A metasurface absorber capable of monitoring two-dimensional (2-d) electric field distributions has been developed, where a matrix of lumped resistors between surface patches formed on a mushroom-type structure works as a 2-d array of short dipole sensors. In this paper absorption and reflection of a spherical wave incident on the metasurface absorber are analyzed by numerical computation by the plane-wave spectrum (PWS) technique using 2-d Fourier analysis. The electromagnetic field of the spherical wave incident on the absorber surface is expanded into a large number of plane waves, for each of which the TE and TM reflection and absorption coefficients are applied. Then by synthesizing all the plane wave fields we obtain the spatial distributions of reflected and absorbed fields. The detailed formulation of the computation is described, and the computed field distributions are compared with those obtained by simulation and actual measurement when the spherical wave from a dipole is illuminated onto a metasurface absorber. It is demonstrated that the PWS technique is effective and efficient in obtaining the accurate field distributions of the spherical wave on and around the absorber. This is useful for evaluating the performance of the metasurface absorber to absorb and measure the spherical wave field distributions around an EM source.

A feasibility study has been made of the detection possibility of radio wave noises, i.e., Martian atmospherics, emitted from discharges in the Martian atmosphere during large dust storms. The spacecraft NOZOMI, which was launched in 1998, is to be placed on an elliptic orbit around Mars with perigee of 150-200 km. An onboard-equipment LFA (Low Frequency Plasma wave Analyzer) has capability to measure the low frequency plasma waves in the frequency range from 10 Hz to 32 kHz. In order to know if the LFA can detect the atmospheric radio noises, the propagation characteristics of electromagnetic waves through the Martian ionosphere are studied theoretically by using a full-wave method. The ionosphere is modeled as a magneto-ionic medium based on the recent observations of magnetic anomaly by Mars Global Surveyor spacecraft, and the atmospheric constituent and electron density by Viking observations. Our calculation shows that the waves at frequencies less than a hundred hertz can propagate with low attenuation and reach to altitudes above 200 km in the whistler-mode in the regions of magnetic anomalies in the dayside ionosphere. It is shown that the radio noises emitted from electric discharge in an intense dust storm, with the intensity over -30 dBV/m/Hz at the ionospheric entry point, can be sensed by the LFA. The observational identification of Martian atmospherics will contribute to the physical study of charge/discharge process in the Martian atmosphere.