(where fp is expressed in kHz and Ne in cm-3). This condition forms the basis for remotely measuring the plasma density from such reflections since Ne can be directly obtained. An analogous cutoff involving both the local plasma and cyclotron frequencies exists for the X mode [Fung and Green, 1996].
As the sounder frequency is increased, the waves penetrate to greater distances, into regions of larger Ne , yielding echoes with larger delay times. By inverting the resulting echo delay as a function of frequency, Ne as a function of distance from the spacecraft can be determined.
As technology advanced, ionospheric swept-frequency sounders were incorporated into satellites in order to obtain Ne(h) profiles of the topside ionosphere (above the density maximum) which is inaccessible to ground-based sounders. Alouette 1 & 2 and ISIS 1 & 2 satellites provided data critical for the success of one of the most long-lasting international space programs, producing more than 50 satellite-years of swept-frequency ionospheric topside-sounder data [Jackson, 1986]. In addition to producing a wealth of information on vertical Ne(h) profiles and orbit-plane Ne images, the Alouette and ISIS satellites also demonstrated that active radio sounders could operate in a manner compatible with other instruments on the same spacecraft. This compatibility was particularly well illustrated with ISIS 2 which included two independent optical instruments and produced the first auroral images from space [Lui and Anger, 1973; Shepherd et al., 1976].
The greatest advance in radio sounding techniques has been in bottomside (ground-based) ionospheric sounders over the last two decades [Hunsucker, 1991]. In addition to measuring the amplitude and time delay of the returned pulse, as done traditionally by analog ionosondes, advanced ionospheric digital sounders measure the frequency, phase, Doppler spectrum, polarization, and direction of arrival of the echoes. Of relevance to the present work is the Digisonde Portable Sounder (DPS) developed at the University of Massachusetts, Lowell [Reinisch et al., 1992]. An important feature of the DPS is the high degree of software controlled flexibility in mode of operation and measurement format. Large scale ionospheric structures can be imaged by the DPS in the form of sky maps that show the location of a multitude of reflection points [Reinisch et al., 1995].
A schematic magnetospheric Ne profile is shown in Figure 1. The IMAGE spacecraft is located at its apogee of approximately 7 RE altitude. At this location, RPI will be able to remotely measure Ne values from 0.1 to 105 cm-3 corresponding to fp (or RPI frequency) values from about 3 kHz to 3 MHz.
The direction of arrival for each echo can be determined from a 3-axis dipole antenna system. The RPI antenna system will consist of two crossed 500 m tip-to-tip thin wire dipole antennas in the spin plane and a 20 m tip-to-tip thin wire dipole along the IMAGE spin axis. IMAGE will have a 2 minute spin period. Studies indicate that the power amplifiers should have a maximum peak power of 10 W and maximum antenna voltage of 3 kV, and be able to drive the spin plane antennas to transmit right/left-hand circularly (perpendicular to the spin plane) or elliptically polarized signals. The echo arrival angles are calculated from the returning signal voltages and phases on the three orthogonal antennas. The accuracy of the measurement of the echo arrival direction varies directly with the signal-to-noise (S/N) ratio of the echo. For example, for a S/N ratio of 40 dB the accuracy is 1° [Calvert et al., 1995; 1997].
To allow the most efficient use of available spacecraft resources, techniques must be utilized to maximize the S/N of the echo. For magnetospheric sounding, the large echo travel distances, low transmitter power, and small antenna length (relative to the wavelength) will require, in some cases, on-board signal processing in order to produce an adequate S/N ratio. Several techniques are in use in ground-based ionospheric sounders [Reinisch et al., 1992]. In addition to a multitude of pulse operation modes, pulse compression and coherent spectral integration (Fourier Transform) techniques in use by the DPS are essential elements of the RPI instrument. Figure 2 provides a brief overview of these signal processing techniques.
Pulse compression, illustrated in Figure 2A, requires the transmission of a phase coded pulse. Each distinctive constant phase portion of the transmitted pulse is called a chip (4 chips are shown). The convolution of the echo with the known transmission code then produces an enhancement in the resulting combined signal when the phases of the echo and code match exactly, while the background noise signals would be minimized by its random phases.
