Radio Imaging of the Magnetosphere

Radio sounding can be used to produce "images" of magnetospheric electron density distributions that could revolutionize research into the magnetosphere and its plasma content, especially when combined with other techniques. Based on more than a half-century heritage of ionospheric sounding combined with digital techniques, the magnetospheric radio sounder is yielding measurements that were once impossible to obtain.

A magnetospheric radio sounder can provide unprecedented global magnetospheric information by providing quantitative electron density profiles simultaneously in different directions. From a sequence of these, a contour plot of the density structure in the orbital plane can be constructed, with some out of plane information as well.

The study of the Earth's magnetosphere has progressed from the era of single-spacecraft, single point measurements to an era of 4-dimensional information, including time variability. To accurately quantify this dynamic system, either a permanent fleet of simultaneously operating spacecraft or a way to remotely sense the plasma domains is required. Multiple spacecraft missions can provide detailed measurements only in specific regions at any given time. One such mission is soon to be launched under the auspices of the International Solar Terrestrial Physics Program (ISTP).

Remote sensing techniques, such as auroral imaging, can increase our understanding by monitoring plasma domains. Auroral images, however, monitor only the precipitating particles; particles trapped in the equatorial plane or those at the magnetospheric boundary layers typically do not reach the atmosphere. Thus a new class of imagers for magnetospheric plasmas is being developed. The first mission planned is the Inner Magnetospheric Imager (IMI) [Armstrong et al., 1991]. Remote sensors being studied for this mission include imagers for observing the geocorona in the far ultraviolet (FUV), the aurora in the FUV and X-ray wavelengths, heavy ions in the plasmasphere, a region of dense cold plasma of ionospheric origin surrounding the Earth from scattered extreme ultraviolet (EUV) radiation, and the ring current by energetic neutral atom (ENA) detection [see Williams et al., 1992].

During the NASA Space Physics Strategy Implementation Study of 1990, we suggested using radio sounders to study the magnetopause and its boundary layers [Reiff, 1991], and the plasmasphere [Green and Fung, 1993]. For more details on the SIS, see "Space Physics Strategy Implementation Study, The NASA Space Physics Program from 1995 to 2010". Both the magnetopause and plasmasphere, as well as the cusp and boundary layers, can be studied if a radio sounder is placed on a high-inclination polar orbiting spacecraft with a reasonable (>=6 R_E) apogee (Figures 1 and 2).

Figure 1. Ray tracing of selected rays from a sounder at 50 kHz in a model magnetosphere, based on computer modeling. The location of the sounder is within the magnetospheric cavity. The blue ray paths are three rays reflected by the magnetopause boundary layer and the red ray paths are reflected by the outer plasmasphere. The contours shown are electron densities at 10, 30, and 100 cm^-3. In each set of three rays, only the central ray is reflected at a plasma frequency that corresponds to the sounder wave frequency and returns along the same path back to the spacecraft and therefore would be observed as an echo. The two outside ray paths in each set are refracted in a way that they do not propagate back to the spacecraft. Focusing of the returns from the concave magnetopause is evident.

Figure 2. Schematic contours of constant plasma density. A radio sounder on a polar orbiting spacecraft can remotely sense the density structures at the magnetopause, the plasmapause, the auroral density cavity, and the magnetospheric cusp, thus creating quantitative Òimages.Ó Lower-frequency waves (dotted) probe the low density surfaces, while higher frequency waves (dashed) are reflected at higher density structures. Waves on the boundaries yield multiple reflecting points, allowing a crude image within a minute and a magnetospheric image as illustrated within the time required for a quarter of a spacecraft orbit. The sounder will also yields reflections from the magnetopause at the dawn and dusk flanks, as well as from the dayside.

