500-m tip-to-tip dipole antennas in the spin plane; these two dipoles are also used for transmission. The third antenna, along the spin axis, is 20 m long and is used for reception only. The three thin-wire dipole antennas and their deployers have been designed and built by Able Engineering Company (AEC). The antenna material is 7-strand BeCu wire with a diameter of 0.4 mm. The IMAGE spacecraft will have a spin rate of 0.5 rotations per minute, sucient to keep the two 500-m dipoles with their tip masses of 50 g in stable positions. A thin-wire antenna is also required for the dipole along the spin axis in order to minimize the photoelectric noise that can aect the quasi-thermal noise spectroscopy measurements (Meyer-Vernet et al., 1998) RPI plans to carry out. Two self-erecting berglass lattice booms extend the two 10-m wires to their measurement positions. Right- and left-hand polarized signals will be transmitted by feeding the currents into the long-wire antennas 90◦ out of phase as discussed by Reinisch et al. (2000) in an RPI instrument paper. This produces a nearly isotropic radiation pattern with maxima in the direction of the spin axis and a 3 dB minimum in the spin plane (Reinisch et al., 1999). In a feasibility paper (Calvert et al., 1995), we have shown that a 10 W transmitter together with low-noise receivers provide adequate signal-to-noise ratios to determine range and direction of echo signals up to a distance of at least 5Re . Unlike radio sounding on the ground, magnetospheric sounding is not limited by the interference from man-made sources, but by natural external noise. Moderately intense to intense natural noises, in RPI’s frequency range, consist of Type III solar-noise bursts and storms, auroral kilometric radiation (AKR), and the non-thermal continuum (escaping

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and trapped). These natural noises have the potential to aect RPI’s ability to clearly distinguish the echoes that will be generated. However, due to several mitigation strategies and a favorable orbit, RPI should be able to operate without signi cant impact from these natural noises. Natural noise takes several forms. Type III solar storms consist of thousands of Type III bursts produced quasi-continuously, resulting in broadband emissions. The frequency range for Type III emissions extends from above the RPI maximum frequency of 3 MHz to the lowest frequencies that can propagate through the earth’s magnetosheath, typically 30 to 100 kHz. Type III bursts are nearly as intense as the most intense AKR, whereas Type III solar storms have typical power uxes only about an order of magnitude above the cosmic noise background (Benson and Fainberg, 1991; Bougeret et al., 1984). Type III bursts will be observed by RPI in any location in its orbit making echo detection extremely dicult while they last; however, they are relatively rare and will not aect detection of echoes at lower frequencies. AKR is associated with auroral arcs and originates above auroral regions at about one half to a few Re altitude. The frequency of the emission is from about 30 to about 700 kHz with peak emission between 100 and 400 kHz, depending on local time and magnetic activity. The maximum power for AKR is many orders of magnitude above the cosmic and receiver noise. Propagation eects (see Green et al., 1977) restrict AKR to higher magnetic latitudes over certain local times. Fortunately, both O and X mode AKR are very narrowband, in the order of 1 kHz or less (Gurnett and Anderson, 1981; Benson et al., 1988). In this case, RPI’s frequency agility capability will be important and it is expected that AKR will not pose a major problem in RPI’s ability to generate and detect echoes between AKR narrowbanded emissions. Continuum radiation has two components, trapped and escaping (Gurnett, 1975). The trapped component ranges in frequency from about 30 kHz to the magnetosheath fp , which is between 30 and 100 kHz. The frequency range of the escaping component varies from the magnetosheath fp ( ∼ 30 kHz) to a few hundred kHz. Continuum radiation is believed to be generated primarily in the O mode. The source region appears to be near the low-latitude plasmapause, primarily on the dawn sector. In a recent study by Green and Boardsen (1999), the angular distribution of the non-thermal continuum radiation was studied with observations from the Hawkeye spacecraft and modeled with ray tracing calculations. From these results it is clear that the trapped continuum radiation does not uniformly illuminate the magnetospheric cavity but is mainly con ned to low latitudes. Thus, RPI should rarely encounter continuum radiation since IMAGE will be in a high-inclination orbit. In comparison to these natural noises, the receiver and cosmic noise are of less importance. Fig. 2 shows the system con guration for RPI. The electronics, including two transmitter exciters, three receivers, and digital control and power circuits, are contained on six printed circuit boards mounted inside the main chassis. The

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Fig. 2. RPI instrument con guration.

