Introduction to Auroral E
Region Irregularities
John D Sahr
Electrical Engineering
University of Washington
21 June 2015
what are they?
• Ion-acoustic plasma turbulence, 95-120
km, found near the Aurora
• driven (mostly) by electron Hall current
• related to equatorial electrojet, nonspecular meteor trail echoes
• scatter radio waves easily, up to 1 GHz
Auroral Oval
The Auroral Ovals form near the boundary between open
and closed magnetic field lines
http://www.swpc.noaa.gov/pmap/pmapN.html
http://www.swpc.noaa.gov/pmap/pmapS.html
Ionospheric currents
www.comet.ucar.edu www.hao.ucar.edu
Why should we care?
• They’re interesting: almost 2D turbulence
• They’re tricky: live in the “ignorosphere”
• They’re dramatic: remarkably loud
• They’re driven: by magnetics storms
• They’re very CEDARy:
coupling (✓), energetic (✓), dynamic (✓), atmospheric (✓)
discovery
• Discovered by mid 1930s (Eckersley and others) with
HF and VHF radio wave backscatter. Altitude around
100 km
• … but that’s a problem. A bit like Rutherford’s alpha
particle scattering experiment. He didn’t expect the
alpha particles to experience large angle scatter, “It was quite the most incredible event that has ever
happened to me in my life. It was almost as
incredible as if you fired a 15-inch shell at a piece of
tissue paper and it came back and hit you.”
discovery (2)
ionospheric plasma
ionosonde
oblique total internal reflection
Physicists had a good idea of basic plasma physics
and understood how/why ionosondes worked,
and understood oblique reflection from the ionosphere.
discovery (3)
E region plasma
…but oblique backscatter was a surprise. The
only reflection mechanism they knew of had to
do with the plasma frequency.
E
E
E
!e =
excess ions
excess electrons
Plasma Frequency
s
N e2
✏0 me
p
fe ⇡ 9 N
ions and
electrons
balance
ion momentum
fe ⇡ 15 MHz
F region
fe ⇡ 2 MHz
E region
electron momentum
anyway…
• E region scatter remained a mystery …
• … until about 1955. Booker noted that the backscatter
was coming from the a place in the E region where the
radio wave propagated exactly perpendicular to the
local magnetic field: a huge clue.
• in 1963 Farley modified the nascent kinetic theory of
incoherent scatter (1961-ish) to provide a very good
explanation for the E region irregularities.
Booker’s observation
scatter, but
not backscatter
backscatter
a theory is born
• Farley’s theory was kinetic (i.e. considered the plasma
as a collection of charged particles)
• However a simpler approach works quite well using
two fluid theory, which treats ionospheric plasma as a
mixture of an electron fluid and an ion fluid.
(see two slides back)
gyro physics…
The Earth’s magnetic field pervades the ionosphere,
charged particles feel Lorentz force, and try to gyrate.
eB
!c =
m
frequency
Vth
mVth
rc =
=
!c
eB
radius
Vth
electron
!ce = 107 rad/s
9 mm
90 km/s
ion
!ci = 180 rad/s
2000 mm⇤
375 m/s
⇤
. . . We’ll see this doesn’t matter
Hall drift
When you apply an Electric Field perpendicular to
the magnetic field, the charged particles drift in
the E x B direction:
E
ion drift
B
(up)
electron drift
Same velocity
thus
no net current
collisions
However, with collisions there is a Hall current:
collision
frequency
electron
ion
10000 s
1000 s
1
1
mean free
path
orbits per
collision
9m
160
0.375 m
1
30
The ions are unmagnetized.
The electrons are magnetized.
Hall Current
So: in the E region, the electrons Hall drift
but the ions don’t
E
slow ion drift
B
(up)
electron drift
Acoustic Instability
When the electron Hall Speed exceeds the
ion-acoustic speed, it’s a sonic boom!
E
B
(up)
,
e
l
tab ited
s
xc
r ly
e
a
line early
lin
n
no
Linearly
unstable
line
a
no
nlin rly s
ta
ea
rly ble,
exc
ite
d
when the electric field exceeds
20 mV/m, that’s enough
to trigger the instability.
electron drift
Acoustic Instability (2)
The sound waves are strongly constrained
to the plane perpendicular to B
ble
linearly sta
ped
m
a
d
y
l
g
n
stro
E
Linearly unstable
(up)
linearly s
table
strongly d
amped
B
about 0.25 degrees!
electron drift
Bragg scatter of radio waves
incident radio wave
incident
Acoustic Waves modulate
the index of refraction
target
scattered
scattered wave
The scatter adds coherently
when radio = 2 target .
