Since plasma waves can be either electromagnetic or electrostatic, it is important that both the electric field and magnetic field of a wave be measured in order to distinguish these two types of waves. The frequency range that must be measured is determined by the characteristic frequencies of the plasma. At the high-frequency end of the spectrum, the highest frequencies of interest are determined by the electron plasma frequency, fpe, and electron cyclotron frequency, fce. These frequencies are largest near perigee, and for the nominal Polar perigee radial distance of 1.8 RE are about 100 to 300 kHz. To make certain that the instrument can cover the spectrum of auroral kilometric radiation, which sometimes extends above 500 kHz, it was decided that the instrument must provide measurements up to 800 kHz. At the low-frequency end of the spectrum, the lowest frequencies of interest are mainly determined by the ion cyclotron frequency, fci. The ion cyclotron frequencies are smallest near apogee, and for the nominal polar apogee radial distance of 9 RE are about 1.5 Hz for H+ ions, and about 0.1 Hz for O+ ions. To provide measurements near the ion cyclotron frequency, it was decided that the instrument must provide measurements down to 0.1 Hz. This low-frequency limit provides some overlap with the static electric field (EFI) and the magnetometer (MAG) instruments which provide quasi-static electric and magnetic field measurements at frequencies up to several tens of Hz.
One of the more difficult decisions that must be made in the design of any plasma wave instrument is the choice of the frequency and time resolution. Our approach has been to provide several different types of receivers, each of which has certain advantages in time and frequency resolution. For example, a sweep frequency receiver is included that has very good frequency resolution (few percent) but relatively poor time resolution (few tens of seconds). To provide improved time resolution, a multichannel analyzer is also included that has very good time resolution (~ 1 second) but relatively poor frequency resolution (4 channels per decade). To provide the highest possible frequency-time resolution, a wideband receiver is included that provides essentially continuous waveforms over a broad bandwidth (up to 90 kHz). However, since the wideband system generates extremely high data rates (249 kbits/sec), wideband waveform data can only be transmitted a small fraction of the time (~ 15%).
To determine the direction of propagation of electromagnetic and electrostatic waves, it is essential that simultaneous 3-axis electric and magnetic field measurements be obtained. Since broadband multi-axis measurements also imply very high data rates, it was decided that these measurements could be obtained on a sampled basis, since continuous wave normal and Poynting flux measurements are not necessary to provide a good understanding of the wave fields.