MEASUREMENTS AND ANALYSIS OF ANTENNA IMPEDANCE ABOARD THE GEOTAIL SPACECRAFT

A practical method of estimating the effective impedance of electric d ipole antennas aboard the scientific spacecraft Geotail has been devel oped. The principle of the newly proposed method is to derive the comp lex impedance of the dipole antennas immersed in the space plasma by m eans of the analysis of signal waveforms captured when a calibration s ignal was imposed directly onto the antennas. The derived complex ante nna impedance is expressed as frequency responses of its real and imag inary pasts. During the flight of Geotail in the Earth's magnetosphere , the impedance measurements were conducted in various regions of the magnetosphere. It has been confirmed, from the analysis of the measure d data. that the impedance of the Geotail antenna forms an equivalent electric circuit consisting: of a resistance and a capacitance connect ed in par allel. It has been found that the resistance value is easily changed by the extremely rare ambient plasma density in the magnetosp here. The validity of the measured values have been examined along a t heory of probes in a sheath region, taking into account antenna potent ials which are lifted up due to the photoelectron emissions from the a ntenna surface. The analyzed result shows that the measured values of the antenna resistance are consistent with those within the sheath cal culated from the theory. Correlation plots of the capacitance values o btained from both the measurements and the calculations have shown tha t the major parts of the measured capacitance represent the ''sheath c apacitance,'' but other parts might result fi om a combination of the sheath impedance and the plasma impedance, in addition to the free-spa ce capacitance.