Cluster observations Ali Varsani

178 Pages·2015·23.7 MB·English

Microscale dynamics of the Earth’s magnetopause and

its boundary layer – Cluster observations

Ali Varsani

A thesis submitted to UCL
for the degree of Doctor of Philosophy

October 2015

UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS
MULLARD SPACE SCIENCE LABORATORY
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‘I, Ali Varsani confirm that the work presented in this thesis is my own. Where information has
been derived from other sources, I confirm that this has been indicated in the thesis.’
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Abstract
This thesis presents original research on the solar wind interactions with the Earth's
magnetosphere, with main focus on processes occurring at the magnetopause. Such processes
are often difficult to fully access observationally as their typical timescales are faster than the
particle data cadence which is usually limited to the spin resolution, typically a few seconds, of
the spacecraft. Here, a novel method is applied to data from the Cluster mission in order to
acquire the pitch angle distribution of particles at 0.125 second per sample, the highest-temporal
resolution available at the time of research described here. Two chapters of this thesis present
results based on the exploitation of this high-temporal resolution dataset.
The first study presents analysis of Cluster observations of the substructure of a flux transfer
event (FTE), which reveals unprecedented details of the FTE. The work resolves previously
unseen layers of the FTE, and provides evidence of recent reconnection in the outer layers
(Varsani et al., 2014). The second study focuses on the microscale dynamics within Kelvin-
Helmholtz waves at the dusk flank of the magnetopause. This analysis reveals the presence of
boundary layer plasma travelling faster than the sheath (BPFTS), which appears to be the result
of magnetic reconnection even though, contrary to earlier suggestions, there is no evidence of
rolled-up KHI vortices.
The final study conducts a survey of Cluster observations to compare the two techniques for
identification of rolled-up KHI vortices: the low-density faster than sheath (LDFTS) and
boundary plasma faster than sheath (BPFTS) methods. The results show that since BPFTS is
more rigorously defined, based on the magnetopause transition parameter (TP), it eliminates
ambiguity in the observation of the plasma depletion layer instead of the boundary layer.
Furthermore, the flow directions identified by BPFTS method, are consistent with the
expectations of the KHI growth criteria.
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Acknowledgements
I would like to express my sincere gratitude to various people, whom their help and support
made this research possible.
First and foremost, I would like to thank my parents for their unceasing encouragement and the
precious support they offered all my life. Their words of inspiration have been the prime reason
for me to follow my dream, by doing a PhD in Space Science.
I take this opportunity to express my profound gratitude to Professor Christopher Owen, my
principal supervisor, for his exemplary guidance and valuable critiques throughout the course of
my PhD programme. My grateful thanks are also extended to Professor Andrew Fazakerley, my
secondary supervisor, for introducing the Magnetospheric Science to me, and for his valuable
advices throughout my research. I wish to thank the academic and research staff in the Space
Plasma Physics Group, and the PEACE operation team, for engaging in remarkable scientific
discussions, which lead to great improvements in the work presented in this thesis.
I would like to express my very great appreciation to everyone at the UCL/MSSL’s general
office and administration, and in particular Libby Daghorn and Jane Salton for facilitating the
accommodation during the time this thesis was being written up, and also Julia Wehrle and
Martin de la Nougerede for providing the transportation to work.
I wish to give my warm thanks to the students at MSSL’s flats, and also my office mates, who
made the years living and working at MSSL, a truly memorable time for me.
Finally, I would like to thank the Faculty of Mathematical and Physical Sciences at UCL, for
the financial support they provided towards my tuition fees.
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Contents
Abstract ......................................................................................................................................... 3
Acknowledgements ...................................................................................................................... 4
Contents ........................................................................................................................................ 5
List of Figures ............................................................................................................................... 7
List of Tables ................................................................................................................................ 9
Chapter 1: Introduction ........................................................................................................ 10
1.1 Plasma Physics Concepts ................................................................................................ 10
1.1.1 Definition of Plasma ............................................................................................. 10
1.1.2 Single Particle motion ........................................................................................... 12
1.1.3 Kinetic theory ....................................................................................................... 14
1.1.4 Magnetohydrodynamics ........................................................................................ 15
1.1.5 Magnetic Reconnection ........................................................................................ 16
1.1.6 Kelvin-Helmholtz Instability ................................................................................ 18
1.2 Solar-Terrestrial Plasma Physics ..................................................................................... 18
1.2.1 Solar Wind ............................................................................................................ 19
1.2.2 The Earth’s Magnetosphere .................................................................................. 21
1.2.3 The Dayside Magnetopause .................................................................................. 21
1.2.4 Flux Transfer Events ............................................................................................. 27
1.2.5 Kelvin-Helmholtz Instability at the Magnetopause .............................................. 32
Chapter 2: Instrumentation and analysis techniques......................................................... 34
2.1 The Cluster Mission ........................................................................................................ 34
2.1.1 The Plasma Electron and Current Experiment: PEACE ....................................... 36
2.1.2 The Cluster Ion Spectrometry Experiment: CIS ................................................... 41
2.1.3 The Fluxgate Magnetometer: FGM ...................................................................... 42
2.1.4 The Electric Field and Waves Experiment: EFW ................................................. 43
2.2 Data analysis techniques ................................................................................................. 43
2.2.1 High-time-resolution Pitch Angle Data ................................................................ 43
2.2.2 The Transition Parameter ...................................................................................... 46
2.2.3 Identification of KHI rolled-up vortices ............................................................... 47
Chapter 3: Cluster Observations of the Substructure of a Flux Transfer Event ............ 53
3.1 Introduction ..................................................................................................................... 53
3.2 Instrumentation ............................................................................................................... 54
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3.3 Orbit and Configuration .................................................................................................. 54
3.4 Observations .................................................................................................................... 56
3.4.1 ACE, Wind and Geotail observations ................................................................... 56
3.4.2 Cluster observations: FGM, PEACE and EFW .................................................... 58
3.4.3 Magnetopause Boundary Observations ................................................................ 62
3.4.4 High-time-resolution Observations ....................................................................... 64
3.5 Discussion ....................................................................................................................... 77
3.6 Summary ......................................................................................................................... 87
Chapter 4: Microscale Dynamics within Magnetopause Kelvin-Helmholtz Waves ........ 89
4.1 Introduction ..................................................................................................................... 89
4.2 Spacecraft Dataset ........................................................................................................... 90
4.3 Spacecraft Position .......................................................................................................... 90
4.4 Solar wind observations .................................................................................................. 91
4.5 Boundary wave observations .......................................................................................... 92
4.6 LDFTS and the possibility of nonlinear K-H waves ....................................................... 96
4.7 Local/global structures within the boundary waves ........................................................ 98
4.8 Reconnection signatures in the BPFTS region .............................................................. 103
4.9 Summary and conclusion .............................................................................................. 105
Chapter 5: A survey of Kelvin-Helmholtz wave structure revealed using the
magnetopause transition parameter ....................................................................................... 109
5.1 Introduction ................................................................................................................... 109
5.2 Spacecraft orbit ............................................................................................................. 110
5.3 Survey dataset ............................................................................................................... 111
5.4 Designation of transition parameter values ................................................................... 114
5.5 LDFTS versus BPFTS................................................................................................... 117
5.6 Velocity space distribution of BPFTS ........................................................................... 120
5.7 Survey Results from Cluster 1 and 3 ............................................................................. 124
5.8 Summary and conclusion .............................................................................................. 126
Chapter 6: Summary and Future work ............................................................................. 130
6.1 Discoveries concerning Flux Transfer Events: ............................................................. 130
6.2 Discoveries concerning the Kelvin-Helmholtz Instability at the Magnetopause: ......... 131
Appendix 1 ................................................................................................................................ 134
List of abbreviations ................................................................................................................ 165
References ................................................................................................................................. 166
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List of Figures