The use of radio sounding techniques dates back to the study of the ionosphere by G. Breit and M. A. Tuve in 1926. Swept frequency ground-based sounders can monitor the electron number density (N_e) structure of the ionosphere up to the F-layer density peak with high time resolution. The early instruments evolved into a global network that produced high-resolution ionograms - displays of echo delay versus frequency - on 35-mm film. The bottomside electron density profiles deduced from these records provided a cornerstone for the success of the International Geophysical Year.

The Alouette/International Satellites for Ionospheric Studies (ISIS) program pioneered the use of spaceborne swept frequency sounders in order to obtain Ne profiles of the topside of the ionosphere, above the density maximum. Using a series of consecutive profiles obtained along the orbit, an orbital plane N_e contours from the satellite altitude to the altitude of the F- layer peak density can be produced.

The error of the Alouette/ISIS-derived N_e values is typically within 10%, even at remote distances. The ISIS satellites also demonstrated that the sounders could operate compatiblibly with imaging instruments on the same spacecraft.

Digital technology was used in later spacecraft sounders, such as in Japan's Ionosphere Sounding Satellite b, Ohzora (also called EXOS-C), and Akebono (or EXOS-D) spacecraft; and in the USSR's Intercosmos 19 and Cosmos 1809 missions.

Bottomside (ground-based) ionospheric sounders have enjoyed great advances over the last few decades. They can now measure the frequency, phase, Doppler shift and spread, polarization, and direction of the echo's arrival. Two examples are the Advanced Ionospheric Sounder, developed at the U.S. Department of Commerce Laboratories in Boulder, and the Digisonde, developed at the University of Massachusetts Lowell. These instruments offer a high degree of flexibility in measurement format since their operations are controlled by software [Reinisch, 1986]. Their scientific capabilities go far beyond the N_e profiles of the standard ionosondes. Results show, for example, turbulence, drifts, winds, and structures. A new generation of spaceborne sounders can use these techniques to make sounding of the magnetosphere possible with modest power requirements.

RADIO WAVE SOUNDING OF THE MAGNETOSPHERE

In contrast, the Z mode has a lower frequency cutoff f_z (when v_ph²/c² = infinity) and an upper frequency resonance (when v_ph²/c² = 0 ) that restricts its propagation and is thus referred to as a trapped mode of the plasma. This wave mode is thus unsuitable for use in direct sounding to great distances. The whistler mode, which is frequently generated by lightning, is also a trapped mode.

Since the X and O modes have no propagation restrictions above their low frequency cutoffs, they are suitable for remote radio sounding. The cutoff for the O mode is the local plasma frequency f_p , where f_p = (N_ee²/e₀ m)^1/2 / (2(pi)) =~ 9 (N_e)^1/2 (kHz), when N_e is expressed in cm^-3; e₀ is the permittivity of free space, e is the electron charge and m is the electron mass. For the X mode, the cutoff f_x is {f_g/2 + (f_p² + f_g/4)^1/2}. Here the gyrofrequency f_g = eB/2(pi)m = 28.0 B (Hz) when B, the magnetic field strength, is expressed in nT. These two modes can propagate freely with group velocity v_g ~ v_ph ~ c when the wave frequencies are well beyond the plasma cutoffs; thus they are called the free-space modes.

A swept-frequency sounder transmits and receives X and O mode pulses at increasing frequencies. When the transmitted waves enter a region of increasing plasma density or magnetic field strength, they are reflected upon encountering their respective plasma cutoff frequencies; that is, when the wave frequency matches the cutoff frequency. In fact, from most of the inner magnetosphere, a satellite lies between increasing density gradients at the magnetopause and the plasmasphere (see Figures 1 and 2).

For most magnetospheric applications, the primary echo paths will be approximately perpendicular to the plasma density contours at reflection (Figure 3). A data record, called a plasmagram, consists of the time delay and amplitude along with complex Doppler and angle of arrival information for each sounding frequency and polarization. The true path length can be inverted from the density along the path and the received time delay. For example, the difference between a density shelf and a density minimum along the line of sight can be determined.