Fig. 3. RPI transmitter and antenna coupler for one antenna element.

two exciters drive four RF ampli ers, each feeding the RF current through a tuning coupler to a 250-m antenna. In Fig. 3, the RF ampli er is identi ed as a transmitter consisting of a variable 6 to 24 V power supply and two FET am-

pli ers driving a step-up transformer. The variable supply voltage is controlled to limit the voltage at the antenna feed point to 1:5 kVrms , i.e., 3 kVrms between antenna terminals, reducing the risk of arcing. For f ¡ 300 kHz, the antenna impedance is capacitive with a reactance of Xa =1=!C where the capacitance of the 500-m dipole is C = 533 pF. The radiated power is Pr = Ia2 Rr where Ia is the antenna current and Rr the radiation resistance. For a short thin-wire dipole the radiation resistance is Rr = 202 (L=)2 (Kraus, 1988 Chapter 5), where L = 500 m for RPI, and  is the wavelength. By measuring the antenna current we were able to determine the radiated power as function of frequency. Fig. 4 shows the power radiated by each 250-m antenna element. A 10 W power maximum is imposed by spacecraft power limitations.

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Fig. 4. Variation with frequency of the RF power radiated by one antenna element. Below ∼ 280 kHz, the power is limited by the imposed voltage maximum of 1:5 kVrms . At the higher frequencies, the power supply voltage is reduced to limit the radiated power to ∼ 10 W.

The entire RPI operation is controlled by the SC7 central processing unit (CPU) in the main chassis. The space quali ed SC7, built by Southwest Research Institute, is based on a Texas Instruments TMS320C30 digital signal processor. The CPU controls all analog and digital operations including frequency synthesis, receiver gain selection, waveform shaping and timing, signal processing, and measurement programming. Each of the six antenna elements drives a high-impedance, low-noise preampli er mounted at the antenna feed point (Fig. 2). The preampli ers have been designed to recover rapidly from the high voltage generated during the transmitter pulses. The 300-Hz receiver bandwidth, matched to the bandwidth of the transmitted waveform (Table 1), provides a range resolution of 480 km. The nal intermediate frequency signal at the receiver output, IF = 45 kHz, is digitized, and digital-signal processing is applied depending on the selected operational waveforms. √ The overall system sensitivity limit is about 8 nV= Hz for the Z receiver connected to the short antenna along the spin √ axis, and 25 nV= Hz for the X and Y receivers connected to the long antennas in the spin plane. The corresponding eld strength sensitivities are therefore ∼ 800 pV=m for the z component and ∼ 100 pV=m for the x and y components. Internal calibration signals are fed to the receiver inputs before the transmission for each selected frequency to calibrate the receiver gains. The RPI receivers are specially designed for tolerance to the high-level transmitter pulse and recovery to full sensitivity within less than 7 ms. This fast recovery time of less than two reciprocal bandwidths is made possible by two special design features. Firstly, the 300-Hz receiver bandwidth is the result of seven tuned stages of receiver IF, each with a bandwidth of 1 kHz. The tuning is performed with ferrite-loaded transformers and inductors, which are critically tuned based on the magnetic permeability of the ferrite core material. When a stage of the receiver is saturated, the

