Magnetosphere-Ionosphere Coupling through Plasma Turbulence at High- Latitude E-Region Electrojet Y. Dimant and M. Oppenheim Tuesday, April 13, 2010 Center for Space Physics, Boston University Dynamical Processes in Space Plasmas Israel, April 2010

Outline Background and motivation Anomalous electron heating Nonlinear current; energy deposition 3-D and 2-D fully kinetic modeling of E-region instabilities Anomalous conductivity Conclusions; future work

Solar Corona Solar Wind Ionosphere Magnetosphere Inner Boundary for Solar-Terrestrial System

Earth’s Ionosphere

What’s going on? Field-aligned (Birkeland) currents along equipotential magnetic field lines flow in and out. Mapped DC electric fields drive high-latitude electrojet (where Birkeland currents are closed). Strong fields also drive E-region instabilities: turbulent field coupled to density irregularities. –Turbulent fields give rise to anomalous heating. –Density irregularities create nonlinear currents. These processes can affect macroscopic ionospheric conductances important for Magnetosphere- Ionosphere current system.

Motivation How magnetospheric energy gets deposited in the lower ionosphere? Global magnetospheric MHD codes with normal conductances often overestimate the cross-polar cap potential (about a factor of two). Anomalous conductance due to E-region turbulence can account for discrepancy!

Strong electron heating Reproduced from Foster and Erickson, mV/m

(Reproduced from Stauning & Olesen, 1989)

Anomalous Electron Heating (AEH) Anomalous heating: Normal ohmic heating by E 0 cannot account in full measure. Farley-Buneman, etc. instabilities generate  E. Heating by major turbulent-field components  E   B is not sufficient. Small  E || || k || || B, |  E || |<<|  E  |, are crucial: –Confirmed by recent 3-D PIC simulations.

Analyitical Model of AEH Dimant & Milikh, 2003: –Heuristic model of saturated FB turbulence (HMT), –Kinetic simulations of electron distribution function. Difficult to validate HMT by observations: –Radars: Pro: Can measure k || (aspect angle ~ 1 o ), Con: Only one given wavelength along radar LOS. –Rockets: Pro: Can measure full spectrum of density irregularities and fields, Con: Hard to measure E || ; other diagnostic problems. Need advanced and trustworthy 3-D simulations!

PIC simulations: electron density E 0 x B direction E 0 direction

3D simulations 256x256x512 Grid256x256x512 Grid Lower Altitude (more collisional)Lower Altitude (more collisional) Driving Field: ~4x Threshold field (150 mV/m at high latitudes)Driving Field: ~4x Threshold field (150 mV/m at high latitudes) Artificial e - mass: m e:sim = 44m e ;Artificial e - mass: m e:sim = 44m e ; ExB direction (m) E 0 direction (m) B 0 direction (m) Potential (x-y cross-section) Potential (x-z cross-section) 4 Billion virtual PIC particles 4 Billion virtual PIC particles 2D looks the same! 2D looks the same!

Higher altitude 3D simulation Ions: First Moment (RMS Of V i ) electrons: First Moment (RMS Of V e ) 3-D Temps 2-D Temps

Anomalous heating [Milikh and Dimant, 2003] E = 82 mV/m (comparison with Stauning and Olesen [1989])

Cross-polar cap potential (Merkin et al. 2005)

Anomalous Electron Heating (AEH) Affects conductance indirectly: –Reduces recombination rate, –Increases density. All conductivities change in proportion. Inertia due to slow recombination changes: –Smoothes and reduces fast variations. Can account only for a fraction of discrepancy. Need something else, but what?

Nonlinear current (NC) Direct effect of plasma turbulence: –Caused by density irregularities,  n. Only needs developed plasma turbulence – no inertia and time delays. Increases Pedersen conductivity (|| E 0 ) –Crucial for MI coupling! Responsible for the total energy input, including AEH.

Characteristics of E-region waves Electrostatic waves nearly perpendicular to Low-frequency, E-region ionosphere (90-130km): dominant collisions with neutrals - Magnetized electrons: - Demagnetized ions: Driven by strong DC electric field, Damped by collisional diffusion (ion Landau damping for FB)

electrons ions Two-stream conditions (magnetized electrons + unmagnetized ions)

Wave frame of reference _ + _ + ___ ___ ____ IonsElectrons

Nonlinear Current

Mean Turbulent Energy Deposit Work by E 0 on the total nonlinear current Buchert et al. (2006): –Essentially 2-D treatment, –Simplified plasma and turbulence model. Confirmed from first principles. Calculated NC and partial heating sources: –Full 3-D turbulence, –Arbitrary particle magnetization, –Quasi-linear approximation using HMT.

Anomalous energy deposition Nonlinear current: is total energy source for turbulence! How 2-D field and NC can provide 3-D heating? Density fluctuations in 3-D are larger than in 2-D!

3-D vs. 2-D, Densities

Nonlinear current (NC) Mainly, Pedersen current (in E 0 direction). May exceed normal Pedersen current. May reduce the cross-polar cap potential. Along with the anomalous-heating effect, should be added to conductances used in global MHD codes for Space Weather modeling

E-region turbulence and Magnetosphere-Ionosphere Coupling Anomalous electron heating, via temperature- dependent recombination, increases electron density. Increased electron density increases E-region conductivities. Nonlinear current directly increases mainly Pedersen conductivity. Both effects increase conductance and should lower cross polar cap potentials during magnetic storms. Could be incorporated into global MIT models.

Conclusions Theory & PIC simulations: E-region turbulence affects magnetosphere-ionosphere coupling: –(1) Anomalous electron heating, via temperature-dependent recombination, increases electron density. Increased electron density increases E-region conductivities. –(2) Nonlinear current directly increases electrojet Pedersen conductivity. Responsible for total energy input to turbulence. –Both anomalous effects increase conductance and should lower cross-polar cap potentials during magnetic storms. Will be incorporated into a global MHD model.

Fully Kinetic 2-D Simulations Simulations Parameters: Altitude ~101km in Auroral region Driving Field: ~1.5 Threshold field (50 mV/m at high latitudes) Artificial e - mass: m e:sim = 44m e ; m i:sim =m i 2-D Grid: 4024 cells of 0.04m by 4024 cells of 0.04m Perpendicular to geomagnetic field, B 8 Billion virtual PIC particles Timestep: dt =  s (< cyclotron and plasma frequencies) E 2 (V/m) 2 Time (s) ExB direction (m) E 0 direction (m)

Threshold electric field 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 High-latitude ionosphereEquatorial ionosphere [Dimant & Oppenheim, 2004]

3-D vs. 2-D, Temperatures 3-D Simulations get hotter!3-D Simulations get hotter! Electron Moments Ion Moments 3-D Temp 2-D Temp Time (s)

‘5-moment’ transport equations Fluid-model equations for long-wavelength waves: they do not include heat conductivity, Landau damping, etc., but contain all essential factors.