Geoscience Reference
In-Depth Information
as darker regions and are usually located between 5
◦
and 30
◦
solar latitude (Hobiger
2005
). A widely used quantity is the Wolf sunspot number R:
R
=
k
(
f
+
10
g
)
(56)
where
f
total number of the observed single spots,
g
number of sunspot groups,
k
constant, depending on the instrumental sensitivity.
Sunspots were first noticed around 325 BC and have been recorded for several
centuries. The spectral analysis of sunspot number time series shows a very promi-
nent period of about 11.1 years. However the typical cycle is not symmetric, the
time from minimum to maximum is about 4.3 years and the time from maximum
to minimum is 6.6 years on average. The latest solar minimum was recorded in the
period 2005-2006. The next solar maximum is expected to occur in 2013. The solar
activity level is also measured by the F10.7 cm solar radio flux which is a proxy for
the solar EUV radiation.
Seasonal variations. In general, the maximum electron density and the total elec-
tron content of the nighttime F region are higher in summer than in winter. However,
in mid latitudes, the peak density of noon profiles is considerably greater in winter
than in summer; this is called winter anomaly. This anomaly is more evident in mid-
latitudes than in low and high latitudes (Feltens et al.
2009
). The winter anomaly
occurs in the daytime only and is thought to be due to a large summer electron loss
rate caused by an increase in the molecular-to-atomic composition of the neutral
atmosphere (Davies
1990
). The night time F2 layer tends to be at higher altitudes
in summer than in winter, the tendency being accentuated in lower latitudes. The
night time F2 layer tends to be thicker when higher. In the summer months, in mid
latitudes, the F layer bifurcates into the F1 and F2 layers. Under these conditions
the F2 peak density is fairly small and is located at a relatively high altitude. The
F1 layer is not so much a distinct layer but rather a minor inflection in the profile at
about 180-220km. However, in the summer or around noon time, it is more liable
to see the F1 layer as an individual layer comparing to winter time or in the sunrise
and sunset.
Diurnal variations. The most apparent effect is seen within the ionosphere as
the Earth rotates around its axis. The ionization increases in the sunlit hemisphere,
reaching a peak shortly after local noon time, and decreases on the shadowed side.
In general, the electron densities are greater at all heights by day than by night and
large diurnal changes occur particularly in the lower ionosphere. While the D layer
causes the weakening of propagated radio waves through the ionosphere during the
day, it almost disappears during the night. The E layer appears promptly at sunrise
and essentially disappears at sunset, except for the residual ionization at night.
