Generation of the transpolar potential

Ramon E. LopezDept. of Physics

UT Arlington

2

How does the solar wind drive convection?

Dungey [1961]ReconnectionMost of the potential -up to hundreds of kV

Axford and Hines (1961)Viscous interaction~20-30 kV

Linear reconnection driving by the

solar wind

so

Transpolar Potential Saturation (storm main phases)

See also Ober et al., (2003), Hairston et al. (2003)

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Linear regime - Geoeffective length

• The solar wind voltage across the 32 Re Y-extent of the dayside magnetopause is 204 KV for every mV/m in the solar wind

• So the actual projection of the solar wind voltage onto the X-line (which extends from terminator to terminator) must be less

• From previous figure we get TP = 46*VBz + 15Solar wind projection is 7.2 Re in Y-extent

• What does the LFM do?

LFM MHD Simulation Potential

Viscous Potential increases with Solar Wind speed

The Potential has 2 parts (for now)Viscous Potential - Φv(V, n, Σp) We determine this for each parameter set ofruns, then subtract it from the total potential

Reconnection Potential - Φr(V, n, Σp, B)

The potential along the mergingline is the rate at which flux crosses the merging line.

LFM MHD Simulation Potential

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The geoeffective

length is directly

confirmed by following

plasma flow streamlines

from the solar wind

See also Merkin et al. (2005)

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What controls the projection of the solar wind on the X-line?

• The flow is determined by the total forces acting in the magnetosheath.

• When B in the solar wind gets large, the nature of the force balance changes from a plasma pressure-dominated flow to a magnetic stress-dominated flow.

• I argue that this transition is what controls the transition to the saturation of the transpolar potential

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Y-extent of streamlines intersecting X-line shrinks for beta<1

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Geoeffective lengths give Reconnection Potential

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Density dependence• Higher density

needs higher Bz to transition to beta<1 in sheath, hence larger potentials in the saturation regime

n = 8/cc, Bz = -10 nT

n = 5/cc, Bz = -10 nT

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Conductivity dependence

• Higher ionospheric conductivity results in greater magnetopause erosion, a thicker magnetosheath, lower beta in the sheath, more diversion of the flow, hence smaller a saturation potential

Σ = 5 mho, Bz = -10 nT

Σ = 10 mho, Bz = -10 nT

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Velocity dependence• Higher solar wind

speed produces a larger pressure force in the magnetosheath

• This reduces the geoeffective length in the solar wind

400 km/s

33.9 kV

8.3 RE

600 km/s

48.7 kV

5.9 RE

800 km/s

101.3 kV

4.0 RE

Solar WindSpeed

ViscousPotential

GeoeffectiveLength

Sound Speed dependence as well!

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LFM shows expected behaviors

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How does this agree/differ with the

Siscoe-Hill model?

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What are these potentials?Φm given by solar wind electric field times the geoeffective length

Φs given by the value of the Region 1 current that weakens the dayside field by about 50%

Region 1 takes over from the Chapman-Ferraro current and exerts force balance with the solar wind

The bow shock current

QuickTime™ and a decompressor

are needed to see this picture.

Where does the current go?

Look at the direction of the current in the volume at Z=0

Bz = -20 nTV = 400 km/sn = 5 Cs = 40 km/s

The magnetic force can be the largest force in the

magnetosheath if beta<1

Now we can understand the dependence on the geoeffective length

on beta and solar wind V

The larger the divergence of the flow, the smaller the geoeffective length.

Larger plasma pressure causes a greater divergence

When JxB takes over, a larger B causes a greater divergence

What about closure of the

bow shock current

through the ionosphere?

Thesecurrents

exist!

Lopez et al., 2008

JASTP

Vx = 400km/s, Vz = -150 km/s, Bz = -15 nT

φnorth > φsouth with Σpconstant. This cannot be due to reconnection!

More current flows to the north!

€

σ p

JyDensity

Driving via the Bow Shock GeneratorThe current in the bow shock is a generator

This dynamo current acts as a source for potential

Bz = -20 nT, V = 400 km/s, n = 5/ccCurrent streamlinesDensity color-coded

Interhemispheric asymmetry and the Convection Reversal Boundary

location for large southward IMF• Summer hemisphere has higher FAC, lower potential

relative to winter hemisphere• Convection reversal boundary in both hemispheres

located in open field line region - not at the boundary between open and closed field lines

• This is necessary since the reconnection potential must be the same in both hemispheres

Halloween storm observations are consistent

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Aug 10,2000

TextText

0 nT

0 nT

-13.5 nT nT

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Good northern hemisphere passClear convection pattern

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66.8˚66.5˚

Upward FAC

Closed 2-cell convection in the polar cap driven by closure of bow shock current

DMSP F13 path

Polar cap

Let’s not restrict ourselves to Bz<0Wilder et al. (2007, 2009) have shown saturation for northward IMF in SuperDarn observations

LFM saturates for large northward IMF

DMSP data do the same thing

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What about large By?

LFM exhibits saturation

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AIME and DMSP confirm it

VBy = 8 mV/mWell withinsaturation

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Sample DMSP Observations

VBy = 8.1 mV/m

ΦF13 = 99.2 kV

ΦF15 = 100.5 kV

F13

F15

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5 mho 20 mhoβ-dependent saturation onset

Reconsider the Siscoe-Hill model

The value of the saturation potential is lower for east-west IMF (and lower still for northward IMF)

Therefore Region 1 currents are lower for a By-saturated potential compare to a Bz-saturated one

Neither force balance nor dayside Region 1 magnetic perturbation control the onset of saturation. However, the transition to a magnetically-dominated magnetosheath does.

What about closure of the

bow shock current for large By?

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OMNI data:Bx = -5.5 nT

By = -13.2 nTBz = -2.1 nT

January 10, 1997CME-driven

storm

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Precipitating electrons - the upward current in the polar cap?

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Convection reversal coincident with the precipitation!

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Lobe cell convection

• Birkeland Current driven by bow shock will drive convection

• All on open field lines• Lobe cell convection

may not be reconnection driven

Bow shock dynamo and coupling to geospace

• The solar wind flow energy dissipated at the bow shock creates a dynamo (J•E<0). This in part powers dayside merging (Siebert and Siscoe, 2002).

• The bow shock current closes in part through the ionospheric load (J•E>0) where it can impose a potential in the polar cap and dissipate solar wind mechanical energy extracted at the shock

• This represents a means of driving ionospheric and magnetospheric convection without reconnection or viscous interaction at the magnetopause - it is a third fundamental mode of driving convection!

Conclusions• The behavior of the reconnection part of the transpolar

potential can be understood in terms of basic physics (Faraday’s Law, MHD momentum equation)

• The divergence of the magnetosheath flow explains the magnitude of the linear potential, the transition to the saturated potential, and dependencies on solar wind

• The closure of the bow shock current in the ionospheric polar cap is distinct from both reconnection and the viscous interaction. It is a fundamental mechanism by which solar wind mechanical energy extracted at the shock is deposited in the geospace system.

• Thus there are three sources of ionsopheric potential: reconnection, viscous interaction, and bow shock current closure