An analysis of the structures of the high latitude ionosphere was conducted using a model of auroral particle precipitation constructed from the DMSP satellites data in both hemispheres17,18. The model is uploaded on the website of the Polar Geophysical Institute (http://apm.pgia.ru). In Fig. S1 (in “Supporting information”), this model is presented for quiet conditions. The model describes three main auroral precipitation zones: diffuse auroral zone I equatorward of the auroral oval, structured auroral precipitation of the auroral oval (region of auroral lights, aurora), and zone II of the soft diffuse precipitation poleward of the aurora.
The boundaries of the precipitation zones in the midnight ionosphere change with longitude18,19, as well as the position of the MIT23. In the southern hemisphere these boundaries were revealed from the TIMED data obtained in 2002–200719. They are presented in Fig. S2 (in “Supporting information”). The equatorward and poleward boundaries of the oval experience synchronous longitudinal variations with an amplitude of ~ 2.5°. Therefore, it is most effective to analyze the structures of the high latitude ionosphere in terms of geomagnetic latitude–geographic longitude. Figure 1 (bottom panel) shows the positions of the different structures in the winter midnight (23-01 LT) ionosphere of the southern hemisphere. To eliminate the dependence on geomagnetic activity, the positions of the MIT, RIT, and HLT were reduced to Kp = 2 according to Λcorr = Λc − a(Kp(τ) − 2), where Λc is the current position of the structure and the a factor is 2.0° for the MIT8, 1.5° for RIT21, and ~ 1.5° for HLT16. The Kp(τ) index was used as it considers the prehistory of geomagnetic activity development11. In Fig. 1, zones I and II of the diffuse precipitation taken from Fig. S1 are shaded. The average (for all longitudes) position of the equatorward boundary of the auroral precipitation oval corresponds to 64° at Kp = 215. The upper curve in Fig. 1 (bottom panel) corresponds to the CHAMP satellite inclination. The satellite inclination of 87° does not limit the observations of the discussed structures, except for the polar hole. But polar hole cases are shown in Fig. 1 solely for the completeness of the pattern; only unambiguous cases were selected.
The black dots in Fig. 1 depict the cases of MIT observations (n = 703). The approximating curve demonstrates the longitudinal effect in the MIT position with an amplitude of ~ 3° and a correlation coefficient of 0.52. The data scatter (standard deviation) is 1.85°, which is less than the 2°–3° value that is usually observed in the statistical processing of the trough data. In the first approximation, the longitudinal variations in the MIT position are consistent with the variations in the position of the precipitation in zone I. The main task was to separate the MIT from HLT (blue squares) at the high latitude boundary of the MIT occurrence region. Figure 2a shows the simplest case when both troughs are observed simultaneously. This case allows us to draw a fundamentally important conclusion: the MIT poleward wall is, as usual, determined by the precipitation in zone I, and the HLT poleward wall is undoubtedly formed by the precipitation in zone II. The latter fact is the key to the identification of the HLT1. The HLT was previously studied in detail from the Ni variations recorded on board OGO-6 at heights of 400–1100 km16 and from EISCAT radar data24. In particular, the statistical position of HLT relative to the auroral oval was determined16. The authors observed the HLT exclusively within the auroral oval and attributed its formation ultimately to the action of electric fields in the zone of the convection of the high latitude ionospheric plasma. These fields cause the frictional heating and upward vertical drift of the plasma. The first process leads to an increase in recombination, the second one to the escape of plasma upward along the magnetic field lines. Since this effect is observed in a limited region, the HLT of this type is usually narrow (3°–5° in latitude). Such a trough is observed in Fig. 2b together with the polar hole. We define such a trough as the HLT2; it is depicted by filled squares in Fig. 1. Figure 2c shows a rather rare example of the simultaneous observation of the three troughs: MIT, HLT2, and HLT1. Figure 1 shows that HLT2 is observed less frequently than HLT1. In Fig. 2, an approximation curve for all high latitude troughs (HLT1 and HLT2) is drawn. For the visibility, the precipitation zones are shown by hatching in Fig. 2. They are located considering the longitude and the Kp index value. However, it should be remembered that the precipitation zones are taken from the model and may not exactly correspond to the trough current position.
