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03.07.2007 | 9:30 am | IUGG | page 1/25 Manoj et al, Evidence for short ....
Evidence for short correlation lengths of the noon-time equatorial electrojet – inferred from a comparison of satellite and ground magnetic data.
C. ManojNational Geophysical Research Institute, Hyderabad, India.
H. LührGeoForschungsZentrum – Potsdam, Germany
S. MausCIRES, University of Colorado, USA
N. NagarajanNational Geophysical Research Institute , Hyderabad, India.
03.07.2007 | 9:30 am | IUGG | page 2/25 Manoj et al, Evidence for short ....
(Figure from Anderson et al, 2002)
Solar tidal effects causes current flow in the day time ionosphere E region (Sq)
Sq current system sustains an eastward directed electrified from dawn-dusk at low latitude.
A Hall current is then generated, carried by the upward moving electrons.
The non-conductive boundaries above and below the lower ionosphere causes large vertical electric field build up.
This vertical electric field (about 5 to 10 times stronger than the eastward electric field that produced it.
This vertical field generates an eastward current called equatorial electrojet (EEJ) in noon-time ionosphere
Equatorial Electrojet - generation
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Simulated horizontal magnetic anomaly at ground due to ionospheric currents (from CM4). Unit - nT
Equatorial Electrojet – magnetic fields
The equatorial electrojet produces strong enhancement of horizontal magnetic intensity within ±3° of the magnetic equator.
EEJ has been studied using magnetometer array, rockets, radar, satellites… etc. etc..
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Equatorial Electrojet – magnetic fields
0 5 10 15 20 25-10
0
10
20
30
40
50
60
LT
ET
T
H -
HY
B
H
A unique way of studying the EEJ is by using the differences in horizontal magnetic variations at an equatorial observatory from another observatory separated by 10°-15° in latitude.
EEJ was also studied by satellite missions like POGO, Magsat, Oersted and CHAMP. LEO satellites, which flies above the ionosphere senses EEJ as negative signal at dip equator.
Lühr et al, 2004
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Some open questions on EEJ
Lühr et al, 2004 reports uncorrelated currentstrength between successive CHAMP passes over EEJ. These passes are separated in space by ~23º and in time by ~93 minutes.
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UT 6
UT 7:30
-40 -30 -20 -10 0 10 20 30 40
23º West and 93 minutes later
Some open questions on EEJ
Is the observed variability in EEJ current strength due to spatial (23º) or temporal (93 minutes) effects ?
180 W 135 W 90 W 45 W 0 45 E 90 E 135 E 180 E
90 S
45 S
0
45 N
90 N
180 W 135 W 90 W 45 W 0 45 E 90 E 135 E 180 E
90 S
45 S
0
45 N
90 N
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Some open questions on EEJ
Are Sq and EEJ current systems coupled ?
EEJ is often modeled as an equatorial enhancement of a coherent, large scale Sq current system (for eg. MacDaugall, 1979, CM4, Sabaka et al, 2004 ). Forbes (1981) concludes that EEJ and Sq are coupled current systems. This finding is also supported by Hesse (1982).
However studies by Mann & Schlapp (1988) and Okeke (2006) shows poor correlation of horizontal magnetic fields between observatories within the equatorial region and outside of it. Also studies by Raghavarao & Anandarao (1987) finds that Sq and EEJ are decoupled.
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While, from the ground, a continuous record of the current-induced magnetic field is obtained, polar orbiting satellites take just a snapshot of the latitudinal current distribution while passing over the equatorial region.
The temporal variations recorded by a ground station can either be caused by a change in current strength or by a displacement of the current axis. Satellite measurements on the other hand give no information on the temporal variation of the EEJ but a good picture of the current geometry.
By combining both data sets the advantages can be used to eliminate several ambiguities and answer the questions we discussed.
How do we go about it ?
03.07.2007 | 9:30 am | IUGG | page 9/25 Manoj et al, Evidence for short ....
Roadmap
Observatory and satellite data.
Data processing
Correlation analysis
Results
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Observatory data
ETTTIR PND
ABG HYB
HUA
FUQMBO
GUI
AAE
QSB
GUA
CBI
ABG ETT HYB TIR HUA FUQ MBO GUI AAE QSB GUA CBI PND0
5000
10000
15000
20000
25000
30000
num
ber
of h
ourly
mea
ns
Distribution of the geomagnetic observatories used for the study.
Hourly means of the horizontal intensities from 13 observatories.
Period: Sep 2000 – Dec. 2002
Screened for Kp ≤ 2 to limit the analysis to magnetically quiet days.
