Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsm51b0378h&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SM51B-0378
Physics
7855 Spacecraft Sheaths, Wakes, Charging, 6939 Magnetospheric Physics, 6964 Radio Wave Propagation, 6969 Remote Sensing, 0609 Antennas
Scientific paper
Characteristics of a wire antenna used for the electric field observation in the magnetized space plasma could be different from those in free space because of the plasma sheath surrounding the antenna. We need to know the antenna characteristics (antenna impedance and effective length) in the magnetized plasma to calibrate the observed data and obtain the absolute intensity of the electric field. In our previous antenna study, we have estimated the effective length from the ratio of the open voltage induced at the antenna terminal and the electric field theoretically calculated from Maxwell's equations using the observed plasma density and geomagnetic field. We have estimated the effective lengths of antennas onboard the GEOTAIL and Akebono satellites. The two types of wire antennas onboard the GEOTAIL are nearly equal to the effective length in free space. However, we see their fluctuations depending on the angle between the antenna and the sunward direction which varies with the satellite spin. The effective lengths of the two wire antennas onboard the Akebono satellite are also nearly equal to those in free space. Again, we see a small variation depending on the angle between the antenna and the geomagnetic field. The impedance of the antennas onboard GEOTAIL has been measured by Tsutsui et al.[1997] by using the calibration function onboard GEOTAIL. They have found that the impedance depends mainly on the ambient electron density and the satellite spin. The impedance onboard Akebono has been measured by Hashimoto et al.[1991], where the results also depends on the electron density and the satellite spin. For the GEOTAIL case, the dependence of the effective length and the impedance measurement on the satellite spin is caused by the fluctuation of the photoelectron emissions from the satellite body and the antenna surfaces. On the other hand, for the Akebono case, the photoelectrons do not play an important role because the spin axis of Akebono always points to the sun. In this study, we estimate capacitance component of the impedance of the two wire antennas onboard Akebono with assuming that the effective length is constant, and discuss the dependence of estimated impedance on the angle between the antenna and the geomagnetic field are found. The fluctuation range of the estimated impedance is in good agreement with the measurement. The theoretical capacitance component would not be found by using a co-axial capacitor model, because the fluctuation depending on the geomagnetic field cannot be explained by using this model. The thickness and/or the dielectric constant of the plasma sheath would change by the effect of the geomagnetic field. So we will need another model to know the accurate characteristics of the antennas in the magnetized plasma.
Higashi Ryoichi
Imachi T.
Kimura Isao
Nagano Isamu
Yagitani Satoshi
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