1 Introduction to Solar Activity.- 1.1 Some Basic Properties of the Sun.- 1.2 Basic Equations of Magnetohydrodynamic.- 1.2.1 Magnetohydrostatics.- 1.2.2 Waves.- 1.2.3 Instabilities.- 1.3 Sunspots.- 1.4 Prominences.- 1.4.1 Prominence Formation.- 1.4.2 Magnetostatic Support.- 1.5 The Corona.- 1.5.1 Models of the Corona.- 1.5.2 Coronal Heating.- 1.6 Solar Flares.- 1.7 Conclusion.- References.- 2 An Introduction to Magnetospheric MHD.- 2.1 Introduction.- 2.2 Why is There a Magnetosphere?.- 2.3 The Open Magnetosphere Morphology.- 2.4 Momentum Transfer.- 2.5 Magnetospheric Substorms.- 2.6 Magnetohydrodynamic Waves.- References.- 3 Magnetohydrodynamic Waves.- 3.1 Structuring and Stratification.- 3.2 Waves in a Magnetically Structured Atmosphere.- 3.3 Waves in a Uniform Medium.- 3.3.1 The Alfven Wave.- 3.3.2 Magnetoacoustic Waves.- 3.4 Waves in Discretely Structured Media.- 3.4.1 Incompressible Medium.- 3.4.2 Compressible Medium.- 3.5 Oscillations in a Low ?-Gas.- 3.5.1 Slab Inhomogeneities.- 3.5.2 Cylindrical Inhomogeneities.- 3.5.3 Impulsively Generated Fast Waves.- 3.6 Damped Alfven Waves.- 3.7 Waves in Stratified Atmospheres.- 3.7.1 Sound Waves.- 3.7.2 The Influence of a Horizontal Magnetic Field.- 3.8 Slender Flux Tubes.- 3.8.1 The Slender Flux Tube Equations: Sausage Modes.- 3.8.2 Pulse Propagation.- 3.8.3 Kink Modes.- 3.8.4 Instabilities in Tubes.- References.- 4 MHD Instabilities.- 4.1 Equilibrium Solutions.- 4.1.1 Introduction.- 4.1.2 Energetics.- 4.1.3 The Lorentz Force.- 4.1.4 Magnetohydrostatic (MHS) Equilibria.- 4.1.5 Cylindrically Symmetric Magnetic Fields.- 4.1.6 2-Dimensional Magnetic Fields.- 4.2 Physical Description of MHD Instabilities.- 4.3 Linearised MHD Equations.- 4.4 Normal Modes Method.- 4.5 Energy (or Variational) Method.- 4.6 The Rayleigh-Taylor Instability.- 4.6.1 Normal Modes - Two Fluids.- 4.6.2 Normal Modes - Continuous Fluid.- 4.6.3 Simple Energy Method - Two Fluids.- 4.6.4 Energy Method - Continuous Fluids.- 4.6.5 MHD Incompressible Rayleigh-Taylor Instability.- 4.7 The Sharp Pinch - Normal Modes.- 4.7.1 Inner Solution r a.- 4.7.3 Matching Conditions at r = a.- 4.8 General Cylindrical Pinch - Energy Method.- 4.8.1 Minimisation of ?2W.- 4.8.2 Suydam's Criterion - A Necessary Condition.- 4.9 Necessary and Sufficient Conditions - Newcomb's Analysis.- 4.10 Resistive Instabilities - Tearing Modes.- 4.10.1 Introduction.- 4.10.2 The Analysis of FKR.- 4.11 Applications of MHD Instabilities.- 4.11.1 Introduction.- 4.11.2 Ideal Kink Instability of Coronal Loops.- 4.11.3 Two-Ribbon Flares.- References.- 5 Magnetic Reconnect.- 5.1 Introduction.- 5.2 Reconnection: What It Is and What It Does.- 5.3 Fluid (MHD) Models of Reconnection.- 5.4 The Single-Particle Approach in a Collision-Free Plasma.- References.- 6 Magnetoconvection.- 6.1 Small Flux Tubes.- 6.2 Convection in a Strong Magnetic Field.- 6.3 Structure of the Large-Scale Magnetic Field.- References.- 7 Aspects of Dynamo Theory.- 7.1 The Homopolar Disc Dynamo.- 7.2 The Stretch-Twist-Fold Dynamo.- 7.3 Behaviour of the Dipole Moment in a Confined System.- 7.4 The Pros and Cons of Dynamo Action.- 7.5 Flux Expulsion and Topological Pumping.- 7.6 Mean-Field Electrodynamics.- 7.7 Some Properties of the Pseudo-Tensors ?ij and ?ijk.- 7.8 The Solar Dynamo.- 7.9 Magnetic Buoyancy as an Equilibration Mechanism.- References.- 8 Solar Wind and the Earth's Bow Shock.- 8.1 The Solar Wind as a Fluid.- 8.1.1 Fluid Models of the Solar Wind.- 8.1.2 Solar Wind Magnetic Fields.- 8.1.3 Mass and Angular Momentum Loss.- 8.1.4 Refinements of Fluid Models.- 8.2 The Solar Wind as a Plasma.- 8.2.1 Why a Plasma Description is Needed.- 8.2.2 Solar Wind Protons.- 8.2.3 Minor Ions in the Solar Wind.- 8.2.4 Waves in the Solar Wind.- 8.3 The Earth's Bow Shock.- 8.3.1 Why a Shock is Needed.- 8.3.2 General Shock Considerations.- 8.3.3 Macroscopic Fields at Collisionless Shocks.- 8.3.4 Particle Dynamics at Collisionless Shocks - Electrons.- 8.3.5 Particle Dynamics at Collisionless Shocks - Ions.- 8.3.6 The Global Structure of the Earth's Bow Shock and Foreshock.- 8.4 Conclusion.- References.- 9 Planetary Magnetospheres.- 9.1 Comparative Theory of Magnetospheres.- 9.1.1 Obstacles in a Flowing Plasma.- 9.1.2 Plasma Sources.- 9.1.3 Magnetospheric Flows.- 9.2 Planetary Magnetospheres.- 9.2.1 Mercury.- 9.2.2 Venus.- 9.2.3 Earth.- 9.2.4 Mars.- 9.2.5 Jupiter.- 9.2.6 Saturn.- 9.2.7 Uranus.- 9.2.8 Neptune and Pluto.- 9.3 Conclusions.- References.- 10 Comets.- 10.1 Introduction to Comet Structure.- 10.2 Interaction between the Solar Wind and the Comet.- 10.3 Production of Neutral Gas.- 10.3.1 Vaporisation.- 10.3.2 Neutral Gas Density.- 10.4 Ionisation.- 10.4.1 Ionisation Processes.- 10.4.2 Size of the Coma.- 10.5 Ion Pick-Up.- 10.5.1 Ion Pick-Up Trajectories.- 10.5.2 Stability of the Distribution.- 10.6 Principal Plasma Regimes.- 10.6.1 Main Regions.- 10.6.2 The Contact Surface.- 10.6.3 Bow Shock.- 10.7 Magnetohydrodynamic Flow at a Comet.- 10.7.1 Numerical Solution of the MHD Equations.- 10.7.2 Validity of the MHD Approach.- 10.8 Special Features of the Morphology.- 10.8.1 Rays, Tail Streamers.- 10.8.2 Disconnection Events.- 10.8.3 Dusty Plasmas.- 10.9 Conclusion.- References.