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0 5 10 15 20 25-10
0
10
20
30
40
50
60
LT
ET
T
H -
HY
B
H
EEJ signals from ground data
ΔHEEJ – ΔHNon-EEJ
ΔH is the variation from midnight level.
Average daily variation of the horizontal components of geomagnetic field observed at ETT with respect to the station HYB.
Typically, the EEJ signal reaches up to 53 nT. The solid line represents a polynomial fit to the data.
EEJ signals for 2000-2002
Time (Years)
UT
(H
ours
)
2000 2001 2002 2003
0
10
20-100
0
100
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Satellite data
L R
Scalar magnetic field data from 2000 to 2002
Local Time : 10 to 13Kp index ≤ 2
Total 1653 crossings
Data reduction
Main field (Pomme 1.4, Maus et al, 2005)Lithospheric field (MF2, Maus et al, 2002)Diamagnetic effect (Lühr et al, 2003)Large-scale magnetospheric fields by polynomial fitting
Current density distribution was modeled by series of EW oriented current lines separated by 0.5º in latitude and located at an altitude of 108 km.
Induction effect conductosphere at depth of 200 km
Re-drawn from Lühr et al, 2004
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-20 -15 -10 -5 0 5 10 15 20-40
-20
0
20
40
60
80
100
Degrees Latitude about dip-equator
nT
AAE
ABG
TIR
HYB
ETT
MBO
HUA
FUQ
GUAPND
Bx
Bz
B
Magnetic profile from CHAMP
Predicted ground magnetic field profile due to the noon time equatorial electrojet from the CHAMP average current profile.
The locations of geomagnetic observatories are plotted with respect to the dip-equator along the magnetic field profile
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0 5 10 15 20 25-10
0
10
20
30
40
50
60
LTE
TT
H -
HY
B
H
Since the satellite crosses the dip-equator at a certain LT and the corresponding observatory data may have a different LT, a correction needs to be applied to make the data set comparable.
A degree-9 polynomial was used to findThe ratio of expected EEJ strength at observatory and satellite local time
LT correction
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5 10 15 200
10
20
30
40
50
LT (Hours)
GU
A
H-C
BI
H (
nT)
50 30 10 -10 -30 -50-50
0
50
100ETT-HYB 2000 /10 /7, 7:30 UT
Geographic Latitude
H
(nT
) CM4 SqHYB H
PND H
ETT HSq at ETT
Sq correction
By subtracting the data from non – equatorial observatory, we remove a part of the Sq variation at the equatorial observatory.
The unresolved part corresponds to the latitudinal slope of the Sq between the observatory pair.
Although none of the two stations is directly below the EEJ a daily variation of more than 50 nT is seen here.
CM4 model (Sabaka et al, 2004) was used to obtain an estimate of the latitudinal slope of the Sq signal between the observatory pairs.
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0 0.1 0.2 0.3 0.40
20
40
60
80
100
H
CC 0.15
-30
0 0.1 0.2 0.3 0.40
20
40
60
80
100
CC 0.49
-20 0 0.1 0.2 0.3 0.4
0
20
40
60
80
100
CC 0.83
-10
0 0.1 0.2 0.3 0.40
20
40
60
80
100
A/m
H
H = 299.6 * I + -9.75 CC 0.94
0
0 0.1 0.2 0.3 0.40
20
40
60
80
100
A/m
CC 0.81
10
0 0.1 0.2 0.3 0.40
20
40
60
80
100
A/m
CC 0.56
20
-40 -30 -20 -10 0 10 20 30 40-0.5
0
0.5
1
Distance from Observatory in Degrees
Cor
rela
tion
Coe
ffic
ient
With LT correctionWithout LT correction
Correlation Analysis
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-40 -20 0 20 40 -0.2
00.20.40.60.8
1
ETT-HYB-40 -20 0 20 40
-0.20
0.20.40.60.8
1AAE-QSB
-40 -20 0 20 40 -0.2
00.20.40.60.8
1
TIR-ABG -40 -20 0 20 40
-0.20
0.20.40.60.8
1MBO-GUI
-40 -20 0 20 40 -0.2
00.20.40.60.8
1
HUA-FUQ -40 -20 0 20 40
-0.20
0.20.40.60.8
1GUA-CBI
ETTTIR PND
ABG HYB
HUA
FUQMBO
GUI
AAE
QSB
GUA
CBI
Correlation Analysis
Without Sq correction
With Sq correction
Correlation coefficients as function of distance from the observatories.
