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General Disclaimer One Or More of the Following Statements May Affect General Disclaimer One or more of the Following Statements may affect this Document This document has been reproduced from the best copy furnished by the organizational source. It is being released in the interest of making available as much information as possible. This document may contain data, which exceeds the sheet parameters. It was furnished in this condition by the organizational source and is the best copy available. This document may contain tone-on-tone or color graphs, charts and/or pictures, which have been reproduced in black and white. This document is paginated as submitted by the original source. Portions of this document are not fully legible due to the historical nature of some of the material. However, it is the best reproduction available from the original submission. Produced by the NASA Center for Aerospace Information (CASI) UNIVERSITY OF ILLINOIS URBANA AERONOMY REPfC".)IRT NO, 67 1 AN INVESTIGATION OF THE SOLAR ZENITH ANGLE VARIATION OF D-REGION IONIZATION (NASA-CF-143217) AN INVE'STIGATICN CF THE N75 -28597 SOIA; ZENITH ANGLE VAFIATICN CF C-FEGICN IONI2ATICN (I11incis Cniv.) 290 F HC iE.75 CSCL 044 U11C13s G3/4b 31113 by ^' x '^^ P. A. J. Ratnasiri o c n C. F. Scchrist, Jr. m y T April I, 1975 Library of Congress ISSN 0568-0581 Aeronomy Laboratory Supported by Department of Electrical Engineering National Science Foundation University of Illinois Grant GA 3691 IX Urbana, Illinois ii ABSTRACT A review of the D- region ionization measurements and its solar zenith angle variation reveals that a unified model of the D region, incorporating both thenetural chemistry and the ion chemistry, is required for a proper understanding of this region of the ionosphere. Model calculations are carried out with a view to interpreting the solar zenith angle variation of D- region ionization as measured on July 24, 1968 at Wallops Island. All input data are taken cor- responding to this day. The model developed for the neutral chemistry includes the trans- port terms relating to molecular and eddy diffusion. It describes the diurnal behavior of the minor neutral constituents formed in an oxygen- hydrogen-nitrogen atmosphere, in the height interval between 30 and 120 km. Computations carried out for two cases of the eddy diffusion coefficient models indicate that the constituents which are important for the D--region positive-ion chemistry do not show a significant variation with zenith angle for values up to 75° over the D-region heights. In the ion chemistry model, ion-pair production rates are calculated for solar X-rays between 1 A and 100 A, EUV radiations from 100 A up to the Lyman--a line, precipitating electrons and galactic cosmic rays. Two cases of 0 2 production rates due to different fluxes of precipitating electrons and X-rays below 3 A have been considered. The model describes the solar zenith angle variation of the positive-ion composition, negative- . 0 ion composition and the electron densities up to 75 zenith angle, in the fi height interval between 60 and 100 km. i'k iii A comparison of the computed electron-density profiles with the rocket measured profiles reveals that good overall agreement is obtained when the neutral chemistry model associated with the low eddy diffusion coefficient values is used. The computed solar zenith angle variation, however, is only a fraction of the measured variation. This discrepancy is probably due to short-term variations in the menopause temperatures and X-ray fluxes below 3 A. iv TABLE OF CONTENTS Page ABSTRACT . ii TABLE OF CONTENTS . iv i LIST OF TABLES . vii LIST OF FIGURES . viii 1. INTRODUCTION . 1 1.1 The Ionospheric D Region . 1 1.2 Methods for D-Region Investigations . 4 1.2.1 Ground-based experiments . 5 1.2.2 Rocket experiments . 8 1.3 Evidence for Diurnal Variation . 13 1.4 Some Outstanding Problems of the D Region . 17 1.5 Objectives and the Outline . 24 2. PHOTOCHEMISTRY OF NEUTRAL SPECIES . 26 2.1 Neutral Atmosphere . 26 2. Photodissociation Coefficients . 28 2.2.1 Solar radiation in the 1000-4000 R region 33 2.2.2 Absorption cross-sections . 34 2.2.3 Solar variation of photodissociation coefficients 42 2.3 Reactions of Oxygen Con4tituents . 46 2.4 Oxygen-Hydrogen Reactions . 49 2.5 Nitrogen Reactions . 55 r 2.6 Carbon Reactions . 63 3. MODEL CALCULATION OF NEUTRAL SPECIES DISTRIBUTION . 68 3.1 Transport Processes in the Upper Atmosphere . 68 t Y Page 3.1.1 Eddy-diffusion coefficients . 69 3.1.2 Molecular diffusion coefficients . 73 3.2 Solution of Continuity Equation with Transport Term 75 3.3 Boundary Conditions . 82 3.3,1 Boundary conditions for lower atmosphere produced constituents . 82 3.3.2 Boundary conditions for upper atmosphere produced constituents . 