The Characteristics of the Long Dipole Antennas on IMAGE at VLF Frequencies Within 
the Plasmasphere

T. W. Chevalier, T. F. Bell, I. S. Inan, U. S. Inan, and B W. Reinisch

Outside the plasmasphere the local plasmafrequency and gyrofrequency are relatively low, and 
at VLF frequencies (3-30 kHz) the refractive index of VLF waves is generally close to unity. 
In this case the long dipole antennas on IMAGE can be successfully  modeled as short antennas 
in free space in order to determine the input impedance of the antenna. Following this step, 
the antennas can be tuned using inductors of known values in order to maximize the radiated 
power from the antennas.

Within the plasmasphere the situation is drastically different since the refractive index of 
VLF waves is much larger there, and the IMAGE antennas are then large compared to the VLF 
wavelength. Furthermore the effects of the ion sheath surrounding the antennas become more 
important because of the higher density of the ambient plasma. As a result of these factors, 
neither the VLF input impedance or radiated power of the dipole antennas is presently known 
within the plasmasphere. However for an untuned antenna, the radiated power depends upon the 
input reactance of the antenna, and this power is an important factor in understanding the 
attenuation, scattering, and mode coupling that affect VLF waves as they propagate to distant 
points and then reflect back to the spacecraft. In the present paper we use finite difference, 
time domaine techniques in order to determine the VLF input impedance and radiated power of 
the IMAGE long antennas within the plasmasphere. We also make application to data obtained by 
the Radio Plasma Imager within the plasmasphere.

_______________
Presented at the Fall American Geophysical Union Meeting, 
San Francisco, CA., December 15-19, 2000