currents, electrojets and...
TRANSCRIPT
![Page 1: Currents, Electrojets and Instabilitiescedarweb.vsp.ucar.edu/wiki/images/7/70/2016CEDAR_Sunday_Sahr.p… · John D Sahr Electrical Engineering University of Washington 19 June 2016](https://reader033.vdocument.in/reader033/viewer/2022060909/60a408b548dc0b7dee562d5d/html5/thumbnails/1.jpg)
Currents, Electrojets and Instabilities
John D Sahr
Electrical Engineering University of Washington
19 June 2016
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Outline• The two main sources of large scale
currents in the ionosphere:
• solar-wind/magnetosphere, and
• dynamo ( ) forces.
• wired together by the magnetic field.
• The resulting fields and currents create meter-scale plasma irregularities
~F ⇥ ~B
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Ionospheric structure!
http://www.nap.edu/read/13060/chapter/12#154
This picture is intended
to scare you.
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Let’s start slowly• The ionosphere is that part of the
atmosphere in which a significant portion of the gas is ionized; you need to include plasma physics to describe what is happening above about 90 km.
• The ionosphere is created (mostly) by UV and x-ray photons from the Sun which partially ionize the neutral atmosphere.
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Where do ionospheric currents come from?
• Distortion of the Earth’s magnetic field due to the buffeting from the solar wind.
• Dynamo situations near the Earth, caused by the neutral winds dragging ionospheric plasma through the Earth’s magnetic field.
� > 1
� < 1
� =NkBT
B2/2µ=
Plasma pressure
magnetic field energy density
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Some basics (1)• Charged particles experience force from Electric and
Magnetic Fields
• If there is an electric field component parallel to , then electrons and ions freely accelerate. An electron exposed to 1 will achieve 90 km/s after 1 second (E region electron thermal velocity).
• Earth’s Magnetic Field lines are ~superconductors
~Fs = qs⇣~E + ~vs ⇥ ~B
⌘
~BµV/m
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Some basics (2)• Charged particles experience force from
Electric and Magnetic Fields
• When the electric field is perpendicular to then the charge particles will gyrate about the magnetic field, and drift at a mean speed. 1 mV/m causes drift of 20 m/s at high latitude, 40 m/s at the equator.
~Fs = qs⇣~E + ~vs ⇥ ~B
⌘
~B
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Some basics (3)
• Charged particles experience force from Electric and Magnetic Fields
• When the ion collision frequency is higher than the gyro frequency, then the ions don’t gyrate about (much); they slowly drift parallel to the electric field .
~Fs = qs⇣~E + ~vs ⇥ ~B
⌘
~B
~E
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Ionospheric currents driven by magnetosphere-solar wind interaction
www.comet.ucar.edu www.hao.ucar.edu
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Those currents …• Field Aligned Currents are electric currents
that flow along the Earth’s magnetic field lines with very little resistance.
• Pedersen Currents are electric currents that flow parallel to the electric field but perpendicular to ,
• Hall Currents are electric currents that flow perpendicular to both and .
~B
~B~E
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Those currents …• The Earth’s nearly static natural magnetic
multipole field would induce no currents in the ionosphere because it is curl-free:
• But when the solar wind presses on the magnetosphere, currents flow to support the perturbed .
r⇥ ~B =@ ~D
@t+ ~J = 0
~B
0
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Magnetospheric currents
https://ww
w.researchgate.net/publication/24014482_TIPO
_Tesla_Interferometric_Planetary_O
bserver
… are not the topic of this talk; the point is that the solar-wind
interacts with the magnetosphere,
and drive currents along the Earth’s magnetic field into
the ionosphere.
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Ohm’s Law
• In simple media, a simple Ohm’s Law:
• In partially ionized plasmas, we need a more complicated Ohm’s Law:
~J = � ~E
2
4Jx
Jy
Jz
3
5 =
2
4�p
�h
0��
h
�p
00 0 �0
3
5
2
4E
x
Ey
Ez
3
5
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What does that mean?
Altitude pedersen hall specific
h < 80 km small:no plasma
small:no plasma
small:no plasma
80 < h < 150 modest:ion collisions
large:magnetized e
unmagnetized i
large:plenty of plasma
150 < hsmall:
magnetized emagnetized i
small:magnetized emagnetized i
large:plenty of plasma
�p �h �0
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Ohm’s Law, again• If you think of those magnetospheric currents
being forced through the lower ionosphere, they will produce an electric field due to the finite resistivity in the ionosphere:
• These electric fields can generate meter scale plasma waves if they are large enough.
2
4E
x
Ey
Ez
3
5 =
$⌃
��12
4Jx
Jy
Jz
3
5
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Ohm’s Law, again• That Ohm’s law is correct in frame drifting
with the neutral gas, but viewed in an Earth-fixed system we need a (relativistic) coordinate change:
• These electric fields can generate meter scale plasma waves if they are large enough.
~E + ~U ⇥ ~B =$⌃ · ~J
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How about the equator?
• The magnetosphere can only (directly) drive ionospheric currents at high latitudes, since the field lines connect the ionosphere to the magnetosphere.
• …Yet there is a large ionospheric current at the magnetic equator; what causes this?
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Neutral Atmosphere vs. the Ionosphere; or
Superman vs. Batman
• The neutral atmosphere has greater mass and number density than the ionosphere below about 2000 km.
