Equatorial coronal holes and their relation to the high-speed solar wind streams [Elektronische Ressource] / vorgelegt von Lidong Xia

Sujets

• Astronomie

Informations

• Publié par: georg-august-universitat_gottingen
• Publié le: 01 janvier 2003
• Nombre de lectures: 10
• Langue: English
• Poids de l'ouvrage: 12 Mo

Extrait

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 Universit at¨ zu Gotting¨ en
vorgelegt von
Lidong Xia
aus
Kaihua/China
Gotti¨ ngen 2003D7
Referent: Prof. Dr. Franz Kneer
Korreferent: Prof. Dr. Eckart Marsch
Tag der mun¨ dlichen Pru¨fung: 22. Mai 2003Dedicated to Yiling and MinlanAbstract
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
5formed at a temperature below 5 10 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 (T ≈ 6.3 10 K) and Mg X (T ≈ 1.1 10 K)e e
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√
2field strength. The spectroscopically obtainable quantity ofv I (withI ∼n ), which ise
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
i3 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