An analysis of the structures of the high-latitude ionosphere was performed using an auroral particle precipitation model constructed from DMSP satellite data in both hemispheres17,18. The model is uploaded to the Polar Geophysical Institute website (http://apm.pgia.ru). In fig. S1 (in “Supporting information”), this model is presented for quiet conditions. The model describes three main aurora precipitation zones: diffuse aurora zone I equatorward of the aurora oval, structured aurora precipitation of the aurora oval (region of aurora light, aurora) and zone II of the soft diffuse precipitation toward the aurora.
The boundaries of the precipitation zones in the midnight ionosphere change with longitude18,19as is MIT’s position23. In the Southern Hemisphere, these boundaries were revealed from TIMED data obtained in 2002–200719. They are presented in fig. S2 (in “Supporting information“). The equatorial and poleward boundaries of the oval experience synchronous longitudinal variations with an amplitude of ~ 2.5°. Therefore, it is most efficient 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 various structures during the winter midnight (23-01 LT) ionosphere in the Southern Hemisphere. To eliminate the dependence on geomagnetic activity, the positions of 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 a the factor is 2.0° for MIT81.5° for RIT21and ~ 1.5° for HLT16. The Kp(τ) index was used because it takes into account the prehistory of the development of geomagnetic activity11. In fig. 1zones I and II of the diffuse precipitation taken from fig. S1 are shaded. The mean position (for all longitudes) of the equatorward boundary of the aurora precipitation oval corresponds to 64° at Kp = 215. The upper curve in fig. 1 (lower panel) corresponds to the CHAMP satellite inclination. The satellite’s 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 for pattern completeness only; only unambiguous cases were selected.
The black dots in fig. 1 depict cases of MIT observations (n = 703). The approximate curve shows the longitudinal effect in the MIT position with an amplitude of ~3° and a correlation coefficient of 0.52. The data spread (standard deviation) is 1.85°, which is less than the 2°–3° value typically observed in the statistical processing of valley data. In the first approximation, the longitudinal variations in the MIT position correspond to the variations in the precipitation location in Zone I. The main task was to separate the MIT from the HLT (blue squares) at the high latitude limit of the MIT occurrence area. Figure 2a shows the simplest case when both troughs are observed simultaneously. This case allows us to draw a fundamentally important conclusion: the MIT polar wall is determined, as usual, by the precipitation in Zone I, and the HLT polar wall is undoubtedly formed by the precipitation in Zone II. The latter fact is the key to the identification of HLT1. HLT has previously been studied in detail from You variations recorded on board OGO-6 at altitudes of 400–1100 km16 and from EISCAT radar data24. In particular, the statistical position of the HLT relative to the auroral oval was determined16. The authors observed the HLT exclusively within the auroral oval and ultimately attributed its formation to the action of electric fields in the zone of high-latitude ionospheric plasma convection. These fields cause frictional heating and upward vertical drift of the plasma. The first process leads to an increase in recombination, the second to plasma flowing upwards along the magnetic field lines. Since this effect is observed in a limited area, 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 HLT2; it is depicted by filled squares in fig. 1. Figure 2c shows a rather rare example of simultaneous observation of the three valleys: MIT, HLT2 and HLT1. Figure 1 shows that HLT2 is observed less frequently than HLT1. In fig. 2, an approximation curve is drawn for all high latitude valleys (HLT1 and HLT2). For visibility, the precipitation zones are shown with dashes in fig. 2. They are positioned 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 current position of the valley.