1.1 Electron trajectory in a uniform magnetic field .............................................................. 13
1.2 The reconnection process ................................................................................................ 17
1.3 Typical values for space plasmas parameters ................................................................. 19
1.4 The Parker Spiral ............................................................................................................ 20
1.5 The 'closed' and 'open' magnetosphere ............................................................................ 22
1.6 The low-latitude dayside magnetopause during steady reconnection ............................. 24
1.7 The behaviour of a particle transmitted through a current sheet ..................................... 25
1.8 The Russel and Elphic FTE model .................................................................................. 28
1.9 Three different models of FTEs ...................................................................................... 30
1.10 Schematic view of the Plasma Depletion Layer (PDL) .................................................. 31
1.11 The Kelvin-Helmholtz instability at flank magnetopause ............................................... 33

2.1 Top hat electrostatic analyser - PEACE instrument ........................................................ 37
2.2 Mounting configuration of LEEA and HEEA on Cluster spacecraft .............................. 37
2.3 Energy sweep modes used in PEACE instrument ........................................................... 38
2.4 On-board selection of PAD by PEACE instrument ........................................................ 41
2.5 PAD calculation per sweep, from 3D data ...................................................................... 45
2.6 Calculation of the Transition Parameter.......................................................................... 46
2.7 Flow patterns in KHI ....................................................................................................... 48
2.8 Example of scatterplots used for identification of LDFTS ............................................. 49
2.9 Location of rolled-up K-H events previously identified via LDFTS .............................. 50
2.10 Velocity disturbance pattern in boundary waves ............................................................ 52

