reconnection and its relation to auroral physics
DESCRIPTION
Reconnection and its Relation to Auroral Physics. Observation and Theory Uppsala, April 2004. Magnetospheric Field Line Structure (Empirical Tsyganenko Model). Solar Wi nd. Magnetosheath. Z (R E ). Bow Shock. Lobes. 3. 2. X-point. 1. 1. 3. Magnetopause. B. X (R E ). - PowerPoint PPT PresentationTRANSCRIPT
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
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
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)
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
Electron Acceleration in Magnetotail Reconnection
Oieroset et al. (2002)
FAC‘s connected to Hall Current
Wrong !No Hall current !
Reconnection Region
Acceleration of Electrons
Distribution Functions
Drake et al. Science 299, 2003Scholer et al. PoP 10, 3521, 2003
With guide field
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
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)
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