Evidence for short correlation lengths of the noon-time equatorial electrojet

1
Evidence for short correlation lengths of the
noon-time equatorial electrojet inferred from a
comparison of satellite and ground magnetic data.
C. Manoj National Geophysical Research Institute,
Hyderabad, India. H. Lühr GeoForschungsZentrum
Potsdam, Germany S. Maus CIRES, University of
Colorado, USA N. Nagarajan National Geophysical
Research Institute , Hyderabad, India.
2
Equatorial Electrojet - generation
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
(Figure from Anderson et al, 2002)
3
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..

Simulated horizontal magnetic anomaly at ground
due to ionospheric currents (from CM4). Unit - nT
4
Equatorial Electrojet magnetic fields
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
5
Some open questions on EEJ
Lühr et al, 2004 reports uncorrelated
current strength between successive CHAMP passes
over EEJ. These passes are separated in space by
23º and in time by 93 minutes.
6
Some open questions on EEJ
UT 6
Is the observed variability in EEJ current
strength due to spatial (23º) or temporal (93
minutes) effects ?
23º West and 93 minutes later
UT 730
7
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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How do we go about it ?
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.
9
Roadmap
Observatory and satellite data. Data
processing Correlation analysis Results
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Observatory data
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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EEJ signals from ground data
?HEEJ ?HNon-EEJ ?H 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
12
Satellite data
Scalar magnetic field data from 2000 to
2002 Local Time 10 to 13 Kp 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
13
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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LT correction
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 find The ratio
of expected EEJ strength at observatory and
satellite local time
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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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Correlation Analysis
1
0.5
Correlation Coefficient
With LT correction
0
Without LT correction
-0.5
-40
-30
-20
-10
0
10
20
30
40
Distance from Observatory in Degrees
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Correlation Analysis
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.
Without Sq correction
With Sq correction
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Low correlation
Is the observed variability in EEJ current
strength due to spatial (23º) or temporal (93
minutes) effects ?
From our ground/satellite comparison performed at
various longitude separations we may conclude
that this is primarily a spatial effect
Reason ?
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.
Implications ?
19
Sq and EEJ
Without Sq correction

20
N
Bombay
Pune
Hyderabad
HYB

15
N
Bangalore
Madras
PND

10
N
Madurai
ETT
SRI LANKA
Colombo

With Sq correction
5
N


70
E
E


90

75
E
80
E
85
E
20
Sq and EEJ
Distance from the observatory in degrees
21
Sq and EEJ
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 gtgt ExB drift monitoring
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Summary of correlation analysis
Station Pair CC 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
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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
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Satellite data.
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
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