The Bimodal Solar Wind-Magnetosphere-Ionosphere System
George SiscoeCenter for Space Physics
Boston University
● Vasyliunas Dichotomization Momentum transfer via dipole interaction Momentum transfer via atmospheric drag
● Dipole Interaction Regime No effect on neutral atmosphereTranspolar potential proportional to IEF Dayside compression
● Atmospheric Drag RegimeCause of neutral flywheelTranspolar potential saturationDayside rarefaction Magnetopause “erosion”
● Summary Dichotomization, transpolar potential saturation, dayside
compression versus rarefaction, magnetopause erosion, and neutral flywheel all part of one story
oPVAε ~ 1
P = ionospheric Pedersen conductanceVA = Alfvén speed in the solar wind ε = magnetic reconnection efficiency
Key Point
By this criterion, the standard magnetosphere is solar wind dominated; the storm-time magnetosphere, ionosphere dominated.
Vasyliunas Dichotomization
Vasyliunas (2004) divided magnetospheres into solar wind dominated and ionosphere dominated depending on whether the magnetic pressure generated by the reconnection-driven ionospheric current is, respectively, less than or greater than the solar wind ram pressure.
The operative criterion is
CMEs
CIRs
IonosphereDominated
Solar WindDominated
Lindsay et al., 1995
Based on the method of momentum transfer between the solar wind and the terrestrial system, they correspond to dipole interaction dominated and atmospheric drag dominated
To emphasize their dynamical difference, we choose “dipole interaction” and “atmospheric drag” to distinguish them.
Alternative Nomenclature
Based on current systems, Vasyliunas’ two cases correspond to Chapman-Ferraro domination and region 1 domination.
Midgley &Davis, 1963
x
z
Chapman &Ferraro, 1931
Chapman-Ferraro Current System
ICF = BSS Zn.p./o
3.5 MA
Pertinent Properties of Dipole Interaction
C-F compression= 2.3 dipole field
2x107 N
Ram Pressure Contribution to Dst
April 2000 storm
Huttunen et al., 2002
GOES 8
A dipole interaction property
Psw compresses the magnetosphere andIncreases the magnetic field on the dayside.
Chapman-Ferraro Compression
V
BE
Interplanetary Electric Field DeterminesTranspolar Potential
A magnetopause reconnection property
● Magnetopause reconnection● Equals transpolar potential● Transpolar potential varies linarly with Ey
(Boyle et al., 1997)● Magnetosphere a voltage source as seen
by ionosphere
IMF = (0, 0, -5) nT
5 10 15 20
100
200
300
400
500
Tra
ns
po
lar
Po
ten
tia
l (k
V)
Ey (mV/m)
Dipole Interaction Dominated MagnetosphereSummary
● Psw compresses the magnetospheric field and increases Dst.
● Ey increases the transpolar potential linearly.
● Magnetosphere a voltage source
Field compression and linearity of response to Ey hold foronly one of the two modes of magnetospheric responsesto solar wind drivers—the usual one.
Key Point
Then Came Field-Aligned Currents
Iijima &Potemra, 1976
Region 1
Region 2
Atkinson, 1978
R 1
C-F Tai
l
Total Field-Aligned Currentsfor Moderate Activity
(IEF ~1 mV/m)
Region 1 : 2 MARegion 2 : 1.5 MA
3.5
MA
5.5 MA
1 MA/10 Re
Question: How do you self-consistentlyaccommodate the extra 2 MA?
Answer: You Don’t. You replace the Chapman-Ferraro current with it.
