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Equatorial Coronal Holes and Their Relation to the High-Speed Solar Wind Streams

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Equatorial Coronal Holes and Their Relation to the High-Speed Solar Wind Streams Equatorial Coronal Holes and Their Relation to the High-Speed Solar Wind Streams Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultaten¨ der Georg-August-Universitat¨ zu Gottingen¨ vorgelegt von Lidong Xia aus Kaihua/China Gottingen¨ 2003 Bibliografische Information Der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbiblio- grafie; detaillierte bibliografische Daten sind im Internet uber¨ http://dnb.ddb.de abrufbar. D7 Referent: Prof. Dr. Franz Kneer Korreferent: Prof. Dr. Eckart Marsch Tag der mundlichen¨ Prufung:¨ 22. Mai 2003 Copyright c Copernicus GmbH 2003 ISBN 3–936586–20–9 Copernicus GmbH, Katlenburg-Lindau Satz & Coverdesign: Lidong Xia Druck: Schaltungsdienst Lange, Berlin Gedruckt mit Unterstutzung¨ des Deutschen Akademischen Austauschdienstes Printed in Germany Dedicated to Yiling and Minlan Abstract The heating of the solar corona and the origin and acceleration of the solar wind are among the important unsolved problems of space plasma and solar physics. Coronal holes (CHs) have been known as the source of the fast solar wind. However, the plasma properties at the base of CHs have not yet been fully understood. The purpose of this thesis work is to study equatorial CHs and their relation to the origin and propagation of the high-speed solar wind streams by combining observations with both space-based (SOHO and WIND) and ground-based (NSO/KP) instruments. With the high spectral and spatial resolution of SUMER on SOHO, the morphology of equatorial CHs was investigated and compared with the quiet-Sun region by deducing 2-D images in ultraviolet emission line parameters (intensity, Doppler shift and width), which provide useful information about the plasma properties in different layers of the solar atmosphere. The relationship between line pa- rameters and the underlying photospheric magnetic field was studied morphologically and statistically. Furthermore, a comparison of coronal and in situ (at 1 AU) observations was made to study the geometrical expansion factor of the solar wind stream tube. The main findings can be summarized as follows: • The bases of equatorial CHs seen in chromospheric lines generally have similar prop- erties as normal QS regions. An obvious difference has been found in the shape of the H I Lβ line, which has very asymmetric profiles (skewed towards the blue side) in CHs. Loop-like fine structures are the most prominent features in the transition region. • Apparent blue shifts are found in Dopplergrams deduced from transition region lines formed at a temperature below 5 105 K (although on average they are red shifted). Struc- tures with bluer shifts usually have also broader line widths. They seem to represent plasma above large concentrations of unipolar magnetic field, without obvious bipolar photospheric magnetic features nearby. 5 6 • Blue shifts deduced from the Ne VIII (Te ≈ 6.3 10 K) and Mg X (Te ≈ 1.1 10 K) lines predominate in the CH region. Larger-scale outflow are mainly associated with the network where unipolar magnetic field dominates (open magnetic funnels). Red or less blue shifts in EUV bright points indicate that they are unlikely the main source of the fast solar wind. • The Mg X line broadening shows a clear trend to increase with√ the increasing magnetic 2 field strength. The spectroscopically obtainable quantity of v I (with I ∼ ne), which is used as a proxy for the coronal mass flux of the nascent fast solar wind, also reveals a clear positive correlation to the magnetic field strength. • Expansion factors of the solar wind stream tube are determined consistently from two independent groups of parameters, relating to the conservation of the mass flux and mag- netic flux, respectively. The observational results concerning the source of the fast solar wind in CHs obtained in this work are expected to provide a clearer physical picture of the plasma conditions prevailing at the coronal hole base, and thus to be important constraints on theory. Publications and contributions Parts of the results of this thesis were taken from the following publications and confer- ence contributions. 1. Publications L. D. Xia and E. Marsch, Equatorial Coronal holes and Their Relation to the High-Speed Solar Wind Streams, in Conference Proceeding: “Solar Wind Ten”, American Institute of Physics (AIP), p.319-322, 2003. L. D. Xia, E. Marsch and W. Curdt, On the Outflow in an Equatorial Coronal Hole, A&A, 399, L5-L9, 2003. 