Slide Number 1The Exploding SunThe History of the Solar WindThe Sun’s Atmosphere Extends far into SpaceThe Sun’s Atmosphere Extends Far into SpaceSlide Number 6Discovery of the Solar WindSlide Number 8Solar Wind ObservationsSlide Number 10Slide Number 11Why is There a Solar Wind?Pressure GradientsEscaping GravitySlide Number 15The Parker ModelSlide Number 17Not all Stars will Have a Solar WindMore Solutions for Other StarsQuantitative Solutions for the Solar WindSlide Number 21The Interplanetary Magnetic FieldSlide Number 23Slide Number 24Slide Number 25Slide Number 26Current SheetsSlide Number 28Slide Number 29The 3D HeliosphereSlide Number 31Slide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39Slide Number 40ShocksSlide Number 42Slide Number 43Cosmic RaysWhen Cosmic Rays Reach EarthAssignmentESS 7ESS 7Lectures 6, 7 and 8Lectures 6, 7 and 8October 8, 10 and 13October 8, 10 and 13The The HeliosphereHeliosphereThe Exploding Sun• We have seen that at times the Sun explosively sends plasma into the surrounding space. • This occurs most dramatically during CMEs.The History of the Solar Wind• 1878 Becquerel (won Noble prize for his discovery of radioactivity) suggests particles from the Sun were responsible for aurora• 1892 Fitzgerald (famous Irish Mathematician) suggests corpuscular radiation (from flares) is responsible for magnetic stormsThe Sun’s Atmosphere Extends far into Space2008 Image1919 NegativeThe Sun’s Atmosphere Extends Far into Space• The image of the solar corona in the last slide was taken with a natural occulting disk – the moon’s shadow. • The moon’s shadow subtends the surface of the Sun.• That the Sun had a atmosphere that extends far into space has been know for centuries- we are actually seeing sunlight scattered off of electrons.• The Earth’s atmosphere is stationary. The Sun’s atmosphere is not stable but is blown out into space as the solar wind filling the solar system and then some.• The first direct measurements of the solar wind were in the 1960’s but it had already been suggested in the early 1900s.– To explain a correlation between auroras and sunspots Birkeland [1908] suggested continuous particle emission from these spots. – Others suggested that particles were emitted from the Sun only during flares and that otherwise space was empty [Chapman and Ferraro, 1931].A Solar Wind not a Stationary AtmosphereDiscovery of the Solar Wind• That it is continuously expelled as a wind (the solar wind) was realized when Biermann [1951] noticed that comet tails pointed away from the Sun even when the comet was moving away from the Sun.• The definitive answer came from spacecraft.• The Mariner 2 spacecraft returned 3 months of continuous solar wind data while traveling to Venus. The Solar Wind is Always ThereSolar Wind Observations• Solar wind speeds (heavy lines) and densities (light lines) [Hundhausen, 1995].• The most detailed observations of the solar wind have been made from spacecraft near the Earth.Observed Properties of the Solar Wind near the Orbit of the Earth (after Hundhausen, [1995])Proton density 6.6 cm-3Electron density 7.1 cm-3He2+ density 0.25 cm-3Flow speed (nearly radial) 450 km s-1Proton temperature 1.2x105KElectron temperature 1.4x105KMagnetic field 7x10-9TSolar Wind Parameters Through a Sphere at 1AUFlux Through a Sphere at 1AU(after Hundhausen, [1995])Protons 8.4x1035 s-1Mass 1.6x1012 g s-1Radial momentum 7.3x1014 N (Newton)Kinetic energy 1.7x1027 erg s-1 (107 erg/J)Thermal energy 0.05x1027 erg s-1Magnetic energy 0.025x1027 erg s-1Radial magnetic flux 1.4x1015 Wb (Weber)Why is There a Solar Wind?• The solar wind exists because the Sun maintains a 2x106K corona as its outer most atmosphere. • The Sun’s atmosphere “boils off” into space and is accelerated to high velocities (> 400 km s-1).• There is a large pressure difference (gradient) with high pressure near Sun and low pressure in space• Causes solar wind to flow from Sun to space• Heating of corona gives the high temperatures and allows escape velocitiesPressure Gradients• Pressure (= nkT) (where n is the number density and T is temperature)• Higher the temperature and density the higher the pressure.• Gradient = spatial difference – (P2 -P1) /(x2 -x1 ) = (DP/Dx)=• This is equivalent to a force– The force is– The force is a vector that is directed from the region with high pressure to the region with low pressure – Force causes an accelerationP∇PF−∇=Escaping Gravity• Gravity holds the atmosphere to Sun (or Earth) against the force due to the pressure gradient.• This is called hydrostatic equilibrium• Equilibrium => balance• Balance of gravity versus pressure gradient• Force balance mg(r) = n(r)kT(r)• Note (r) means that the parameter is a function of distance (i.e., gravity falls off with height)• If the temperature is high enough the gas can escape• Parker [1958] proposed that the solar wind was the result of the high temperature corona and developed a hydrodynamic model to support his idea. – Simplifying assumptions:1. The solar wind can be treated as an ideal gas.2. The solar wind flows radially from the Sun.3. Acceleration due to electromagnetic fields is negligible.4. The solution is time stationary (i.e. the time scale for solar wind changes is long compared to the time scale for solar wind generation).A Simple Model of the Solar WindThe Parker Modelwhere v is the velocity of the solar wind, r is the radial distance from the Sun, G is the gravitational constant, MSun is the mass of the Sun, and the speed of sound squared cs2=γkT/m (γ=5/3, k is the Boltzman constant, and m is the mass of a proton).[]2222sSunscvrGMcrvdrdv−⎥⎦⎤⎢⎣⎡−=• The transition from subsonic to supersonic occurs at a critical radius rc where • In order for a real continuous solution to exist at rc• The form of solutions for the expansion of the solar windscv=22sSunccGMr =The Observed Solar Wind is SupersonicNot all Stars will Have a Solar Wind• Solution A is the “observed” solar wind. It starts as a subsonic flow in the lower corona and accelerates with increasing radius. At the critical point the solar wind becomes supersonic.• For solution F the speed increases only weakly with height and the critical velocity is not reached. For this case the solar wind