Halloween Storm, 2003
Doppler Velocity, m/s
96.5 MHz radio waves (3 meter wavelength)
scattering from 1.5 meter ion sound waves
+1200
Cascades
E region turbulence
+300
-300
Doppler
upshift
Doppler
up and downshift
-1200
150 300
600
Range, km
900
1200
A little math …
Continuity Equation
@t ns + r · ns ~us = P
L⇡0
Momentum Equation
⇣
qs
~ + ~us ⇥ B
~
@t ns ~us + r · ns ~us ~us =
ns E
ms
Gas Law
ps =
s k B T s ns
⌘
quasi neutrality
ne = ni
or
Poisson’s Equation
~ = qe (ni
r · ✏E
ne )
1
rps
ms
… gives the dispersion relation
for an acoustic wave with wave number ~
k
this wave is a solution: A exp(j!r t + t)
~ ⇥B
~
E
Vd =
B2
~k · V
~d
!r =
1+
1
=
1+

⌫i
2
!r
k
2
⌫e ⌫i
=
⇡ 0.1
⌦e ⌦i
2
Cs
density gradients modify this…
2
Cs
⇡
kB
e Te
+ k B i Ti
me + mi
E direction
numerical simulation:
Oppenheim, Otani, Ronchi
“Saturation of FarleyBuneman …” JGR v101
N A8, August 1996
meteor trail
Meteor Trails have similar physics,
driven by density gradients
Why 95 — 120 km?
• Above 120 km
temperature
increases so Cs
increases
• Above 120 km ions
become more
magnetized, so Vd
decreases
• Below 95 km
electron and ion
collisions increase
• so
increases
What don’t we know?
• We don’t understand the precise shape of the
power spectrum (unlike incoherent scatter).
• We don’t know the spatial spectrum very
well, except for some (strong) hints from
sounding rockets.
• We don’t know the precise alignment of the
turbulence and auroral arcs.
• Spatial resolution is challenging, especially at
high latitudes.
Same time, different TX
96.5
MHz
98.9
MHz
Why the difference? We don’t know.
references
Eckersley, T. L. "Irregular ionic clouds in the E layer of the ionosphere." Nature 140 (1937): 846-847.
Farley, D. T. "A plasma instability resulting in field-aligned irregularities in the ionosphere." Journal of Geophysical Research
68.22 (1963): 6083-6097.
Fejer, Bela G., and M. C. Kelley. "Ionospheric irregularities." Reviews of Geophysics 18.2 (1980): 401-454.
Hamza, A. M., and J. P. St-Maurice. "A turbulent theoretical framework for the study of current-driven E region irregularities at
high latitudes: Basic derivation and application to gradient-free situations." Journal of geophysical research 98.A7 (1993).
Kelley, Michael C. The Earth's Ionosphere: Plasma Physics & Electrodynamics. Vol. 96. Academic press, 2009.
Oppenheim, Meers, Niels Otani, and Corrado Ronchi. "Saturation of the Farley-Buneman instability via nonlinear electron E× B
drifts." Journal of Geophysical Research: Space Physics (1978–2012) 101.A8 (1996): 17273-17286.
Pfaff, R. F., et al. "The E-region rocket/radar instability study (ERRRIS): Scientific objectives and campaign overview." Journal of
atmospheric and terrestrial physics 54.6 (1992): 779-808.
Sahr, John D., and Bela G. Fejer. "Auroral electrojet plasma irregularity theory and experiment: A critical review of present
understanding and future directions." Journal of Geophysical Research: Space Physics (1978–2012) 101.A12 (1996):
26893-26909.
Schlegel, K., E. C. Thomas, and D. Ridge. "A statistical study of auroral radar spectra obtained with SABRE." Journal of
Geophysical Research: Space Physics (1978–2012) 91.A12 (1986): 13483-13492.
Sudan, R. N. "Unified theory of type I and type II irregularities in the equatorial electrojet." Journal of Geophysical Research:
Space Physics (1978–2012) 88.A6 (1983): 4853-4860.
Questions?
Big thanks
To my students:
Weiwei Sun, Marcos Iñonan
To my past students/present colleagues:
Frank Lind, Melissa Meyer, Andy Morabito, Cliff Zhou, Laura Vertatschitsch
To my advisors and mentors:
Don Farley, Sunanda Basu, Wes Swartz, Bela Fejer, Jason Providakes
To my sponsors
NSF, AFOSR, NATO, Boeing, Xilinx