• The neutral atmosphere is massive compared to the ionosphere below about 500 km; where the neutral atmosphere goes (perp to B), that’s where the ionosphere goes.
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Tides and Winds
• When you heat a gas, it expands; when you heat the atmosphere, it expands in the only direction that it can: up. And it drags the ionosphere along with it (especially the ions).
• Of course, the Earth is also spinning, and once you put gas in motion around the Earth, Coriolis forces will push it around.
• You’ll hear a lot about tides and (thermospheric) winds at CEDAR.
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E region dynamo
• If you just think about the Earth getting heated on the dayside, and cooled on the night side, you can figure out the basic equatorial current (the Equatorial Electrojet).
B
day night
U Ueastward E, J westward E, J
heating cooling
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Tides (2)
• The daytime eastward electric field creates an upward E x B drift, but the ions are collisional; only the electrons E x B drift, separating the charges in the E-region slab.
B
day timenight time
EE
+++
��� +++
���
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Tides (3)• This secondary Electric field induces its own E x B
drift; westward during the day, eastward during the night.
• Again, the current is carried mostly by electrons, eastward during the day, westward at night.
B
day timenight time
EE
Ve VeJ J
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a little differently• a small Ex (East) is created by neutral wind/tide
• it generates a Pederson Current and a Hall current (down, Jy)
• … but the Jy current runs out of conductivity, charging the top and bottom of the electrojet, and Jy goes to zero.
Jx
Jy
�=
�p
�h
��h
�p
� E
x
Ey
�
�h
Ex
= �p
Ey
Jx
= �p
Ex
+ �h
Ey
=
✓�p
+�2
h
�p
◆
| {z }Cowling conductivity, �c
Ex
�c � �h
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Equatorial Electrojet
• When you fold in other details — such as the descent of field lines to lower altitudes — you find that this large current is restricted to flow within a few degrees of the magnetic equator, peaking at about 105 km altitude.
• When the electrojet velocity exceeds C_s, then meter-scale, field aligned, plasma sounds waves are generated.
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Equatorial Electrojet
http://geomag.org/info/equatorial_electrojet.html
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F region dynamo
• At higher altitudes the Hall conductivity is quite small because both electrons and ions ExB drift (low collisions).
• The neutral atmosphere, although still dense, begins to exchange momentum with the F-region plasma.
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F region dynamo
• The equatorial F region field lines descend into the E-region at mid latitudes: the F-region dynamo is connected to the E region dynamo at mid latitudes.
• The E region Dynamo can enhance or suppress the F region Dynamo, depending upon which way the winds are blowing.
• In the upper F region, the ions can start to exert noticeable force on the neutrals.
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F region dynamo, seasonal effects
• At equinoxes, both E-region “footprints” of the equatorial F region are “on” or “off” together.
• In January, the southern footprint is on longer
• in June, the northern foot print is on longer
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Field Aligned Irregularities
• E and F region irregularities are highly field aligned, because high electron mobility (large ) along B “shorts out” any parallel E fields that could form.
• Thus, the density irregularities look like long columns of high and low density that are aligned with B.
�0
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What causes E region irregularities?
• In the E region, electrons drift in the ExB direction, while the ions barely move.
• When the electron drift exceeds the plasma sound speed, it creates a plasma “sonic boom”, strong perturbations in the plane perpendicular to B, and strongest in the ExB direction
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numerical simulation:
E di
rect
ion
Oppenheim, Otani, Ronchi “Saturation of Farley-
Buneman …” JGR v101 N A8, August 1996
B
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Halloween Storm, 2003
Cascades E region turbulence
Range, km150 300 600 900 1200
+1200
+300
-300
-1200
Dop
pler
Vel
ocity
, m/s
96.5 MHz radio waves (3 meter wavelength) scattering from 1.5 meter ion sound waves
Doppler upshift
Doppler up and downshift
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Other E region things …
• Quasi-Periodic Echoes
• Sporadic E
• 150 km echoes (at the equator) closely track the day time vertical E field.
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What causes F region irregularities?
• In the F region, the whole plasma moves in the ExB direction. However the Rayleigh-Taylor instability works.
• In the F region the peak density is around 350 km, with less density below. Thus, there’s a heavy fluid above a light fluid — the heavy fluid tries to fall down, and the light fluid should try to bubble up.
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F x B drifts
• In studying plasmas E x B drifts are a special case of F x B (Force x B) drifts.
• Pressure gradient produces a force in the same direction for electrons and ions, so they drift in opposite directions, thus producing a non zero current.
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F x B drifts
more plasma
less plasma
B
ionelectronrP
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F x B drifts with perturbation
more plasma
less plasma
B
ionelectron E
E x B
downward perturbation
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Find JRO plumes
From Jorge Chau: https://en.wikipedia.org/wiki/Jicamarca_Radio_Observatory#/media/File:Esf.jpg
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Air glow plumes
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summary
• Two main sources of ionospheric currents:
• neutral winds (low latitude), and
• magnetosphere currents (high latitude)
• sufficiently strong currents cause E region irregularities
• sufficiently strong gradients cause F region irregularities
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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
Mike Kelly, and his book!
To my sponsors NSF, AFOSR, NATO, Boeing, Xilinx, Washington Research Foundation