The Polar PWI uses seven antennas for detecting the electric and magnetic field components of plasma waves. Detailed characteristics of the electric and magnetic antennas are listed in Table 3. These antennas consist of a pair of orthogonal two-sphere electric antennas in the spin plane of the spacecraft with sphere-to-sphere separations of 130 meters (Eu) and 100 meters (Ev), respectively, a short two-sphere electric antenna (Ez) aligned along the spacecraft spin axis with a sphere-to-sphere separation of 14 meters, a triaxial search coil magnetic antenna system mounted on the end of a 6-meter rigid boom, and a magnetic loop antenna mounted on the same boom with its axis oriented parallel to the Eu electric antenna. The mounting arrangement of the antennas on the spacecraft is illustrated in Figure 1.
Figure 1: A schematic drawing of the Polar spacecraft, illustrating the orientation of the three orthogonal electric antennas, the magnetic loop antenna, and the triaxial search coil antennas.
The electric field antennas and associated electronics are provided by the Electric Field Instrument (EFI) team, and are used by both PWI and EFI. The spheres on the electric antennas are 8 cm in diameter and contain high-impedance preamplifiers that provide signals to the main electronics package. (For a detailed description of the EFI antennas, see Harvey et al. [1994].) Buffer amplifiers in the deployment mechanisms provide independent signals to EFI and PWI. Each of the three search coil antennas consists of two coils mounted on a high-permeability µ-metal core 40 cm long. Each coil consists of 10,000 turns of No. 40 AWG wire. The voltage from the coils is amplified by a preamplifier to provide a low-impedance signal to the main electronics box. The preamplifiers are mounted in the search coil housing. The sensitivity constant of the search coil is 70 µV/nT-Hz and the resonance frequency is approximately 8 kHz. The axes of the Bu, Bv, and Bz sensors are oriented within one degree of the Eu, Ev, and Ez electric antenna axes. A photograph of one of the Polar search coil antennas is shown in Figure 2.
Figure 2: A photograph of a magnetic search coil antenna.
The search coil antenna design is based on similar sensors flown on the ISEE 1 and 2 spacecraft. The loop antenna is similar to the magnetic loop antenna flown on the Dynamics Explorer 1 spacecraft and is designed to detect magnetic fields over a frequency range from 25 Hz to 800 kHz. The loop sensitivity constant is 110 µV/nT-Hz and the resonance frequency is approximately 50 kHz. A photograph of the magnetic loop antenna is shown in Figure 3.
Figure 3: A photograph of the magnetic loop antenna.
