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

       Data for orbits 1 and 4 have been obtained for approximately identical conditions: longitudes of 36^o and 25^o, Kp ~ 1, and t ~ t* ~ 0.8. However, the character of electron concentration variations at a height of 1000 km differs dramatically for these two cases. Discussing this feature, we note that the lower working frequency of an ionosonde is 0.3 MHz. Therefore, the Ne(1000) values below a threshold level of about 103 cm^-3 formally correspond to zero value in Fig. 9. Figure 9 shows that for the orbits in question Ne(1000) < 103 cm^-3 at high latitudes and, therefore, a pronounced MIT polar wall is almost not distinguished here. Consequently, at any concentration at a trough minimum, its ratio to the concentration at the polar wall would be much less than to the concentration at the equatorial wall. This statement contradicts the conclusions drawn by Ben'kova et al. [1993] but agrees with the commonly accepted conclusion that the polar wall becomes less pronounced with increasing altitude. On the contrary, a knee in the latitudinal behavior of the electron concentration, which is the ionospheric projection of the old plasmapause, is clearly detected at lower latitudes (about 55^o). The new position of the plasmapause is apparently located at = 65^o. This corresponds to the plasmapause position after a long undisturbed period. Data of orbits 1, 2, and 3 demonstrate the effect of gradual filling of the shells between the new and old plasmapauses. According to the theory (see, e.g., Krinberg and Tashchilin [1984]) mainly relatively low L shells are filled up. The knee in Ne(1000) is located at = 54^o, 57^o, and 62^o for orbits 1, 2, and 3, corresponding to t ~ 0.7, 1.8, and 2.9 days after the magnetic disturbance, respectively. However, such a shift apparently mismatches the plasmapause shift somewhat in the equatorial plane. The process of filling proceeds in such a way that the plasmapause acquires a two-step structure in the course of t 0.7 days and only the polar step approximately corresponds to the knee in Ne(1000), since the time of complete filling of the plasmasphere is substantially longer than 3 days. All parameters of orbit 4 formally coincide with those of orbit 1; however, the knee in Ne(1000) is located at = 65^o rather than 55^o. This can apparently be explained by the fact that the disturbance of June 7-8 was relatively short and the L shells were depleted not so intensely as in the previous series of storms between the new and old plasmapauses. Mainly hydrogen ions maintain the process of filling of the geomagnetic field tubes with plasma. This process is substantially more durable than the NmF2 relaxation to the undisturbed state [Krinberg and Tashchilin, 1984]. Nevertheless, the old plasmapause, which actually existed in all cases, affected the position of the base of the MIT equatorial wall, which always remained near = 55^o. The relation between the position of the old plasmapause and the base of the MIT equatorial wall is evidently provided by the residual ring current, which exists near the old plasmapause for a long time after the magnetic storm (see, e.g., Burke et al. [1979]). The processes proceeding at latitudes of the ring current lead to heating of the ionospheric plasma [Cornwall and Coroniti, 1971], enhancement of O+ recombination near the F2 layer maximum, and, consequently, depletion of NmF2.
       Thus, at low solar activity, the dynamics of the equatorial wall at high altitudes sharply differs from its dynamics at altitudes of the F2 layer. Consequently, the MIT characteristics (depth and width) will be also considerably different. For example, the trough depth counted relative to the equatorial wall varies at heights of the F2 layer from 2 to 5 for different cases. The plasma concentration at a trough minimum at a height of 1000 km is not determined; however, the trough depth for orbits 1 and 4 exceeds 6, which is certainly higher than at altitudes of the F2 layer. This fact agrees with the conclusions drawn by Ben'kova et al. [1993]. Such trough behavior at low solar activity is adequately explained in terms of depletion of the subauroral geomagnetic tubes [Krinberg and Tashchilin, 1984]. At high solar activity, the background concentration is much higher, geomagnetic tubes are filled up much more rapidly, and so the trough shape remains almost unchanged with height.
       Remind that the above conclusions have been drawn on the whole for an increased level of geomagnetic activity. However, during the observations the Kp index varied from 0+ to 2-. Thus, the trough shape largely depends on the prehistory of geomagnetic activity. Sivtseva and Ershova [1989] have drawn similar conclusions. They have shown that the MIT polar and equatorial walls at heights of the topside ionosphere are more closely related to the dynamics of O+ and H+ ions, respectively. The total concentration of ions in the MIT region depends on the ratio of concentrations of these two ions, which can be widely variable at different latitudes in the process of disturbance development.