IMF = (0, 0, -5) nT
Chapman-Ferraro System
Region 1 System
(JxB)x
This is the usual case
Pure Region 1 Current System
IMF = (0, 0, -20) nT
Region 1 Current System Fills Magnetopause
Region 1 CurrentContours
X=+25X= -70
S= ρVV + p I + B2/2μo I - BB/μo
Net Force on Terrestrial SystemIntegrate x-component of momentum stress tensor over a
surface containing the terrestrial system
Net Force = 1.2x108 N
IMF = (0, 0, -20) nT
Net Force = 2.4x107 N
IMF = (0, 0, 0) nT
Drag Amplification
I1xBPCxl = 2x108 N/MA
I1xBMPxl = 1x107 N/MA
Back of the envelope estimate
i.e., roughly an order of magnitude amplification
Region 1 Current Contours
Region 1 Current Streamlines
Region 1 Force on the Atmosphere
5x
10
8 N
IMF = (0, 0, -20) nT
Atmospheric Reaction
● Region 1 current gives the J in the JxB force that stands off the solar wind
● And communicates the force to the ionosphere
● Which communicates it (amplified) to the neutral atmosphere as the flywheel effect
● Sometimes more than 200 m/s in the E region
Bow Shock
Streamlines
Region 1Current
ReconnectionCurrent
RamPressure
Cusp
Richmond et al., 2003
Goncharenko et al., 2004
25 Sept. 1998
Elementary Dynamics
● The force on the neutral atmosphere is total region 1 current times polar magnetic field strength times length across polar cap: or (qualitatively) I1xBPxl
● The mass of the atmosphere in and above the E region over the polar cap ~ 1010 kg.
● This gives an acceleration of ~ 7 m/s/hr/MA
● For example, 5 MA region 1 current applied for 10 hours gives a speed of ~350 m/s in the E region for the flywheel
Key Point
In establishing the neutral flywheel, duration of current might count for more than strength of
ram pressure.
Other Properties of Pure Atmospheric Drag Coupling
● Most region 1 current closes on bow shock (Alfvén wings)
● Reason: small field strength difference between tail and magnetosheath
● Low-latitude cusp and equatorial dimple
Zero IMF
IMF Bz = -20 nT
X = 0
0o 5 nT45o 5 nT
90o 5 nT
180o 2 nT 180o 10 nT 180o 20 nT
180o 30 nT
Cahill & Winckler, 1999
Dipole Field
Dayside Magnetic Decompression
IMF = 0
Chapman-Ferraro
Region 1
IMF Bz = -30
Transpolar Potential Saturation
IR
IRH
Where:H is the transpolar potential.R is the potential from magnetopause reconnection. I is the potential at which region 1 currents generate . a significant perturbation magnetic field at the reconnection site.
01.0
6.572/1
31
swEosw
P
swP
swE
H
/
IMF = 0
Chapman-Ferraro
Region 1
IMF Bz = -30
Baseline (PSW=1.67, Σ=6)
10 20 30 40 50
50
100
150
200
250
300
350
Ey (mV/m)
Tra
nsp
ola
r P
ote
nti
al (
kV)
PSW=10
Σ=12
Transpolar Potential Saturation
o
swP
H
/
314608
Saturation regimeLinear regime
61
6.57/
swPswE
H
Evidence of Two Coupling Modes
• Transpolar potential saturation
Instead of this
You have this• Reduced dayside compression seen at synchronous orbit
Instead of this
You have this
Hairston et al., 2004
5 10 15 20
100
200
300
400
500
Tra
ns
po
lar
Po
ten
tia
l (k
V)
Ey (mV/m)April 2000 storm
Huttunen et al., 2002
GOES 8Mühlbachler et al., 2003
ΔB = “erosion” contribution to Btot
Dipole Interaction Dominant
1. Dominant current system Chapman-Ferraro
2. Magnetopause current closes on magnetopause
3. Magnetopause a bullet-shaped quasi-tangential discontinuity
4. Force transfer by dipole Interaction
5. Transpolar potential proportional to IEF
6. Solar wind a voltage source for ionosphere
7. Compression strengthensdayside magnetic field
8. Minor magnetosphere erosion
Atmospheric Drag Dominant
1. Dominant current system Region 1
2. Magnetopause current closesthrough ionosphere and bow shock
3. Magnetopause a system of MHDwaves with a dimple
4. Force transfer by atmospheric dragDrag amplification and neutral flywheel
5. Transpolar potential saturates
6. Solar wind a current source for ionosphere
7. Stretching weakens daysidemagnetic field
8. Major magnetosphere erosion
Summary
Dichotomization, transpolar potential saturation, weak Dst responseto ram pressure, magnetopause erosion, neutral flywheel effect
all part of one story.
The Bimodal SWMIA System
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