Effective lengths of crossed wire antennas onboard Akebono

The VLF instruments onboard Akebono are designed to observe VLF/ELF plasma waves and to determine wave normal direction of the waves. Two pairs of dipole antennas crossing each other are used for VLF electric field. The absolute intensity of the electric field at each dipole antenna is calculated from the induced voltage at the base of the antenna and its effective length. However it is not easy to estimate the values of effective lengths in plasmas, and a few investigations about it have been made. In these investigations, the effective lengths of two dipole antennas have been assumed the same. In this study we obtain the effective lengths of each of dipole antennas separately, which is very important to determine the wave normal direction and Poynting flux. We evaluate the actual effective lengths by making use of OMEGA signals observed as whistler mode waves in the magnetosphere. The frequency and transmitting sequence of OMEGA signals are exactly known. For an OMEGA signal, we can theoretically calculate its electric field components from its observed magnetic field components and plasma parameters using the dispersion equation of cold plasma and Maxwell's equations. By comparing such calculated values with actually observed electric fields by the antennas, we can determine the actual effective lengths of each dipole antennas separately. Results have shown that each of their effective length is almost half of the actual lengths of the dipole antennas.