In Figure 2B the spectral integration technique requires a Fourier analysis of multiple pulses and their associated echoes. The result produces a weighted sum of the echo amplitudes with the same Doppler shift. Repeated echoes from the remote Ne structure are distinguished from other echoes by (1) their direction of arrival, (2) by their different Doppler shifted echo frequencies, and (3) by their different echo delay times. This technique is routinely used during ground-based sounding to map the motions of Ne irregularities in the polar cap and auroral regions of the ionosphere [Reinisch et al., 1995; Reinisch, 1995]. It is believed that these techniques will perform even better in space, since the Doppler spreading of multiple sources will be enhanced by the satellite motion.
The combination of these two techniques produces a S/N gain described by:
where m is the number of coded pulses, n the number of chips, and No is the original noise before digital integration [Calvert et al., 1995]. A 16 chip pulse compression and a 8 point spectral integration would yield S/N improvement by a factor of 11.3 and hence a 21 dB increase in S/N. Assuming receiver noise plus cosmic noise levels for the RPI instrument, the calculated worst case signal-to-noise ratio before (right axis) and after (left axis) digital processing magnetopause, plasmapause, and plasmasphere echoes are shown in Figure 3 [after Calvert et al., 1995]. The calculations assumed RPI at 6 RE, the magnetopause at 10 RE and the plasmapause at the L=4 dipole L shell. The curves indicate a range of possible conditions and assume total reflection, e.g., no O mode echoes are returned at frequencies above fp at the target location.
The number of sounding frequencies for a given measurement together with the coherent integration time for each frequency determine the total measuring time and hence the time resolution of a single complete measurement of the echoes from a target. The characteristics for the RPI instrument to be flown on IMAGE are summarized in Table 1.
|RF Power||10 W||10 W|
|Pulse Width||53 msec||3.2 to 125 ms|
|Receiver Bandwidth||300 Hz||fixed at 300 Hz|
|Pulse Rate||2 pps||1 to 5 pps|
|Frequency Range||10 to 100 kHz||3 kHz to 3 MHz|
|Frequency Steps||5%||>=100 Hz|
|Integration Time||8 seconds||> 2 seconds|
Table 1. RPI instrument characteristics.
Based on these instrument characteristics, Table 2 provides the expected measurement capabilities. While IMAGE is in the magnetospheric Ne cavity the RPI should be able to provide unprecedented global scale magnetospheric observations, detecting the locations and motions of important boundaries and their motions several RE from the spacecraft.
|Echo Range||500 km||0.1 to 5 RE|
|Angle-of-Arrival||1° at 40 DB S/N||resolution = 2/[S/N]|
|Doppler||0.125 Hz||75 Hz|
|Time||8 sec./freq step||4 sec./freq step|
Table 2. RPI measurement capabilities.
Type III solar noise bursts and storms are radiation near the solar wind fp, generally believed to be excited by outward propagating energetic electrons from the sun. Since the solar wind density decreases outward from the sun, the resulting spectrum is a band of emissions with decreasing frequency with increasing time. Type III 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 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 fluxes only about an order of magnitude above the cosmic noise background [Benson and Fainberg, 1991; Bougeret et al., 1984].
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 kHz to about 700 kHz with peak emission between 100 to 400 kHz, depending on local time and magnetic activity [Kaiser and Alexander, 1977]. The maximum power for AKR is many orders of magnitude above the cosmic and receiver noise. Propagation effects [see Green et al., 1977] restrict AKR to higher magnetic latitudes over certain local times. Importantly, both O and X mode AKR are very narrowband, on the order of 1 kHz or less [Gurnett, et al., 1979; Gurnett and Anderson, 1981; Benson et al., 1988].
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. The trapped component is also observed as a broadband emission believed to be the result of frequency diffusion of multiple reflections off the magnetopause [Kurth et al., 1981]. The radiation pattern of the escaping continuum [Morgan and Gurnett, 1991] has been found to be from 40° to 60° about the magnetic equator which will typically not interfere with RPI measurements. There are no reports of trapped continuum radiation, at the same intensity as observed near the equator, at the high latitudes and high altitudes characteristic of IMAGE's orbit.
