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Ionospheric Electric Fields, Currents, and Resulting Magnetic Fields Variations

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Ionospheric Electric Fields, Currents, and Resulting Magnetic Fields Variations Ionospheric Electric Fields, Currents, and Resulting Magnetic Fields Variations Junhu Du A thesis submitted for the degree of Doctor of Philosophy . Ill - The Faculty of Science and Technology The University of New South Wales Sydney, 1998 CERTIFICATE OF ORIGINALITY 1 lleteby declare that this submission is my own work and to die best of my knowledge it conllins no materials previously published or wriam by -tbcr person, nor material which ID a substantial extent bas been accepted for die awanl of any other degree or diploma at UNSW or any other educational insti1ution, except where due acknowleclgement is llllde in the thesis. Any contribution lllldc ID the research by odlers, with whom I bave worlted at UNSW or elsewllcre, is explicitly acknowledged in lbe thesis. l also declare that lbe intellec1ull content of this thesis is lbe product of my own ..t, except ID die extent that ISSis1ance from others in lbe project's design 111d c:mc:eption or in style, ~talion and linguistic expression is acknowledged. (~) ....... Abstract This thesis uses an equivalent circuit model to calculate ionospheric electric fields, current densities and introduced magnetic fields variations on the ground. The winds used as input are adjusted to yield values of the calculated parameters close to those which have been measured experimentally. The role of the field aligned current is examined. MSIS-90 and IRl-86 are adopted in the program to represent the neutral atmosphere and the ionosphere. The flux tube integrated conductivities in the E and F region are compared. It is found that the ionospheric electric dynamo process is controlled by the E region during daytime but by the F region during nighttime. The F region has a larger effect on the dynamo processes during solar maximum than at solar minimum, and during equinox than in solstice. The different wind models are included and their contributions to the ionospheric electric fields are discussed. We studied the electric field variations with altitude, season and solar activity. In equatorial area, the ionospheric eastward electric field changes very little within the whole ionosphere. The southward(equatorward) electric field is large and changes quickly with height in the E region although it is nearly constant in the F region. The prereversal enhancement of the eastward electric field is produced by the F region dynamo. We conclude that the Forbes and Gillette tidal wind omitting the semidiurnal eastward component can repro­ duce most features of the Jicamarca experiment and the AE-E and DE-2 satellite observations of the electric fields. The HWM90 empirical wind model failed to produce the observed electric field and it seems the semidiurnal wind in HWM90 is too strong. The changes of the field aligned current with altitude, UT, season and solar ac­ tivity are examined. We find that the field aligned current is sensitive to the arithmetic method used and is located mainly in the E and low F region. The non-coincidence of the geomagnetic and geographic equators has a strong effect on the field aligned current in the equatorial zone. The solar activity does not much influence the field aligned current distribution pattern but changes its magni­ tude and introduces considerable field aligned current at night. The field aligned currents driven by Forbes' winds for March equinox and December solstice flow mainly from the southern to northern hemisphere in the morning and vice versa in the afternoon at F region heights. The observed magnetic field variations on the ground are well reproduced in our simulations. It was shown that the field aligned current is the main contributor to the eastward magnetic field component in the equatorial zone. The longitudinal inequality of the northward magnetic field is introduced mainly by the variations of the local magnetic field intensity. The electric field variations have only a mi­ nor effect. The northward magnetic field variations with the solar activity are introduced by changes of the E region equatorward electric field and the Hall conductivity. The difference in the seasonal dependence between simulations and observations in the equatorial zone is pointed out. It seems that a more accurate wind model is needed. Acknowledgments The work described in this thesis was carried out in the School of Physics at the University of New South Wales under the direction of Associate Professor Robert J. Stening. I would like to take this opportunity to express my most heartfelt gratitude to my supervisor, Associate Professor Robert J. Stening, for allowing me use his equivalent circuit codes, for his unflagging encouragement, constant reassurance, careful supervision, and patient guidance both during the research and prepara­ tion of this thesis. Thanks must also go to my fellow student Scott, who has provided me with mis­ cellaneous help and useful discussions concerning the ionospheric physics. Great thinks also go to Weihong Zheng, who has helped me with computer graphing. I am very grateful to my wife Wei Sun and my son Johnny Du for their support and understanding throughout the course of my study. I also acknowledge the financial support provided by an Australia Commonwealth Government Overseas Postgraduate ·Research Scholarship scheme, and another scholarship provided by Associate Professor Robert J. Stening which makes my research possible. Contents 1 Introduction 1 1.1 Introduction . 