Reconnection and its Relation to Auroral Physics

Observation and Theory

Uppsala, April 2004

Magnetospheric Field Line Structure (Empirical Tsyganenko Model)

X (RE)

Z (R

E)

Solar Wind

B

X-point

Magnetopause

Magnetosheath

Bow Sho

ck

Lobes

1

3

2

1

3

The Meaning of Reconnection

Axford 1984

Generalized Ohm´s Law(Fluid Approach)

E + v B - j = (0pe2)-1t j + (jv + vj – (en)-1j j)} + (en)-1{ j B - Pe + Fepmf

Inertial term Hall term Wave pmf

In quasi-equilibrium the electron pressure gradient term is the ion pressure term, for then:

j B - Pe ·Pi

Assumptions: two-fluid (protons/electrons)

ideal conditions ~ collisionless

me/mi <<1, 0

[ Wave ponderomotive force usually neglected without justification (?)

May be important in a turbulent plasmasheet ]

Dispersion Relations

No guide field:Alfvén whistler

With guide field:Kinetic Alfvén wave

Wang et al. JGR 105, 2000

Estimates of Reconnection Rate

No guide field: HALL With guide field: Pressure

Reconnection Models

• Sweet-Parker resistive• Petschek resistive• Hill variant of Petschek • Sonnerup mixed non-resistive (Hall)• Simulations

– Resistive – Collisionless

• Hybrid – Vlasov – Full-Particle

Magnetospheric Requirements

• Location outside ionosphere• Total non-collisionality mfp ~ 1 AU• No Parker-Sweet• Petschek only if anomalous an=e2n/mean

– Localized resistivity– Problem of generation of anomalous collisions – No strong wave activity observed so far !– Reconnection is (probably) collisionless

• Bursty Bulk Flows |v| ~ vA• Generation of Field-Aligned Currents• Acceleration of Ions and Electrons < 300 keV

• Fast reconnection (electron scales)

Runov et al. 2003

Jetting and Field Line Curvature(Cluster Tail Observations)

Frey et al. (2003)

Poleward Reconnection for Northward IMF

(Cluster Observations)

Geotail/Equator-S Conjunction

Phan et al., Nature 404, 848, 2000

Magnetopause Reconnection

Phan et al., Nature 404, 848, 2000

Conditions for Hall Effect• Hall effect exists only in region with distinct separation of electron and

ion motion• Hence in region where by some external means (e.g. geometry) the

ions remain unmagnetized while the electrons are magnetized• The required motion is the normal E B drift in the collisionless case• Otherwise also pressure gradient drifts contribute when the

transverse pressure gradient generates a transverse electric potential • RECONNECTION IS IDEALLY SUITED FOR HALL EFFECT IN RANGE

e < L < i

around the X-lineas scales imposed by reconnection geometry herei.e. ions do really decouple from electron motion

with electrons remaining frozen-in and moving inward towards the X-linewhere they locally decouple on scale

L < e

Reconstruction of Hall Current System in the Magnetotail (Nagai et al., 1998, 2001)

Electron Hall Current System i

Unmagnetised Ions

Unmagnetised Electrons

e

Hall-Current System

jH = 0

jH = 0

jH = 0

jH 0

Hall Currents

Closure of Hall Currents Via Field Aligned Currents

O

O

Relation between Hall/FACs and Field-aligned Electron Fluxes in Tail Reconnection

Vin = E B

vout ~ vA

Hall Current jH

FACsdownward

upward

no FAC

upward Electrons

downward Electrons

Slow EB inflow implies narrow region of downward FAC/upward e-

Fast reconnection outflow implies broad region of upward FAC/

downward e- - Fluxes (in this model) equatorward

B

Hall-Effect in Magnetotail 1

Nagai et al., JGR 106, 25929, 2001

Received 12. July 2000

Hall-Effect in Magnetotail 2

Oieroset et al., Nature 412, 416, 2001 Received 1. May 2001

Hall-Electron Distribution

Asano et al., JGR 109, A02212, 2004

Schematics of Tail-Hall-Region

Magnetopause Reconnection

Mozer et al., PRL 89, 2002

Electron Acceleration in Magnetotail Reconnection

Oieroset et al. (2002)

FAC‘s connected to Hall Current

Wrong !No Hall current !

