Solar Wind-Magnetosphere Interaction for Northward Interplanetary Magnetic Field

Paul SongCenter 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 MagnetopauseNorthward IMF Southward IMF

Distribution Functions Across the Magnetopause

Summary of LLBL Observationsfor 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]

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

o Through reconnection at two hemispheres the magnetosphere captures a segment of a solar wind flux tube

o 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

• 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?

Challenges

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., 1984]

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 SimulationNoon-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)