generation of the transpolar potential ramon e. lopez dept. of physics ut arlington
TRANSCRIPT
Generation of the transpolar potential
Ramon E. Lopez
Dept. of Physics
UT Arlington
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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 nTBy = -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