The Optical and Spectral Measurements of Low Pressure Air Discharges as Sprite Models

The measured results for the optical and spectral characteristics of the long gap low pressure air discharge as the model of red sprite or blue jet have been reported and compared with the observed data of natural sprites by Mende et al..Model experiments have been done at filling air pressure between 7.5 mTorr and 20 Torr correspond to the altitude between 80 km and 25 km. INTRODUCTION Red sprites and blue jets are middle-atmospheric electric discharges linked to lightning. These discharges have been reported episodically for many years, but systematic investigation has begun in the last decade (Lyons,1996, Suszcynsky et al.,1999, Wescott et al.,2001). Red sprites have been large red luminous structures centered in the mesosphere but possessing upward and downward appendages spanning the altitude range 40-80 km (the air pressure range 4.0 Torr7.5 mTorr). Blue jets have been narrow upward directed cones of blue light emanating directly from the tops of active thunderstorm systems and upward to altitude 40 km (Sentman et al.,1996). They are very important themes to clarify the characteristics of those discharges and to investigate the potential for sprite and jet interference with aerospace activities in the 20 to 100-km layer, including research balloons, military aviation, the next generation high-speed civil transport, and the Spade Shuttle. To clarify such the air breakdown characteristics at the uniform electric field in the range of air pressure like as middle atmospheric discharges above thunderstorm systems, air breakdown voltages for D.C. voltage and switching impulse voltage at low air pressure and long gap have been measured in the high voltage laboratory of Tohoku Gakuin University(Goto et al.,1996). In this paper, measured results for the optical and spectral characteristics of the long gap low pressure air discharges as the models of red sprites or blue jets have been reported. APPARATUS AND PROCEDURE The block diagram of the experimental apparatus is shown in Fig. 1. An aluminum disk rounded edge of 3.2 m mean diameter as the high voltage electrode is suspended by an insulator string above a grounded aluminum plate to form a parallel plane gap and a low pressure discharge tube exhausted by an oil rotary vacuum pump has been set up co-axially for the gap. The used discharge tube is a pyrex glass tube 1.5 m length with 0.15 m in diameter. The used high voltage supplies are 50 kV (C=2.2 uμF) and 100 kV(C=.12 μF) D.C. high voltage sources and the current limited resistances used 1.5 kOhm and 3.0 kOhm respectively. The measurements of the discharge current waveform and the total light waveform have been done with a digital oscilloscope TDS 220 and a shunt resistance (R=0.82 Ohm) as the current sensor or a head-on type photomultiplier 7102 (light sensitivity: S1) as the total light sensor respectively. Moreover , the colors and the states of optical emissions from the low pressure air discharges have been observed with a still camera. The pressure dependence of spectral distributions from those discharges have been measured with Hamamatsu Photonics multi-channel analyzer PMA 11(band width of wave length: 300-800 nm). Fig.1 The block diagram of the experimental apparatus EXPERIMENTAL RESULTS AND DISCASSION As the observed results, the colors and states of the low pressure and long gap air discharges with 1.5 m glass tube by a still camera are purple red and glow type like as the red sprite in low pressure range and purple blue and streamer type like as the blue jet in higher pressure range, as shown in Fig.2. Those colors and states may be change with the discharge current density. So,we have measured the spectroscopic distribution of the light emission from the low pressure air discharges. The obtained pressure dependence of spectral distributions are shown in Fig. 3 (a)(d) for each air pressure respectively. The obtained spectra of the low pressure air discharges as the model for sprite discharges are same to the observed spectra of natural sprite in the range from 550 nm to 800 nm and there are N2(B-A) and N2 (A-X). In Fig. 4 , comparison of synthetic spectra of N2(B-A) and N2 (A-X) emission are shown(Green et al.,1996, Heavner et al.1996). For the pressure dependence of spectral distribution, Hα line(656.2 nm) intensity decreased with air pressure, otherwise, O line(777.6 nm) intensity increased with air pressure. The intensity of red range emission is the highest at 40 mTorr(70 km), According to air pressure, spectral line intensity of O and N increased and the color of discharges tend to blue-purple. 7.5mTorr 0.15Torr 2.0Torr 7.0Torr