Keywords:Arase, Whistler mode wave, electric field sensor, calibration
The Plasma Wave Experiment (PWE) on board the Arase satellite measures electric field from DC to 10 MHz, and magnetic field from a few Hz to 100 kHz. The waveform capture (WFC) is one of subsystems of the PWE and is dedicated to measuring waveform for the two electric components and three magnetic field components. The WFC aims at detailed measurements of plasma waves in the VLF range such as chorus, hiss and lightning whistler, but it is necessary to be corrected for gain and phase according to the frequency responses of sensors and receivers. In particular, the electric field data needs to be corrected in consideration of the antenna impedance of the electric field sensor. The characteristics of the impedance strongly depend on the electron density and temperature at the observation point. We have been operating the onboard calibration system regularly, and the frequency response is statistically derived as a function of electron density. In the present study, we evaluated the frequency response by deriving the refractive indices of lightning whistlers or chorus emissions in two different ways; one from E/B ratio of the waveform data measured by the WFC and the other from dispersion relation of whistler mode wave.
First, we select the events when wave normal directions of chorus or lightning whistlers were almost parallel (< 30 degrees) against the ambient magnetic field. Second, we derive refractive indices by calculating the E/B ratio from the waveform data. On the other hand, we also derive the refractive indices theoretically by the dispersion relation of whistler mode wave using in situ cyclotron frequency, plasma frequency and wave normal direction. These refractive indices shall essentially match when the electric field is appropriately calibrated. In other words, we estimate the quantitative error of frequency response of electric sensor comparing the difference between them. In the present study, we report the current status of the calibration process and show the reliability of the waveform data in electric field after calibration.