In the Eastern Hemisphere, at longitudes 30°–90°E, the MIT is located at the highest latitudes so that the region of its existence overlaps the Zone I precipitation 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 valley observations in this region were thoroughly analyzed. The upper panel of Fig. 1 shows the longitudinal variations in the size of the polar wall (PW) derived from CHAMP data for the quiet period 15–24 August 2000 (dots and approximation line). The longitudinal effect is detected with certainty, which is quite surprising, given the extremely irregular nature of diffuse precipitation. The dashed line shows the longitudinal variations in the mean precipitation energy flux derived at latitude – 65° GMLat from the colored Fig. S219. As might be expected, the variations in the magnitude of PW coincide completely with variations in 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, precipitation in the Western Hemisphere forms a pronounced PW of MIT, which is always clearly determined. This illustrates the latitudinal e.g cross section in fig. 2d, representing the MIT recorded on 9 August 2000, at longitude 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, either MIT or HLT can form. For example curve 2 in fig. 2d represents the latitudinal e.g cross section obtained on 7 August 2000, at longitude 100° E at 0.5 LT and Kp = 1+. Latitude profile 2 shows a weak electron density peak at the same latitudes as profile 1, i.e. at latitudes in zone I of precipitation. Therefore, we can talk about the formation of weakly expressed MIT. Latitude profile 3 was also recorded on 9 August 2000, but at longitude 92° E. Here there is neither a peak nor a minimum of electron density at MIT’s latitudes, so MIT is not identified in this case. The minimum of the electron density is observed far poleward at latitude – 68°, and it indeed belongs to HLT1 because its PW is formed by the precipitation in Zone II. Note that this trough can easily be confused with MIT in a cursory analysis. Finally, if both zones have no precipitation, a monotonic decrease of the electron density to the pole without peaks and valleys is recorded. Such cases correspond to e.g values close to 0 in the upper panel of Fig. 1.
The red dots in fig. 1 depict the RIT cases observed against MIT. RITs form during the recovery phase of a geomagnetic storm and even a weak substorm due to the decay of the magnetospheric ring current. The dynamics of this midlatitude trough have been described in detail previously20,21. When MIT and RIT are observed simultaneously, their identification is not difficult; The MIT position corresponds to the model8 and precipitation in zone I, at which time the equatorward trough is RIT (Fig. 2e). But during a storm any situation can be observed: both valleys, a MIT or a RIT. Additionally, MIT can be identified on one path and RIT on the next path. Therefore, the main method of MIT and RIT separation is an analysis of the prehistory of the development of geomagnetic disturbances20,21. Herein, weak geomagnetic disturbances for the current period were also analyzed to distinguish RIT from MIT. An example of such an analysis is applied below in the discussion of Fig. 3.
Numbers 2f,g show examples of structures that can be defined as quasi-troughs. Figure 2fshows the latitudinal e.g Cross section typical of the Americas and Atlantic longitudes: steep poleward wall (PW) of the trough, surface electron density minimum slightly equatorward (at -65.5°), and deep and broad minimum at -55°. How is MIT’s position determined in this case? The latitude − 65.5° for Kp = 1− rather corresponds to the PW of MIT, and the latitude − 55° goes completely outside the range of existence of “normal” MIT. Similarly, the position ofNo minimum at latitude − 60.5° for Kp = 1− in fig. 2g is definitely lower than the “normal” position of the MIT at longitude 29° E (Fig. 1). The trough’s well-defined PW allows us to solve this problem. During the midnight hours, the base of the PW usually coincides with the boundary of diffuse precipitation towards the equator25. The MIT minimum is within 5° equatorward of this boundary4and the minimum distance is about 2°26; therefore, the MIT minimum is typically 3°–4° equatorward of the PW. The minimal trough determined in this way in fig. 2f,g coincide with the average position of the MIT (Fig. 1). As for the reason for the formation of an additional electron density minimum, we should note that the geomagnetic latitude of − 56° at longitude 285° roughly corresponds to the geographic latitude − 66°, that is, the Arctic Circle. The Arctic Circle limits the region of the polar night under winter conditions, where there is no solar ionization and the electron density decreases. 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 electron density minimum recorded on 29 August 2001, at latitude −50.2° and longitude 194° E. Several well-pronounced LLTs were observed at latitudes −50° and the equator (not shown in Fig. . 1). They apparently belong to the class of LLTs discovered earlier27.
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