Transcript Acceleration of Coronal Mass Ejection In Long Rising Solar

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 ,nu s  0
 // 
e2n
me e ,i
 
E // B
Force equilibrium:
Electric force = frictional force
No Lorentz force
For plasmas (without neutral), Coulomb collision
 //  8103 (Te[k ])3/ 2 / ln 
Ionosphere Conductivity
Transverse conductivity
 
EB
  

qs ( E  us  B)  ms s,nus  0
Force equilibrium:
Electric force + magnetic force=
frictional force
Ionosphere Conductivity
 
EB
Transverse conductivity
P  {
 e ,n B e
H  {
( B e ) 2
en
B ( ) 2  ( e ) 2
e ,n
B
en
B ( ) 2  ( e ) 2
e ,n
B
 (
 i , n B i
i ,n )
 (
2
 ( B )
i 2
}
( B i ) 2
i ,n )
2
 ( B )
i 2
}
Maximum conductivity:    i
i ,n
B
Transverse conductivity, especially Hall, confines to a
rather narrow range of height (~ 125 km), the so called
dynamo layer
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

I(θ,φ, ) =  ε(z,θ,φ, )
0
dz
cosθ
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
3
E
B
uD  L
2
gr
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