To embark on the new thrust of global imaging science,
NASA has recently selected the Imager for Magnetopause-to-
Aurora Global Exploration (IMAGE) mission as the first of a
new series of mid-size explorers (MidEx) to be flown in 1999
(NASA press release no. 96-68, April 10, 1996). IMAGE will
carry a number of global imaging instruments to study the
global response of the magnetosphere to the changes in the
solar wind. Information on IMAGE can be obtained via the
world wide web at:
Many of the techniques suitable for global magnetospheric imaging have been reviewed by Williams et al. . We will provide a brief summary below on those techniques and some of the global science objectives which can be achieved by them. Among the most innovative ones are perhaps the energetic neutral atom (ENA) [Roelof, 1987] and the radio plasma (RPI) [Green et al., 1993; Reiff et al., 1994a,b] imaging techniques. In the balance of the paper, we will discuss an innovative application of the radio sounding technique and in particular, the roles that the radio and energetic neutral imaging techniques can play in providing information on the dynamics of storm-time ring currents and their contributions to the structure of the radiation belts.
The Earth's magnetosphere is extremely dynamic, with large-scale changes in size and shape in response to interplanetary conditions, and major internal reconfigurations. Unlike single-spacecraft measurements, which allow only sampling of magnetospheric conditions in specific regions at any given time, global remote sensing techniques can provide simultaneous viewing of various plasma domains such as the plasmasphere, ring current, and radiation belts.
As described by Williams et al., , there are a number of remote sensing and imaging techniques which can produce global images of the magnetosphere. They include the measurements of (1) the fluxes of energetic neutral atoms resulting from charge exchange reactions between energetic ions and the cold geocorona, (2) the solar extreme-ultraviolet radiation resonantly scattered by He+ and/or O+, (3) the emissions in far ultraviolet, visible, and X-ray wavelengths caused by the precipitating auroral particles, and (4) the echoing of variable-frequency radio signals from different density levels within large-scale magnetospheric structures using a magnetospheric radio sounder.
Neutral atom imaging measures the neutrals resulting from the change-exchange reactions between the neutral hydrogen geocorona and the energetic particles in the inner magnetosphere. This technique will provide important information on storm-time ring-current dynamics and trapped radiation distributions. In addition, imaging the resonantly scattered solar emission by the cold He and O ions at 304 angstroms and O+ at 834 angstroms will yield important information about how these cold ions are distributed in the plasmasphere.
Auroral imaging provides a measure of the energy deposition by precipitating particles as well as the global morphology of a substorm. In the far-ultraviolet (FUV) wavelength range the aurora is typically brighter than the dayside atmospheric background [Rairden et al., 1986] and can be used to monitor the auroral oval during orbital night and day. Ultraviolet (UV) images can provide information on the mean energies and energy fluxes of the precipitating electrons and their relationship with the global structure of the aurora. In addition, imaging the x-ray radiation resulting the Bremsstrahlung emissions of more energetic (up to hundreds of keV) precipitating electrons will reveal information of the losses of energetic electrons from the ring current and radiation belts.
Finally, radio wave imaging involves the measurements of transmitted radio wave pulses which are reflected at propagation cutoffs in the structured magnetospheric plasma. Operating in the frequency range from 3 kHz to 1 MHz, a radio sounder can remotely measure a wide range of densities while determining the positions (i.e., ranges from the transmitter) of different critical plasma boundaries, such as the plasmapause and magnetopause, on time scales of a few minutes.
Clearly, the strength of a global imaging or remote sensing mission lies in combining different instrumentation techniques within a single mission. In the remainder of this paper, we will discuss the use of the radio imaging technique to measure various aspects of the dynamics of the high energy particle environment of the inner magnetosphere. More information on the neutral atom imaging technique can be found in the paper by Beutier et al. in this volume.
The feasibility of magnetospheric imaging and radio remote sensing using advanced radio sounding technique has been extensively studied by Green et al., , Calvert et al.,  and Green et al., . Both the magnetopause and plasmasphere, as well as the cusp and boundary layers, can be observed by a radio sounder in a high-inclination polar orbit with an apogee greater than 6Re. Radio imaging will provide measurements of magnetospheric densities with unprecedented precision and coverage in the plasmasphere, inner magnetosphere and magnetopause, such that the structure and dynamics of different magnetospheric plasma regions can be determined.
Like a radar, a radio sounder transmits and receives coded electromagnetic radio pulses. A basic radio sounder measures the time delay between the transmitted pulse and the echo. The time delay measurement is then converted into a distance. In a magnetized plasma the reflection location depends on the wave frequency and its mode (or polarization), the ordinary (O) or extraordinary (X) mode.
Reflection of the O mode occurs at the plasma cutoff where the sounder wave frequency equals the local electron plasma frequency fp, which is determined by the local electron density, fp~ 9(Ne)1/2 kHz (with Ne in cm-3). This condition forms the basis for measuring plasma density at a remote location. As the sounder frequency is increased, the waves penetrate to greater distances, into regions of larger plasma density, yielding echoes with successively larger delay times. From the echo delays as a function of sounding frequency, the electron density profile from the spacecraft can be determined.
For the X mode, the cutoff frequency is given by fx=(fb/2)+[fp2 + fb2/4]1/2, where fb = 2.8B MHz (where B is the magnetic field strength in gauss). The O and the X modes can propagate freely at or near the speed of light when the wave frequencies exceed the plasma cutoffs and hence are called the free-space modes. It is the X mode that is the most important in measuring the effect of the ring current, as we will demonstrate in the next section.
