Introduction to Auroral E Region Irregularities. John D Sahr Electrical Engineering University of Washington 21 June 2015

Transcription

1 Introduction to Auroral E Region Irregularities John D Sahr Electrical Engineering University of Washington 21 June 2015

2 what are they? Ion-acoustic plasma turbulence, 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

3 Auroral Oval The Auroral Ovals form near the boundary between open and closed magnetic field lines

4 Ionospheric currents

5 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 ( )

6 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.

7 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.

8 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.

9 Plasma Frequency excess electrons E E E excess ions s Ne 2! e = 0 m e p f e 9 N f e 15 MHz F region f e 2 MHz E region ions and electrons balance ion momentum electron momentum

10 anyway E region scatter remained a mystery until about 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.

11 Booker s observation scatter, but not backscatter backscatter

12 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)

13 gyro physics The Earth s magnetic field pervades the ionosphere, charged particles feel Lorentz force, and try to gyrate.! c = eb m r c = V th = mv th! c eb frequency radius Vth electron! ce = 10 7 rad/s 9 mm 90 km/s ion! ci = 180 rad/s 2000 mm 375 m/s...we ll see this doesn t matter

14 Hall drift When you apply an Electric Field perpendicular to the magnetic field, the charged particles drift in E the E x B direction: B (up) ion drift electron drift Same velocity thus no net current

15 collisions However, with collisions there is a Hall current: electron collision frequency mean free path s 1 9m orbits per collision 160 ion 1000 s m 1 30 The ions are unmagnetized. The electrons are magnetized.

16 Hall Current So: in the E region, the electrons Hall drift E but the ions don t slow ion drift B (up) electron drift

17 Acoustic Instability When the electron Hall Speed exceeds the ion-acoustic speed, it s a sonic boom! E linearly stable, nonlinearly excited when the electric field exceeds 20 mv/m, that s enough to trigger the instability. B (up) linearly stable, nonlinearly excited Linearly unstable electron drift

18 Acoustic Instability (2) The sound waves are strongly constrained to the plane perpendicular to B linearly stable strongly damped E (up) Linearly unstable linearly stable strongly damped about 0.25 degrees! electron drift B

19 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

20 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

21 A little math Continuity t n s + r n s ~u s = P L 0 Momentum t n s ~u s + r n s ~u s ~u s = q s n ~E s + ~us B ~ m s 1 m s rp s Gas Law quasi neutrality p s = s k B T s n s n e = n i or Poisson s Equation r ~ E = q e (n i n e )

22 gives the dispersion relation for an acoustic wave with wave number ~ k this wave is a solution: A exp(j! r t + t)! r = ~ k ~V d 1+ V d = ~ E ~ B B 2 = 1 1+ apple i! 2 r k 2 C 2 s = e i e i 0.1 density gradients modify this C 2 s k B et e + k B i T i m e + m i

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

24 meteor trail Meteor Trails have similar physics, driven by density gradients

25 Why 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

26 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.

27 Same time, different TX 96.5 MHz 98.9 MHz Why the difference? We don t know.

28 references Eckersley, T. L. "Irregular ionic clouds in the E layer of the ionosphere." Nature 140 (1937): Farley, D. T. "A plasma instability resulting in field-aligned irregularities in the ionosphere." Journal of Geophysical Research (1963): Fejer, Bela G., and M. C. Kelley. "Ionospheric irregularities." Reviews of Geophysics 18.2 (1980): 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, 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 ( ) 101.A8 (1996): 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): 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 ( ) 101.A12 (1996): Schlegel, K., E. C. Thomas, and D. Ridge. "A statistical study of auroral radar spectra obtained with SABRE." Journal of Geophysical Research: Space Physics ( ) 91.A12 (1986): Sudan, R. N. "Unified theory of type I and type II irregularities in the equatorial electrojet." Journal of Geophysical Research: Space Physics ( ) 88.A6 (1983):

29 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