2. Data of the Measurements
[3] The distributions of
hmF2 in the belt of the invariant latitudes
between
40o and
65o according to the Intercosmos 19 data were
built. The data cover conditions of the local summer in the
Northern and Southern hemispheres during the solstices for the
period with high solar activity ( F10.7
200 ) since 1979 till 1981.
The data for very quiet conditions ( AE < 300 nT) were chosen in
order to minimize the influence of the electric fields and acoustic
gravity waves (AGW) and therefore to reduce the data scatter and
to increase the accuracy of the representation of the
hmF2 distribution. As a result, the stable background state of the quiet
ionosphere determined (as one of the factors) by the undisturbed
wind system was found. About 100 and more than 60 orbits were
chosen in the Northern and Southern hemispheres, respectively.
The satellite orbits in both hemispheres are oriented in such a way
that the local time increases with a latitude increase from
2300 LT (40o
L ) to
0100 LT (65o
L ).
The local time at the fixed
invariant latitude changes with the longitude also. In order to
eliminate this weaker dependence, the data were corrected taking
into account the diurnal variations in
hmF2 from the
International Reference Ionosphere
(IRI) model.
The correction was carried out to the nearest hour in local time:
2300 LT for 40o
L, 2330 LT for 50o
L,
2400 LT for 60o
L, and 0100
LT for 65o
L.
Thus the obtained distribution is not a LT map in a
literal sense; however, all data fall into a narrow interval of the
near-midnight hours, and this does not complicate strongly the
analysis. The other circumstance is more important: the auroral
ionosphere at 0100 LT at ~70% of longitudes is sunlit, so in the
period when at a latitude of 40o
L purely night conditions are
realized, some intermediate conditions are maintained at 65o
L,
that
should be taken into account in the calculations.
[4] The data on the electron temperature
Te obtained from in situ
measurements on board the Cosmos 900 satellite were also used in
calculations. The measurements were conducted almost in the
same conditions as the measurements of
hmF2 at altitudes of
~370 km and ~470 km in the Northern and Southern hemispheres,
respectively
[Karpachev et al., 1997].
A correction of the
Te values
on the altitude using the IRI model was performed in the Southern
Hemisphere. The
Te distribution in the latitude belt from 40o
L to
65o
L was obtained by averaging of 70 and 100 satellite orbits in
the Northern and Southern hemispheres, respectively.
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Figure 1
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[5] Let us consider longitude-latitudinal variations in
hmF2 at
middle (40o
L and 50o
L ),
subauroral (60o
L ), and auroral (65o
L )
latitudes (see Figure 1). One can see in Figure 1 that the mean
value of
hmF2 decreases at the transition from 40o
L to 65o
L.
This
decrease would be even stronger at a fixed local time, because at
its change from 2300 to 0100 LT the
F2 layer (at all latitudes) vice
versa ascents by ~10 km compensating the
hmF2 decrease with
latitude. The analysis of the Intercosmos 19 data shows that the
character (amplitude and shape) of the averaged longitudinal
variations in
hmF2 is very stable at the fixed latitude in spite of
quite large day-to-day variations. The stability of LE is mainly a
manifestation of the undisturbed wind system stability. At a
transition from middle latitudes to auroral ones LE changes weakly
by the amplitude which is about 45-50 km and 65-70 km in the
Northern and Southern hemispheres, correspondingly. The changes
in the shape of the LE are also small and manifest in an eastward
shift of the phase. Let's compare the longitudinal variations in
hmF2 obtained from the Intercosmos 19 data with the
hmF2 variations according to the IRI model
[Bilitza, 1990]
obtained for
the same conditions from the ground-based sounding data. The
zonally averaged values of
hmF2 obtained from the topside and
ground-based sounding differ only slightly (no more than 5 km).
However, at some particular longitudes the difference may be
considerable. The strongest discrepancies (20-30 km) occur in the
Southern Hemisphere at longitudes of the Indian Ocean and Pacific
Ocean, where there are no ionospheric stations. As a result IRI
model does not adequately reproduce the shape of the longitudinal
variations in
hmF2 in both hemispheres for the considered
conditions, therefore analyzing these variations one cannot
determine their causes. This can be done only using the
Intercosmos 19 data what cover homogeneously all longitudes and
latitudes in both hemispheres. It is worth noting, however, that
both data sets distinctly reproduce a local maximum in
hmF2 at
longitudes of about
150-210o in the Southern Hemisphere.
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