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currents are sucient to depress the eective permeability of the tuned circuit in that stage. This core saturation detunes the stage thereby reducing the amplitude of the signal passed on to the next stage. During the transmitter pulse, all stages are bordering on saturation and it is important to discharge the tuned circuits as quickly as possible after the transmit pulse in order to be ready for echo signal reception. Secondly, a xed receiver gain is set before any pulses are transmitted, eliminating any consideration of an automatic gain control (AGC) response time. The receivers have an instantaneous dynamic range of 60 dB. In addition, the computer controlled AGC can vary the receiver gain by 66 dB, resulting in an overall signal operating range of 126 dB, from 0 to −126 dB m. The sensitivity is increased further by the pulse compression and signal integration with a typical signal processing gain of ∼ 20 dB, resulting in a system sensitivity of −146 dB m or about 12 nVrms . This sensitivity will enable RPI to detect pulse echoes from ranges in excess of 5Re . The output of the X, Y and Z receivers is a 45 kHz intermediate frequency (IF) signal, which is digitized taking in-phase and quadrature samples every 1.6 s. Table 1 summarizes the speci cations for the RPI instrument. 2.2. Waveforms and signal processing Because of the largely unknown magnitude of transport velocities of plasma structures or wave velocities in the magnetopause, the coherence and the Doppler characteristics of the expected RPI echoes can only be estimated. Therefore, RPI was designed with several waveforms of widely varying characteristics. The capability of the various waveforms is intimately related to the algorithms used to process the echo returns. 2.2.1. Coherent integration The critical issue in selection of a waveform is the coherence time of the medium (Fung et al., 2000). The coherence time is the interval during which the phase of each of the sinusoidal components in an echo signal does not signi cantly change, that is, the complex amplitudes of successive samples of the same echo will sum in phase when accumulated over the integration period. If the signal samples are not coherent then the mean amplitude of the accumulated sum of N samples will be larger than a single sample by the square root of N , which is the same increase that is achieved when integrating random noise. However, if the multiple samples are coherent, the amplitude of the accumulated sum is N times a single sample, which is root N larger than the accumulated noise component of the signal, and therefore increases detectability, even for signals which are weaker than the received noise. It would appear then that any Doppler shift large enough to change the phase of a signal by more than 90◦ during the integration would ruin the coherence. Spectral integration, however, corrects this phase shift for each resolvable Doppler frequency, thereby allowing coherent integration for any signal that has a constant Doppler

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Table 1 RPI speci cations System parameter

Nominal

Limits

Rationale

Radiated power

10 W per antenna element

Required for adequate SNR

Frequency range

10 W at 5 to 20% duty cycle 3 kHz–3 MHz

Freq. accuracy

1 × 10−5

Freq. steps

5% steps

¿ 100 Hz

Measurement duration

1 s to ∼ minutes

50 ms

Maximum virtual range Minimum virtual range Range increments Pulse rep rate Pulse width Receiver bandwidth Receiver sensitivity

120,000 km

300,000 km

980 km with 3.2 ms short pulses 240 km 1 s−1 3.2 ms 312 Hz√ 25 nV=√ Hz (X & Y); 8 nV= Hz (Z) 8s

0 km for passive modes 240 or 480 km 0.5 –20 s−1 3.2 ms–1.9 s

Coherent integration time Doppler resolution Receiver saturation recovery Doppler range Amplitude resolution Angle-of-arrival resolution Antenna length Processing gain Mass incl. antennas Average power

125 ms– 64 s

125 mHz 6 ms ±2 Hz 3 dB

±150 Hz 3=8 dB

2◦

1◦ when SNR is 40 dB or better

10 21 56 32

and 250 m dB kg W

0 –33 dB

over the integration period. The characteristics that limit coherent integration time have to do with the extent to which the observed object is accelerating, since such acceleration leads to glinting, blinking or twinkling. The RPI instrument uses a variety of waveforms to ensure target detection in the presence of high Doppler shifts, acceleration or rapidly changing objects. 2.2.2. Natural noise mitigation Natural noise, including auroral kilometric radiation (AKR) will interfere with the reception of weak echoes. Since the frequency spectrum of AKR noise is highly variable it will generally be possible to nd quiet frequencies with low noise. A “frequency search” technique is therefore used at each nominal sounding frequency just prior to the transmission of the pulse or the sequence of pulses. Five frequencies spaced by 300 Hz are tested around the