1 1.2 Simulation models 2 1.2.1 No feedback between the electrodynamics and the neutral motions .............. 3 1.2.1.1 Equivalent Circuit Network 3 1.2.1.2 Differential Equation .... 4 1.2.2 Feedback between the Electrodynamics and the Neutral Motions 8 1.3 Experiments . 10 1.3.1 Rockets 10 1.3.2 Incoherent Scatter Radar . 10 1.3.2.1 Equatorial Region. 10 1.3.2.2 Low and Middle Latitude 14 1.3.2.3 Empirical Drift Model 15 1.3.3 HF Coherent Scatter Radar 15 1.3.4 Satellite .......... 17 2 Simulation Model 21 2.1 Introduction .. 21 2.2 Equivalent Circuit 21 2.3 Perpendicular Current Density . 26 2.3.1 No Dynamo in F Region 27 2.3.2 No Dynamo in E Region 28 2.4 Scale Factor . 28 2.5 Equipotential Lines • • I • • • • • • • • • • • • • • • • • • • • • 31 2.6 Field Aligned Current 32 2.7 Magnetic Field Variations on the ground 33 3 Conductivity and Background Atmosphere 38 3.1 Conductivity .... 38 3.2 Collision Frequency . 39 3.3 Electron Temperature 42 3.4 Neutral Atmosphere . 44 11 3.5 Ionosphere . 46 3.6 Internal Magnetic Field . 47 3. 7 Atmospheric Wind 48 3.7.1 History ... 48 3.7.2 The (1, -2) Tidal Winds in E Region and Measured F Re­ gion Zonal Winds . 49 3.7.3 Forbes Diurnal and Semidiurnal Tidal Winds 51 3. 7.4 HWM90 . 53 4 Flux Tube Integrated Conductivity 55 4.1 Introduction . 55 4.2 Equations .. 57 4.3 Flux Tube Integrated Conductivity 58 4.3.1 Altitude distribution . 58 4.3.2 Local Time Variation . 60 4.3.3 Solar Activity Dependence . 60 4.3.4 Longitudinal Variation 70 4.3.5 Seasonal Variation 70 4.4 Discussion 71 4.5 Summary 72 lll 5 Electric Fields and Currents 74 5.1 Introduction . 74 5.2 Electric Field and Current Produced by the (1, -2) mode in the E region and the F Region Winds 75 5.2.1 E Region alone . 76 5.2.2 Whole Ionosphere . 77 5.2.2.1 Dynamo in the E region only 77 5.2.2.2 Dynamo in the F Region only 78 5.2.2.3 Dynamo Operating Throughout the Whole Ionosphere 79 5.2.3 Altitude Variation 80 5.2.3.1 Daytime . 80 5.2.3.2 Nighttime 82 5.2.4 Solar Activity Dependence . 83 5.3 Electric Field Produced by Forbes' Tidal Winds 84 5.3.1 Altitude Variation 86 5.3.2 Seasonal Variation 86 5.3.3 Solar Activity Dependence . 87 5.4 Electric Field Produced by HWM90 Winds . 88 5.5 Comparison and Conclusion ........ 88 lV 6 Field Aligned Current 111 6.1 Introduction .... 111 6.2 Altitude Variation 112 6.3 Universal Time Effect . 115 6.4 Solar Activity Effect 119 6.5 Seasonal Variation . 120 6.6 Discussion and Summary . 121 7 Magnetic Fields Variations on the Ground 136 7.1 Introduction ................. 136 7.2 Magnetic Fields Variations without Field Aligned Current 137 7.2.1 Latitudinal Variation . 137 7.2.2 Longitudinal Variation 139 7.2.3 Seasonal Variation . 140 7.2.4 Solar Activity Effect 140 7.3 Magnetic Fields Variations Introduced by Field Aligned Current 141 7.4 Conclusion . 145 V Chapter 1 Introduction 1.1 Introduction Models of the ionospheric wind dynamo are useful for several purposes. They can help us to elucidate the characteristics of the dynamo mechanism, and show where and how the electric field and current were generated in the electrically conducting ionosphere. The dynamo model can also be used to infer properties of the local and global scale thermospheric winds by comparing calculated electric fields, currents and generated magnetic field variations with experimental values. The dynamo model certainly has the potential to predict upper atmospheric electrodynamic properties which we have not measured yet, such as field aligned currents. In this chapter, we will review the dynamo simulation models and discuss the experiments concerning the electric fields and currents measured by incoherent and coherent scatter radars, rockets and satellites. 1 CHAPTER 1. INTRODUCTION 2 1.2 Simulation models The electric fields and currents produced by dynamo action in the ionosphere have been investigated and computed by many workers. It is not easy to solve the relevant equations without any assumptions and simplifications since we have the wind dynamo process, the anisotropic inhomogeneous conductivity of the ionosphere plasma, and the feedback between the electrodynamics and the neutral winds all involved in the process.
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