Solar Wind-Magnetosphere Interaction for Northward Interplanetary Magnetic Field Paul Song Center for Atmospheric Research University of Massachusetts Lowell LLBL formation Global model Summary Acknowledgments: C. T. Russell, T.I. Gombosi, D.L. DeZeeuw

Structure of the Magnetopause Northward IMF Southward IMF

Distribution Functions Across the Magnetopause

Summary of LLBL Observations for Northward IMF Density and temperature change in steps: against diffusion to be important Indication of mixtures of plasmas of magnetosphere and magnetosheath origins at different ratios Thicker and faster on the nightside Smaller density gradient and velocity shear on the nightside

Northward IMF [Dungey, 1963] Southward IMF [Dungey, 1961] Northward IMF [Dungey, 1963]

Song and Russell Model [1992] Reconnection takes place on the stagnant field line at regions of high field shear

After Cusp Reconnection As Alfvenic kink propagates to lower latitudes, the newly reconnected field line “sinks” into the magnetosphere Note the foot of the field moves sunward

NBZ Model Entry Mechanism Through reconnection at two hemispheres the magnetosphere captures a segment of a solar wind flux tube The newly captured flux tube sinks into the magnetosphere via propagating Alfven waves.

Formation of the LLBL After the captured flux tube becomes a magnetospheric flux tube The original flux tube is compressed and shortened (magnetic volume decreases =>B and  increases) Total pressure of the flux tube increases. The flux tube expands (increase in length or volume) along the magnetopause to the flank via interchange instability Ionospheric dissipation drags the motion Successive reconnection events form multiple layers of LLBL Interpenetration and mixing of plasmas of two origins result in decreased ratio of magnetosheath-to-magnetosphere population: an aging process How can the flux tube flow back?

Global Modeling the Solar Wing-Magnetosphere-Ionosphere System Challenges The topological status of the magnetosphere: open or closed? Driver(s) of ionospheric sunward flow Source(s) of NBZ currents Key problem: are “viscous cells” driven by viscosity?

Ionospheric Observations for NBZ Field-aligned current Precipitation particles [Ijima and Potemra, 1978] [Newell and Meng, 1994]

Ionospheric Convection and Field Perturbations for NBZ [Potemra et al

Ogino’s code, NBZ, [Ogino and Walker, 1984] Cusp reconnection Closed magnetosphere

Rice Model, NBZ [Usadi et al., 1993] Cusp merging Closed magnetosphere Shorter tail for large IMF magnitude

Fedder and Lyon (1995), NBZ MHD Simulation Noon-midnight meridian Equatorial Plane Cusp merging Closed magnetosphere 4-cell ionosphere convection NBZ currents Flow diversion at 95 Re

Raeder’s Model, NBZ [Raeder et al., 1995] Cusp reconnection Tail reconnection Open tail No ionospheric convection is shown

Ogino’s code, NBZ, [Bargatze et al., 1999] Cusp reconnection Closed magnetosphere

Global MHD Simulation For Northward IMF Reconnection Tail Tail-length Ionosphere Ogino-Walker cusp closed ~1/B Wu cusp closed ~ 1/B Usadi et al. cusp closed ~ 1/B Fedder-Lyon cusp closed ~1/B 4-cell/NBZ Raeder cusp+tail open Michigan cusp closed ~ 1/B 4-cell/NBZ Bargatze cusp closed ~ 1/B 4-cell/NBZ ISM cusp closed 4-cell/NBZ

Raeder’s Model, NBZ [Raeder et al., 1995] Cusp reconnection Tail reconnection Open tail No ionospheric convection is shown

Global Picture Solar wind and magnetosphere are coupled through high latitude reconnection. For due NBZ, the magnetosphere is closed except the cusps Three topological boundaries and regions. Outer magnetosphere: two convection channels and two cells. LLBL is driven by pressure gradients. “Viscous” cells are driven at ionosphere by Pedersen currents. A region of stagnant flow near midnight in the tail between 20-50 Re depending on the IMF strength: cold-density plasma sheet. Ionosphere: 4-cell convection. NBZ, Region I, and (Region II currents, not modeled). Polar caps, although closed, see solar wind particles

NBZ MHD Simulation (Michigan Code)

Summary Chris and I first proposed a model of formation of LLBL for northward IMF We then collaborated with Michigan group and developed a self-consistent global model for northward IMF: Solar wind entry: reconnection. LLBL flow: driven by pressure force. Magnetotail length: increases with 1/BIMF, NSW, MSW. Reverse cells: driven by reconnection and LLBL. “Viscous cells”: driven at ionosphere by Pedersen currents. Magnetopause definition: the magnetopause currents may differ from the topological boundary. Stagnation line/point dilemma: No stagnation region in the magnetosheath. A stagnation line occurs in the magnetospheric field. Ionosphere: Precipitation within (outside) the polar cap is of solar wind (magnetospheric) origin (mistaken by some people as evidence of an open region). The most important things I learned from Chris: A positive view toward referees and referee’s reports There are “only” 3 ways to prove truth! (simulation is NOT among them!) Can you summarize your thesis in one sentence, or two sentences, or … (an anti- correlation between the number of sentences with the significance of work)