Covers expected range of plasma densities. Accurately measures observed plasma densities. 5% in frequency gives 10% in plasma density resolution. Dierent spatial and temporal requirements along orbit Extent of expected magnetospheric echo ranges Pulse width + receiver recovery time is 7 ms Required sampling resolution Sets unambiguous range Provide 480 km range resolution Consistent with 3.2 ms pulse width Keeps receiver noise below cosmic noise Provides both processing gain & Doppler resolution Determined by coherent integration time Specially designed monostatic radar receiver To measure expected plasma velocities Data format allows 3=8 dB, but typical display is 3 dB Identify echo direction with required accuracy SNR required To enhance weak echoes

nominal frequency, and the one with the lowest noise level is selected for sounding. 2.2.3. Evenly spaced pulse sequences and their limitations To provide a detection range of 60,000 or 120,000 km requires a pulse repetition rate of 2 or 1 Hz, respectively, in order to avoid range aliasing. With such a slow repetition rate, several seconds are required to integrate repeated pulse transmissions. For instance, the integration of 16 pulses spaced by 0.5 s, requiring 8 s of integration time, provides a 12 dB signal-to-noise enhancement if the signal remains coherent for this time. Since this is not assured we have provided the 16-chip complimentary phase code and the FM chirp waveforms that provide 12 and 18 dB of signal enhancement, respectively, within one second (see below). A serious disadvantage of evenly spaced pulses is the maximum Doppler frequency that can be unambiguously

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sampled, which is half the repetition rate. Very fast moving structures, such as plasmoids and waves in the magnetopause, are estimated to produce tens of Hz Doppler shifts leading to aliasing, i.e., fold-over in the computed Doppler spectrum. The pulse repetition rate is equal to the data-sampling rate, since for each range one data sample is obtained after each transmitted 3.2 ms pulse. If the echo frequency is shifted by tens of Hz, it becomes quite meaningless to produce a Doppler spectrum from the repeated pulse echoes. For such high Doppler conditions, the FM chirp pulse can be used to achieve a similar processing gain using only a single pulse. This single-pulse technique does not, however, provide a Doppler spectrum. On the other hand, Doppler processing is desired to determine the radial velocity of the observed structures; also, as discussed earlier, Doppler separation is a prerequisite for arrival angle measurements.

of the echo. A spectral analysis of the down-converted signal produces a range pro le of all echoes received (Poole, 1985). Since the receiver is linear, overlapping reception of multiple echoes from dierent ranges are resolved since they produce dierent Fourier components.

2.2.4. Long pulse To measure the true radial velocity of very fast moving plasma irregularities, RPI can use the long-pulse waveform. The long pulse of either 125 or 500 ms duration is long enough to provide a Doppler spectrum with a resolution somewhere between 2 and 8 Hz, and a Doppler range of ±150 Hz, limited only by the 300 Hz analog bandwidth of the receiver. Of course, these long pulses provide no useful range resolution.

2.3.1. Plasmagrams Like the ionogram for ionospheric sounding, the plasmagram gives the most complete visualization of the received signals for magnetospheric sounding. It presents all signals received in a frequency-versus-range frame. Fig. 5 shows a simulated plasmagram with simulated noise. The echo ranges were calculated using ray tracing through a plasma density model that includes an ionosphere, plasmasphere, polar cap, cusp, magnetosphere and magnetopause. The echo amplitudes were derived from the radiated power (see Fig. 3) and a eld strength inversely proportional to the distance. The actual eld strengths may dier by ∼ ± 6 dB since the virtual range is larger than the distance, and because of focussing and defocussing (Calvert et al., 1995). The noise consists of the band near the electron plasma frequency, denoted by Fpe on the frequency axis, and the instrument noise that was measured as a function of frequency. The echoes at a range of about 3Re are from the magnetopause. The echoes which extend from about 3Re and 6 kHz to about 5:5Re and 80 kHz are from the cusp, and the remaining echoes are from the magnetosphere, plasmasphere and ionosphere below the spacecraft. Actual plasmagrams can be expected to be much more complicated, containing spread traces, echoes from plasma irregularities, and multi-hop echoes. From the variation of the virtual echo ranges R0 (f) with frequency, one can calculate the actual (or “true”) ranges R(f) and the plasma concentration pro le using the techniques developed for ionospheric ionograms (Jackson, 1969; Huang and Reinisch, 1982).