Figure 3. A schematic of multiple reflections from waves at the low-latitude boundary layer. From these reflections, the sounder should be able to infer the scale lengths of the boundary variations. The thickness of the boundary layer is enhanced for clarity.

Using the swept frequency measurements, the electron plasma density profiles of remote plasma regions can be derived. The density structures sensed by a magnetospheric sounder are dynamic in space and in time. Thus each transmission will result in more than one echo (Figures 2 and 3). Returns are usually obtained from several directions almost simultaneously, but with different time delays, return angles, and Doppler shifts. This allows determination of one or more N_e profiles along different directions, and a rough cross-section can be mapped within minutes.

From a sequence of such profiles taken during a portion of a single orbit, it is possible to produce two-dimensional, cross-sectional magnetospheric images (Figure 2). Raytracing studies indicate that from a dayside position much as in Figure 2, signal returns would also be received from the dawn and dusk magnetopause. Thus information on the 3-dimensional magnetospheric N_e structures will be obtained.

One result of reflecting from nonplanar surfaces is the focusing or defocusing of the echo wave amplitude by a factor of F = 1 / (1 + r/R), where r is the range and R (negative for a concave surface) is the reflecting surface radius of curvature. Since the magnetopause is concave as seen from near-earth spacecraft, this focusing effect enhances the signal return, making it possible to routinely sound the magnetopause and its boundary layers at great distances (Figure 1). Although sounding the plasmasphere may be more difficult, since the reflection is from a convex surface, the distances are shorter. A 10-W sounder transmitter is adequate for this purpose.

Electron densities in the magnetosphere range from approximately 0.1-1 cm^-3 in the auroral plasma cavity, plasma sheet and magnetotail lobes; 1-20 cm^-3 in the boundary layers, 1-10 cm^-3 in the plasmatrough, and 10²-10⁶ cm^-3 in the plasmasphere and ionosphere. To sound most of the magnetosphere and plasmasphere, one would use frequencies from 3 kHz (0.1 cm^-3) to 3 MHz (10⁵cm^-3). The corresponding echo delays for a satellite sounder at a few Earth radii above the plasmapause or inside the magnetopause will only be a few tenths of a second. Angle of arrival information can best be derived from the signals simultaneously received on 3 orthogonal antennas.

Natural interference in the frequency range of the magnetospheric sounder is caused by cosmic radio noise, type III solar radio bursts, the magnetospheric continuum radiation, and the auroral kilometric radiation (AKR). All but the first of these emissions are highly variable, with the last two also consisting of narrowband discrete components. For a magnetospheric sounder with an antenna length of 500 m or more, the noise margin overcomes interference by the cosmic noise, the continuum radiation, and all but the strongest and fortunately infrequent solar radio bursts.

AKR, on the other hand, dominates the spectral range of the sounder waves from 50 to 800 kHz. Fortuitously, the discrete AKR emission spectrum allows us to choose the sounding frequencies after listening for the presence of the natural emission within a narrow frequency range. As modern ground-based ionospheric sounders avoid frequencies used by broadcast stations, the magnetospheric sounder will search for a frequency range in which the natural emission is absent before it transmits in that frequency range. If it detects natural emissions in one narrow frequency band, it will switch to a neighboring frequency before transmitting, thereby setting the transmitter frequency to an interference free channel.

Even though the ISIS topside sounder did not have the clear channel search capability, it received a number of sounder echoes during intense AKR as it passed through the source regions in the auroral zone. This has been attributed to the very narrow band AKR emission that only affects a few selected frequency channels at a time.

It is unlikely that the short sounder pulses, of 1 to 3 ms, last long enough to trigger natural emissions like AKR, although such triggering may occur if the sounder transmits continuously at pertinent frequencies during transits of the AKR source region. Such triggering should not interfere with sounding, since the timescale for triggering is tens of seconds or more, longer than the sounding time at a given frequency.

