The dependence of the main ionospheric trough shape on longitude, altitude, season, local time, and solar and magnetic activity
A. T. Karpachev
Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation, Russian Academy of Sciences, Troitsk, Moscow region, 142190 Russia
To simplify an analysis, we compare three most typical longitudinal sectors
in the Northern (210, 240, and 300o
) and Southern (60, 180,
) hemispheres (see Fig. 3). The concentration in
the entire region of the trough is maximum in the Northern Hemisphere at a longitude of
. A trough minimum shifted poleward over the largest
distance, and a general decrease in the plasma concentration is most considerable. Therefore,
the most developed trough with a width of 12o
and a depth
of 1.7 is detected here. This is also evident in Fig. 2a, which shows the variations in the
trough depth and width. On the contrary, at longitudes of about 210o
the plasma concentration
in the trough is minimum and slightly varies with latitude; therefore, the trough is wide
) and shallow (1.25) here. The trough is exhausted at
a longitude of 240o
because the amplitudes and phases of the
NmF2 longitudinal variations in the Northern Hemisphere vary in such a way that the NmF2
profiles intersect at longitudes of about 240o
(see Fig. 2a).
In the Southern Hemisphere, the concentration in the trough region is maximum at longitudes
of about 180o
and a trough minimum is shifted poleward.
Therefore, the MIT characteristics are similar to the characteristics of the analogous sector
in the Northern Hemisphere. On the contrary,
at a longitude of about 330o
the concentration in the trough
is low and a trough minimum is located most closely to the equator. Here the electron
concentration varies with latitude in such a way that a narrow
shallow (1.4) trough is finally formed.
Variations in the MIT shape in three most typical longitudinal sectors in (a) the
Northern and (b) the Southern hemispheres according to the data of topside sounding
(NmF2) onboard the Intercosmos-19 satellite under quiet winter near-midnight conditions.
The longitudinal sector 30-60o
in the Southern Hemisphere
is peculiar. The trough depth increases sharply (up to 3.0) in this sector as compared to
the adjacent longitudes, again due to the latitudinal changes in the character of LE on
NmF2 (see Fig. 2b). Thus, the features revealed in the MIT behavior are related to the
manifestation of global LE rather than to the trough formation mechanism. The LE character
depends mainly on the geomagnetic field parameters and on the neutral wind [Deminov and
Karpachev, 1988]. The velocities and phases of the meridional and zonal components of the
neutral wind vary with local time; therefore, the character of LE should also vary. The
conclusions drawn above hold true only for near-midnight hours. The errors in determining the
trough depth and width are related to the same factors because Intercosmos-19 satellite data
were averaged over a rather wide time interval (0000-0200 LT).
Thus, the trough depth in the Northern and Southern hemispheres varies from 1.1 to 1.7 and
from 1.4 to 3.0, respectively. The trough width counted from the inflection point varies
Northern and Southern hemispheres, respectively.
4. SEASONAL VARIATIONS IN THE NIGHTTIME MIT SHAPE
Seasonal variations in the MIT shape (or speaking more generally, the Ne variations in the
MIT region) were considered in detail by Besprozvannaya  and Tulunay .
They determined that nighttime MIT exists regularly under winter equinoctial conditions
but is formed only at isolated longitudes under summer conditions. Besprozvannaya 
stated that MIT is observed at western longitudes of the Northern Hemisphere. Tulunay
 analyzed separately longitudinal zones of 0-180o
in either hemisphere. The reasons for such
a choice are obscure because it is independent of the LE character in the Northern and
Southern hemispheres. Intercosmos-19 and Cosmos-900 data make it possible to determine
the most characteristic longitudinal sectors in both hemispheres. These longitudes are
Northern Hemisphere, where Ne in all seasons is minimal and maximal, respectively and the
distance of a trough minimum from the pole is the largest and the smallest, respectively.
Similar longitudinal sectors in the Southern Hemisphere are
E, as well as
E (in winter) and 240-300o
E (in summer).
