Geoscience Reference
In-Depth Information
3.1.1
. Nocturnal variation
A typical photocount profile is plotted in Fig. 1(a). The profile was obtained
at Hefei during the night of 23 December 2005. The resonant scattering from
the Na layer between 80-105 km is clearly evident. The wavelike structure in
the layer is caused by the wind perturbation associated with low-frequency
internal gravity waves. The non-zero count level above 110 km is caused
primarily by background noise from scattered moonlight and starlight.
1
Fig. 1(b) shows the sodium density variations with the local time and
altitudes from 20:15LT 22 December 2005 to 04:00LT 23 December 2005.
We can find during the midnight of 22 December, the sodium density
increased to a very high value (
7000 cm
−
3
), which is called sporadic
sodium layer and considered to be linked to the formation of sporadic E,
2
or the atmospheric dynamic procession (tide, gravity waves).
∼
3.1.2
. Seasonal variation
To investigate the seasonal variation, 53 nights' observational data were
selected (exclude the period of sporadic sodium layer). It has been proved
that the unperturbed sodium layer is a Gaussian profile:
exp
−
z − z
Na
)
2
2
A
Na
√
2
(
ρ
Na
=
σ
Na
πσ
Na
where
A
Na
is column abundance,
z
Na
is centroid height and
σ
Na
is RMS
width.
Seasonal variations of sodium column abundance (a), centroid height
(b) and RMS width (c) are given in Fig. 2. Triangles stand for daily
mean, error bars are daily standard deviations and dash lines are monthly
mean values. Sodium column abundance reaches a maximum value of
6
10
9
cm
−
2
in
June. It reveals a significant summertime depletion followed by a maximum
concentration in wintertime, especially in December. The monthly mean
column abundance profile shows a minimum value in April due to that
there is only one night data which lasted for 1 h in April, and we use it to
fix the data gap during April. The same seasonal variations in abundance
have been reported in other lidar sites,
3
,
4
and our results are consistent
with those previous studies. Daily centroid height varies from 91.5 km to
92.5 km and has no obvious seasonal variation generally. It is higher in
summertime and lower in wintertime. RMS width has a semiannual vari-
ation. It increases in summer and wintertime, decreases in spring and
10
9
cm
−
2
in December and a minimum value of 1
.
014
×
.
126
×