2.2.5. Staggered pulse sequence The staggered pulse sequence (SPS) provides the same Doppler range, but also provides range information with 480-km resolution. It consists of 212 pseudo-randomly spaced 3.2-ms pulses transmitted in about 2.5 s (Reinisch et al., 2000). Echoes from previously transmitted pulses are received in the listening time between transmissions. An echo received during a given listening time may have come from any of the preceding transmitted pulses, therefore the processing algorithm provides discrimination of one range versus another, but there is some inevitable leakage of energy from one range to the others. A random phase shift is applied to the transmitted signals and is removed upon reception, so successive echoes will coherently integrate only for those echoes that result from the correct transmitted pulse, that is, those that experienced the time delay that corresponds to the range currently being processed. 2.2.6. FM chirp For the detection of high Doppler echoes, the FM chirp waveform can be used. The chirp waveform has a RF carrier that is linearly increasing in frequency (Barry, 1971) modulated by a rectangular pulse. The RPI chirp waveform has a sweep rate of 244 Hz in 0.125 s. The received signals are mixed with a local oscillator signal which sweeps at the same rate used for the transmitted pulse, and the dierence frequency is therefore linearly proportional to the time delay

2.3. RPI data products It is dicult to present RPI’s multi-dimensional sounding data in one image since, for each sounding frequency, echoes can arrive from all directions with dierent delays (ranges). The most direct display is the plasmagram, which is similar to the ionogram for ionospheric sounding. The plasmagram does not show, however, from what direction the echoes are arriving, and a complementary display is required, the echo-map, to assess the 3-D plasma distribution in the magnetosphere.

2.3.2. Echo-maps The arrival angle (see Section 4) and range of an echo at frequency f determine the location and plasma frequency of the re ection point. The echo-map is a 2-D cross-section of the 3-D space, with all echoes projected into the orbital plane. The intersection of this plane with the magnetopause and plasmapause (Fung and Green, 1996) is shown to aid the interpretation of the echoes. (Fig. 6). The echo range is

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Fig. 5. Simulated plasmagram showing the signal amplitudes as function of range and frequency. Signal and noise amplitudes use the color code shown on the right.

Fig. 6. Simulated echo-map. The echo locations are projected into the orbital plane conserving echo range and azimuth angle. The echo colors identify the sounding frequency.

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conserved by projecting each echo along an arc with radius R0 into the echo-map. Thus, the echo locations on the 2-D plane present both azimuth and range information. The colors of the echoes represent the sounding frequency, i.e., the plasma frequency of the re ecting structure. Inherent in the construction of the echo-maps is the 180◦ ambiguity of the echo location as discussed in Section 4. We therefore show the echo together with a “ghost” echo assumed to arrive from the opposite hemisphere. The browse display assumes one answer, shown in color, and indicates the ghost location in gray. In most instances it will be possible to resolve the 180◦ ambiguity by inspecting the plasmagram traces and comparing the deduced echo locations with predictions based on magnetospheric Ne models.

3. Topside sounder Originally planned for the Ukrainian satellite mission WARNING, the design of a topside automated sounder (TOPAS) followed the concepts developed for UML’s successful ground-based ionosonde, the Digisonde Portable Sounder (DPS) (Reinisch et al., 1997). The radio imaging technique uses the single point method described in Section 4. The instrument con guration is very similar to RPI’s shown in Fig. 2. The instrument speci cations, summarized in Table 2, dier of course from RPI’s. The frequency range is from 0.5 to 30 MHz covering densities from 109 to 1013 m−3 . Three orthogonal 10-m antennas will be used for reception. To keep the total mass down, only one transmitter will be used to drive one of the antennas, which will be electronically tuned to optimize the transmission eciency. Ten watts of radiated power will produce signal-to-noise ratios of 30 to 40 dB, making use of the digital processing gain obtained by applying the pulse compression and Fourier analysis techniques developed for the DPS. An additional waveform, a staggered pulse sequence with 33.3 s pulses will expand the Doppler frequency range to ±1 kHz, large enough to avoid aliasing caused by the satellite motion. By measuring the arrival angles and the Doppler shift of the ionospheric echoes it will be possible to determine the vertical electron density pro les in real time, and also to calculate the velocity of ionospheric irregularities (Fig. 1b). After identifying and disregarding the o-vertical echoes, the vertical echo traces can be automatically scaled and the Ne pro le calculated (Reinisch and Huang, 1982; Huang and Reinisch, 1982). The availability of real time topside pro les will be a valuable input to space weather forecasting. By analyzing the Doppler frequencies for the dierent echoes it will also be possible to determine the drift velocity of the topside plasma irregularities similar to the Digisonde’s IDI technique (Cannon et al., 1991; Scali et al., 1995; Smith et al., 1998).

