Thursday, May 14, 2009Cluster Workshop – UppsalaR. J. Strangeway – 1 The Auroral Acceleration...
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Transcript of Thursday, May 14, 2009Cluster Workshop – UppsalaR. J. Strangeway – 1 The Auroral Acceleration...
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 1
The Auroral Acceleration Region: Lessons from FAST, Questions for
Cluster
Robert J. StrangewayInstitute of Geophysics and Planetary Physics
University of California, Los Angeles
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 2
Outline
Overview of our understanding of M-I coupling from a FAST perspective
• Force balance and field-aligned currents (FACs)
Magnetosphere – MHD force balance
Vorticity and Alfvén waves
Ionosphere – vorticity again
• Auroral Acceleration Region
The three types of aurora
• FAST-THEMIS – Substorm current wedge
Substorm current wedge consists of multiple FACs
Flow channel between FACs may be westward traveling surge
• Cluster-THEMIS – possible conjunction April 7, 2009
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 3
MHD Force Balance – Lorentz Force
€
Collisionless MHD Momentum: ρ dUdt
= j×B−∇P
€
Frozen−in Ion Condition: E+U×B=0
€
Ionosphere : n miνinUi−Un( )+meν en Ue −Un( )
⎡
⎣ ⎢
⎤
⎦ ⎥=F+ j×B
€
Maxwell Stress : j×B=−∇ B2
2μ0+B⋅∇ ⎛ ⎝ ⎜ ⎞
⎠ ⎟B
μ0
€
Frozen−in Electron Condition: E+U×B− j×B/ne=0
but j⋅j×B=0, Hall term does no work
⎡
⎣
⎢ ⎢ ⎢
⎤
⎦
⎥ ⎥ ⎥
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 4
Field-Aligned Currents – Magnetosphere
Plasma momentum equation – force balance – leads to a fundamental source of field-aligned currents
Following Hasegawa and Sato [1979], and D. Murr, Ph. D. Thesis “Magnetosphere-Ionosphere Coupling on Meso- and Macro-Scales,” 2003:
Assumptions: •j = 0, E + UxB = 0•j = 0 implies that displacement current can be neglectedE + UxB = 0 only enters through the vorticity terms
B•∇j•BB2 =2
B•∇P×∇BB3
+ 1B2 B×ρ
dUdt •∇VA
2
VA2
+ ρB2 B•dωdt −ω•dBdt
Vasyliunas’ pressure gradient term
Inertial term
Vorticity dependent terms (ω =U)
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 5
Vorticity – Alfvén Wave
If we assume in the general FAC equation that field-aligned vorticity is the dominant term and linearize, then:
€
∂j||∂z ≈ ρ
B∂ω||∂t
From curl of Faraday’s law and Ampere’s law:
€
∇×∇×E ⎛ ⎝ ⎜ ⎞
⎠ ⎟=−μ0
∂j∂t
Frozen-in condition:
€
−∇×E=∇× u×B ⎛ ⎝ ⎜ ⎞
⎠ ⎟= B⋅∇ ⎛
⎝ ⎜ ⎞
⎠ ⎟u−B∇⋅u( )
€
−∇×∇×E ⎛ ⎝ ⎜ ⎞
⎠ ⎟=μ0
∂j∂t
= B⋅∇ ⎛ ⎝ ⎜ ⎞
⎠ ⎟ω+∇×B∇⋅u( )
Parallel component:
€
μ0∂j||∂t =B
∂ω||∂z ∴ VA
2∂2ω||∂z2 =
∂2ω||∂t2
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 6
Field-Aligned Currents – Ionosphere
€
j×B=ρν in(U−Un)
Lorentz force moves plasma against frictional drag
€
(B⋅∇)j−(j⋅∇)B=ρν in(ω−ωn)−(U−Un)×∇ρν in
Taking curl:
FAC small vorticity conductivity gradients
Strong relationship between field-aligned vorticity and field-aligned currents
In ionosphere relationship depends on conductivity – impedance mismatch
Parallel to B
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 7
Ionospheric Current Closure
Iijima & Potemra [1978] published maps of the currents flowing into and out of the ionosphere
Higher latitude currents are called “Region 1”
Lower latitude currents are “Region 2”
Ionospheric Pedersen closure currents (red) provide the Lorentz force that moves plasma (and magnetic flux) over the polar cap, and returns flux at lower latitudes
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 8
Auroral Acceleration Region
Electron Energy
Electron Pitch Angle
Ion Pitch Angle
Ion Energy
Magnetic Field Arrows show FAC
Electrons carry FACs
Magnetic field deviations correspond to ionospheric flows
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 9
Implied Flow Pattern
Delta-B’s are projected to the ionosphere
Delta-B’s correspond to bending of field lines, pulling ionosphere through atmosphere
In the northern hemisphere flows are anti-parallel to the delta-B’s
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 10
FAST Observations – Three Types of Aurora
Auroral zone crossing shows:
Inverted-V electrons (upward current)
Return current (downward current)
Boundary layer electrons
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 14
Haerendel [2008] – Current TransformersModified version of Cowling conductivity
Some Issues –
Sp,out = jH•Ei, balanced by excess Hall current in direction of auroral electrojet (AEJ) and enhanced Sp,in, no net change
AEJ is assumed to be a Pedersen current
But I agree with the idea that Hall currents and conductivity gradients can result in additional FACs – part of the feedback process
Haerendel has also emphasized the “fracture” zone associated with parallel potentials
Note: j|| in this sketch are secondary FACs (!)
