The Bimodal Solar Wind-Magnetosphere- Ionosphere System George Siscoe Center for Space Physics Boston 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

 o  P V A ε ~ 1  P = ionospheric Pedersen conductance V A = 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 Ionosphere Dominated Solar Wind Dominated 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 I CF = B SS Z n.p. /  o  3.5 MA Pertinent Properties of Dipole Interaction C-F compression = 2.3 dipole field 2x10 7 N

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

V B E 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 Transpolar Potential (kV) Ey (mV/m)

Dipole Interaction Dominated Magnetosphere Summary ●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 for only one of the two modes of magnetospheric responses to 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 Tail Total Field-Aligned Currents for Moderate Activity (IEF ~1 mV/m) Region 1 : 2 MA Region 2 : 1.5 MA 3.5 MA 5.5 MA 1 MA/10 Re Question: How do you self-consistently accommodate 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 Current Contours

X=+25 X= -70 S= ρVV + p I + B 2 /2μ o I - BB/μ o Net Force on Terrestrial System Integrate x-component of momentum stress tensor over a surface containing the terrestrial system Net Force = 1.2x10 8 N IMF = (0, 0, -20) nT Net Force = 2.4x10 7 N IMF = (0, 0, 0) nT

Drag Amplification I 1 xB PC x l = 2x10 8 N/MA I 1 xB MP x l = 1x10 7 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 5x10 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 1 Current Reconnection Current Ram Pressure Cusp Richmond et al., 2003 Goncharenko et al., 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) I 1 xB P x l ●The mass of the atmosphere in and above the E region over the polar cap ~ 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

0 o 5 nT45 o 5 nT 90 o 5 nT 180 o 2 nT 180 o 10 nT 180 o 20 nT 180 o 30 nT Cahill & Winckler, 1999 Dipole Field Dayside Magnetic Decompression

IMF = 0 Chapman- Ferraro Region 1 IMF Bz = -30 Transpolar Potential Saturation 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 /1 31  sw E o P P E H /  

IMF = 0 Chapman- Ferraro Region 1 IMF Bz = -30 Baseline (P SW =1.67, Σ=6) Ey (mV/m) Transpolar Potential (kV) P SW =10 Σ=12 Transpolar Potential Saturation Saturation regime Linear regime / sw P E 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., Transpolar Potential (kV) Ey (mV/m) April 2000 storm Huttunen et al., 2002 GOES 8 Mühlbachler et al., 2003 ΔB = “erosion” contribution to B tot

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 strengthens dayside magnetic field 8.Minor magnetosphere erosion Atmospheric Drag Dominant 1.Dominant current system Region 1 2.Magnetopause current closes through ionosphere and bow shock 3.Magnetopause a system of MHD waves with a dimple 4.Force transfer by atmospheric drag Drag amplification and neutral flywheel 5.Transpolar potential saturates 6.Solar wind a current source for ionosphere 7.Stretching weakens dayside magnetic field 8.Major magnetosphere erosion Summary Dichotomization, transpolar potential saturation, weak Dst response to ram pressure, magnetopause erosion, neutral flywheel effect all part of one story. The Bimodal SWMIA System