In the eastern hemisphere, at longitudes of 30°–90° E, the MIT is located at the highest latitudes so that the region of its existence overlaps with the precipitation in zone I and the region of HLT existence. In the region of the intersection of the two sets of troughs, the problem of separation becomes particularly acute. Therefore, all cases of trough observations in this region were analyzed thoroughly. The top panel in Fig. 1 shows the longitudinal variations in the magnitude of the poleward wall (PW) derived from the CHAMP data for the quiet period of August 15–24, 2000 (dots and approximation line). The longitudinal effect is detected confidently, which is quite surprising, bearing in mind the extremely irregular character of diffuse precipitation. The dashed line depicts the longitudinal variations in the average precipitation energy flux derived at latitude of − 65° GMLat from the colored Fig. S219. As one might expect, the variations in the magnitude of the PW completely coincide with variations in the precipitation. However, the high degree of coincidence is also surprising. Electron precipitation is much stronger in the western hemisphere than in the eastern hemisphere. Therefore, in the western hemisphere, the precipitation forms a pronounced PW of the MIT, which is always clearly determined. This illustrates the latitudinal fp cross-section in Fig. 2d, which represents the MIT recorded on August 9, 2000, at longitude of 286° E at 0.6 LT and Kp = 2−. In the eastern hemisphere at problematic longitudes different scenarios can be realized. If the precipitation in zones I and II is still quite intense, they form (weak) peaks of electron density, and both troughs are observed. If the precipitation in one of the zones is very weak, then either the MIT or the HLT can be formed. For example, curve 2 in Fig. 2d represents the latitudinal fp cross-section obtained on August 7, 2000, at longitude of 100° E at 0.5 LT and Kp = 1+. The latitudinal profile 2 shows weak electron density peak at the same latitudes as profile 1, i.e. at latitudes of zone I of precipitation. Hence, we can talk about the formation of weakly expressed MIT. The latitudinal profile 3 was also recorded on August 9, 2000, but at longitude of 92° E. Here is neither a peak nor a minimum of electron density at the latitudes of the MIT, therefore the MIT is not identified in this case. The minimum of the electron density is observed much poleward at latitude of − 68°, and it certainly belongs to HLT1 because its PW is formed by the precipitation in zone II. Note that this trough can be easily confused with the MIT in a cursory analysis. Finally, if both zones have no precipitation, then a monotonous decrease is recorded in the electron density to the pole without peaks and troughs. Such cases correspond to the fp values close to 0 on the top panel in Fig. 1.
The red dots in Fig. 1 depict the RIT cases that were observed equatorward of the MIT. The RIT forms during the recovery phase of a geomagnetic storm and even a weak substorm because of the decay of the magnetospheric ring current. The dynamics of this mid-latitude trough was described in detail earlier20,21. When the MIT and RIT are simultaneously observed, their identification is not difficult; the MIT position corresponds to the model8 and precipitation in zone I, at that time, the equatorward trough is the RIT (Fig. 2e). However, during a storm, any situation can be observed: both troughs, one MIT, or one RIT. Moreover, the MIT can be identified on one path, and the RIT on the next path. Therefore, the main method of MIT and RIT separation is an analysis of the prehistory of geomagnetic disturbance development20,21. Herein, even weak geomagnetic disturbances for the period under consideration were analyzed to separate the RIT from the MIT. An example of such an analysis is applied below in the discussion of Fig. 3.
Figures 2f,g show examples of structures that can be defined as quasi-troughs. Figure 2fshows the latitudinal fp cross-section typical for the longitudes of America and the Atlantic: steep poleward wall (PW) of the trough, shallow electron density minimum slightly equatorward (at − 65.5°), and deep and wide minimum at − 55°. How is the position of the MIT determined in this case? The latitude of − 65.5° for Kp = 1− corresponds rather to the PW of the MIT, and the latitude of − 55° completely goes beyond the existence region of “normal” MIT. Similarly, the position of Ne minimum at latitude of − 60.5° for Kp = 1− in Fig. 2g is definitely lower than the “normal” position of the MIT at longitude of 29° E (Fig. 1). The well-defined PW of the trough allows us to solve this problem. In the midnight hours, the base of the PW usually coincides with the equatorward boundary of diffuse precipitation25. The MIT minimum is located within 5° equatorward of this boundary4, and the minimum distance is about 2°26; therefore, the MIT minimum is usually 3°–4° equatorward of the PW. The minimum of the trough determined in this way in Fig. 2f,g coincides with the average position of the MIT (Fig. 1). As for the reason for the formation of an additional minimum of electron density, we should note that the geomagnetic latitude of − 56° at longitude of 285° approximately corresponds to the geographical latitude of − 66°, that is, the Polar Circle. The Polar Circle limits the area of the polar night in winter conditions, wherein there is no solar ionization and the electron density decays. The influence of the polar night affects a fairly wide range of longitudes from 120° W to 30° E.
Finally, Fig. 2h shows an example of a clearly defined minimum of electron density recorded on August 29, 2001, at latitude of − 50.2° and longitude 194° E. Several well-expressed LLTs were observed at latitudes − 50° and equatorward (not shown in Fig. 1). They apparently belong to the class of LLTs discovered earlier27.
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