The central bin gives a high correlation between the satellite and ground data. However, the correlation decays very fast, when the satellite passes further away from the station longitude.
Statistically significant correlation lengths of ~± 15º is observed in Indian and American sectors.
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Low correlation
From our ground/satellite comparison performed at various longitude separations we may conclude that this is primarily a spatial effect
The driving electric fields has large spatial scales (~ 30º)
Since we have excluded the electric field, the conductivity may be responsible for the short-range coherence of the EEJ.
A promising candidate for local conductivity modulation is plasma instability within the Cowling channel.
Is the observed variability in EEJ current strength due to spatial (23º) or temporal (93 minutes) effects ?
Reason ?
Implications ?
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Sq and EEJ
Without Sq correction
-40 -20 0 20 400
0.2
0.4
0.6
0.8
1
Distance from observatory in degrees
Cor
rela
tion
coef
ficie
ntETT-PNDPND-HYB
-40 -20 0 20 400
0.2
0.4
0.6
0.8
1
Distance from the observatory in degrees
Cor
rela
tion
coef
ficie
nt
PND-HYBETT-PND
With Sq correction
Bangalore
Bombay
Colombo
Hyderabad
Madras
Madurai
Pune
SRI LANKA
70 E 75
E 80
E 85
E 90
E
5 N
10 N
15 N
20 N
PND
ETT
HYB
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Bamako
Nouakchott
GUINEA
MAURITANIA
MOROCCO
WES
TER
N S
AH
AR
A
20 W 15 W 10 W 5 W
10 N
15 N
20 N
25 N
30 N
MBO
GUI
-40 -20 0 20 40 -0.2
0
0.2
0.4
0.6
0.8
1
Distance from the observatory in degrees
Sq and EEJ
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The uncorrelated variations in the Sq and EEJ signals show that the temporal variations of EEJ and Sq are decoupled.
Reason ?
A possible cause for the latitudinally very confined variations of the EEJ can be the penetrating electric field associated with DP2 fluctuations (e.g. Kikuchi et al., 1996, 2000).
The amplitude of these magnetic signatures is at dip-latitudes sometimes 10 times larger than at stations outside the Cowling channel (see Kikuchi et al., 1996, Fig. 2).
The Sq system, on the other hand, is driven primarily by tidal winds which do not show short-period variations
Implications ?
Monitoring of EEJ should be done with the reference observatory 4° to 5° apart from the dip latitude
>> ExB drift monitoring
Sq and EEJ
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Station PairCC without Sq correction
CC with Sq correction
Distance between the station pair (degrees)
ETT-HYB 0.93 0.94 10.26
TIR-ABG 0.94 0.94 13.4
HUA-FUQ 0.8 0.76 16.47
AAE-QSB 0.69 0.56 29.94
MBO-GUI 0.51 -0.02 18.45
GUA-CBI 0.163 -0.12 14.6
ETT-PND 0.97 0.97 3.35
PND-HYB 0.53 0.30 6.91
Summary of correlation analysis
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Conclusions
Combined analysis of satellite and ground magnetic data gave new insights on the noon-time EEJ.
The uncorrelated EEJ current strengths observed by CHAMP in its successive passes are caused by short longitudinal correlation lengths of EEJ. A suggested reason is the conductivity discontinuities in the Cowling channel due to plasma instabilities
The uncorrelated variations in the Sq and EEJ signals show that the temporal variations of EEJ and Sq are decoupled. Possibly, the penetrating electric fields from high latitude regions are responsible for the uncorrelated, short period fluctuations of current strength in EEJ
Satellite data along with data from a dedicated, a dense NS magnetometer array near geomagnetic dip-equator would be ideal to further probe EEJ
03.07.2007 | 9:30 am | IUGG | page 24/25 Manoj et al, Evidence for short ....
Satellite data.
Observatory data.
Organization / Institute Country Observatories
Instituto Geográfico Agustín Codazzi COLOMBIA FUQ
Addis Ababa University ETHIOPIA AAE
Institut Français de Recherche Scientifique pour le Développement
FRANCE MBO
Indian Institute of Geomagnetism INDIA ABG, PND, TIR
National Geophysical Research Institute INDIA ETT, HYB
Japan Meteorological Agency JAPAN CBI
National Centre for Geophysical Research LEBANON QSB
Instituto Geográfico Nacional SPAIN GUI
US Geological Survey UNITED STATES GUA
Instituto Geofisico del Peru PERU HUA
The operational support of the CHAMP mission by the German Aerospace Center (DLR) and the financial support for the data processing by the Federal Ministry of Education and Research (BMBF) are gratefully acknowledged