Earth Planets Space, Vol. 58 (No. 9), pp. 1147-1154, 2006
M. Wakabayashi and T. Ono
Planetary Plasma Physics Laboratory, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
(Received November 15, 2005; Revised June 26, 2006; Accepted July 3, 2006; Online published September 29, 2006)
The direct observation of electron number density was successfully carried out using an impedance probe on-board the S310-35 sounding rocket during the DELTA (Dynamics and Energetics of the Lower Thermosphere in Aurora) campaign, which provided the altitude profile along the rocket trajectory in a diffuse aurora. The plasma density profile showed clear density enhancements due to the auroral precipitation as well as artificial ionizations induced by the electron beam injection. The enhanced ionization due to the electron beam experiment was too high to be determined by means of the detection of the UHR (Upper Hybrid Resonance) frequencies. To overcome this difficulty, SHR (Sheath Resonance) frequency and sheath capacitance values were used to deduce the plasma density in the artificially ionized region. Based on the SHR analysis method, we were able to determine the maximum density to be 2 · 106 cm-3 at the altitude of 98.9 km; this measurement was ascribed to the artificial ionization by the 1-keV electron beam in comparison with the results of particle detector instrument on-board the S310-35. The data analysis of the sheath capacitance was able to distinguish the natural ionization region from the artificial one. The impedance probe measurement was able to evaluate the ionospheric conductivities which have important properties for evaluating the Joule heating process in the thermosphere. Both Pedersen and Hall conductivities showed distinctive enhancement around an altitude of 128 km which were due to the auroral particle precipitation.
Key words: Impedance probe, electron density, auroral precipitation, electron beam injection, polar ionosphere.