Geoscience Reference
In-Depth Information
period II (from November 1, 2002 to June 4, 2003) are investigated, and
about 50% (1,678) profiles present a detectable secondary layer.
The peak density of the secondary layer for both periods decrease
with the increase of the solar zenith angle, and can also described by
N
m
=
N
m0
cos
k
χ
just like that for the primary layer. However, the SZA
dependence of the peak height is different for each period: for period I, the
peak height increase with SZA, similar to that for the primary layer; on
the contrary, for period II the peak height is nearly constant when SZA is
increasing (Fig. 2).
The peak density of the secondary layer is obviously modulated by the
solar radiation (Fig. 3) just like the case for the primary layer. The peak
density is positively correlated to the solar radiation (Fig. 4). Again, there
are apparent differences between the solar radiation dependence for the two
periods.
Since the variations of SZA,
L
s
and the latitude coverage for the two
datasets are quite similar, the disagreement of the behaviors of the sec-
ondary layer for both periods needs to be attributed to other reason(s).
The first candidate is the solar irradiance. Table 2 summarizes the solar
irradiance conditions during these two periods. The average E10.7 is 172.7
during period I, and 148.4 during period II. These correspond to E10.7*
value of 67.8 and 61.5, respectively. Obviously, the solar activities are dif-
ferent for these two periods.
In this paper, the E10.7 index is used as a measure of the solar irra-
diance input to the Martian secondary layer. In fact, however, one of the
dominant ionization source for the Martian ionospheric secondary layer
is soft X-rays.
12
,
14
The increase in the fluxes of soft X-rays is not linear
with the increase of average E10.7 and the secondary peak behaves dif-
ferently at different solar radiation conditions.
12
,
13
These could directly
contribute to the disagreement of the secondary peak's behaviors at dif-
ferent time. A reasonable way to improve is to use direct X-ray flux mea-
surements for further investigation; however, it is beyond the goal of this
paper.
Table 2.
Summary of solar fluxes conditions of two periods.
Minimum
Maximum
Average
Deviation of
Period (days)
E10.7*
E10.7*
E10.7*
E10.7*
Period I (209)
51.3
113.1
67.8
10.4
Period II (216)
40.1
79.4
61.5
8.6
