Development of a Fast Wideband Self-Impedance Probe for Ionospheric Research

In situ measurements of the local plasma impedance provide highly accurate measurements of the absolute electron density as well as information concerning the plasma temperature, neutral densities, and collision frequencies. When immersed in a plasma, a probe will source or sink a current to or from the plasma when driven with a voltage signal, depending on its excitation frequency. Theoretical models predict that at low frequencies the probe couples capacitively to the plasma. At very high frequencies (above the upper hybrid frequency) the probe again couples capacitively. In between, the probe couples inductively. By measuring the impedance as a function of frequency, it is possible to determine the resonances where the imaginary part of the impedance changes sign. From the shape of the impedance curve as a function of frequency, and from the location of these resonances, it is possible to determine the electron density, and in principle, the electron temperature and collision frequency. We present a new design for a self-impedance probe that uses a pseudo-white-noise generator to measure the impedance at all frequencies simultaneously, allowing for high spatial resolution (40 m) on typical sounding rocket flights. This impedance probe measures the complex impedance between 7 kHz and 4 MHz, corresponding to plasma densities up to 1.8 × 105 cm-3. We present diagnostic measurements, theory of operation, and results from the first flights of this instrument on two sounding rockets launched during the NASA EQUIS-II campaign from Kwajalein Atoll. During these flights, the electron density profile was determined for densities greater than about 1 × 103 cm-3. The low-frequency "series" resonance, which contains information about the electron temperature and collision frequencies, was also identified. We will also discuss instrument developments that promise to improve the instrument's performance in the future.