Figure 1 illustrates the difference in the reflection locations of the O and X mode for a radio imager located on the magnetic equator outside of the plasmasphere with sounder frequency of 150 kHz. Contour plots of the fx (top half) and fp (bottom half) cutoff frequencies are shown in Figure 1. Radio waves propagating at frequencies above the cutoff frequencies in either of the two free-space modes will travel at nearly the speed of light in a straight line over most of their trajectories. Upon encountering a plasma cutoff (location where the wave frequency equals the fp), as shown in the lower portion of Figure 1, the echoes will be specularly reflected. From knowledge of the local fp the plasma density Ne (in cm-3) can be calculated. From knowledge of fp in the vicinity of the fx reflection point fb can be calculated and hence the magnetic field can be determined.
As motions of charged particles are controlled by the background magnetic field, much of the radiation belt and ring current dynamics can be elucidated by observing the variations in the global geomagnetic field caused by the rise and decay of the ring current. Figure 2 depicts the situation in which a net ring current flows westward in the nightside magnetosphere. The magnetic perturbations due to the current will decrease the geomagnetic field in the region between the earth and the ring current, while the earth's field is enhanced by the current's field in the region beyond the ring current L-shells.
Since the X-mode cutoff frequency depends on the magnitude of the local background magnetic field (and density), an increase or decrease in the background field will cause the X- mode cutoff surfaces (Figure 1) to shift farther away from or closer to Earth, respectively. To investigate the detectability of the changes in the geomagnetic field caused by the ring current, we have performed a ray- tracing modeling study.
For illustrative purposes, we have adopted for the ray tracing calculations a diffusive equilibrium plasma model based on those given by Aikyo and Ondoh  and Angerami and Thomas , and a background (unperturbed) magnetic field given by a dipole. Figure 3 shows the ring current density profile obtained by using the Hilmer and Voigt model . The gross features (the main positive and negative component currents) of this profile resemble those of a storm-time ring current observed on September 5, 1984 (see Figure 4 in Lui et al. ).
Effects of the magnetic perturbations of the net westward ring current (c.f., Figure 2) are clearly seen in Figure 4, in which decreased and increased field regions (compared to the earth's dipole field) are observed. However, the presence of an eastward (negative) current (see Figure 3) at the lower L-shell range of the ring current region effectively causes the net westward (positive) current to center at higher L values (near L=8).
Using the ray tracing modeling code developed by Green , we have modeled the propagation of both the X and O mode waves with frequencies in the range 160 < f < 200 kHz, launched earthward along the equator from a sounder located at L=10. The X and O mode waves are reflected upon encountering their respective plasma cutoffs and return as echoes when they reach the sounder location. The total echo delay times for the various frequency waves are plotted in a plasmagram shown in Figure 5. The O mode echoes have longer delay times than the X mode echoes at a given frequency because the O mode cutoff surfaces are located closer to Earth and thus farther from the sounder at 10 RE.
The X mode echoes for both the case of a pure dipole and the case of a dipole plus ring current perturbations are also shown in Figure 5. The X-mode calculations show that the echoes would suffer slightly longer time delays when the ring current is present because their cutoff surfaces have shifted earthward (farther away from the sounder). For a nominal time delay of 0.2 s, the change in the time delay is of the order of a few percent, about 25 ms. With advanced digital sounding techniques [Calvert et al., 1995; Green et al., 1996], the changes in the delay times should be readily measurable.
Although significant changes in the magnetospheric cold plasma density structure are known to occur during magnetic storms and substorms [e.g. Carpenter et al., 1993; Carpenter, 1995] (changes that are themselves important topics of study by the EUV photon imager and the radio sounder), the density model in our example was held constant so that the O mode trace in Figure 5 was unchanged. This can be justified by the fact that in a number of important observing situations, the density profile is not expected to depart significantly from its quiet time levels as the dilation of the geomagnetic field by the ring current proceeds. One such important situation should occur wihin the outer storm-time plasmasphere at longitudes where there has not been a significant storm-time loss of flux tube electron content and where during a storm the equatorial density profile preserves its quiet time property of varying roughly inversely with flux tube volume [e. g. Chappell et al., 1971; Carpenter and Anderson, 1992]. Within longitudinally localized sectors, depletion of flux tube electron content by a factor of up to 3 does occur within the outer storm-time plasmasphere [e. g. Park, 1974, Carpenter et al., 1993], the occurrence of corresponding density profile changes could be identified by the sounder and EUV imager and an appropriately varying density model applied to the ring current detection problem.
In particular, both the energetic neutral atom and radio plasma imaging techniques will be important in that they provide complementary measurements of the ring current and the radiation belts. Energetic neutral particle imaging measures the previously trapped energetic ions which make up the bulk of the ring current, yielding information such as pitch angle distributions [Fok et al., 1995]. On the other hand the radio imaging technique, while measuring the cold plasma component, will provide critical information on the changes in the geomagnetic field and in the plasmaspheric density structures as a function of the phases of a storm or substorm, and on the effects of those changes on the dynamics of the trapped particles in the radiation belts, as depicted in Figure 6. These techniques are therefore useful for investigating substorm injection boundaries and the inward motion as well as the rise and decay of the storm-time ring current.
In conclusion, global imaging measurements, while complementing in situ observations, will provide space physics research with new perspectives. Missions such as IMAGE should lead to significant advancement of our understanding of global magnetospheric structures and dynamics.
Acknowledgments. The authors gratefully acknowledge the early inspiration from the late Dr. S. D. Shawhan and useful discussions with the MI Science Definition Team. We also thank Dr. D. L. Carpenter for helpful discussions and insightful comments.
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