Introduction to Space Weather
Ionosphere II:
Radio Waves
April 19, 2012
Jie Zhang
Copyright ©
CSI 662 / PHYS 660
Spring, 2012
Roadmap
•Part 1: Sun
•Part 2: Heliosphere
•Part 3: Magnetosphere
•Part 4: Ionosphere
•Part 5: Space Weather
Effects
CH10: Ionosphere I
CH11: Ionosphere II
CSI 662 / PHYS 660
CH11: Ionosphere II
11.1 Radio Waves in the Ionosphere
11.2 Ionosphere Currents
Plasma-15: Radio Waves in the Ionosphere
Apr. 19, 2012
CH11: Ionosphere II
References and Reading Assignment:
•PRO CH 4.7
•KAL CH 8.3.3
(on radio waves)
(on currents)
CH11.1 Radio Waves in the
Ionosphere
• Radio wave is altered during its passage through the
ionosphere
– Propagation direction changes:
• Refraction
• Reflection
– Intensity changes:
• Attenuation
• Absorption
Radio Wave
Ionosonde
A special radar to examine
ionosphere from ionogram:
Elapsed time  height
Frequency  electron density
ionosonde
Plasma Frequency:
Natural Oscillation in a Plasma:
nme
d 2 ( x )
dt
2

e2n2
0
x
x  (x) 0 sin( p t )
p 
e2n
 0 me
 p [ s ]  56.4 n[m ]
1
3
3
fp[ Hz]  9 n[m ]
Forced Oscillation in a Plasma:
d 2 ( x )
nme
dt
2
 nme
d ( x )
dt

e2n2
0
x  en 0 sin(t )
x  (x) 0 sin(t   )
  (x) 0 sin(t   )   (x) 0 cos(t   ) 
2
 p (x) 0 sin(t   )  (e / me ) 0 sin(t )
2
Ionosphere as a Dielectric
• Interaction depends on frequency
   p
  
phase  0
conductivity  0
p 2

nref  1  ( )
• Nref < 1, radio wave will be refracted according to
the familiar Snell’s law. Θ2 > Θ1
sin  2 
sin 1
nref
Ionosphere as a Dielectric
Wave damping due to electron interaction with neutral
particles
 Pfr 
e 0
2 me
2
2
n e , n
2
Radio wave (e.g., 5 Mhz) refraction and damping
usually occur in the upper D region and lower E
region
Reflection:
Ionosphere as a Conductor
   p (h)
• Wave interacts strongly with plasma, inducing a
large current. Ionosphere acts like a conductor
• Radio wave is reflected
• This often occurs in the F-region
(Continued on April 26, 2012)
CH11.2. Ionosphere Currents
Polar Upper Atmosphere
• Polar Cap: ~ 30°
• Polar oval: ~5° (noon)
to ~10° (night)
• Subpolar latitude: < 65°
Polar Upper Atmosphere
Magnetic field connection
• Polar Cap: open field connecting to magnetotail lobe region
• Polar oval:
• night side: quasi-closed field connecting to plasma sheet
• day side: open field connecting to solar wind – the cusp
• Subpolar latitude: closed dipole field
Polar Upper Atmosphere
• 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
Dusk
Dawn
The circular cells are for the pattern of the dynamo velocity
Convection and Electric Field
• Polar cap electric field Epc (from measurement)
• Dawn to dusk direction
• Epc = 10 mV/m = 10 V/km
• Polar cap potential: ~ 30 kV from 6 LT to 18 LT, over 3000
km
• Polar oval electric field
• Dawn sector: equatorward
• Dusk sector: poleward
• Epo =30 mV/m = 30 V/km
• Potential drop: ~ 30 kV, counterbalance of the polar cap E
• Subpolar region electric field
• < 5 mV/m
Convection and Electric Field
• Polar cap convection
• Caused by E X B drift
• anti-sunward
• Drift time scale cross the polar cap ~ 2 hours
• Polar oval convection
• Sunward convection
• Form a close loop with the polar cap convection
• Two convection cells
Drift velocity = 500 m/s,
when
E=10 mV/m, and
B=20000 nT
UD  E / B
Ionosphere Current
• Burkeland current: parallel to B;
;vertical
• Pederson current: perpendicular B, parallel E ; horizontal
• Hall current:
perpendicular B, perpendicular E ; horizontal
Ionosphere Current
• Birkeland current: Field-aligned current
• Region 1 current: on the poleward side of the polar oval,
connecting to the magnetopause curret
• Region 2 current: on the equatorward side of the polar oval,
connecting to the plasma sheet current
Ionosphere-Magnetosphere Coupling
• Region 1 current
• Plasma sheet current
is re-directed to the
ionosphere
• Current flows into the
ionosphere in the dawn
sector
• Current flows out the
ionosphere in the dusk
section
Ionosphere-Magnetosphere Coupling
• Region 2 current
• Associated magnetic
field lines end in the
equatorial plane of the
dawn and dusk
magnetopause at a
geocentric distance of
L ≈ 7-10
Ionosphere Conductivity
(optional)
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, perpendicular to B
• Hall σ: perpendicular to both E and B
Ionosphere Conductivity
 
E // B
Parallel conductivity


qs E  ms s ,nu s  0
 // 
e2n
me e ,i
Force equilibrium:
Electric force = frictional force
No Lorentz force
For plasmas, 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 End