Transcript Slide 1
Anomalous ionospheric conductances caused by plasma turbulence in highlatitude E-region electrojets Y. S. Dimant and M. M. Oppenheim Center for Space Physics, Boston University [email protected] Session SA33A: Anomalous ionospheric conductances caused by plasma turbulence in high-latitude E-region electrojets Wednesday, December 15, 2010 1:40PM – 6:00PM Paper SA33A-2165 2012 AGU Fall Meeting Monday–Friday, December 3–7, 2012, San Francisco, California, USA 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. 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. Location: Lower Ionosphere Energy flow in Solar-Terrestrial System Solar Corona Solar Wind Magnetosphere Ionosphere Magnetosphere-Ionosphere Coupling 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 conductances important for MagnetosphereIonosphere current system. Anomalous electron heating During magnetospheric storms/substorms, Eregion 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 (at higher latitudes) 125 mV/m (Foster and Erickson, 2000) Recent observation: Te > 4000K at E0=160 mV/m (Bahcivan, 2007) (Stauning & Olesen, 1989, E0=82 mV/m) Characteristics of E-region Waves • Electrostatic waves nearly perpendicular to B0 , k|| k • Low-frequency, en • E-region ionosphere (90-130km): dominant collisions with neutrals - Magnetized electrons: e en (E x B drift) - Unmagnetized ions: i in (Attached to neutrals) • Waves are driven by strong DC electric field, E0 B0 • Damped by collisional diffusion (ion Landau damping for FB) 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! Threshold electric field Equatorial ionosphere High-latitude ionosphere FB: Farley-Buneman instability 1: Ion magnetization boundary IT: Ion thermal instability 2: Combined instability boundary ET: Electron thermal instability CI: Combined (FB + IT + ET) instability [Dimant & Oppenheim, 2004] AEH: Heuristic Model of Turbulence (comparison with Stauning and Olesen [1989]) 3500 3000 radar eff T 2500 Ti 2000 1500 Te 1000 500 T0 100 105 110 115 120 125 130 135 h, km E = 82 mV/m [Milikh and Dimant, 2003] Plasma Heating (PIC simulations) 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! Test LFM Simulation with Modified Conductivities: Cross-Polar Cap Potential (Merkin et al. 2005) ANEL: ANomalous ELectron heating recombination-density effect on conductivities Non-Linear Current 1. FB turbulence: electron density perturbations (ridges and troughs) with oppositely directed turbulent electrostatic fields. 2. E x B drift of magnetized electrons has opposite directions in ridges and troughs. 3. 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! Quasi-stationary waves B0 Ions E n 0 _ + _ _+ E0 Electrons E _ _+ VePed + _ + n 0 V n,i V Ph _ + _ + n 0 _ + _ _ + _+ + n 0 2 V0 E0 B0 B0 e E e 2 , mee ViPed eE mi i Farley-Buneman Turbulence (PIC simulations) E Non-Linear Current E e - E0 E 0 B0 e J NL - NC and M-I Energy Exchange (including Anomalous Heating) • Energy deposition for E-region turbulence and heating: – Total energy input from fields to particles: E j – Normal Joule heating: W0 E0 j0 – Saturated turbulence in a periodic box: E j 0 – Turbulent energy: work by external field E0 on waveinduced nonlinear current, WAEH E0 jNL j j0 jNL jNL e n Vi Ve • Small turbulent fields parallel to B0 are crucial for anomalous electron heating! 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] 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] 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!