2. Conference contributions L. D. Xia, E. Marsch, I. E. Dammasch and K. Wilhelm, SUMER Observations of Coronal Holes on the Disk, XXVI General Assembly of the European Geophysical Society, Nice, France, 25 - 30 March, 2001 (oral report). L. D. Xia and E. Marsch, Equatorial Coronal Holes and Their Relation to Solar Wind High-Speed Streams, Solar Wind 10, Pisa, Italy, 17-21 June, 2002 (poster). Contents Contents i List of Figures iv List of Tables viii List of Abbreviations xi 1 Introduction 1 1.1 The Sun and solar wind . 1 1.2 Great puzzle: coronal heating and solar wind acceleration . 4 1.3 Outline of the thesis . 4 2 Review: Coronal Holes and Origin of the Fast Solar Wind 5 2.1 Overall description of coronal holes . 5 2.1.1 Introduction . 5 2.1.2 Coronal holes seen in various wavelengths . 6 2.1.3 The morphology of coronal holes . 7 2.1.4 Underlying photospheric magnetic fields . 9 2.1.5 Fine structures in coronal holes . 10 2.2 FUV/EUV observations of coronal holes . 15 2.2.1 Observations of FUV/EUV radiance . 15 2.2.2 Underlying chromosphere and transition region . 16 2.3 Plasma parameters deduced from observations . 17 2.3.1 Plasma velocity inferred by Doppler shifts . 17 2.3.2 Non-thermal velocity . 20 2.3.3 The density and temperature . 21 2.4 Coronal holes and the fast solar wind . 22 2.4.1 The fast solar wind . 22 2.4.2 Coronal holes: sources of the fast solar wind . 26 2.5 Heating and acceleration mechanisms in coronal holes . 27 2.5.1 The mass and energy balance . 27 2.5.2 Heating and acceleration mechanisms . 29 2.5.3 Recent modelling studies of the coronal funnels . 31 i 3 Instrumentation and Diagnostic Principles 33 3.1 Introduction . 33 3.1.1 Overview of observations at FUV/EUV wavelengths . 33 3.1.2 SOHO mission . 34 3.2 The SUMER instrument . 36 3.2.1 Scientific goals of SUMER . 36 3.2.2 The SUMER spectrometer . 37 3.2.3 Calibrations and corrections . 42 3.3 Additional instruments . 45 3.3.1 EIT instrument . 45 3.3.2 MDI instrument . 45 3.3.3 NASA/NSO Spectromagnetograph . 45 3.3.4 SWE instrument . 46 3.3.5 MFI instrument . 46 3.4 Diagnostic methods with FUV/EUV lines . 46 3.4.1 Atomic processes in the upper solar atmosphere . 46 3.4.2 Formation of line and continuum emission . 48 3.4.3 Diagnostics with FUV/EUV lines . 49 4 Observations and Methods of Data Analysis 55 4.1 Description of observations . 55 4.2 Identification of coronal holes . 58 4.3 Identification of lines . 61 4.4 Determination of line parameters: intensity, position and width . 62 4.4.1 Discussion of estimating errors . 64 4.5 Additional geometrical correction . 65 4.6 Wavelength calibration . 66 5 Morphology of the Equatorial Coronal Holes 69 5.1 Introduction . 69 5.2 Data selection . 69 5.3 Spectroheliograms . 71 5.3.1 Chromospheric lines and continua . 71 ii 5.3.2 Transition region lines (Te < 0.5 MK) . 72 5.3.3 Upper transition region and coronal lines (Te > 0.5 MK) . 73 5.4 Doppler shifts . 79 5.4.1 Chromospheric and transition region lines (Te < 0.5 MK) . 79 5.4.2 Upper transition region and coronal lines (Te > 0.5 MK) . 81 5.5 Network, bright structures and underlying magnetic fields . 81 5.6 Outflow at the coronal base . 87 5.7 Coronal holes seen in the O VI and H I Lβ lines . 90 5.7.1 Analysis of the H I Lβ line . 90 5.7.2 Loop-like structures seen in the O VI line . 95 5.8 Rapid time variations . 101 5.9 Case study of a transient event . 103 5.10 Summary and discussion . 105 6 Statistical and Quantitative Analysis 109 6.1 Introduction . 109 6.2 Data selection and analysis . 110 6.3 Statistical studies of line parameters . 111 6.3.1 Distribution of line intensity, Doppler shift and width . 111 6.3.2 Relationship between line parameters and the chromospheric net- work ................................ 116 6.3.3 Relationship between Doppler shifts of different lines . 121 6.4 Average intensity, Doppler shift and line width . 122 6.4.1 Lines with formation temperatures Te > 0.5 MK . 123 6.4.2 Transition region lines . 127 6.4.3 Chromospheric lines . 135 6.5 Temperature dependence of intensity, Doppler shift and line width . 138 6.5.1 Intensity ratio between the CH and QS regions . 138 6.5.2 Average Doppler shift . 139 6.5.3 Average line width . 141 6.6 Plasma parameters deduced from coronal lines and relation to the under- lying magnetic field . 143 6.7 Summary and discussion . 145 iii 7 Comparison of Coronal and in situ Observations 147 7.1 Introduction . 147 7.2 Data selection . 148 7.3 Equatorial coronal holes and high-speed solar wind streams . 148 7.4 Diagnostic of electron density in coronal holes . 152 7.4.1 Line intensity ratios . 152 7.4.2 Line profiles deduced from SUMER data . 153 7.4.3 Electron densities deduced from SUMER data . 155 7.5 Comparison of coronal and in situ observations .
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