Onsets and Flapping Studied using a Dynamic Harris Sheet Model

Onsets and Flapping Studied using a Dynamic Harris Sheet Model

M. Connors1, R. Lerner1, G. Jaugey2,

B. Lavraud3, M. Volwerk4, R. L. McPherron5

1Athabasca U., 2U. Joseph Fourier, 3LANL, 4Österreichische Ak. Wissensch., 5UCLA

STAMMS-2 Orleans, France September 2007 Image: Mikko Syrjäsuo

Athabasca University Geophysical Observatory (AUGO)

54.72 N, 246.7 ECGM (2005) 62.0, 306.5L=4.55

Founded 2002(UCLA mag 1998)

A comprehensive observatory in western Canada ideally located for THEMIS conjunctions (see poster)

Harris modelHarris model

Magnetic field :

where By=Bz=0

and Bo = Blobes

Current density :

derived from

Plasma sheet - Harris modelPlasma sheet - Harris model

0 zx JJ

Plasma sheet : hot plasma

n = 3 cm-3

Eth ~ 4200 eV

B ~ 10 nT

β ~ 6

z>0 => Bx>0 , z<0 => Bx<0

)tanh(L

ZoZBB ox

)²(sec)(0

0

L

ZoZh

L

BJy

)(0 BJ curl

Bo

Method for Harris sheet fitting

• The Levenberg-Marquardt nonlinear fitting method was used for BX(Z0,L,B0) during the slow traversal of Cluster through the plasma sheet

• The Levenberg-Marquardt nonlinear fitting method was used for BX(Z0,L) with B0 held constant during flapping

• Thompson et al. (2005 JGR) used a similar fitting technique

Cluster Plasma sheet crossing September 22th, 2001

Plasma sheet crossing at Plasma sheet crossing at ~ 5 UT~ 5 UT

B reverse

low |B|

more energized particles

Harris form

Harris fitHarris fitAugust 3August 3rdrd, 2003, 2003

Results

Bo = B(lobes) = 26.8 nT

=> realistic ( Theory ~30 nT )

Zo = -1.64 Re

=> not centred (z axis)

=> movement of the plasma sheet in the Z direction => flapping

L = 0.63 Re

=> thickness of the plasma sheet

Questions

Are the parameters related ?

What influences the parameters ?

Bo

Zo

)tanh(L

ZoZBB ox

Flapping studyFlapping study

flapping real crossing

Harris flapping study :

fit again using Z separation of the spacecraft rather than the Z motion over time

Bo is known ( Harris crossing study)

Harris fit to find L, Zo

awaited results : L constant and Zo function of time

Flapping affects fits during slow passage of Cluster through plasma sheet

BUT,

Plasma sheet crossing is too long flapping are very fast => Good Harris study could come from flap fits

Flapping studyFlapping studyAugust 23August 23rdrd , 2002 , 2002

L ~ cst during flapping

Zo(t) => velocity of the flapping Vz

ResultsResults

L ~ 0.89 Re

Zo is a function of time => Vz

Questions

Vz same as Cluster data ?

The current is Harris current ?

=> Vz and J

Flapping studyFlapping studyAugust 23August 23rdrd , 2002 , 2002

Vz (cluster data) > Vz fit

11 UT => +60 km.s-1

Current fit realistic (compared to the data in black)

Jfit(blue)

Vy ≈ or > Vz !! => First hypothesis (flap only in Z direction) false

our fit needs some changes !!

Conclusions on FlappingConclusions on Flapping

Our method for the fit gives good results for the plasma sheet thickness, the Zo evolution and also the current.

Nevertheless, our hypothesis of only Z motion of the plasma sheet is false. It’s moving also at least in the Y direction.

We are working to improve our fit assuming that the plasma sheet can be tilted yet planar (3-d Harris fit).

This approach would differ from least-variance methods

Statistical Trends

• 71 plasma sheet crossings in the public data set were fit

• The only significant trend was of B0 vs P

• L(B0) seemed to show the opposite behaviour than expected, thicker for higher lobe field

• The lack of relationships is likely due to use of overly long time periods

Pressure Variation

• Beff is the lobe field as calculated from the measured B and particle data

• Ptotal also arises from this and shows an influence on B0

Pressure Statistics

• The lobe field varies with solar wind pressure (ACE)

• We did not find other strong dependencies on solar wind parameters

August 21 2002 ca. 8 UT Canada Conjugacy Event

Active Period with good conjugacy to Churchill meridian at onset at 7.9 UT

Ground Pi2 also starts 7.9 UT (MCMU is to west)

Cluster Pi2 starts at 8.1 UT

Features of 21 Aug 2002 event

• Plasmoid signature in Bz at onset time

• Flow reversal from –X bursts to +X bursts indicating tailward motion of the X-line

• Flapping after onset

• Current enhancement at onset

• Very clear activations of all signatures at onset

B signatures: activation, flapping

Plasmoid: bipolar BZ

Flow direction changes

as neutral line

moves outward

July 27 2003: Inference from Flapping

• Unusual evening Ps6/omega band event recorded by magnetometer and THEMIS test camera at Athabasca

• Cluster in morning sector; ground data sparse; spacecraft spacing very close

• Multiple flappings where BX goes to 0 but not beyond

Bifurcated Current Sheet?

• Bx several times went to 0 but not beyond

• Could bifurcated current sheet be related to Ps 6?

Summary

• Enhanced facilities in Canada benefit THEMIS and Cluster

• 1-d Harris fit can be applied for PS crossing or for flapping

• 3-d Harris fit may be needed and is being worked on

• Conjugate studies can now be more frequent and with better data

Acknowledgements

• Canadian Space Agency and University of Alberta for CARISMA data

• A. Balogh, ICSTM, for Cluster FGM data

• This work funded by Canada Research Chairs, Canada Foundation for Innovation, AU, and NSERC