The Auroral Acceleration Region: Lessons from FAST, Questions for Cluster

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


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


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

⎡

⎣

⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥


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)


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


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


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


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


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


FAST Observations – Three Types of Aurora

Auroral zone crossing shows:

Inverted-V electrons (upward current)

Return current (downward current)

Boundary layer electrons


Upward Current


Downward Current


Polar Cap Boundary


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 (!)


Substorm Current Wedge – FAST/THEMIS

Substorm current wedge and partial ring current [Sergeev et al., 1996]


Westward Traveling Surge

Akasofu [1964] Marklund et al. [1998], Freja data


FAC Structure in the Bulge

Hoffman et al. [1994] – DE-2 Observations

Where are the substorm current wedge currents?


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


FAST Observations

• Westward flow channel

• Inverted-V electrons carry upward current

• Low energy electrons carry downward current

• Net current upward, away from ionosphere


MHD FAC and Precipitation


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


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])


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?


Feb 16, 2006 Pi2 Data


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?


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


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)


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

Documents

The Auroral Acceleration Region: Lessons from FAST, Questions for Cluster