A technique that involves the use of crossed magnetic antennas in a natural waveguide has been proposed for tracking Martian dust storms from a single observing station; that is, without having to triangulate from multiple observing stations. The technique is applicable to tracking thunderstorms on Earth.
An electromagnetic wave propagating in a waveguide exhibits dispersion; that is, the components of the wave at different frequencies propagate at different speeds and arrive at a receiving site at different times. Because the degree of dispersion is cumulative with distance, a measurement of dispersion can be used to estimate the distance a wave has traveled.
A highly electrically conductive ionosphere and the ground below it constitute a natural waveguide. Within this waveguide, powerful low-frequency electromagnetic signals, like those generated by intense electrical discharges (e.g., lightning strokes) can travel great distances. Provided that a sufficiently sensitive radio spectrometer is used to measure the dispersion of a wave, the distance from a receiver to a lightning stroke or other source of the wave can be estimated fairly precisely from the measured dispersion, even if the distance is thousands of kilometers. Furthermore, by using two crossed magnetic antennas (search coils) to measure mutually perpendicular horizontal components of the magnetic field of the wave, one obtains the information needed to calculate the azimuth of the source. Thus, the location of source, projected onto the ground surface, can be fully determined.
Electrical discharges are expected to occur on Mars. These discharges are expected to arise from dust storms, instead of from thunderstorms as on Earth and elsewhere in the Solar system. Electrically charged dust storms may act to transfer electrical currents across long filamentary paths and may thereby radiate at frequencies<10 kHz. By exploiting the propagation of such waves within the natural waveguide between the Martian ionosphere and ground surface, one would determine the locations of the discharges according to the principle described above and would thus be able to track the dust storms from one site on the surface. The range of detectability for the instrument is determined, in part, by the ground conductivity (i.e., conductivity of lower boundary of the waveguide) with reduced attenuation associated with higher conductivities. Thus, an estimate of Martian subsurface conductivity can also be derived by the variation of discharge signal strength with distance.
An incoming electromagnetic wave would have magnetic vector components Bxand By where x and y denote mutually perpendicular coordinate axes aligned approximately with corresponding mutually perpendicular axes of sensitivity of two search coils. Measurements of the waveforms and analyses of the spectra of both Bx and By would be needed to determine the degree of dispersion and the azimuth. The results of the dispersion and azimuth calculations would be used, in turn, to estimate the distance and direction to the source of the wave. Waveform analysis would require sampling of Bx and By at a rate of approximately 20 kS/s.
The figure is a system-level block diagram of an instrument that would perform the necessary measurements and would not impose excessive demands on telemetric resources. The instrument would include two search coils mounted with their axes of sensitivity orthogonal to each other in a horizontal plane. The outputs of the coils would be fed to a waveform-capturing system (WCS).
WCS would accumulate data continually in a circular buffer which would pass the data to a telemetry buffer on command. The command would be issued in response to a trigger signal generated by an external sensor whenever the sensor detected a discharge event. (The external sensor could be a photometer, vertical electric-field sensor, or other device that is particularly sensitive to broadband signals from lightning-like discharges.) Thus, the only data returned by the instrument would be those obtained around the time of a discharge, and the telemetric data rate averaged over long observing time would thereby be kept low. Each set of data thus returned would be used to compute the distance and direction to a source.
This work was done by W. Farrell, M. Desch, M. Kaiser, and J. Houser of Goddard Space Flight Center. No further documentation is available. GSC-13976