Ionospheric Current and Aurora

CSI 662 / ASTR 769 Lect. 12 Spring 2007

April 24, 2007

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 AtmosphereMagnetic 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

Eswsw BUE

Convection and Electric Field• Polar cap convection

• Caused by EXB drift• anti-sunward• Drift time scale cross the polar cap ~ 2 hours

BEUD /

Drift velocity = 500 m/s, whenE=10 mV/m, andB=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

Euuen

uuenj

Ej

ei

ei

/)(

)(

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 BE

//

ieemne

snsss umEq

,

2

//

, 0

For plasmas (without neutral), Coulomb collision

ln/])[(108 2/33// kTe

Force equilibrium:Electric force = frictional forceNo Lorentz force

Ionosphere Conductivity

Transverse conductivity BE

0)( , snssss umBuEq

Force equilibrium:Electric force + magnetic force= frictional force

Ionosphere Conductivity

Transverse conductivity BE

}{

}{

22,

2

22,

2

22,

,

22,

,

)()(

)(

)()(

)(

)()()()(

iBni

iB

eBne

eB

iBni

iBni

eBne

eBne

Ben

H

Ben

P

Maximum conductivity: Transverse conductivity, especially Hall, confines to a rather narrow range of height (~ 125 km), the so called dynamo layer

iBni ,

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• Collisional excitation

• Secondary process• Secondary ionization• Secondary dissociation• Secondary scattering• Charge exchange• Dissociation exchange• Excitation exchange• Dissociative recombination• Radiative recombination• Collisional quenching

Energy conversion:•1% radiation•50% heating•30% chemical energy•Other: scatter back to magnetosphere

The Rayleigh (R): A Basic Unit for measuring Aurora-Airglow Emissions

0

dzI(θ,φ, ) = ε(z,θ,φ, )

cosθ

• 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

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

Lu

u

D

BEE

D

ED

grD

gr Lu

Lu

1

2

3

• At small L, curvature-gradient drift dominates

• Particles can only drift to within a certain distance of the dipole

• Ions and electrons drifts in different direction along the dipole

• There is a forbidden zone for ions (electrons)

• Excess charges accumulate

The End