Y. S. Dimant and M. M. Oppenheim Center for Space Physics, Boston University dimant@bu

1. Anomalous ionospheric conductances caused by plasma turbulence in high-latitude E-region electrojets Y. S. Dimant and M. M. Oppenheim Center for Space Physics, Boston University dimant@bu.edu Session SA33A: Anomalous ionospheric conductances caused by plasma turbulence in high-latitude E-region electrojets Wednesday, December 15, 20101:40PM – 6:00PM Paper SA33A-2165 2012 AGU Fall Meeting Monday–Friday, December 3–7, 2012, San Francisco, California, USA

2. Abstract During periods of intense geomagnetic activity, electric fields penetrating from the Earth's magnetosphere to the high-latitude E-region ionosphere drive strong currents named electrojets and excite plasma instabilities. These instabilities give rise to plasma turbulence that induces nonlinear currents and strong anomalous electron heating observed by radars. This plays an important role in magnetosphere-ionosphere coupling by increasing the ionospheric conductances and modifying the global energy flow. The conductances determine the cross-polar cap potential saturation level and the evolution of field-aligned (Birkeland) currents. This affects the entire behavior of the near-Earth plasma. A quantitative understanding of anomalous conductance and global energy transfer is important for accurate modeling of the geomagnetic storm/substorm evolution. Our theoretical analysis, supported by recent 3D fully kinetic particle-in-cell simulations, shows that during strong geomagnetic storms the inclusion of anomalous conductivity can more than double the total Pedersen conductance - the crucial factor responsible for magnetosphere-ionosphere coupling through the current closure. This helps explain why existing global MHD codes developed for predictive modeling of space weather and based on laminar conductivities systematically overestimate the cross-polar cap potentials by a factor of two or close.

3. Motivation • Global magnetospheric MHD codes with normal conductances often overestimate the cross-polar cap potential (up to a factor of two). • During magnetic (sub)storms, strong convection DC electric field drives plasma instabilities in the E region • E-region instabilities create turbulence: density perturbations coupled to electric field modulations • Anomalous conductance due to E-region turbulence could account for the overestimate of the cross-polar cap potential.

4. Location: Lower Ionosphere

5. Energy flow in Solar-Terrestrial System Solar Corona Solar Wind Magnetosphere Ionosphere

6. Magnetosphere-Ionosphere Coupling

7. Anomalous conductivity • Instability-driven plasma density irregularities coupled to turbulent electrostatic field: • 1: Turbulent field gives rise to anomalous electron heating (AEH). Reduced recombination leads to plasma density increases. • 2: Electron density irregularities and turbulent electrostatic fields create wave-induced nonlinear currents (NC). • Both processes affect macroscopic ionospheric conductancesimportant for Magnetosphere-Ionosphere current system.

8. Anomalous electron heating During magnetospheric storms/substorms, E-region turbulence at the high latitude electrojet heats up electrons dramatically, affecting ionospheric conductance. This temperature elevation is induced mainly by turbulent electric fields. The small turbulent field component parallel to B0 plays a crucial role. 25 mV/m 125 mV/m Recent observation: (at higher latitudes) Te > 4000K at E0=160 mV/m (Bahcivan, 2007) (Foster and Erickson, 2000)

9. (Stauning & Olesen, 1989, E0=82 mV/m)

10. Characteristics of E-region Waves • Electrostatic waves nearly perpendicular to • Low-frequency, • E-region ionosphere (90-130km): dominant collisions with neutrals • - Magnetized electrons: (ExB drift) • - Unmagnetized ions: (Attached to neutrals) • Waves are driven by strong DC electric field, • Damped by collisional diffusion (ion Landau damping for FB)

11. Major E-region instabilities Driven by large-scale DC electric field • Farley-Buneman (two-stream) instability Caused by ion inertia • Gradient drift (cross-field) instability Caused by density gradients • Thermal (electron and ion) instabilities Caused by frictional heating Ion kinetic effects are crucial: need PIC simulations Small parallel fields are important: need 3-D simulations!

12. Threshold electric field Equatorial ionosphere High-latitude ionosphere FB: Farley-Buneman instability IT: Ion thermal instability ET: Electron thermal instability CI: Combined (FB + IT + ET) instability 1: Ion magnetization boundary 2: Combined instability boundary [Dimant & Oppenheim, 2004]

13. AEH: Heuristic Model of Turbulence (comparison with Stauning and Olesen [1989]) E = 82 mV/m [Milikh and Dimant, 2003]

14. Plasma Heating (PIC simulations)

15. Ionization-Recombination Mechanism • Turbulent electric fields heat electrons. • Elevated electron temperature does not affect conductivities directly, but … • Hot electrons reduce plasma recombination rate. • Reduced recombination (presumed given ionization source) increases E-region plasma density. • Higher plasma density increases all conductivities in proportion. • Not sufficient and slowly developing (tens of seconds) mechanism!

16. Test LFM Simulation with Modified Conductivities: Cross-Polar Cap Potential (Merkin et al. 2005) ANEL:ANomalous ELectron heating recombination-density effect on conductivities

17. Non-Linear Current • FB turbulence: electron density perturbations (ridges and troughs) with oppositely directed turbulent electrostatic fields. • E x B drift of magnetized electrons has opposite directions in ridges and troughs. • More electrons drift in ridges than in troughs. • This forms an average DC current, mainly in the Pedersen to E0 direction. • The modified Pedersen conductivity is most important for current closure. • Fast-developing and robust mechanism!

18. Quasi-stationary waves Ions Electrons _ _ _ _ + + + _ _ _ _ + + + + _ _ _ _ + + + +

19. Farley-Buneman Turbulence (PIC simulations)

20. Non-Linear Current

21. NC and M-I Energy Exchange (including Anomalous Heating) • Energy deposition for E-region turbulence and heating: • Total energy input from fields to particles: • Normal Joule heating: • Saturated turbulence in a periodic box: • Turbulent energy: work by external field E0 on wave-induced nonlinear current, • Small turbulent fields parallel to B0 are crucial for anomalous electron heating!

22. Anomalous Pedersen Conductivity (extreme convection field) 0: Undisturbed (“normal”) conductivity 1: Anomalous conductivity with nonlinear current (NC) 2: Anomalous conductivity with NC + AEH effect [Dimant and Oppenheim, 2011]

23. Anomalous Pedersen Conductivity (strong convection field) 0: Undisturbed (“normal”) conductivity 1: Anomalous conductivity with nonlinear current (NC) 2: Anomalous conductivity with NC + AEH effect [Dimant and Oppenheim, 2011]

24. Conclusions • Convection field drives E-region instabilities: • Turbulent fields cause anomalous heating • Irregularities and fields create nonlinear current • Both anomalous effects lead to increased conductances • Can explain lower than in conventional models values of cross-polar cap potentials • Should be included in global MHD models!