9. CONCLUSIONS

       The large data set of Intercosmos-19, Cosmos-900, and Cosmos-1809 satellite data makes it possible to analyze the variations in the MIT shape based on a unified point of view. It is also of importance to accurately take into account considerable longitudinal variations in the position and shape of the trough. The revealed dependences of the MIT shape on longitude and hemisphere make it also possible to eliminate some contradictions between different studies based on different data sets. This allows us to come very close to constructing a MIT shape model. It is crucial to solve this problem for the subauroral ionosphere. The following particular results have been obtained:
       (1) We have demonstrated that the longitudinal variations in the MIT shape are determined by the changes in the form, amplitude, and phase of the NmF2 longitudinal variations at different latitudes in the trough region. The deepest trough is observed at longitudes of 300-330^o (1.7) and 30-60^o (3.0) in the Northern and Southern hemispheres, respectively. In the Southern Hemisphere the trough is least pronounced at longitudes of about 330^o. The trough is completely exhausted in the vicinity of 240^o longitudes in the Northern Hemisphere. The trough width in both hemispheres varies with longitude within 7-17^o.
       (2) We have demonstrated that an analysis of the seasonal variations in the MIT shape without taking into account longitude and hemisphere results in contradictions. Under summer conditions (at altitudes of 350-550 km) insignificant MIT (with a depth of <1.4) exists only at a minimum of the background concentration, i.e., at longitudes of 200-240^o and 30-90^o in the Northern and Southern Hemispheres, respectively. Under the midnight equinoctial conditions, a pronounced trough is formed at all longitudes of both hemispheres. It has a width of 8-11^o and a depth of 1.4-2.5. In winter the MIT depth increases up to 2-5^o everywhere except the longitudinal sector 210-240^o in the Northern Hemisphere, where the trough is absent.
       (3) The diurnal variations in the MIT shape at a height of about 500 km have been distinctly detected from Cosmos-900 data. MIT is most developed in the dusk sector. Its depth and width at longitudes of 300-330^o in the Northern Hemisphere are 3-4 and about 3^o, respectively. Closer to midnight the MIT depth decreases to 2 and the width increases to 5^o. MIT becomes shallow (1.5) and wide (~11^o) by the morning. The electron concentration at a trough minimum varies weakly during the night, in the evening the concentration being even slightly lower than at midnight.
       (4) If the Kp index increases from 2 to 5, the trough width at longitudes of 300-330^o in the Northern Hemisphere, on the average, decreases from 3.5-5^o to 2.5-4^o and the trough depth increases from 2.3 to 3. However, if a Te peak is developed at the bottom of the MIT equatorial wall, the trough width may increase up to 6-10^o. Moreover, the situation depends on how the positive and negative phases of a storm develop at the walls and at a minimum of MIT. This is the cause of principal discrepancies arising in an analysis of the MIT shape variations during a storm. Thus, the problem of MIT shape variations with magnetic activity cannot be solved merely by averaging data.
       (5) At very high solar activity, MIT is determined mainly by the presence of the highly developed polar wall at heights of the F2 layer. If solar activity decreases from F_10.7 ~ 180 to ~ 90, the MIT depth at isolated latitudes may increase from 1.5 to 5 and the width may increase slightly. As a result, at low solar activity both MIT walls become almost equally steep. This is a rather seldom example, when the conclusions of different studies agree with one another.
       (6) An analysis of Cosmos-1809 data for winter conditions at low solar activity shows that the MIT shape depends on the ionospheric dynamics (related to disturbances) much stronger than at high activity:
   - The MIT polar wall is more pronounced than the equatorial wall at heights of the F2 layer. The position of the polar wall and the concentration within the trough depend mainly on the DPB variations during a storm. The position of the bottom of the equatorial wall of the trough in NmF2 is determined by the residual ring current and old plasmapause, whose positions remain almost unchanged in the course of about 3 days after a storm.
   - At altitude of 1000 km, the polar wall is insignificant and, vice versa, the equatorial knee in the Ne latitudinal distribution is clearly defined. The position of the base of this knee is determined by the degree of depletion (depending on the time lapsed after a disturbance) of the geomagnetic tubes between the old and new plasmapauses after a series of magnetic storms. As a result, the MIT shape at low solar activity varies strongly with height.
       ACKNOWLEDGMENTS. This work was supported by the Russian Foundation for Basic Research, project no. 01-05-64155. I thanks V.V. Afonin for Cosmos-900 satellite data.

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