| Large-scale TEC perturbations/enhancements observed during the day sectors
of major storm periods, 12-13 February 2000, 23 September 1999, 29 October
2003, and 21 November 2003, were studied using a high resolution GPS network
over Japan. TEC enhancements described in the present study have large
magnitudes (≥25×1016 electrons/m2) compared to the
quiet-time values and long periods (≥120 min). The sequential manner of
development and the propagation of these perturbations show that they are
initiated at the northern region and propagate towards the southern region of
Japan, with velocities >350 m/s. On 12 February 2000, remarkably high
values of TEC and background content are observed at the southern region,
compared to the north, because of the poleward expansion of the equatorial
anomaly crest, which is characterized by strong latitudinal gradients near
35° N (26° N geomagnetically). When the TEC enhancements, initiating
at the north, propagate through the region 39-34° N (30-25° N
geomagnetically), they undergo transitions characterized by a severe decrease
in amplitude of TEC enhancements. This may be due to their interaction with
the higher background content of the expanded anomaly crest. However, at the
low-latitude region, below 34° N, an increase in TEC is manifested as an
enhanced ionization pattern (EIP). This could be due to the prompt
penetration of the eastward electric field, which is evident from high
values of the southward Interplanetary Magnetic Field component (IMF Bz)
and AE index. The TEC perturbations observed on the other storm days also
exhibit similar transitions, characterized by a decreasing magnitude of
the perturbation component, at the region around 39-34° N. In addition to
this, on the other storm days, at the low-latitude region, below 34° N,
an increase in TEC (EIP feature) also indicates the repeatability of the above
scenario. It is found that, the latitude and time at which the decrease in
magnitude of the perturbation component/amplitude of the TEC enhancement are
matching with the latitude and time of the appearance of the high background
content. In the present study, on 12 February 2000, the F-layer height
increases at Wakkanai and Kokubunji, by exhibiting a typical dispersion
feature of LSTID, or passage of an equatorward surge, which is matching with
the time of occurrence of the propagating TEC perturbation component.
Similarly, on 29 October 2003, the increase in F-layer heights by more than
150km at Wakkanai and 90 km at Kokubunji around 18:00 JST, indicates the
role of the equatorward neutral wind. On that day, TEC perturbation
observed at the northern region, after 18:30 JST, which propagates towards south,
could be caused mainly by the equatorward neutral wind, leading to an F-layer
height increase. These observations imply the role of the equatorward neutral
wind, which increases the F-layer height, by lifting the ionization to the
regions of lower loss during daytime, when production is still taking
place, which, in turn, increases the TEC values.
Large-scale traveling ionospheric disturbances (LSTIDs) are considered as
ionospheric manifestations of the passage of Atmospheric Gravity Waves
(AGWs) that are generated at the high latitude by energy input from
the magnetosphere to the low-latitude ionosphere. This study shows that
large-scale TEC perturbations observed here are produced at the northern
region due to the combined effects of the equatorward neutral wind, the subsequent
F-layer height increase, and LSTIDs. When these perturbation components
propagate through the region, 39-34° N, they undergo transitions
characterised by a decrease in magnitude. Also, at the low-latitude region,
below 34° N, an increase in the TEC exhibits EIP feature, due to the prompt
penetration of the eastward electric field. |