WFC

The WFC data are used for the detailed analysis of the wave characteristics, such as determination of the wave vector, polarization, Poynting flux, and antenna sheath impedance. The system has five (two electric and three magnetic) receivers each connected to a sensor through each front end circuit. The WFC has two different operation modes: Memory mode and Direct mode.
In the Memory mode, wave signals are measured simultaneously as E_U, E _V , B _\alpha, B _\beta, and B_\gamma. These are fed to a gain-controller followed by an anti-aliasing filter (LPF) with an upper frequency cutoff of 4 kHz, then through high pass filters (HPF) which have a low frequency cutoff of 10 Hz. An alternate HPF with a low frequency cutoff of 100 Hz can be used with the electric field measurements. Wave forms of the analog signal are sampled then converted into 12 bit data by an A/D converter with a frequency of 12 kHz. The 12 bit digital data are compressed into 8 bit by a quasi-logarithmic compression method. The compressed 8 bit data are then stored into onboard memory with a storage of 512 kBytes for a period of 8.7sec. The stored wave form data are read out of memory and telemetered to the ground using the PCM telemetry during a uninterrupted period of 275 seconds in telemetry Format 2 and 375 seconds in telemetry Format 3. The start timing for initiating wave form capture is controlled by either free running timing pulse (INTER. mode) or when the instrument receives one of eight different sources for triggering signals. The instrument is triggered when one of the following measurements exceeds a preset level:

1. The intensity of Z axis component of the DC magnetic field as measured by the outboard fluxgate magnetometer of the MGF.
2. The amplitude of the plasma turbulence detected by a single probe of the EFD.
3. The magnitude of ion and/or electron moments as measured by the electrostatic analyzers of the LEP-EA.
4. The amplitude of the electric field measured on the 100 kHz channel of the MCA.
5. The amplitude of the electric field measured on the 316 Hz channel of the MCA.
6. The amplitude of the magnetic field measured on the 10 Hz channel of the MCA.
7. The integral amplitude of the electric field over the entire frequency range of Band 1 (frequencies less than 250 Hz) of the SFA.
8. The integral amplitude of the magnetic field over the entire frequency range of both Bands 1 and 2 (frequency less than 1.875 kHz) of the SFA.

The selection among the possible eight triggering sources and its preset level or amplitude is made via telemetry commands.
When the WFC instrument is operated in the Memory mode the following three modes of operation are available for storing and reading the data:

1. AFT: Storage of 512 kByte of data after instrument triggering.
2. MID: Storage of two successive 256 kByte sequences of data, one before and the other after instrument triggering.
3. BEF: Storage of 512 kByte of data before instrument triggering.

In the last two modes, a continuous sequence of signal sampling and storage into the memory is repeated until a triggering signal is detected by the instrument.
A common problem of noise interference in the plasma wave instruments on spinning occurs as each group of solar cell circuits cycles on/off when the solar cell surface of the group faces to the sun-lit side then to the shadow side of the spacecraft. This repetition of the circuit from on to off to on again causes radiation of electromagnetic noises. To mitigate this problem, a high pass filter (HPF) with a cutoff frequency of 10 Hz is inserted in the receivers in order to suppress the detection of low frequency noise from the solar cell circuits. For the electric field receivers, we prepared an option for changing the HPF cutoff frequency from 10 Hz to 100 Hz in case the noise extended to higher frequencies then expected. The selection of the lower frequency cutoff is based upon the observed insitu noise level. These noise suppression measures limit frequency range of the observed wave forms to a band from 10 Hz (or 100 Hz) to 4 kHz.
Besides the Memory Mode, the WFC can be operated in a Direct mode, with two possible variations: one is the Single channel mode in which only one field component is measured continuously and telemetered to ground on a real-time basis. In this operation mode the instrument can measure continuous wave forms of one electric or one magnetic component with an upper limit in frequency of 640 kHz in telemetry Format 2 (or 470 Hz in telemetry Format 3). The other method of instrument operation is in the Dual channel mode where two field components are measured and telemetered alternately. In this mode the upper limit in frequency is 320 Hz in telemetry Format 2 (or 235 Hz in telemetry Format 3) for each component. For both of these mode, the data sampling frequency is three times the upper limit in frequency.
The WFC system can measure the wave amplitude with a dynamic range of 66 dB. The receiver gain keeping its dynamic range can be stepped up in the gain controller "H/L" by 40 dB and 20 dB in electric and magnetic channels to amplify the weak signal, respectively.