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 15
Substorm Current Wedge – FAST/THEMIS
Substorm current wedge and partial ring current [Sergeev et al., 1996]
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 16
Westward Traveling Surge
Akasofu [1964] Marklund et al. [1998], Freja data
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 17
FAC Structure in the Bulge
Hoffman et al. [1994] – DE-2 Observations
Where are the substorm current wedge currents?
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 18
March 23, 2007 Substorm
• FAST and THEMIS in same local time sector ~ 21 MLT
• THEMIS footprint in the south
• FAST footprint in the north
THE (P4)
THA (P5)THB (P1)THD (P3)
THC (P2)
FAST
-x
z
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 19
FAST Observations
• Westward flow channel
• Inverted-V electrons carry upward current
• Low energy electrons carry downward current
• Net current upward, away from ionosphere
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 21
THEMIS Observations
Green trace shows westward component
Two FACs, with net positive change
EquatorTo Earth
dB
s/c
Balanced currents, s/c can cross in either direction
LHS consistent with FAST, and movement of plasmasheet over the s/c – net outward FAC
Unbalanced currents, s/c direction matters
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 22
WTS and Substorm Current Wedge
• THEMIS observed pair of field-aligned currents, with net current out of the ionosphere, resulting in dipolarization – western leg of substorm current wedge
• FAST observed pair of FACs and a flow channel. Dominant signature, but there is a small net outward current
• MHD simulations show that the paired FACs are also associated with a rapid westward motion of the region of electron precipitation, is this the WTS?
• Implication: The upward current for the substorm current wedge is not a single FAC, but is more complex (See also Hoffman et al. [1994])
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 23
Another WTS Example
Electron Energy > 30 keV
Upgoing electrons
Change in plasmasheet ion energy flux, boundary?
Peak in magnetic field perturbation
Polar cap Eastward flow
W’ward flow
Eastward flow
Strong downward current
Low latitude eastward flow, note FAC at low latitude edge
What drives this flow?
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 25
Cluster Results – Temporal Evolution
Marklund et al. [Nature, 414, 724-727, 2001] present Cluster results showing temporal evolution of downward current region
Top panel shows electric field – Bipolar structure that grows and then decays away, width constant
Bottom panel shows downward current density – Peak of current density co-located with electric field reversal, but current widens as a function of time
How does this evolve in the ionosphere? What happens in the magnetosphere?
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 26
Cluster-THEMIS Conjunction
April 7, 2009, 05:05 UT
THEMIS-A, -D, -E ~ 22 LT, near apogee
Cluster-1, -2, -4 near perigee, roughly same local time sector
Possible conjunction
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 27
THEMIS overview, April 7, 2009
Psuedo-AE
Athabasca Keogram
Magnetic Field (GSE coords)
Flow velocity, blue positive sunward
Ions (ESA + SST)
THEMIS sees an activation signature around 05:05 UT
Electrons (ESA + SST)
Thursday, May 14, 2009 Cluster Workshop – Uppsala R. J. Strangeway – 28
Summary
• Multi-platform studies are essential in understanding the role of field-aligned currents and the auroral acceleration region in magnetosphere-ionosphere coupling
• Lessons from FAST:
Aurora come in a variety of forms – probably a signature of different stages of the M-I coupling process
Understanding the substorm current wedge and westward traveling surge benefits from a combination of multi-platform observations and simulations
• Questions for Cluster
Cluster may be able to take on the role of FAST in M-I coupling studies
Candidate conjunction event April 7, 2009