Vasyliunas Dichotomization Momentum transfer via dipole interaction

1. The Bimodal Solar Wind-Magnetosphere-Ionosphere SystemGeorge SiscoeCenter for Space PhysicsBoston University • Vasyliunas Dichotomization Momentum transfer via dipole interaction Momentum transfer via atmospheric drag • Dipole Interaction Regime No effect on neutral atmosphere Transpolar potential proportional to IEF Dayside compression • Atmospheric Drag Regime Cause of neutral flywheel Transpolar potential saturation Dayside rarefaction Magnetopause “erosion” • Summary Dichotomization, transpolar potential saturation, dayside compression versus rarefaction, magnetopause erosion, and neutral flywheel all part of one story

2. Vasyliunas Dichotomization CMEs Solar Wind Dominated Ionosphere Dominated CIRs 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 • oPVAε ~ 1 • P = ionospheric Pedersen conductance VA = 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. Lindsay et al., 1995

3. Alternative Nomenclature Based on current systems, Vasyliunas’ two cases correspond to Chapman-Ferraro domination and region 1 domination. 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.

4. Chapman & Ferraro, 1931 Midgley & Davis, 1963 z C-F compression = 2.3 dipole field 2x107 N x Pertinent Properties of Dipole Interaction Chapman-Ferraro Current System ICF = BSS Zn.p./o  3.5 MA

5. GOES 8 April 2000 storm Huttunen et al., 2002 Ram Pressure Contribution to Dst A dipole interaction property Psw compresses the magnetosphere and Increases the magnetic field on the dayside. Chapman-Ferraro Compression

6. V E B 500 400 300 Transpolar Potential (kV) 200 100 5 10 15 20 Ey (mV/m) Interplanetary Electric Field Determines Transpolar 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

7. Dipole Interaction Dominated Magnetosphere Summary • Psw compresses the magnetospheric field and increases Dst. • Ey increases the transpolar potential linearly. • Magnetosphere a voltage source Key Point Field compression and linearity of response to Ey hold foronly one of the two modes of magnetospheric responsesto solar wind drivers—the usual one.

8. 1 MA/10 Re 5.5 MA Iijima & Potemra, 1976 Region 1 Atkinson, 1978 Region 2 3.5 MA R 1 Tail C-F Total Field-Aligned Currents for Moderate Activity (IEF ~1 mV/m) Region 1 : 2 MA Region 2 : 1.5 MA Then Came Field-Aligned Currents Question: How do you self-consistently accommodate the extra 2 MA?

9. Chapman-Ferraro System Region 1 System (JxB)x Answer: You Don’t. You replace the Chapman-Ferraro current with it. IMF = (0, 0, -5) nT This is the usual case

10. Pure Region 1 Current System IMF = (0, 0, -20) nT

11. Region 1 Current Contours Region 1 Current System Fills Magnetopause

12. X= -70 X=+25 Net Force on Terrestrial System Integrate x-component of momentum stress tensor over a surface containing the terrestrial system S= ρVV + p I + B2/2μo I - BB/μo Net Force = 1.2x108 N Net Force = 2.4x107 N IMF = (0, 0, -20) nT IMF = (0, 0, 0) nT

13. I1xBMPxl = 1x107 N/MA I1xBPCxl = 2x108 N/MA Drag Amplification Back of the envelope estimate i.e., roughly an order of magnitude amplification

14. Region 1 Current Contours Region 1 Current Streamlines 5x108 N Region 1 Force on the Atmosphere IMF = (0, 0, -20) nT

15. 25 Sept. 1998 Bow Shock Streamlines Region 1 Current Cusp Goncharenko et al., 2004 Ram Pressure Reconnection Current 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 Richmond et al., 2003

16. 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.

17. Zero IMF X = 0 IMF Bz = -20 nT 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

18. 45o 5 nT 0o 5 nT Cahill & Winckler, 1999 Dipole Field 180o 30 nT 90o 5 nT 180o 20 nT 180o 2 nT 180o 10 nT Dayside Magnetic Decompression

19. 1 3 / 57 . 6 E P sw sw F = H 1 / 2 + x S P 0 . 01 E sw o sw Chapman- Ferraro Region 1 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. IMF = 0 IMF Bz = -30 Transpolar Potential Saturation

20. Chapman- Ferraro Region 1 350 PSW=10 300 250 Baseline (PSW=1.67, Σ=6) 200 Transpolar Potential (kV) 150 Σ=12 100 50 57 . 6 E sw F = IMF = 0 IMF Bz = -30 H 1 6 / P 10 20 30 40 50 sw Ey (mV/m) Linear regime Saturation regime Transpolar Potential Saturation

21. GOES 8 Mühlbachler et al., 2003 April 2000 storm Hairston et al., 2004 Huttunen et al., 2002 500 400 300 Transpolar Potential (kV) 200 100 5 10 15 20 Ey (mV/m) 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 ΔB = “erosion” contribution to Btot

22. The Bimodal SWMIA System Dipole Interaction Dominant Dominant current system Chapman-Ferraro Magnetopause current closes on magnetopause Magnetopause a bullet-shaped quasi-tangential discontinuity Force transfer by dipole Interaction Transpolar potential proportional to IEF Solar wind a voltage source for ionosphere Compression strengthensdayside magnetic field Minor magnetosphere erosion Atmospheric Drag Dominant Dominant current system Region 1 Magnetopause current closesthrough ionosphere and bow shock Magnetopause a system of MHDwaves with a dimple Force transfer by atmospheric dragDrag amplification and neutral flywheel Transpolar potential saturates Solar wind a current source for ionosphere Stretching weakens daysidemagnetic field Major magnetosphere erosion Summary Dichotomization, transpolar potential saturation, weak Dst response to ram pressure, magnetopause erosion, neutral flywheel effect all part of one story.

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