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

e-mail: karp@izmiran.ru

Pages: 1 | 2 | 3 | 4 | 5

      To simplify an analysis, we compare three most typical longitudinal sectors in the Northern (210, 240, and 300^o) and Southern (60, 180, and 330^o) hemispheres (see Fig. 3). The concentration in the entire region of the trough is maximum in the Northern Hemisphere at a longitude of 300^o. 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 12^o 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 210^o the plasma concentration in the trough is minimum and slightly varies with latitude; therefore, the trough is wide (17^o) and shallow (1.25) here. The trough is exhausted at a longitude of 240^o 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 240^o (see Fig. 2a). In the Southern Hemisphere, the concentration in the trough region is maximum at longitudes of about 180^o and a trough minimum is shifted poleward. Therefore, the MIT characteristics are similar to the characteristics of the analogous sector near 300^o in the Northern Hemisphere. On the contrary, at a longitude of about 330^o 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 (9^o) and shallow (1.4) trough is finally formed.

Fig. 3.

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 30-60^o 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 from 7^o to 17^o and from 9^o to 18^o in the 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 [1975] and Tulunay [1973]. They determined that nighttime MIT exists regularly under winter equinoctial conditions but is formed only at isolated longitudes under summer conditions. Besprozvannaya [1975] stated that MIT is observed at western longitudes of the Northern Hemisphere. Tulunay [1973] analyzed separately longitudinal zones of 0-180^o, 260-320^o, 320-360^o and 180-260^o 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 210-240^o and 300-330^o in the 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 30^oW-90^oE, as well as 150-210^o E (in winter) and 240-300^o E (in summer). Therefore, in the Southern Hemisphere, we will consider the 30-60^o and 210-240^o longitudinal sectors (see Fig. 4).

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, respectively.

       Cosmos-900 data shown in Fig. 4 correspond the near-midnight (2300-0100 LT) quiet (Kp <= 2) conditions of high solar activity (F_10.7 ~ 200). They were obtained in December 1978 and January 1979. A characteristic feature of the 210-240^o 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 50-55^o, which changes into the sharp decrease in Ne towards the auroral oval. The level of Ne at longitudes of 300-330^o 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 ~63^o and 62^o 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 ~8^o and 8^o 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 (5^o) and wider (12^o) at longitudes of 300-330^o and 210-240^o in the Northern and Southern hemispheres, respectively.
       The longitudinal sectors 210-240^o in the Northern Hemisphere and 30-60^o 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 (11^o and 10^o, 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 5^o and 17^o, respectively. As mentioned above, the trough is absent under winter conditions at longitudes of 210-240^o 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 200-240^o and 30-90^o in the Northern and Southern hemispheres, respectively. Under midnight equinox conditions, a pronounced trough is formed at all longitudes of both hemispheres. It is 8-11^o 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 30-60^o. In the Northern Hemisphere the latitudinal- longitudinal variations at longitudes of ~210-240^o are such that the trough is exhausted here.

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