Ground GNSS Ionosphere Sounding - GNSS Remote Sensing: Theory, Methods and Applications - page 90

4.4.5 3-D ionospheric Behaviours to Storms

4.4.5.1 Geomagnetic Conditions

A strong geomagnetic storm commenced on 20 November 2003, as confirmed by
the Dst index time series in Fig. 4.7 (upper) obtained from the World Data Center in
Kyoto ( http://wdc.kugi.kyoto-u.ac.jp/ ) . The Dst or disturbance storm time index in
nanoteslas (nT), is a measure of geomagnetic activity used to assess the severity
of magnetic storms based on the average value of the horizontal component of
the Earth's magnetic field measured hourly at four near-equatorial geomagnetic
observatories. The storm started with the arrival of the shock at about 8:00 UT on 20
November and reached a minimum Dst value of 472 nT at 20:00 UT, accompanied
with an AE index up to 2,600 nT and a maximum Kp value of 9 at about 16:00
UT. Here the AE index is the auroral electrojet index obtained from a number of
stations distributed in local time in the latitude region that is typical of the northern
hemisphere auroral zone, which provides a measure of the overall horizontal current
strength. The planetary K-index (Kp) is a disturbed level of geomagnetic field,
which can be obtained from a number of magnetometer stations at mid-latitudes.
As can be seen from these indices, the magnetic activity returned to normal level by
22 November.

4.4.5.2 Disturbance of F2-Layer Parameters

As the F2-layer peak electron density (NmF2) and its height (hmF2) are of great
influence on the shape of the ionospheric electron density profile Ne (h), the NmF2
and hmF2 are key parameters for monitoring ionospheric conditions in order to
understand the nature of ionospheric activities. Therefore, the behaviour of the
ionospheric F2-layer was investigated in detail over South Korea in terms of the
NmF2 and hmF2 during the geomagnetic storm (20 November 2003). The peak
density (NmF2) and its corresponding height (hmF2) can be easily obtained from
the ground-based GPS reconstructed results of electron density profiles with the
ionospheric height. By checking the solar geophysical index series (Dst, Kp and
AE) from the World Data Center in Kyoto in November 2003, the days 1-19 and
22-30 November are magnetically quiet. The monthly median value of electron
density profiles during these quiet days is taken as the reference and the deviation
can be regarded as the variation of ionospheric F2 peak density and height during
the geomagnetic storm.
The NmF2 and hmF2 hourly time series of monthly median, ionosonde data
and GPS observations are analyzed at the grid site (37.5 ı N, 127.0 ı E) on 20
November 2003, where the monthly median is calculated from 1 month of GPS-
derived electron density profiles. It can be seen that there are anomalous variations
during this storm, which is also confirmed by independent ionosonde observations at