Reconnection Region

Acceleration of Electrons

Lower-hybrid Waves at Magnetopause

Bale et al., GRL 24, 2180, 2002

Lower-Hybrid-Drift Instability

Shinohara et al., PRL 87, 2001

Lower-hybrid Drift Waves without and with Guide Field

Scholer et al. PoP 10, 3521, 2003

Normal Magnetic Component in 3D

Scholer et al. PoP 10, 3521, 2003

no guide field with guide field

3D-Tail-Reconnection

Pritchett & Coroniti JGR 109, 2004 Scholer et al. PoP 10, 3521, 2003

Distribution Functions

Drake et al. Science 299, 2003Scholer et al. PoP 10, 3521, 2003

With guide field

Pritchett‘s 3D Simulation DistributionsStack plot of E||

Propagating waves Heating and acceleration

Electron Velocity and E||

Pritchett & Coroniti 2004

3D-Reconnection Electron Distributions

Pritchett & Coroniti 2004

outside X-line in X-line

Guide Field SimulationDrake et al. Science 299, 2003

Electric fields in guide field caseDrake et al. Science 299, 2003

Non-Hall Reconnection me=mi

Schematic view Initialization

z

y

x

Lx

Ly

0

Jaroschek et al. 2004

Reconnection Without Hall Effect: The Case mi = me

Magnetic Field Electric Induction Field Wave Electric Field

— Evolution of magnetic islands (primary and secondary x-points)

— Evolution of DC electric induction fields in regions of field conversion

— Finite extent of DC electric field in the third (y) dimension

— Evolution of Buneman and Drift Modes in the xy-plane

— Particles accelerated in induction and wave electric field

xz-plane xz-plane xy-plane

Jaroschek et al. 2004

Acceleration in No-Hall 3D-reconnection

Jaroschek et al. PoP 11, 2004

3D Fields in Reconnection

Jaroschek et al. PoP 11, 2004

Auroral zone physics

Ergun et al. PoP 9, 2002

Auroral zone physics

Ergun et al. PoP 9, 2002

Electric field Electron distribution

Evidence for Hall Region-Aurora Coupling

• Observed sequence in auroral current and flux

• Narrow upstream (downward current) electron flux regions versus broad (upward current) downstream (inverted V-event) regions

• Downward electrons High energies (accelerated)

• Upward electrons Low energies (ionospheric)

Ionospheric Signature of FA-Currents An Example from FAST

B

Field-aligned Currents

Electron Flux

downward downwardupward

upward upwarddown

{e-

80 seconds

Ionospheric Signature of FA-Currents An Example from FAST

J

J

J

Low (ionospheric) energies

High (accelerated) energies

No flux-no FAC

Electron Distributions

Oieroset et al., PRL 2002

Treumann et al., PoP 2004

Hu and Sonnerup JGR 108, 2003

Magnetopause Reconstruction

Lyon, Science 288, 2000

Nagai et al. (2002)

Tail-Hall-Reconnection Parameters

Kink-Mode Formation in Reconnection

Collisionless Reconnection Scaling

2D-Current Layers in Reconnection

Lyon, Science 288, 2000

Ion current

Electron current

Dispersive Waves in Reconnection

Rodgers et al., PRL 87, 2001

Lower-hybrid Driven Reconnection

Shinohara et al. PRL 87, 2001

Hall-MHD-Simulations

Wang et al. JGR 105, 2000

J||

E||

Reconnection with guide field

Ey

Jy

Guide field in the simulation

By

Bz

Nonsymmetric MP Reconnection

Ergun et al. PoP 9, 2002

Auroral zone physics

Wind Observations of Hall Effect

Øieroset et al. (2001)

Hall Field

Particles in Hall-Reconnection

Asano et al., 2004

3D Hall Region at Magnetopause

Mozer et al. (2002)

Hall-By Field

3D-Signature in Hall Ex

Polar