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4. Spaceborne radio imaging Radio imaging from a single spacecraft can be done by using three orthogonal receiver antennas. Since the transmission for low and high frequencies is nearly omnidirectional, the echo direction must be determined by measuring the arrival angle of the received signals. Morgan and Evans (1951) had proposed the use of space-quadrature sinusoidal components of the electric- eld vector to determine the polarization plane of an arriving plane wave for groundbased HF measurements. Afraimowich et al. (1999) experimentally veri ed this single-point technique using the method of spectral decomposition of the received signals introduced by Bibl and Reinisch (1978). Shawhan (1970) proposed the use of orthogonal magnetic antennas for direction nding in space. Both sounders described in this paper use electric antennas to measure the wave normal direction. Since for most echoes fNS f with fNS being the plasma frequency at the spacecraft location, the E eld component in the direction of the wave normal is negligible, and the normal to the wave front is the wave direction. To nd the polarization plane, the instruments determine two vectors Ei and Eq (Fig. 7) from the quadrature samples of the three antenna signals. We can write for the voltages at the three receiver outputs: Vx (t) = Vˆx ei(!t+x ) ; Vy (t) = Vˆy ei(!t+y ) ; Vz (t) = Vˆz ei(!t+z ) :

(1)

The peak voltages Vˆm and the phases m (m = x; y; z) are determined from the quadrature samples Im and Qm taken at times !t = 0 and !t = =2: Im = Vˆm eim ; Vˆm =

p

Qm = Vˆm ei(m +=2) :

Im2 + Qm2 ;

m = tan−1

Qm : Im

(2a) (2b)

The eective receiver gains need to be carefully adjusted so that the ratios Vm =Em = ÿm are the same for the three components. The ratio ÿm can be expressed as the product of the eective antenna length L0 ≈ 0:5 La and the receiver voltage gain G, where La is the actual length of the antenna. For RPI, L0x = L0; y ≈ 250 m; L0z ≈ 10 m; Gx = Gy , and Gz = 25 Gx . For TOPAS, L0x = L0; y = L0z ≈ 5 m, and Gx = Gy; = Gz . Careful calibration is required to correct the digital data for dierences in ÿm for the three channels. If we de ne the quadrature vectors I = (Ix ; Iy ; Iz ), and Q = (Qx ; Qy ; Qz ) we can write for the wave normal n=

Ei × Eq I×Q = : |Ei × Eq | |I × Q|

(3)

This vector is parallel to the wave normal, but has a 180◦ ambiguity in its pointing direction that must be resolved with the help of the plasmagram and echo-map for the RPI observations. This complication does not arise in the case

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Table 2 TOPAS speci cations System parameter

Nominal

Limits

Rationale

Radiated power

10 W

Produces adequate SNR

Frequency range

¡ 10 W, 5% or 10% duty cycle 1–18 MHz

0.5 – 40 MHz

Frequency steps

2.5% steps

1 Hz–500 kHz

Time=ionogram or echo-map Maximum range Minimum range

10 s

¿4 s

Measure in situ and F-layer plasma densities Provides 5% in plasma density resolution 75 km along orbit path

3000 km 40 km

Range increments

256 × 2:5 km

3000 km 10 km using short pulses 2.5, 5, 20 km

Waveforms Pulse rep rate

Compl. phase code; staggered pulses; chirp 200 Hz or SPS

Vertical and ducted echoes No reception is possible during transmission Sample period = range resolution

