Alfvén Wave Generation and Dissipation

Leading to High-Latitude Aurora

W. LotkoDartmouth College

Genesis

Fate

Impact

A. Streltsov, M. WiltbergerDartmouth College

SM 52B-08

20

60

182

549

1657

4999

Rayleighs

75

ILA

T

70

65

30 Jan 1998

Substorm Onsets

Rankin & Gillam MPA

1 3 5 7 9 11 13

UT, hours

557.7 nm

VIS Low-Resolution Camera, 557.7 nm

Lyons et al. ‘01

Equatorial Noon-Midnight

ExEx

Power at 1.3 mHz in electric field Ex (GSM) from LFM global MHD.

Fourier transforms are computed from time interval 0900-1200 UT.

Wiltberger et al. ‘02

10 Jan 1997

Goodrich et al. ‘98

1

0 “Fast Mode” Energy

zzmp

zzmp

6 5 4 3 2 1 00

1

“Alfvénic” Energy

x/zmp Earthward

Allan and Wright ‘00

t/mp

vz

10

Disturbance

Time Step t = 6 mp

Earthward Propagation of “Plasma Sheet” Disturbances

Characteristics Parameters

vLobe = 2600 km/s

zmp = 25 RE

mp = 1 min

Fast-Alfvén mode coupling: ky = 1.3

Plasma = 0 !

0

1

10 0.5vA/vLobe

zzmp

AlfvénSpeedProfile

Coupling Efficiency

Allan–Wright Simulation

0 2 4 6 8 10

t/tmp

0

.08

EA

T/E

FT

0.5

00 1 2

Abso

rpti

on

Kivelson and Southwood ‘86

L y

15 R

E

L y

60 R

E

2 2 2 3y Ak (k )Coupling

Parameter,

Phase Mixing, Dispersion and E||

Dispersive Alfvén Waves

/e E||/E

2 2

i e

2 2

i i

k k ρ T

k 1 k ρ T

2 2

e

2 2

e

k k λ

k 1 k λ

>> 1

Kinetic

<< 1

Inertial

Dispersion Lengths

Phase mixing: Lph

Ion gyroradius: = i(1+Te/Ti)

Inertial Length: e = c/pe

Phase Mixing Length

A

ph

2k (z)[x-v (z)t]

L (x,z,t) z

0 10

.001

100

1

5

Altitude, RE

2/

e2

0 10.1

100

z/zmp

L ph,

RE

PSBL LOBELysak and Carlson ‘81

Allen and Wright ‘98

x/zmp = 4, t/tmp = 6

Low-Altitude Dissipation

Streltsov et al. ‘01

= 0.4ci (1 – vc/|v||e|), |v||e| > 0 = 0

Lysak and Dum ‘83

100

10E, m

V/m

100 5 15

Altitude, RE

Low-Altitude Intensification

Streltsov et al. ‘01

Reflection Coefficient

ref 2 20 Ai P

inc 2 20 Ai P

E 1 μ v Σ (1 k d )

E 1 μ v Σ (1 k d )R

-2 2 2 2 ½0 Am pe

k c ωd μ v K 1 /( ) J|| = K ||

J = PE

inc

ref

0.1 1 10 100 1000

Wavelength, km

1

-1

0

Ref

lect

ion

C

oeff

icie

nt

Abs

orp

tion,

%

0

100Insulator

Conductor

vAm

vAi

d

2 RE

Vogt and Haerendel ’99Lysak and Carlson ‘81

Alfvén Wave

Absorption vs

Wavelength

Observed

Width of

Auroral Arcs

0.1 1 10 100

Arc Width, km

Knudsen et al. ‘01Maggs and Davis ‘68

Num

ber

of

Arc

s

0.1 1 10 100 1000

1

-1

0

Ref

lect

ion

C

oeff

icie

nt

Abs

orp

tion,

%

0

100

Wavelength, km

?

North-South Electric Field

East-West Magnetic Field

2 mho 5 mho

M-I Interaction

Alfvén wave FAC exceeds current- carrying capacity of lower m’sphere

E|| is induced to boost electron parallel flux

Accelerated electrons nonuniformly ionize E-layer

Gradients in induce quasi-electrostatic, inertial Alfvén waves at low altitude

Ionospheric Alfvénic fluctuations enhance Joule heating PE2, ion outflow

Reactive Ionosphere

Lotko and Streltsov ‘99

Ionosphere

Equator

Ponderomotive Ion Upwelling

via Alfvén Waves

ap|| = ¼||(E/B0)2

ap|| > ag at 1000 km altitude

when

E > 200 mV/m

Inertial M-I Coupling

Strangeway et al. ‘00

Li and Temerin ’93

SUMMARY

Genesis (magnetotail)

– CPS compressional disturbances shear Alfvén waves in PSBL

– Phase mixing in PSBL gradient creates smaller scale structure

Fate (low-altitude magnetosphere)

– Small k Ionospheric penetration, reflection

– Moderate k Strong absorption in collisionless E|| layer

– Large k Reflection at E|| layer, momentum transfer to electrons

Impact (ionosphere/thermosphere)

– Enhanced Joule heating

– Electron acceleration, 10-km scale auroral arcs

– Ionospheric activation Small-scale resonator Alfvén waves

– Ponderomotive lifting of ionospheric ions