Ground GNSS Ionosphere Sounding - GNSS Remote Sensing: Theory, Methods and Applications - page 89

Fig. 4.15 Averaged diurnal
variations of the seasonal
median of the EST
(equivalent slab thickness)

600

Winter
Spring
Summer
Autumn

550

500

450

400

350

300

250

0 5 10 15 20 25

Local Time (hours)

D TEC = NmF2 (4.34)

where Nm F2 is the maximum electron density of the F-region in el / m 3 .TheESTis
then obtained as a function of local time at intervals of 1 h for each day separately.
The averaged diurnal medians for December-January-February (DJF), March-April-
May (MAM), June-July-August (JJA) and September-October-November (SON)
have been considered to represent the winter, spring, summer and autumn seasons,
respectively. Here some days with no or bad observation data are not used.
Using the Eq. ( 4.34 ), the equivalent slab thickness (EST) was determined
from the GPS-derived TEC and NmF2 (Jin et al. 2007 ). Figure 4.15 shows the
corresponding diurnal variation of EST in four seasons. An oscillating hourly
variability is observed. A clear diurnal variation as well as considerable variability
is observed. The large scatter at any fixed local time shows the importance of
significant day-to-day variability of values between 250 and 580 km. It can also
be noted that the daytime (07-20 LT) EST shows seasonal variations, with higher
values during summer and equinox as compared to the winter, and their magnitude
of diurnal variation is also reflected by the EST ratio. In addition, the nighttime (00-
07 LT and 20-24 LT) values, however, do not show significant seasonal variations.
During 07-20 LT in daytime, in general the larger EST values are observed in
summer while the smaller ones are in winter, and the lower values for all seasons
are observed during 0-07 LT and 20-24 LT in nighttime. The EST is significantly
smaller in winter than in summer but with a consistent variation pattern. During 13-
16 LT, in general the larger EST values are observed in spring and autumn while the
smaller ones are in summer and winter. In addition, two peaks appear to dominate
the EST diurnal variation around 10-18 LT, especially strong dual peaks in summer.
The first peak is around 10-14 LT (pre/postnoon) and the second peak is around
17-18 LT (postsunset).