IAGA2005-A-01304

Variations of field-aligned currents on thenightside inferred from mid-latitude groundbased geomagnetic data: Relationships with theIMF and substorms

S. Nakano(The Institute of Statistical Mathematics; JST CREST)
T. Iyemori(Data Analysis Center for Geomagnetism and SpaceMagnetism, Kyoto University)

Introduction
 Our knowledge about field-alignedcurrents (FACs) has been mostly provided
by statistical analyses of spacecraft data.
 However, it is difficult to observe thetemporal variation of FACs by spacecraft.
We have little knowledge about thecharacteristics of temporal variation ofFACs during disturbed times.

Introduction
 Mid-latitude east-west magnetic variations

on the ground are mostly due to FACs,especially on the nightside.

Mid-latitude magnetic data on thenightside enable us to obtain information
about the temporal variation of FACs.

In this study...
 We examined temporal variations of theeast-west component of mid-latitude
geomagnetic field on the nightside, which
is attributed to the FACs.
 In particular, we shall report thecharacteristics of mid-latitude east-west
geomagnetic variations during the main
phase of magnetic storms.

Geomagnetic observatories

Data Processing
 We used the eastward component in the dipole coordinate

system in order to avoid a mixing of the effect from thering current etc. (Hereafter we denote the eastward
component in the dipole coordinate system by "D"component. ) The signs of D for the observatories in the southernhemisphere were inverted.
And we defined “positive” disturbances as the eastward/westward
disturbances in the northern/southern hemisphere. Positive
disturbances correspond to the effects of upward FACs at higherlatitudes.

Data Processing The Sq field for each month was subtracted to obtain theeastward disturbance field, ΔD , for each mid-latitude
observatories.
 Latitudinal dependence of magnetic effects of FAC wasremoved. (Here the ΔD for each observatory wasnormalized in order that its standard deviation obtained
within 2100-0300 MLT is 10.0 nT. ) Then the MLT profile of ΔD for each minute was derived
by linear interpolation in MLT from the mid-latitude
magnetic data.

The MLT-UT map of eastwest disturbances ΔD

Dst field (SYM-H index)

The z component of the IMF
observed by ACE
ΔH in the auroral region
22-1 MLT and 1-6 MLTWhen the IMF is southward, positive east-west disturbances due to upward
FAC are observed on the almost entire nightside.
We shall refer to this disturbances as NPED (Nightside Positive
East-west Disturbance) in the following.

To summarize the phenomena, we defined two parameters:
ψ: The average of ΔD within 2100<MLT<0300.
ζ: The average of ΔD within 2100<MLT<0000 minus theaverage of that within
0000<MLT<0300

The MLT-UT map of eastwest disturbances ΔD

Dst field (SYM-H index)

The z component of the IMF
observed by ACE
Mid-latitude geomagnetic activity
ψ: NPED
ζ: Positive - R1 sense
ΔH in the auroral region
22-1 MLT and 1-6 MLT

Comparison between ψ and the merging
electric field
Corr. = 0.67

The merging electric field was calculated from OMNI solar wind data
by using the equation(Kan and Lee [1979]).

The MLT-UT map of eastwest disturbances ΔD

Dst field (SYM-H index)

Two step development

The z component of the IMF
observed by ACE
Mid-latitude geomagnetic activity
ψ: NPED
ζ: Positive - R1 sense
ΔH in the auroral region
22-1 MLT and 1-6 MLT

The MLT-UT map of eastwest disturbances ΔD

Dst field (SYM-H index)

The z component of the IMF
observed by ACE
Mid-latitude geomagnetic activity
ψ: NPED
ζ: Positive - R1 sense
Green: ΔD at 0230 MLT
ΔH in the auroral region
22-1 MLT and 1-6 MLT

The MLT-UT map of eastwest disturbances ΔD

Dst field (SYM-H index)

The z component of the IMF
observed by ACE
Mid-latitude geomagnetic activity
ψ: NPED
ζ: Positive - R1 sense
ΔH in the auroral region
22-1 MLT and 1-6 MLT

Results
 When the IMF is southward, positive east-west disturbances due toupward FACs are observed on the nightside.
 While the nightside positive east-west disturbances (NPED) are

principally associated with the southward IMF, they can also beintensified by substorm expansions especially in the post-midnight if theIMF is southward. This suggests that substorms can result in thedevelopment of the upward currents in the post-midnight.
(The positive disturbances after substorm onsets exhibit essentially different
behavior from the current wedge signature.)Effect of the development ofmagnetospheric convection(e.g. Vasyliunas 1972)Effect of substorms
What causes the development of the upward FACs on thenightside at substorm onsets?
 The NPEDs continue even after the decay of the current

wedge as long as the IMF is directed southward.
This fact should be interpreted to indicate that thedevelopment of the upward FACs is not a transientphenomenon such as the formation of the wedge
current system.
But it suggests that a change of themagnetospheric and ionospheric conditions would
cause the development of the upward FACs.

Effect of substorms
What causes the development of the upward FACs on thenightside at substorm onsets?
 The injection of energetic particles into the innermagnetosphere is one of possible crucial changes
causing the development of the upward FACs atsubstorm onsets.
However, the electric and magnetic fields would highly change in the innermagnetosphere in a substorm expansion phase. This could complicate themechanism of the generation of the upward FACs.

Summary We examined temporal variations of the east-west geomagnetic field atmid-latitudes. The results are summarized as follows: When the IMF is southward, positive east-west disturbances due toupward FACs are observed on the nightside. However, the positive
disturbances are also often intensified by a substorm expansion
especially in the post-midnight if the IMF is southward.
 The positive disturbances after substorm onsets exhibits essentiallydifferent behavior from the current wedge signature.
These fact suggests that the upward Region 2 currents in the postmidnight are mainly controlled by the convection electric field andthat they can be also intensified by a substorm expansion.

 The development of the upward currents at substorm onsets

possibly results from the injection of energetic particles from themagnetospheric to the inner magnetosphere.

Acknowledgements
The geomagnetic data and the geomagnetic indices used in thispaper were obtained through Data Analysis Center for
Geomagnetism and Space Magnetism, Kyoto University. We
appreciate all the staffs of INTERMAGNET who kindly provided thegeomagnetic data. The OMNI solar wind data were obtained
through the NSSDC OMNIWeb database. We also thank the ACE
Magnetic Field Instrument team and the ACE Science Center forproviding the ACE data. One of the author (S. Nakano)acknowledges support from Japan Science and Technology Agency(JST) CREST.