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Spectral Irradiance Measurements of Simulated Lightning in Planetary AtmospheresOptical emissions from lightning provide information to estimate the altitude, latitude and longitude of lightning storms, the energy of the discharges, and the production of important trace species such as HCN and C2H2. Knowledge of the spectra of planetary lightning is needed to deduce the total energy dissipated by lightning (and thereby, the production of trace gases) and to help design experiments to detect and track lightning storms. Measurements of the spectral irradiance from approximately 380 to 820 nm are reported for laboratory simulations of lightning in the atmospheres of Venus, Jupiter, and Titan. In our laboratory, laser-induced plasmas (LIP) are used to simulate lightning discharges. This technique avoids contamination of the spectra by elect-ode material and maintains a safe environment while allowing the use of flammable gases, such as hydrogen and methane, found in outer planet atmospheres. The observations were made at I and 5 bars pressure for Venus and Jupiter and at 1 bar for the Titan mixture. At a pressure of one bar, our results show prominent lines from H(sub alpha), H(sub beta), H(sub gamma), and H(sub delta) lines of the Balmer Series of atomic hydrogen, a single line from Helium at 588 nm and strong continuum radiation. At pressures of 5 bars, the H(sub alpha) and H(sub beta) lines are wider, the H(sub gamma), and H(sub delta) lines merge into the continuum because of pressure broadening, and the helium line at 588 nm is no longer visible. The observed spectra of simulated lightning in the venusian atmosphere at 1 and 5 bars shows that the OI multiplet at 777.7 nm dominates the spectra, but weak features due to atomic carbon and singly excited and singly ionized oxygen atoms are also visible. Although lightning has not yet been observed on Titan, it conceivable that some form of lightning discharge could be occurring. Therefore experiments on a Titan atmosphere mixture were conducted. The most prominent features seen in the simulated spectra are the H(sub alpha) at 656 nm. from the dissociation of the methane and two strong features at 740 and 820 nm from singly excited nitrogen atoms. Most of the features between 400 and 656 nm are due to ionized nitrogen atoms. A broad, highly structured feature centered near 389 nm is molecular band radiation from CN molecules formed in the cooling plasma of atomic nitrogen, carbon, and hydrogen. It is observed in the lightning simulations of both the venusian and Titan atmospheres, For Venus, Jupiter, and Titan, narrow spectral features that are ideal for detecting lightning in the presence of sunlight are present in the spectra. These features could allow lightning to be detected and tracked by Earth-based instruments.
Document ID
20020038882
Acquisition Source
Ames Research Center
Document Type
Conference Paper
Authors
Borucki, W. J.
(NASA Ames Research Center Moffett Field, CA United States)
McKay, C. P.
(NASA Ames Research Center Moffett Field, CA United States)
Jebbens, D.
(San Jose State Univ. CA United States)
Lakkaraju, H. S.
(San Jose State Univ. CA United States)
Vanajakshi, C. T.
(Sterling Software, Inc. Palo Alto, CA United States)
Cuzzi, Jeffrey
Date Acquired
August 20, 2013
Publication Date
January 1, 1996
Subject Category
Lunar And Planetary Science And Exploration
Meeting Information
Meeting: URSI Inter Union of Radio Science Conference
Location: Lille
Country: France
Start Date: August 28, 1996
End Date: September 5, 1996
Sponsors: International Scientific Radio Union
Funding Number(s)
PROJECT: RTOP 185-52-62
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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