Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsm11c1186h&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #SM11C-1186
Physics
2700 Magnetospheric Physics, 2712 Electric Fields (2411), 2794 Instruments And Techniques
Scientific paper
The characteristics of a wire antenna used for electric field observation in plasma could be different from the case of that in free space because of plasma sheath surrounding the antenna. We need the characteristics of the antennas (antenna impedance and effective length) in plasma to calibrate the observed data and know the absolute intensity of the electric field. We can measure the antenna impedance with the instruments onboard a spacecraft, but it is difficult to measure the effective length of the wire antenna. So far, we have used the assumed effective length which is a half of the antenna physical length for calibration in the case of observing VLF electric field with the antennas of several tens of meters. A few investigations about the effective lengths have been made, and the results have been the same as the assumed length. In this study, we estimate the effective length and/or the impedance of two wire antennas onboard Akebono satellite. We estimate the effective length by comparing the theoretical electric field and the open voltage induced on the terminal of the wire antenna. The theoretical electric field is calculated from the magnetic field of an OMEGA signal, the geomagnetic field and electron density with the dispersion equation and Maxwell's equations of cold plasma. The results by this estimation method are nearly equal to 30m which is a half of the physical length of the wire antenna with a little fluctuation. The open voltage induced on the terminal of the wire antenna is calibrated by using the antenna impedance assumed to be constant for any spin angles of Akebono. If the angle between the direction of the geomagnetic field and the direction of the wire antennas change with the spin, the antenna impedance would fluctuate with the angle. The impedance measured by an instrument onboard Akebono has been reported to be affected by the spin. Here we estimate dependence of the antenna impedance on the angle between the direction of the geomagnetic field and the direction of the antennas, with assuming the effective length be a half of the physical length.
Higashi Ryoichi
Imachi T.
Kimura Isao
Nagano Isamu
Yagitani Satoshi
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