Generation of the transpolar
potential
Ramon E. Lopez
Dept. of Physics
UT Arlington
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
2
Linear
reconnection
driving by the
solar wind
so
Transpolar Potential Saturation (storm
main phases)
See also Ober et al., (2003), Hairston et al. (2003)
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
5
• 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 of
runs, then subtract it from the total potential
Reconnection Potential - Φr(V, n, Σp, B)
The potential along the merging
line is the rate at which flux
crosses the merging line.
LFM MHD Simulation Potential
The
geoeffective
length is
directly
confirmed by
following
plasma flow
streamlines
from the solar
wind
See also10Merkin
et al. (2005)
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
11
Y-extent of streamlines
intersecting X-line
shrinks for beta<1
12
Geoeffective lengths give
Reconnection Potential
13
n = 8/cc, Bz = -10 nT
Density
dependence
• Higher density
needs higher Bz to
transition to beta<1
in sheath, hence
larger potentials in
the saturation
regime
n = 5/cc, Bz = -10 nT
14
Σ = 5 mho, Bz = -10 nT
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 = 10 mho, Bz = -10 nT
15
Velocity dependence
Solar Wind Viscous
Speed
Potential
Geoeffective
Length
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
• Higher solar wind
speed produces a
larger pressure
force in the
magnetosheath
• This reduces the
geoeffective length
in the solar wind
Sound Speed dependence as well!
16
LFM shows expected behaviors
17
How does this agree/differ with
the
Siscoe-Hill model?
18
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 ChapmanFerraro current and exerts force balance with
the solar wind
19
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 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
Vx = 400km/s, Vz = -150 km/s, Bz = -15 nT
Density
Jy
p

More current flows to the north!
 north >  south with Σp constant.
This cannot be due to reconnection!
Driving via the Bow Shock Generator
The current in the
bow shock is a
generator
Bz = -20 nT, V = 400 km/s, n = 5/cc
Current streamlines
Density color-coded
This dynamo
current acts as a
source for
potential
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
0 nT
0 nT
Aug 10,
2000
Text
Text
-13.5 nT
nT
31
Good northern hemisphere pass
Clear convection pattern
32
Upward FAC
66.5˚
66.8˚
33
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<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
What about
large By?
LFM exhibits
saturation
36
AIME and DMSP confirm it
VBy = 8 mV/m
Well within
saturation
37
Sample DMSP Observations
VBy = 8.1 mV/m
F13
ΦF13 = 99.2 kV
F15
ΦF15 = 100.5 kV
38
5 mho
β-dependent saturation onset
20 mho
39
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 Bysaturated 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?
January 10, 1997
CME-driven
storm
OMNI data:
Bx = -5.5 nT
By = -13.2 nT
Bz = -2.1 nT
42
Precipitating electrons - the
upward current in the polar cap?
43
Convection reversal coincident
with the precipitation!
44
45
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
46
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