3.1 Position of Cluster and Geotail, during the 12-02-2007 FTE event ................................ 55
3.2 Upstream solar wind conditions ...................................................................................... 57
3.3 Cluster four-spacecraft observations ............................................................................... 59
3.4 Motion of the magnetopause boundary ........................................................................... 64
3.5 High-time-resolution electron observations by Cluster 1................................................ 65
3.6 Pitch angle distributions of the FTE layers identified in Fig. 3.5 ................................... 66
3.7 High-time-resolution ion observations by Cluster 1 ....................................................... 71
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3.8 Ion pitch angle distribution inside the FTE ..................................................................... 73
3.9 High-time-resolution electron observations by Cluster 2................................................ 76
3.10 High-time-resolution electron observations by Cluster 3................................................ 76
3.11 A schematic interpretation of the FTE structure ............................................................. 79
3.12 Cooling model versus Cluster observations of the FTE .................................................. 82

4.1 Position of Cluster and Geotail, during the 29-12-2006 KHW event ............................. 91
4.2 Integrated plots for IMF and boundary waves ................................................................ 93
4.3 Schematic view of the boundary waves observed by Cluster 1 ...................................... 96
4.4 Scatterplots of the faster than sheath flows ..................................................................... 97
4.5 VDF plots for the ions as observed by from Cluster 1, 3 and 4 .................................... 101
4.6 Pitch angle plots for the electrons as observed by from Cluster 1, 3 and 4 .................. 102
4.7 LDFTS prediction in KHI versus Cluster observations ................................................ 107

5.1 Trajectory of Cluster spacecraft during a survey on the magnetopause flanks ............. 110
5.2 Calculation of Transition Parameter (TP) from two different data products ................ 113
5.3 Reordered plasma parameters based on the calculated Transition Parameter ............... 114
5.4 Identification of the faster-than-sheath population, LDFTS versus BPFTS ................. 119
5.5 Velocity space distribution of ion population, projected onto LM plane ...................... 121
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List of Tables

2.1 List of instruments aboard each Cluster spacecraft ......................................................... 35
2.2 PEACE sensors LEEA and HEEA - The specifications ................................................. 38
2.3 List of energy sweep modes used in PEACE instrument ................................................ 39
2.4 Telemetry modes for PEACE instrument........................................................................ 39
2.5 List of FGM data resolution modes ................................................................................ 42
2.6 List of FGM time resolution based on the telemetry mode ............................................. 43
2.7 Dayside magnetopause events with high-time-resolution PAD for electrons ................. 44

3.1 High-time-resolution E×B drift velocities from Cluster 1 .............................................. 74
3.2 High-time-resolution E×B drift velocities from Cluster 2 and 3 .................................... 77

4.1 Orientation of magnetopause within boundary waves, observed by Cluster 1 ............... 95

5.1 Statistics of the faster-than-sheath population, the 20-11-2001 KHI event .................. 119
5.2 Statistics of the BPFTS population - The velocity space distribution ........................... 121
5.3 Cluster 1’s survey results, LDFTS vs. BPFTS method ................................................. 122
5.4 Cluster 3’s survey results, LDFTS vs. BPFTS method ................................................. 123
5.5 Statistics of BPFTS population for the dawn and dusk sectors ..................................... 125
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Chapter 1
Chapter 1: Introduction

1.1 Plasma Physics Concepts
1.1.1 Definition of Plasma
The observable matter in our universe is known to be predominantly made up of plasma, a form
of ionized gas, which is often referred to as fourth state of matter. In other words, from stellar
interiors and interstellar medium, to the Sun and the interplanetary medium in our solar system,
the space is occupied with various plasmas. A plasma is a quasi-neutral gas of charged, and
possible neutral particles that exhibit collective behaviour (Chen, 1974). In order to understand
the behaviour of particles in plasma, it is necessary to discuss how a plasma maintains its
quasi-neutral state, and what leads particles to follow a collective behaviour.
1.1.1.1 Quasi-neutrality
A plasma in steady state, is considered to be quasi-neutral, if there are about equal number of
positive and negative charges within volumes significantly smaller than the characteristic
lengths for variations of macroscopic parameters of plasma (e.g., density and/or temperature).
This way, the electrical charge of individual ions and electrons would be canceled over large
scales and provide macroscopic charge neutrality for plasma.
The electric Coulomb potential field of an individual charged particle 𝑞, is given by
𝑞
∅ = (1.1)
𝐶 4𝜋𝜖 𝑟
0
where 𝜖 is the free space permittivity, and 𝑟 is the distance from the charge. Now, if this
0
charged particle is placed inside the plasma, the oppositely charged and similarly charged
particles will be respectively attracted and repelled, until the charge on that particle is shielded
by them. This process, which restores the quasi-neutrality of plasma on the large scale, is known
as Debye shielding. Accordingly, the Debye potential has the form
𝑞 −𝑟
∅ (𝑟)= exp( ) (1.2)
𝐷 4𝜋𝜖 𝑟 𝜆
0 𝐷