PHYSICS AND ENGINEERING PHYSICS

The Disruption Zone Model of Magnetospheric Substorms

George Sofko, Kathryn McWilliams, Chad Bryant

ISuperDARN 2011 Workshop, May 30 – June 3

Dartmouth College, Hanover, NH

University of University of SaskatchewanSaskatchewan

April 5/10 Storms: Rankin shows strong flows from dayside rxn and onsets @ ~0505 & 0852 UT

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Substorm Current Wedge and Current Disruption (Aubry, McPherron & Russell, JGR, 1973)

Current Disruption Zone

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Current Disruption: a Natural Result of Stretched B Geometry Causing Eastward CG Drift in the Midtail

YZ plane view of the magnetotail looking toward Earth from the deep tail. Note that the stretched field region in the mid-tail consists of a neutral sheet bordered by two stretched line zones which we call the DISRUPTION ZONES (DZN, DZS). The eastward DZ current “disrupts” the “normal” westward dawn-dusk current.

University of University of SaskatchewanSaskatchewan

Streatched Magnetic Field Configuration – Cluster and THEMIS results

• Recent Cluster (Shen et al., JGR, 2003) and THEMIS (Saito et al., GRL, 2010) studies have confirmed that the MTPS (MidTail PlasmaSheet for |Y| <~ 8 RE) shows 3 important features:

(i) a deep minimum in B (<~ 2 nT) in the central neutral sheet (NSh) region;

(ii) a gradient outward from the NSh;

(iii) outward curvature of the field lines in the DZ regions neighbouring the NSh to

the north and south.

B

B

K

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Neutral Sheet and Midtail PS (MTPS) Disruption Zones (DZs)

2

2ˆ ˆv = CG

WWb B b K

qB qB

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The DZ ion flows consist of eastward CG drifts and ExB drift toward the NSh

DZN

DZS

NSh

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The eastward CG drifts in the DZs have important consequences

• They lead to strong shears between the westward-flowing NSh and eastward-flowing DZ ions. These shears lead to easy activation of the Kelvin-Helmholtz (KHI) instability at the NSh/DZ interfaces.

• They lead to a two-celled current system, one cell between DZN and the northern half of the NSh, the other cell between DZS and the southern half of the NSh.

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The 2CND (two-cell NSh/DZ) Current system

ˆ(p -p ) b× pˆJ= b×K+B B

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Impact of the 2CND currents

• They produce more flux near the NSh/DZ interfaces, and the extra magnetic pressure thins the NSh.

• The extra flux moves the region of stretched lines earthward, so there is equatorward expansion of the growth phase.

• The currents are inductive and oppose changes in flux – this could trigger onset when a northward turning occurs (Lyons, 1995).

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IMP 6 – Duskward-moving waves in the NSh region

Data obtained by IMP 6 at Z ~ -20 RE show that the Z-component in the NSh can reverse direction as a result of waves in the dawn-dusk direction. In the 3 cases analyzed, two were like (c), one was (a). In all cases, the waves moved westward (duskward). Nakagawa and Nishida assumed the waves were KH kink-mode waves.

Nakagawa and Nishida (Southward Magnetic Fields in the NS produced by Wavy Motions Propagating in the Dawn-Dusk Direction, GRL, 16, 1205, 1989) use the KHI formulation by Lee et al., JGR, 93, 7354, 1988.

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Lee et al. (JGR 1988) - KHI mode sequence – kink => sausage => sausage-tearing

Lee et al. used an MHD approach to the KHI, assuming no flows other than in the NSh – i.e. no help from CG drifts in the DZs.

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Shear at NSh/DZ boundaries leads to the KHI modes: kink => sausage => sausage-tearing

Lee et al. (JGR, 1988) assumed only NSh ion motions and current in their KHI model. AboveFigure adapted from Nakagawa and Nishida (GRL, 1989) paper based on Lee et al paper.

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Fully kinetic 3D model of the NSh by Buchner & Kuska

This model shows structure that is similar to the KHI and led B&K to call it the kinetic “sausage mode”. The only velocity considered was the NSh itself.

Sausage mode instability of thin current sheets as a cause of magnetospheric substorms, Ann, Geophys., 17, 604, 1999.

University of University of SaskatchewanSaskatchewan

YZ view of tail looking toward Earth

The midtail region of stretched closed lines consists mainly of the DZN, NSh and DZS. Outside the midtail are the CPS quasi-dipolar field regions adjacent to the dawn & dusk LLBL regions. Transition zones separate the QD and stretched lines.

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The Substorm Current Wedge – a natural consequence of the DZs

View at ~ Z=+2 RE

McPherron-Russell SCW

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Conclusions

• The Disruption Zones (DZs) arise as a natural result of field line stretching caused by dayside reconnection.

• The eastward drifts in the DZs result in eastward current that “disrupts” the westward plasmasheet current and leads to the Substorm Current Wedge.

• The 2CND inductive current system adds flux that thins the current sheet, extends the midtail earthward, and responds to changes in B such as northward turnings.

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Conclusions (cont’d)• The strong shear at the NSh/DZ interfaces leads to

the progression from kink to sausage to sausage-tearing mode. Ultimately, the latter reconnection causes onset and dipolarization.

• The CG drift is very important, but IT DOES NOT MAP FROM LOW TO HIGH LATITUDES, so there is DECOUPLING between the magnetospheric and ionospheric (auroral) substorms.

• Many key aspects of magnetospheric substorms arise naturally from the formation of the DZs.