NTV (Nitrogen temperature instrument)

Objective

Thermal structure of the lower thermosphere is still poorly known due to the lack of a reliable neutral temperature measurement technique for this region. The existing empirical models are inadequate for detailed discussion of aeronomical phenomena under various atmospheric conditions such as during auroral disturbances. The main objective of this rocket experiment is to investigate energetics and dynamics of the lower thermosphere in aurora by making accurate neutral temperature measurements at 100-150 km altitude using the NTV instrument.

Instrumentation

The NTV was designed to measure vibrational temperature, rotational temperature, and number density of molecular nitrogen (N2) by applying the Electron Beam Fluorescence (EBF) technique. The EBF technique uses a high-energy electron beam to excite a gas molecule by an inelastic collision with an electron. Spectrum of subsequent fluorescence by the excited molecule consists of many vibrational bands, and each band has a fine rotational structure. If the excitation-emission process is known precisely, the analysis of the vibrational-rotational band provides properties of the initial state of molecules.

The NTV consists of an electron gun to excite and ionize ambient N2 and a sensitive spectrometer to detect the fluorescence of the N2+ first negative (1N) system. Figure 1 shows potential energy curves for the corresponding states of N2 and N2+. The electron beam from the electron gun is emitted in a direction perpendicular to the rocket axis and the line of sight of the spectrometer is tilted away from the rocket axis, thus the measurement volume is located at the intersection of the electron beam and the field of view of the spectrometer as shown in Figure 2. Figure 3 shows the electron beam emission test in the space chamber. A beam collector supported by the boom will be deployed at around 80 km altitude and collect the electron beam. The collector is used as a return electrode because serious charging up of the rocket body potential is caused without it.

Figure 1. Potential energy curves for the corresponding states.

Figure 2. Schematic diagram of the NTV instrument.

Figure 3. Photograph of the experiment in the space chamber at ISAS. The violet emission is the fluorescence of the N2+ 1N system.

The measured rotational temperature of N2 can be assumed to be equal to the kinetic (neutral) temperature in the lower thermosphere. Aerodynamic effects on the N2 density measurement caused by the rocket flight are quantitatively correctable using Direct Simulation Monte Carlo (DSMC) method. The great advantage of this type of instrument is that the neutral temperature and density are observed simultaneously and the consistency between the two measurements can be checked assuming hydrostatic equilibrium.

Team

J. Kurihara (ISAS/JAXA), K.-I. Oyama (ISAS/JAXA), AD Co., Ltd., YS Design, Hamamatsu photonics K. K.