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

Anomalous ionospheric conductances caused by plasma turbulence in high-

latitude E-region electrojets

Y. S. Dimant and M. M. OppenheimCenter for Space Physics, Boston University

dimant@bu.edu

2012 AGU Fall Meeting Monday–Friday, December 3–7, 2012, San Francisco, California, USA

AbstractDuring 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

Solar CoronaSolar Corona Solar WindSolar WindIonosphereIonosphereMagnetosphereMagnetosphere

Energy flow in Solar-Terrestrial System

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 Magnetosphere-Ionosphere current system.

Anomalous electron heating

(Foster and Erickson, 2000)

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.

125 mV/m25 mV/m(at higher latitudes)

Te > 4000K at E0=160 mV/m (Bahcivan, 2007)

Recent observation:

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

Characteristics of E-region Waves

• Electrostatic waves nearly perpendicular to

• Low-frequency,

• E-region ionosphere (90-130km): dominant collisions with neutrals

- Magnetized electrons: (E x B drift)

- Unmagnetized ions: (Attached to neutrals)

• Waves are driven by strong DC electric field,

• Damped by collisional diffusion (ion Landau damping for FB)

0 ||, k kB

ene

ini

0 0E B

en

Major E-region instabilities

• 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

Driven by large-scale DC electric field

Ion kinetic effects are crucial: need PIC simulations

Small parallel fields are important: need 3-D simulations!

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]

100 105 110 115 120 125 130 135h,km

500

1000

1500

2000

2500

3000

3500radareffT

AEH: Heuristic Model of Turbulence

eT

iT

0T

[Milikh and Dimant, 2003] E = 82 mV/m

(comparison with Stauning and Olesen [1989])

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 Current1. 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

0E

_+_

+_ __

++ + +

_ _ _

+

+ + +

+

__ _ _

E

E

0n

20000 BBEV

Ions

0n

0n

0n

Phin, VV

Electrons

0B

iii

eee m

e

m

e

E

VE

V e

Ped2Ped ,

Farley-Buneman Turbulence (PIC simulations)

00 BE

0E

E

E

-e

-eNLJ

Non-Linear Current

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!

jE

000 jE W

eine VVjjjj NLNL0

0 jE

NL0AEH jE W

Anomalous Pedersen Conductivity

0: Undisturbed (“normal”) conductivity

1: Anomalous conductivity with nonlinear current (NC)

2: Anomalous conductivity with NC + AEH effect

[Dimant and Oppenheim, 2011]

(extreme convection field)

Anomalous Pedersen Conductivity

0: Undisturbed (“normal”) conductivity

1: Anomalous conductivity with nonlinear current (NC)

2: Anomalous conductivity with NC + AEH effect

[Dimant and Oppenheim, 2011]

(strong convection field)

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!