Local-time distribution of net field-aligned currents
derived from high-altitude satellite data
Nakano S. and T. Iyemori(Graduate School of Science, Kyoto University)

AbstractN
3 - 15 MLT

The dawn-dusk asymmetry of the north-south geomagnetic disturbances at low andmid latitudes has been indicated by many studies. Some researchers have attributed
such asymmetrical geomagnetic disturbances to net field-aligned currents, flowing
into the ionosphere on the dayside and flowing out of the ionosphere on thenightside. However, those net field-aligned currents have not been established by
low-altitude spacecraft observations yet. We statistically investigated the global
structure of the geomagnetic disturbances generated by the field-aligned currents in
the inner magnetosphere by using the magnetic field data obtained by the DE-1
satellite. On the basis of the results, local-time dependence of the net field-aligned
currents was estimated. As a result, net downward currents are found in the prenoon sector and net upward currents are found in the pre-midnight sector. In thepost-noon and the post-midnight sector, there exist only small amounts of the net
field-aligned currents. Considering that Region 2 currents are at lower latitudes thanRegion 1 currents, the result of this study can explain the features of geomagnetic
disturbances at low and mid latitudes.

45Fig. 1: Meridional distribution of
azimuthal magnetic disturbances, ΔBφon the meridional planes of 3-15 MLT(top panel) and 9-21 MLT (bottompanel), under the condition ofAE > 300. Eastward disturbances arerepresented by red and westward
disturbances are represented by blue.
(Nakabe et al. 1997)

A (nT)
1850100

9 - 21 MLT021912345-100-50

050

100

S(nT)

100

50500-50CDFig. 2: Considering the Ampere integral of magnetic
field of the closed path ABCD, the azimuthal
magnetic field proves to be approximately inversely
proportional to distance from the dipole axis of theearth (r cos ) along a terrestrial field line.

Westward

50-50

70

80

5060

Downward

8090

Magnetic Lat. (deg.)

20-21 MLT
150

R2R1

Polar cap

10050A (nT)

50A (nT)

2570-5090150

Eastward
0(nT)

10FAC density

60100-25

Polar cap-100

Polar cap

15-16 MLT60

R10Magnetic Lat. (deg.)(kA/deg)

0-50

0-50-100-100-150-1505060708090

Magnetic Lat. (deg.)

Polar cap

50R25060

R1708090

Magnetic Lat. (deg.)

Fig. 4: The latitudinal profile of the averages of A under thedisturbed condition at the meridian of 3-4 MLT (upperright), 8-9 MLT (upper left), 15-16 MLT (lower left), and20-21 MLT.

Results
Structure of azimuthal magnetic disturbances
At low latitudes, eastward disturbances are predominanton the nightside, and westward disturbances are
predominant on the dayside.

0-100612MLT

1824Upward

Fig. 5: The local-time profile of the net fieldaligned currents.

Dusk

R2-15050

Local-time distribution of net field-aligned currents

whereandare the latitudes of thefootprints.
Fig. 5 shows the local-time
distribution of net field-aligned currents derived
from latitudinal profiles of A as shown in Fig. 4.

R1-1506Fig. 3: The latitudinal and local-time distributionof A under the condition of AE > 300. Positive A
represented by red denotes eastward
disturbances and negative A represented by blue
denotes westward disturbances in the northernhemisphere. At low latitudes, eastward
disturbances are predominant on the nightside,
and westward disturbances are predominant on
the dayside especially in the afternoon sector.

r2 cosλ2 dφ

Suppose that the field-aligned currents are in
sheet-like structures aligned at L-shells and the
disturbance fields in other components than theazimuthal component are negligible, we canderive net field-aligned current density perlongitude J at a meridian as:

R270A

3 - 4 MLT
15010080

BΔB20N150-100
900FAC-50(nT)

8 - 9 MLT: dipole latitude

ΔB1-100

Sr : radial distance from the center of the earth

r1 cosλ1 dφEarth312A is approximately constant along a terrestrial field line. For
the data in the southern hemisphere, the sign is inverted. The
data in low latitudes ( | | < 20°) are not used in this analysis.

dφ2Azimuthal magnetic disturbances in the inner magnetosphere

In order to treat data at different altitudes equally, it isassumed that magnetic disturbances parallel to the terrestrialfield line is negligible. Then when we define a parameter A by,

31A (nT)

Method015

DawnFig. 6: A possible current system which explain our results.

In the polar cap, uniform sunward disturbances areobserved.

Local-time distribution of net field-aligned currents
Net downward currents are mainly seen in the pre-noon
sector, and net upward currents are mainly seen in the
pre-midnight sector on the average.
In the post-noon and the post-midnight, net field-aligned
currents are relatively small, which means that Region
1 currents are almost balanced with Region 2 currents.

Effect on geomagnetic disturbances at mid-latitudes
ΔH

Poleward sheet

FAC density(kA/deg)
2012NetΔD(nT)
10Equatorward sheet

Summary

51018900

There are net downward currents in the pre-noon sector
and net upward currents in the pre-midnight sector.

6080
7060

-10-2006121824In the post-noon and the post-midnight sector, there exist
only small amounts of the net field-aligned currents.

-5500Poleward sheet
Equatorward sheet

MLT-100

6121824MLTFig. 7: Model field-aligned current distribution for the modelcalculation. The top panel indicates the local-time dependenceof the field-aligned current density per longitude for each of
double sheet currents in this model. The bottom panel
indicates the spatial distribution of the field-aligned currents.

Fig. 8: Local-time dependences of themagnetic effects of the field-aligned currents
on geomagnetic disturbances at 30°in dipole
latitude derived from the model calculation.

Considering that Region 2 currents are at lower latitudes
than Region 1 currents, the result of this study canexplain the features of geomagnetic disturbances at low
and mid latitudes.