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