85 3.4 CaZeulation of the Initial Values . 87 3.5 Constant Zenith Angle Calculations . 98 3.5.1 Oxygen and oxygen-hydrogen constituents . 98 3.5,2 Nitrogen and oxygen-nitrogen constituents . 104 3.6 DiurnaZ Variation of NeutraZ Species . 107 3.6.1 Oxygen constituents . 109 3.6.2 Oxygen-hydrogen constituents . 119 3.6.3 Nitrogen and oxygen-nitrogen constituents . , . 129 4. ION-PAIR PRODUCTION RATES . 138 4.1 Solar EUV Radiations . 139 4 .1.1 NO ionization . 139 1 4.1.2 0 2 ( og] ionization . , 143 s 4.1.3 0 2 ionization . 144 4.2 So Zar X Rays . ... , . 148 4.2.1 X-rays below 10 A . 150 4.2.2 30 - 100 A X--nays . 161 4.3 High-Energetic ParticZes . 163 4.3,1 Precipitati::g electrons . 163 F i 1 d ^: 1 i Vi 7 Page 4.3.2 Galactic cosmic rays . 167 4.4 Discussion . 170 f S. D-REGION ION CHEMISTRY . 176 5.1 Measurement of Charged Species Distribution . 176 5.1.1 Positive-ion composition . 176 5.1.2 Negative-ion measurements . 180 5.1.3 Electron-density measurements . 181 5.2 Ionic-Reaction Schemes . 183 5.2.1 Positive-ion reactions . 183 5.2.2 Negative ion reactions . , . 188 5.3 Reaction Rate Constants . 197 6. ION COMPOSITION AND ELECTRON-DENSITY PROFILES . 207 6.1 Ion-Composition Model . 207 6.1.1 Continuity equations . 208 6.1.2 Methods of composition . 212 6.2 Positive-Ion Composition . • . 214 3 6.2.1 Molecular-ion concentrations . 214 6.2.2 Ctuster-ion concentrations . 220 6.3 Negative-Ion Composition 224 ? 6.4 Electron Density . 231 j 6.4.1 Comparison with measured profiles . , . 231 6.4.2 solar zenith angle variation . , . 239 . 244 { t 7. SUMMARY AND CONCLUSIONS . APPENDIX I. NUMERICAL SOLUTION OF PARTIAL DIFFERENTIAL EQUATIONS 249 f REFERENCES . 253 1 vii LIST OF TABLES Table Page 2.1 ''Neutral atmosphere composition at ground level . 27 2.2 Photodissociation reactions . 44 2.3 Oxygen reactions . 48 2.4 Oxygen-hydrogen reactions . 54 2.5 O-N and O-N-H reactions . 62 2.6 C-O-H reactions . 66 4.1 Ionization potential and threshold wavelengths of ionization for atmospheric gases . 140 5.1 Positive-ion reactions . lgg 5.2 Negative-ion reactions . 194 5.3 Recombination coefficients . 202 5.4 Photodetachment rates . 206 viii LIST OF FIGURES Figure Page 1.1 Structure of the day-time electron-density profile of the earth's ionosphere . 2 1.2 Diurnal variation of 'phase height' of reflection l ! for L.F. radio waves propagated to the distance r a marked 19631 . 14 [BeZrose, . a 1.3 Rocket measured D region electron-density profiles at 1 solar zenith angles 90 0 , 84 0 , 18 0 , and 60° on July 24, 1968 [Mechtty and Smith, 1970] . 18 1.4 Rocket measured daytime positive-ion composition in the D region [Narcisi and BaiZey, 1965] . 20 2.1 The temperature and density profiles of the neutral atmo- sphere over Wallops Island on July 24, 1968. Values up to 90 km are based on rocket measurements by Smith et aZ, [1970] and above 90 km, on empirical formulas given by 1 Jacchia [1971] . 29 2.2 The penetration depths of solar radiation incident normally on the atmosphere. 63% of the incident energy in a given wavelength interval is 'rst above the altitude shown. 31 2.3 The solar spectrum between 1000 and 4000 A, based on the measurements by DetwiZer et aZ. [1961], Parkinson Q and Reeves [1969] and Tousey [1963] . 35 2.4 Absorption cross .sections of neutral constituents e having relatively high dissociation energies . 37 I }i t f- ix Figure Page 2.5 Absorption cross sections of neutral constituents having relatively loin dissociation energies . 40 2.6 Photodissociation coefficients of neutral constituents having high dissociation energies, for solar zenith angles 18' and 60 0 . 43 2.7 Photodissociation coefficients of neutral constituents having low dissociation energies, for solar zenith angles 180 and 60 0 . 45 3.1 Eddy diffusion coefficient models used in this study. Molecular diffusion coefficient for oxygen is shown forcomparison . 74 3.2 The height distributions of oxygen constituents ..t noon obtained as initial values . 92 3.3 The height distributions of hydrogen and oxygen--hydrogen constituents at noon, obtained as initial values . 93 3.4 The height distributions of nitrogen and oxygen-nitrogen constituents at noon, obtained as initial values . 96 3.5 The height distributions of carbon constituents at noon, obtained as initial values . 97 3.6 The height distributions of oxygen constituents at noon, calculated using the transport model corresponding to a constant zenith angle and high eddy diffusion coefficient profile. 99 3.7 The height distributions of hydrogen and oxygen hydrogen constituents at noon, calculated using the transport model 1 x t Figure Page corresponding to a constant zenith angle and high eddy i' diffusion coefficient profile .
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