Therefore, in the Southern Hemisphere, we will consider the
longitudinal sectors (see Fig. 4).
Seasonal variations in the MIT shape in the longitudinal sectors (a)
300-330o and (b) 210-240o in
the Northern Hemisphere and (c) 210-240o and (d)
30-60o in the Southern Hemisphere according to Cosmos-900
data satellite for quiet nighttime conditions at high solar activity. The top, middle,
and bottom curves in each figure correspond to summer, equinoctial, and winter conditions,
Cosmos-900 data shown in Fig. 4 correspond the near-midnight (2300-0100 LT)
quiet (Kp <= 2) conditions of high solar activity (F10.7 ~ 200).
They were obtained in December 1978 and January 1979. A characteristic feature of the
210-240o longitudinal sector in the summer Southern
Hemisphere is the presence of the so-called ionization "cliff", i.e., the increase in Ne at
latitudes of 50-55o, which changes into the sharp decrease
in Ne towards the auroral oval. The level of Ne at longitudes of
300-330o in the Northern Hemisphere is also high in summer;
however, "cliff" is not formed, and Ne monotonously decreases towards high latitudes.
The causes of such an Ne behavior were considered by Karpachev [1995, 1996]. In the equinox
conditions, the background concentration decreases not very strongly as compared to the
summer conditions. Nevertheless, a pronounced trough with a minimum at latitudes of
~63o and 62o in the Northern
and Southern hemispheres, respectively, is formed at the longitudes in question, since in the
nonsunlit period the "length" of the shadow becomes considerably larger than in summer
and the trough has got enough time to be formed. The MIT width in near-midnight hours is
~8o and 8o and the depth
is 1.6 and 1.4 in the Northern and Southern hemispheres, respectively. In winter the
background concentration in both hemispheres decreases at all latitudes but especially
strongly at a trough minimum. Therefore, MIT becomes deeper: up to 2 and 2.6 in the
Northern and Southern hemispheres, respectively. MIT becomes narrower
(5o) and wider (12o) at
longitudes of 300-330o and 210-240o in
the Northern and Southern hemispheres, respectively.
The longitudinal sectors 210-240o in the Northern Hemisphere
and 30-60o in the Southern Hemisphere are poorly sunlit at
midnight even under summer conditions. The neutral wind behaves in such a way that the
background concentration is minimal and a trough is as a rule formed [Karpachev, 1995, 1996].
The summer trough is not distinct even in the region of total shadow and, therefore, its
depth does not exceed 1.3 in both hemispheres. During equinoxes, when the nonsunlit period
increases, the concentration in MIT more intensely and MIT becomes deeper (1.4 and 2.3 in the
Northern and Southern hemispheres, respectively) and wider
(11o and 10o, respectively). In
winter the concentration is lower at all latitudes and MIT is observed from sunset to sunrise,
it is pronounced during the entire this period, and its depth and width increase in the
Southern Hemisphere up to 5o
and 17o, respectively. As mentioned above, the trough is
absent under winter conditions at longitudes of 210-240o in the
Northern Hemisphere. Thus, the seasonal variations in the MIT shape differ strongly in
different longitudinal sectors and hemispheres. There is no sense in analyzing these variations
on the basis of onboard data without taking into account the longitude and hemisphere. In
summer weakly pronounced MIT (with a depth of 1.4) exists only at a minimum of the background
concentration at longitudes of 200-240o and
30-90o in the Northern and Southern hemispheres, respectively.
Under midnight equinox conditions, a pronounced trough is formed at all longitudes of both
hemispheres. It is 8-11o wide and 1.4-2.5 deep. In winter
the MIT depth increases up to 2-5 and, as a rule, MIT becomes even wider. In all seasons
in the Southern Hemisphere, the MIT is most pronounced at longitudes of about
30-60o. In the Northern Hemisphere the latitudinal-
longitudinal variations at longitudes of ~210-240o are such
that the trough is exhausted here.
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