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CSI 662 / ASTR 769 Lect. 12 Spring 2007 April 24, 2007 Ionospheric Current and Aurora References: •Prolss: Chap. 7.1-7.6, P349-379 (main) •Tascione: Chap. 8, P. 99 – 112 (supplement) Topics • • • • Polar Upper Atmosphere Ionospheric Currents Aurorae Ionosphere and magnetosphere coupling Ionosphere Currents Polar Upper Atmosphere • Polar Cap: ~ 30° • Polar oval: a few degree • Subpolar latitude Polar Upper Atmosphere Magnetic field connection • Polar Cap: magnetotail lobe region, open • Polar oval: plasma sheet, open • Subpolar latitude: conjugate dipole field, closed Convection and Electric Field • Polar cap electric field Epc • Dawn to dusk direction • Epc = 10 mV/m • Polar cap potential: ~ 30 kV from 6 LT to 18 LT, over 3000 km Convection and Electric Field • Polar cap electric field originates from solar wind dynamo electric field • Same direction • Same overall electric potential drop • Electric field is ~ 40 times as strong as in solar wind Esw U sw BE Convection and Electric Field • Polar cap convection • Caused by EXB drift • anti-sunward • Drift time scale cross the polar cap ~ 2 hours UD E / B Drift velocity = 500 m/s, when E=10 mV/m, and B=20000 nT Convection and Electric Field • Polar oval electric field Eo • Dusk to dawn direction, opposite to polar cap field • E0 = 30 mV/m • Counter-balance the polar cap field • Polar oval convection • Sunward convection • Form a close loop with the polar cap convection • Two convection cells Convection and Electric Field • Polar oval electric field Eo • Dusk to dawn direction, opposite to polar cap field • E0 = 30 mV/m • Counter-balance the polar cap field • Polar oval convection • Sunward convection • Form a close loop with the polar cap convection • Two convection cells Ionosphere Current • Pederson current: perpendicular B, parallel E ; horizontal • Hall current: perpendicular B, perpendicular E ; horizontal • Burkeland current: parallel to B ; vertical Ionosphere Current • Birkeland current: Field-aligned current • Region 1 current: on the poleward side of the polar oval • Region 2 current: on the equatorward side of the polar oval Ionosphere Current • Pederson current flows from dawn to dusk in the polar cap • Pederson current flows radially in the polar oval, dusk to dawn • Pederson current forms a closed loop with Burkeland currents in the two boundary regions: region 1 and 2 • Hall current direction is opposite to the convection, because ions drift slower than the electrons • Westward at the dawn sector • Eastward at the dusk sector Ionosphere Conductivity j E j en(u u ) i e en(u u ) / E i e Deriving conductivity σ is to find the drift velocity under the E in the three components: • Birkeland σ: parallel to B • Pederson σ: parallel to E, E per B • Hall σ: per E and B Ionosphere Conductivity Parallel conductivity qs E ms s ,nus 0 // e2n mee ,i E // B Force equilibrium: Electric force = frictional force No Lorentz force For plasmas (without neutral), Coulomb collision // 8 10 (Te [k ]) 3 3/ 2 / ln Ionosphere Conductivity Transverse conductivity EB qs ( E us B) ms s ,nus 0 Force equilibrium: Electric force + magnetic force= frictional force Ionosphere Conductivity EB Transverse conductivity P { ( en B e , n B e e ,n ) H { ( en B 2 ( B ) e 2 ( B e ) 2 e ,n ) 2 ( B ) e 2 ( i ,n B i i ,n ) ( 2 ( B ) i 2 } ( B i ) 2 i ,n ) 2 ( B ) i 2 } Maximum conductivity: i ,n B Transverse conductivity, especially Hall, confines to a rather narrow range of height (~ 125 km), the so called dynamo layer i Aurora Image taken near Richmond VA, Oct 29, 2003 Akasofu, Secrets of the Aurora Patches and Bands Akasofu, Secrets of the Aurora Aurora • Form • Discrete: arcs, bands, rays, patches • Diffuse • Height: > 100 km • Orientation • Vertical: along the magnetic field line • Horizontal: primarily east-west direction • Colors and emitting elements • O: red (630.0 nm, 630.4 nm), yellow-green (557.7 nm) • N2+: blue-violet (391.4 nm – 470 nm) • N2: dark red (650 nm – 680 nm) • Intensity: up to a few 100 kR (kilo Rayleigh) Aurora • Aurorae are caused by the incidence of energetic particles onto the upper atmosphere • Particles move-in along the open polar magnetic fields • The particles are mostly electrons in the energy range of ~100 ev to 10 kev. • Ions are also observed Aurora Processes • Primary collision • Scattering (elastic collision) • Collisional ionization • Collisional dissociation Energy conversion: • Collisional excitation •1% radiation • Secondary process •50% heating • Secondary ionization •30% chemical energy • Secondary dissociation •Other: scatter back to • Secondary scattering magnetosphere • Charge exchange • Dissociation exchange • Excitation exchange • Dissociative recombination • Radiative recombination • Collisional quenching The Rayleigh (R): A Basic Unit for measuring Aurora-Airglow Emissions • One R corresponds to the emission rate of 106 photons per second radiated isotropically from an atmospheric column with a base area of 1 cm2 • Brightness of the Milky Way Galaxy: 1 kR dz I(θ,φ, ) = ε(z,θ,φ, ) cosθ 0 Auroral Particles • Not solar wind particles • Particles are from magnetotail plasma sheet, with which the polar oval is magnetically connected • Diffuse aurora • convection and subsequent pitch angle diffusion of plasma sheet particles • Discrete aurora • Produced by higher energy electrons (Ee > 1 keV) • Plasma sheet electron (Ee < 1 keV) • Additional acceleration is needed • Acceleration along magnetic field-aligned electric fields • Double layer • Plasma instability produces localized potential differences Ionosphere-Magnetosphere Coupling • Region 1 current • Magnetotail current is re-directed to the ionosphere • Also produce auroral oval electrojet • Energy is from solar wind dynamo • Energy is dissipated in the ionosphere through Joule heating Ionosphere-Magnetosphere Coupling • Region 2 current • Associated magnetic field lines end in the equatorial plane of the dawn and dusk magnetosphere at a geocentric distance of L ≈ 7-10 • Driven by excess charge in the dawn and dusk sectors of the dipole field, caused by different particle paths of electrons and ions Ionosphere-Magnetosphere Coupling • Drift of particles from the plasma sheet uD L E E B 3 u D gr L2 u D gr uD E L1 • Ions and electrons drifts in different • At small L, curvaturedirection along the dipole gradient drift dominates • Particles can only drift to • There is a forbidden zone for ions within a certain distance of (electrons) • Excess charges accumulate the dipole The End