Frequency avoidance is a technique in which the instrument avoids measurement at a sounder frequency where natural wave emissions are too intense. This technique, commonly used by ground-based ionosondes, could be used in specific regions along an orbit where characteristic natural emissions of sufficiently high intensity are known to exist.
The second frequency control technique is frequency agility. If the RPI receiver determines that, for example, three of the five adjacent frequencies have high signal levels and one is very low, the latter would be chosen for transmission and reception. This technique would be very effective for sounding during periods of narrowband AKR or escaping continuum.
Polarization and spatial discrimination techniques can also be used in RPI operations. In polarization discrimination, differences in polarization between noise and expected signals will be used to increase S/N ratios. For example, AKR is known to be strongly polarized in the X mode. In these cases, RPI signals in the other mode will have a higher S/N ratio. In spatial discrimination, RPI would be able to avoid or reduce the effects of strong emissions in certain locations in the magnetosphere. AKR, for example, is absent or much weaker in the low latitude, dayside [Green et al., 1977] and intense continuum radiation has not been observed over the polar cap. Table 3 summarizes the applicable RPI noise mitigation techniques.
Table 3. Mitigation techniques expected to be significant for RPI.
The plasmagram in Figure 4A shows the O mode expected echo amplitudes and delay times as a function of frequency. It is important to note that only receiver noise and cosmic noise have been considered in this simulation. In these calculations, we have included the effects of the RPI antenna length, antenna tuning and matching, and focusing caused by target curvature. The satellite was in the noon meridian, at a latitude of 25° and a geocentric distance of 7 RE. The Ne profile was calculated from the change of echo delay with frequency using the technique developed by Huang and Reinisch, , and is shown in Figure 4B. The calculated Ne profile is in excellent agreement with the model Ne profile used in the ray tracing calculations.
Based on the DPS proven technologies and the simulations of RPI in the proposed IMAGE orbit, RPI should provide information on the location of the magnetopause and plasmapause and also their respective Ne values. It should be possible to deduce global scale boundary structures using the directional and range measurements, as well as from a sequence of many plasmagrams along the IMAGE orbit.
RPI data can also be displayed in several types of image formats. For example an image can be created by combining a series of one dimensional Ne profiles into a two dimensional Ne contour image. The image will be formed as a function of time as the spacecraft moves along its orbit and the image plane is then the satellite orbital plane. A second example is an image created by determining the direction of echoes at a single frequency from a specific magnetospheric feature. Plotting the echo directions could give, for example, an image of a surface wave on the magnetopause. The image plane in this case is perpendicular to the line of sight. Other types of images can also be created [Reiff et al., 1995].
The RPI instrument on the IMAGE mission is a swept in frequency adaptive sounder, with on-board signal processing which transmits and receives coded electromagnetic pulses over the frequency range from 3 kHz to 3 MHz. The pulses propagate as free space waves through the magnetosphere and are reflected upon encountering their plasma cutoff frequencies. The RPI will measure the amplitudes, Doppler shift, and direction of arrival as a function of frequency and echo delay. The echo arrival angles will be calculated from the amplitudes and phases of the signals from three orthogonal receiving dipole antennas. Nearly all the sounding techniques utilized on DPS will be used in the RPI.
Situated in the density cavity of the magnetosphere, the RPI should be able to simultaneously determine the location and dynamics of remote boundaries such as the plasmapause and magnetopause. In addition, the RPI should be able to provide Ne profiles in different directions on time scales of a few minutes or less. At specific sounder frequencies, characteristic of remote plasma regions where large scale oscillations are occurring within the range of RPI, images of the magnetospheric structures should be possible.
In summary, the RPI should be able to provide unprecedented global magnetospheric observations.
Acknowledgments. The authors gratefully acknowledge useful discussions with the late Dr. S. D. Shawhan during early stages of magnetospheric sounder development and with the IMI Science Definition Team. The work at Rice University was supported by NASA under grant NAGW 1655, and at Hughes STX under NASA contract NASW-5016.
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