Currents, Electrojets and Instabilities. John D Sahr Electrical Engineering University of Washington 19 June 2016

Transcription

1 Currents, Electrojets and Instabilities John D Sahr Electrical Engineering University of Washington 19 June 2016

2 Outline The two main sources of large scale currents in the ionosphere: solar-wind/magnetosphere, and dynamo ( ~F ~ B ) forces. wired together by the magnetic field. The resulting fields and currents create meter-scale plasma irregularities

3 Ionospheric structure! This picture is intended to scare you.

4 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.

5 Where do ionospheric currents come from? Distortion of the Earth s magnetic field due to the buffeting from the solar wind. > 1 Dynamo situations near the Earth, caused by the neutral winds dragging ionospheric plasma through the Earth s magnetic field. < 1 = Nk BT B 2 /2µ = Plasma pressure magnetic field energy density

6 Some basics (1) Charged particles experience force from Electric and Magnetic Fields ~F s = q ~E s + ~vs B ~ If there is an electric field component parallel to ~B, then electrons and ions freely accelerate. An electron exposed to 1 µv/mwill achieve 90 km/s after 1 second (E region electron thermal velocity). Earth s Magnetic Field lines are ~superconductors

7 Some basics (2) Charged particles experience force from Electric and Magnetic Fields ~F s = q ~E s + ~vs B ~ 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. ~B

8 Some basics (3) Charged particles experience force from Electric and Magnetic Fields ~F s = q ~E s + ~vs B ~ When the ion collision frequency is higher than the gyro frequency, then the ions don t ~B gyrate about (much); they slowly drift parallel to the electric field. ~E

9 Ionospheric currents driven by magnetosphere-solar wind interaction

10 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 ~B perpendicular to, Hall Currents are electric currents that flow perpendicular to both and. ~E ~B

11 Those currents The Earth s nearly static natural magnetic multipole field would induce no currents in the ionosphere because it is curl-free: 0 r ~ B ~ + ~ J =0 But when the solar wind presses on the magnetosphere, currents flow to support the ~B perturbed.

12 Magnetospheric currents 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.

13 Ohm s Law In simple media, a simple Ohm s Law: ~J = ~ E In partially ionized plasmas, we need a more complicated Ohm s Law: 2 4 J x J y J z 3 5 = 2 4 p h 0 h p E x E y E z 3 5

14 What does that mean? Altitude pedersen hall specific p h 0 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 < h small: magnetized e magnetized i small: magnetized e magnetized i large: plenty of plasma

15 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: 2 4 E x E y 3 2 apple $ 1 5 = 4 J x J y 3 5 E z J z These electric fields can generate meter scale plasma waves if they are large enough.

16 Ohm s Law, again That Ohm s law is correct in frame drifting with the neutral gas, but viewed in an Earthfixed system we need a (relativistic) coordinate change: ~E + ~ U ~ B = $ ~J These electric fields can generate meter scale plasma waves if they are large enough.

17 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?

18 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.

19 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.

20 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). U eastward E, J B westward E, J U day heating night cooling

21 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. E +++ B +++ E day time night time

22 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. Ve J J Ve E B E day time night time

23 a little differently a small Ex (East) is created by neutral wind/tide it generates a Pederson Current and a Hall current (down, Jy) apple apple apple Jx p h Ex = J y but the Jy current runs out of conductivity, charging the top and bottom of the electrojet, and Jy goes to zero. h p E y J x = p E x + h E y = he x = p E y p + 2 h p {z } Cowling conductivity, E x c c h

24 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.

25 Equatorial Electrojet

26 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.

27 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.

28 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

29 Field Aligned Irregularities E and F region irregularities are highly field aligned, because high electron mobility (large 0 ) 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.

30 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

31 numerical simulation: B E direction Oppenheim, Otani, Ronchi Saturation of Farley- Buneman JGR v101 N A8, August 1996

32 Halloween Storm, MHz radio waves (3 meter wavelength) scattering from 1.5 meter ion sound waves Doppler Velocity, m/s Cascades Doppler upshift E region turbulence Doppler up and downshift Range, km

33 Other E region things Quasi-Periodic Echoes Sporadic E 150 km echoes (at the equator) closely track the day time vertical E field.

34 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.

35 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.

36 F x B drifts B more plasma electron ion rp less plasma

37 F x B drifts with perturbation B more plasma downward perturbation electron E less plasma ExB ion

38 Find JRO plumes From Jorge Chau:

39 Air glow plumes

40 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

41 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