50, 100, 200 Hz, or SPS

Pulse width

33:3 s

1 or 8 × 33:3 s

Rec. bandwidth

30 kHz

30 kHz

Rec. sensitivity

500 nV

Coherent integration

80 ms

40 ms–10 s

Doppler resolution Doppler range

12 Hz ±100 Hz or ±1 kHz

0.1 Hz ±1 kHz

Rec. saturation Recovery time Amplitude resolution Angle-of-arrival res. Mass incl. antennas Average power

100 s

Provides required unambiguous range and Doppler range 5 km resolution= 40 km resolution Consistent with XTR pulse width Keeps receiver noise below cosmic noise Provides processing gain & Doppler resolution Coherent integration time SPS waveform eliminates Doppler ambiguities Specially designed receiver

3=8 dB 2◦ 13 kg 18 W

of topside sounding where all echoes arrive from the ionosphere. Incidentally, the direction of the vector n directly gives the sense of rotation of the E vector. If the condition fNS f is not satis ed, one can calculate the displacement vector D = E where the local dielectric constant  can be calculated from the electron plasma frequency fNS and the geomagnetic eld BS at the location of the satellite. From the resonances in the plasmagram or ionogram, one can nd fNS and BS , a B eld model can be used to determine the direction of BS . The direction of the wave vector is then given by Di × Dq . Electromagnetic wave propagation in a magnetoionic medium is anisotropic and only waves with the so-called characteristic wave polarizations can propagate (Stix, 1992; Rawer and Suchy, 1967; Budden, 1985). The two characteristic polarizations are generally right- and left-hand

3=8 dB 1◦ if SNR ¿ 40 dB

typical display is 3 dB Identi es ehco direction

elliptical, and the two characteristic waves, the O (ionic) and X (electronic) modes, propagate with dierent phase and group velocities, leading to Faraday rotation eects, dierent plasma cuto frequencies (re ection levels), and possibly dierent echo delay times. In general, the eld at the spacecraft location will be the sum of the O and X wave modes E = E O + EX :

(4)

The two modes can be separated using the eld measurements and the local plasma parameters. As mentioned earlier, the local plasma frequency can generally be deduced from the plasma resonances observed by the sounder, and the local magnetic eld can be obtained with fair accuracy from models and the measured resonance at the gyrofrequency. It is then possible to calculate the axial ratio  of the

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97

Acknowledgements Parts of this research were supported by NASA subcontracts 83822 to UML and 83814 to Raytheon ITSS from SwRI, by AF contract F19628-96-C-0159 to UML, and by NASA contract NASW-97002 to Raytheon ITSS.

References

Fig. 7. The polarization ellipse with the semi-major axis, a and the semi-minor axis b = a. The vectors Ei and Eq are obtained from the quadrature samples at times !t = 0 and =2.

characteristic polarization ellipses (Kelso, 1964). Reinisch et al. (1999) expressed the eld in the polarization plane x0 y0 in terms of the semi-major axes aO and aX of the polarization ellipses of the O and X modes, and the phase difference  between them. Equating the resulting expression to the eld measured on the x; y, and z antennas yields E = [(aO + aX ei )x0 + (aO  − aX ei )y0 ]ei!t = [Ex x + Ey y + Ez z]ei!t :

(5)

Ex ; Ey and Ez are the measured eld components, and the parameters aO ; aX , and , characterizing the polarization ellipses, can therefore be calculated from the three component equations in (5). 5. Summary Modern radio sounders in space open new possibilities for the exploration of space plasmas. Alternate waveforms and advanced signal processing have reduced the power and mass requirements previously associated with radio sounding. The use of three orthogonal receiver antennas and in-phase and quadrature sampling of the three antenna signals, combined with complex spectral analysis, make it possible to determine the arrival angles of the echoes thus “imaging” the plasma boundaries and irregularities. The Doppler measurements give information about the velocities of the plasma structures. Both of the two instruments described will provide new and much needed information. The magnetospheric RPI will directly determine the response of the magnetopause and the plasmasphere to changes in the solar wind, and the ionospheric topside automated sounder will be able to provide the topside pro les down to the F2 layer peak in real time.

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