[P-CG17] Future missions and instrumentation for space and planetary science

[PCG17-13] Evaluation of thermal noise spectra in unmagnetized and magnetized plasmas for development of future Thermal Noise Receiver

*Tomoya Ito¹, Hirotsugu Kojima¹, Satoshi Kurita¹, Takahiro Zushi² (1.Research Institute for Sustainable Humanosphere, Kyoto University, 2.National Institute of Technology, Nara College)

Keywords:thermal noise, Thermal Noise Receiver

Electron temperatures and densities as measurement parameters by satellite observations in space plasma are important in understanding space electromagnetic environments. Electron detectors on board scientific satellites directly pick up electrons and provide a velocity vector of each detected electron. The observed data are used for deriving phase space densities. However, it is difficult to obtain the temperature and density of cold electrons affected by satellite charging. On the other hand, observations of plasma waves are effective tools for measuring plasma densities and electron temperatures including cold components. Kinetic motions of electrons induce current fluctuations on electric field sensors. The fluctuations are so-called thermal noise. The thermal noise shows a distinctive structure of its frequency spectrum around plasma frequencies or Upper Hybrid Resonance frequencies (UHR). Since the spectral structure reflects kinetic motions of electrons, the derivation of electron temperatures and densities is possible by spectral fitting in theoretical models of the thermal noise. A plasma wave receiver dedicated to observing the thermal noise is called “Thermal Noise Receiver.” The Thermal Noise Receiver needs to observe faint voltage fluctuations of thermal noise picked up by electric field sensors. Noise levels of the receiver should be much lower than the voltage fluctuation of thermal noise to precisely identify its spectral shape in the frequency space. In order to reduce the noise level of the receiver, a low noise amplifier and a narrow band filter are essential. However, commonly used low noise amplifier and narrow band filter make the total size of the receiver large. The purpose of this research is to reduce the size of the thermal noise receiver by utilizing the ASIC technology so that the receiver can be installed to miniaturized spacecraft. In this paper, we estimate the signal levels of the thermal noise by theoretical and numerical analyses under certain plasma conditions. Based on the estimation, we propose the specifications for future miniaturized thermal noise receiver. Then we design a chip for the thermal noise receiver that satisfies the proposed specifications.
The chip consists of a preamplifier, bandpass filters, an automatic gain control loop (AGC loop), and an A/D converter. Since the levels of the thermal noise is much lower than those of other natural plasma waves, it is important to avoid “contaminations” from other plasma waves considering the relation of gains of amplifiers with a dynamic range of an A/D converter. To remove the contaminations, the design of bandpass filters is crucial. We adopt the 5^th-order Butterworth filter to obtain the enough reduction for natural waves other than the thermal noise. Since the frequencies of the thermal noise vary depending on plasma densities, the designed bandpass filter has the capability to change its central frequency and pass band by external signals. Furthermore, gains of main amplifiers are also variable based on the AGC depending on the signal levels to keep appropriate observation range comparing with the dynamic range of the A/D converter.