# Generation of the transpolar potential Ramon E. Lopez Dept. of Physics UT Arlington.

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Generation of the transpolar potential Ramon E. Lopez Dept. of Physics UT Arlington

2 How does the solar wind drive convection? Dungey [1961] Reconnection Most 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)

5 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 + 15 Solar 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 - Φ v (V, n, Σ p ) We determine this for each parameter set of runs, then subtract it from the total potential Reconnection Potential - Φ r (V, n, Σ p, B) The potential along the merging line is the rate at which flux crosses the merging line.

LFM MHD Simulation Potential

10 The geoeffective length is directly confirmed by following plasma flow streamlines from the solar wind See also Merkin et al. (2005)

11 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

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

13 Geoeffective lengths give Reconnection Potential

14 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

15 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

16 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 R E 600 km/s 48.7 kV 5.9 R E 800 km/s101.3 kV 4.0 R E Solar Wind Speed Viscous Potential Geoeffective Length Sound Speed dependence as well!

17 LFM shows expected behaviors

18 How does this agree/differ with the Siscoe-Hill model?

19 What are these potentials? Φ m given by solar wind electric field times the geoeffective length Φ s 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

Where does the current go?

Look at the direction of the current in the volume at Z=0 Bz = -20 nT V = 400 km/s n = 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?

These currents exist! Lopez et al., 2008 JASTP

V x = 400km/s, V z = -150 km/s, B z = -15 nT  north >  south with Σ p  constant. This cannot be due to reconnection! More current flows to the north! JyJy Density

Driving via the Bow Shock Generator The 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/cc Current streamlines Density 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

31 Aug 10, 2000 Text 0 nT -13.5 nT nT

32 Good northern hemisphere pass Clear convection pattern

33 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 B z <0 Wilder 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

36 What about large B y ? LFM exhibits saturation

37 AIME and DMSP confirm it VB y = 8 mV/m Well within saturation

38 Sample DMSP Observations VB y = 8.1 mV/m Φ F13 = 99.2 kV Φ F15 = 100.5 kV F13 F15

39 5 mho20 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 B y - saturated potential compare to a B z -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 B y ?

42 OMNI data: B x = -5.5 nT B y = -13.2 nT B z = -2.1 nT January 10, 1997 CME-driven storm

43 Precipitating electrons - the upward current in the polar cap?

44 Convection reversal coincident with the precipitation!

45

46 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 (JE<0). This in part powers dayside merging (Siebert and Siscoe, 2002). The bow shock current closes in part through the ionospheric load (JE>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

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