UNIQUE SCIENTIFIC MEASUREMENTS

With this technology, the distribution and movement of dense plasmas eroded from the main plasmasphere during substorms can be observed, thus permitting study of the mechanisms by which "detached" as opposed to "connected" outlying cold plasma regions develop. Through correlative studies, questions about the possible decoupling of the high-altitude and low-altitude convection regimes will be investigated.

Radio sounding measurements can also be used to determine the extent of density cavities - regions of anomalously low electron density - associated with auroral acceleration regions. Density cavities in the auroral zone play a major role in auroral plasma dynamics, the auroral acceleration processes, and in the generation and propagation of many auroral zone plasma and radio emissions such as the auroral kilometric radiation. The extent of these cavities can be monitored by a radio sounder at high altitudes over the poles.

A sounder is ideal for investigating the evolution of the density structures of the magnetopause boundary layers, to determine the variability of plasma mantle density and thickness in response to the southward and westward components of the IMF, and the passage of Kelvin-Helmholtz wave structures in the low-latitude boundary layer. A sequence of sounder measurements can be used to determine whether the inner edge of the boundary layer moves only in phase with the magnetopause motion (Figure 4, left), or whether the thickness of the layer varies in time as the plasmas move downstream (Figure 4, right).

Figure 4. A sequence of reflections from density gradients at the magnetopause and the inner edge of the boundary layer. Contours are labeled a, b, c and d in increasing density. From ISEE 1 and 2, we know that the boundary layer has a sharp density gradient at the magnetopause and a sharp gradient at the inner edge, with a "ledge" in between, and that these boundaries are constantly in motion. It is not known, however, if the thickness of the boundary layer is nearly constant (left) or variable (right). This question should b resolved by successive soundings at low (dotted) and high (dashed) frequencies as the structures pass by the spacecraft.

When used as a ranging instrument, a radio sounder can determine the distance from the spacecraft to several locations on the magnetopause simultaneously. By comparing the sounder observations with simultaneous solar wind data from WIND and IMP-8, magnetopause models can be tested. A determination of the variation of the magnetopause shape with varying Z-component of the Interplanetary Magnetic Field (IMF) can also be made.

In the case of spacecraft near apogee in the tail or a lunar-based instrument when the Moon is deep in the magnetotail, the reflected signals from both the dawn and duskside equatorial magnetopause and north and south high-latitude magnetopause can be received nearly simultaneously. Then a determination can be made as to whether the magnetospheric tail is flattened and/or twisted by the IMF, as has been proposed.

By being able to determine the size of the magnetotail and its field strength, the magnetic flux in the magnetotail can be monitored and thus the effects of dayside and tail magnetic merging rates. An instrument in the high-latitude lobes can sound through the low-density plasmasheet to the distant magnetopause. A new perspective on substorm studies may be gained if the plasma sheet density proves sufficient to obtain radio echoes.

ACKNOWLEDGMENTS

REFERENCES

Carpenter, D. L., B. L. Giles, C. R. Chappell, P. M. E. Decreau, R. R. Anderson, A. M. Persoon, A. J. Smith, Y. Corcuff, and P. Canu, Plasmasphere dynamics in the duskside bulge region: A new look at an old topic, J. Geophys. Res., in press, 1994.

Green, J. L., and S. F. Fung, Radio sounding of the magnetosphere from a lunar-based VLF array, Adv. Space Res., in press, 1994.

Reiff, P. H., Magnetopause Mapper (Ionosonde), Section 6.5, Space Physics Missions Handbook, edited by R. A. Cooper and D. H. Burks, NASA OSSA, Washington, D.C., 1991.

Reinisch, B. W., New techniques in ground-based ionospheric sounding and studies, Radio Sci., 21, 3, 331,1986.

Williams, D., E. C. Roelof, and D. G. Mitchell, Global magnetospheric imaging, Rev. Geophys., 30, 183, 1992.

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NASA Approval: J. L. Green, green@nssdca.gsfc.nasa.gov
Last Revised: 13 April 1998, DRW