Currents, Electrojets and...

transcript

Currents, Electrojets and Instabilities

John D Sahr

Electrical Engineering University of Washington

19 June 2016

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

Ionospheric structure!

http://www.nap.edu/read/13060/chapter/12#154

This picture is intended

to scare you.

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.

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

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

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

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

Ionospheric currents driven by magnetosphere-solar wind interaction

www.comet.ucar.edu www.hao.ucar.edu

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 .

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

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.

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

0��

00 0 �0

What does that mean?

Altitude pedersen hall specific

h < 80 km 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

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.

��12

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

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?

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.

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.

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).

day night

U Ueastward E, J westward E, J

heating cooling

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.

day timenight time

�� +++

��

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.

day timenight time

Ve VeJ J

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.

��h

= �p

+ �h

✓�p

| {z }Cowling conductivity, �c

�c � �h

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.

Equatorial Electrojet

http://geomag.org/info/equatorial_electrojet.html

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.

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.

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

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.

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

numerical simulation:

Oppenheim, Otani, Ronchi “Saturation of Farley-

Buneman …” JGR v101 N A8, August 1996

Halloween Storm, 2003

Cascades E region turbulence

Range, km150 300 600 900 1200

96.5 MHz radio waves (3 meter wavelength) scattering from 1.5 meter ion sound waves

Doppler upshift

Doppler up and downshift

Other E region things …

• Quasi-Periodic Echoes

• Sporadic E

• 150 km echoes (at the equator) closely track the day time vertical E field.

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.

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.

F x B drifts

more plasma

less plasma

ionelectronrP

F x B drifts with perturbation

more plasma

less plasma

ionelectron E

downward perturbation

Find JRO plumes

From Jorge Chau: https://en.wikipedia.org/wiki/Jicamarca_Radio_Observatory#/media/File:Esf.jpg

Air glow plumes

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

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

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