space and plasma physics school of electrical engineering royal institute of technology stockholm...
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Space and Plasma PhysicsSchool of Electrical Engineering
Royal Institute of TechnologyStockholmSweden
Investigation of temporal stability of field-aligned
currents in the auroral ovalTomas Karlsson, Johanna Myde
Nordic Cluster Meeting 2011
IntroductionBoudouridis and Spence, 2007
DMSP particle data
Le, Wang, Slavin, and Strangeway, 2007
Space Technology 5, magnetic field data
Nordic Cluster Meeting 2011
Current sheet approximation and Ampére’s law
Ampére’s law (no time dependence):
j
zy
x
jB 0
0, , , ,y yx xz zx y z
B BB BB Bj j j
y z z x x y
But 0x
and 0
z
0, 0, 0, 0,xzz
BBj
y y
eller
y
Bj x
z
0
1
Nordic Cluster Meeting 2011
Magnetic field data from auroral oval passage
t (s)
ILAT(S/C 1)=65 ILAT(S/C 1)=80
DB
EA
ST (
nT)
Nordic Cluster Meeting 2011
Cross correlation
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
AGU Chapman Conference 2011
Magnetic field data from auroral oval passage
t (s)
ILAT(S/C 1)=65 ILAT(S/C 1)=80
DB
EA
ST (
nT)
Nordic Cluster Meeting 2011
Cross correlation
lag (s)
Cro
ss c
orr
elat
ion
co
effi
cien
t
141312
23 24 34
Nordic Cluster Meeting 2011
Raw correlations 2002-20097938 correlations, 1323 oval crossings
lag (s)
Cro
ss c
orr
elat
ion
co
effi
cien
t
nightday
Nordic Cluster Meeting 2011
Correlations, full oval, 2002-2009
lag (s)
Cro
ss c
orr
elat
ion
co
effi
cien
t
Nordic Cluster Meeting 2011
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
t = 546 s
Fit equation:
Nordic Cluster Meeting 2011
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
Nordic Cluster Meeting 2011
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)
Nordic Cluster Meeting 2011
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
Nordic Cluster Meeting 2011
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
Nordic Cluster Meeting 2011
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
Nordic Cluster Meeting 2011
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
Nordic Cluster Meeting 2011
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
Nordic Cluster Meeting 2011
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
Nordic Cluster Meeting 2011
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
Nordic Cluster Meeting 2011
Conclusions• 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
Nordic Cluster Meeting 2011
Future work
• 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
Nordic Cluster Meeting 2011
Physics questions
Nordic Cluster Meeting 2011
Changes in FAC Alfvén wave
FAC zero frequency Alfvén wave
Is the auroral acceleration electric field explainable by AW physics, or is other physics needed?
Temporal behaviour of E and B may give clues.
E E
J
B
S S
Energetic importance of processes with different temporal and spatial scales.
Large-scale SW-M-I coupling time scales.
Differences in temporal scales of upward and downward currents Implications for MI coupling modes, e.g. ionospheric plasma evacuation.
Is temporal variation of currents due to motion of current systems or change of shape and magnitude? (Relation to Maria’s results.)
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