Space and Plasma PhysicsSchool of Electrical Engineering

Royal Institute of TechnologyStockholmSweden

Cluster Workshop 2010-05-20

Investigation of temporal stability of field-aligned currents in the auroral

ovalTomas Karlsson, Johanna Myde

Introduction

Cluster Workshop 2010-05-20

Boudouridis and Spence, 2007

DMSP particle data

Le, Wang, Slavin, and Strangeway, 2007

Space Technology 5, magnetic field data

LS = 10 minMS = 1 min

Magnetic field data from auroral oval passage

Cluster Workshop 2010-05-20

t (s)

ILAT(S/C 1)=65 ILAT(S/C 1)=80 B

EA

ST (

nT)

3200 s

B

Eastj

Cross correlation

Cluster Workshop 2010-05-20

t

ILAT=65 80

ILAT=65 80

S/C

1

S/C

2

lag

1

0

1 12 2

0 0

1

1 1

N L

k L kk

xy N N

k kk k

x x y yN

CCC L

x x y yN N

cross-covariance

auto-covariances (for L=0)

lag

Magnetic field data from auroral oval passage

Cluster Workshop 2010-05-20

t (s)

ILAT(S/C 1)=65 ILAT(S/C 1)=80

DB

EA

ST (

nT)

Cross correlation

Cluster Workshop 2010-05-20

lag (s)

Cro

ss c

orr

elat

ion

co

effi

cien

t

141312

23 24 34

Cluster Workshop 2010-05-20

Raw correlations 2002-20097938 correlations, 1323 oval crossings

lag (s)

Cro

ss c

orr

elat

ion

co

effi

cien

t

nightday

Cluster Workshop 2010-05-20

Correlations, full oval, 2002-2009

lag (s)

Cro

ss c

orr

elat

ion

co

effi

cien

t

Cluster Workshop 2010-05-20

Correlations, full oval, 2002-2009

lag (s)

Cro

ss c

orr

elat

ion

co

effi

cien

t /0

lagy Ce C

C = 0.37C0 = 0.53

= 546 s

Fit equation:

Cluster Workshop 2010-05-20

Correlations, full oval, 2002-2009

Kp dependence

lag (s)

Cro

ss c

orr

elat

ion

co

effi

cien

t

0 < Kp ≤ 11 < Kp ≤ 33 < Kp ≤ 9

Cluster Workshop 2010-05-20

Correlations, full oval, 2002-2009

MLT dependence

lag (s)

Cro

ss c

orr

elat

ion

co

effi

cien

t

EveningNight MorningDay

(18-20)(22-00)(01-03)(11-13)

Cluster Workshop 2010-05-20

Sliding Cross Correlations

t

pre-lag

ILAT=65 80

ILAT=65 80

S/C

1

S/C

2

ILAT=65 80S/C

1

ILAT=65 80

S/C

2

winsize

Cluster Workshop 2010-05-20

Correlations 2002-2007, n=95 154 Window size = 3°

ILA

T

lag (s)

cros

s co

rrel

atio

n c

oef

ficie

nt

1.0

0.5

0.0

Cluster Workshop 2010-05-20

Correlations, 2002-2007

ilat

lag (s)

Window size = 1° Window size = 3°

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rre

latio

n co

effic

ienr

t

Cluster Workshop 2010-05-20

Correlations, 2002-2007 Window size = 3°

ML

T

lag (s)

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

Cluster Workshop 2010-05-20

Correlations, 2002-2007M

LT

lag (s)

Window size = 1° Window size = 3°

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rre

latio

n co

effic

ienr

t

Cluster Workshop 2010-05-20

Correlations, 2002-2007 Window size = 3°

ilat

MLT

lag < 10 s 10 s < lag < 30 s

30 s < lag < 100 s 100 s < lag < 300 s

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rre

latio

n co

effic

ienr

t

Cluster Workshop 2010-05-20

Correlations, 2002-2007 Window size = 1°

ilat

MLT

lag < 10 s 10 s < lag < 30 s

30 s < lag < 100 s 100 s < lag < 300 s

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rre

latio

n co

effic

ienr

t

Conclusions

Cluster Workshop 2010-05-20

• Cross correlation method is a useful method for studying the temporal variations in the auroral oval,

• FAC on the scale of the oval size have a characteristic life-time of around 9 min, consistent with Le et al. (10 min).

• Meso-scale current life-time dependence on MLT and scale size:

• Low-latitude currents somewhat more temporally stable than high-latitude currents.

Morning/day Evening

1° 10 s 30-100 s

3° 30-100 s 100-300 s

Future work

Cluster Workshop 2010-05-20

• Modify normalization in in cross correlation function

• Study scale-size dependence with proper cross-spectral analysis (coherence function)

• Study temporal characteristics of electric fields

• Study energetic importance by calculating Poynting flux

Coherence function

Cluster Workshop 2010-05-20

2

2 2, 0 1xy

xy xyxx yy

G ff f

G f G f

*

1

2

1

2

1

2

0

2

0

2, ,

2,

2,

,

,

d

d

d

n

xy j jjd

n

xx jjd

n

yy jjd

Ti f t

j j

Ti f t

j j

G f X f T Y f Tn T

G f X f Tn T

G f Y f Tn T

X f T x t e dt

Y f T y t e dt

Cluster Workshop 2010-05-20

Correlations 2002-2007, n=109 542 Window size = 1°IL

AT

lag (s)

cros

s co

rrel

atio

n c

oef

ficie

nt

1.0

0.5

0.0

Cluster Workshop 2010-05-20

Correlations, 2002-2007 Window size = 1°

ML

T

lag (s)

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

cros

s co

rrel

atio

n co

effic

ient

1.0

0.5

0.0

This and that

Cluster Workshop 2010-05-20

Smooth window in dB calculation: 400 s = 200 km