An evaluation of the constructions of the excessive latitude ionosphere was carried out utilizing a mannequin of auroral particle precipitation constructed from the DMSP satellites knowledge in each hemispheres17,18. The mannequin is uploaded on the web site of the Polar Geophysical Institute (http://apm.pgia.ru). In Fig. S1 (in “Supporting data”), this mannequin is introduced for quiet situations. The mannequin describes three important auroral precipitation zones: diffuse auroral zone I equatorward of the auroral oval, structured auroral precipitation of the auroral oval (area of auroral lights, aurora), and zone II of the mushy diffuse precipitation poleward of the aurora.
The boundaries of the precipitation zones within the midnight ionosphere change with longitude18,19, in addition to the place of the MIT23. Within the southern hemisphere these boundaries have been revealed from the TIMED knowledge obtained in 2002–200719. They’re introduced in Fig. S2 (in “Supporting data”). The equatorward and poleward boundaries of the oval expertise synchronous longitudinal variations with an amplitude of ~ 2.5°. Due to this fact, it’s handiest to research the constructions of the excessive latitude ionosphere by way of geomagnetic latitude–geographic longitude. Determine 1 (backside panel) exhibits the positions of the completely different constructions within the winter midnight (23-01 LT) ionosphere of the southern hemisphere. To eradicate the dependence on geomagnetic exercise, the positions of the MIT, RIT, and HLT have been lowered to Kp = 2 in accordance with Λcorr = Λc − a(Kp(τ) − 2), the place Λc is the present place of the construction and the a issue is 2.0° for the MIT8, 1.5° for RIT21, and ~ 1.5° for HLT16. The Kp(τ) index was used because it considers the prehistory of geomagnetic exercise improvement11. In Fig. 1, zones I and II of the diffuse precipitation taken from Fig. S1 are shaded. The common (for all longitudes) place of the equatorward boundary of the auroral precipitation oval corresponds to 64° at Kp = 215. The higher curve in Fig. 1 (backside panel) corresponds to the CHAMP satellite tv for pc inclination. The satellite tv for pc inclination of 87° doesn’t restrict the observations of the mentioned constructions, apart from the polar gap. However polar gap instances are proven in Fig. 1 solely for the completeness of the sample; solely unambiguous instances have been chosen.
The black dots in Fig. 1 depict the instances of MIT observations (n = 703). The approximating curve demonstrates the longitudinal impact within the MIT place with an amplitude of ~ 3° and a correlation coefficient of 0.52. The information scatter (commonplace deviation) is 1.85°, which is lower than the two°–3° worth that’s often noticed within the statistical processing of the trough knowledge. Within the first approximation, the longitudinal variations within the MIT place are according to the variations within the place of the precipitation in zone I. The primary activity was to separate the MIT from HLT (blue squares) on the excessive latitude boundary of the MIT incidence area. Determine 2a exhibits the best case when each troughs are noticed concurrently. This case permits us to attract a essentially necessary conclusion: the MIT poleward wall is, as typical, decided by the precipitation in zone I, and the HLT poleward wall is undoubtedly fashioned by the precipitation in zone II. The latter truth is the important thing to the identification of the HLT1. The HLT was beforehand studied intimately from the Ni variations recorded on board OGO-6 at heights of 400–1100 km16 and from EISCAT radar knowledge24. Particularly, the statistical place of HLT relative to the auroral oval was decided16. The authors noticed the HLT completely inside the auroral oval and attributed its formation finally to the motion of electrical fields within the zone of the convection of the excessive latitude ionospheric plasma. These fields trigger the frictional heating and upward vertical drift of the plasma. The primary course of results in a rise in recombination, the second to the escape of plasma upward alongside the magnetic subject strains. Since this impact is noticed in a restricted area, the HLT of this sort is often slender (3°–5° in latitude). Such a trough is noticed in Fig. 2b along with the polar gap. We outline such a trough because the HLT2; it’s depicted by crammed squares in Fig. 1. Determine 2c exhibits a slightly uncommon instance of the simultaneous statement of the three troughs: MIT, HLT2, and HLT1. Determine 1 exhibits that HLT2 is noticed much less often than HLT1. In Fig. 2, an approximation curve for all excessive latitude troughs (HLT1 and HLT2) is drawn. For the visibility, the precipitation zones are proven by hatching in Fig. 2. They’re situated contemplating the longitude and the Kp index worth. Nonetheless, it ought to be remembered that the precipitation zones are taken from the mannequin and should not precisely correspond to the trough present place.
Within the jap hemisphere, at longitudes of 30°–90° E, the MIT is situated on the highest latitudes in order that the area of its existence overlaps with the precipitation in zone I and the area of HLT existence. Within the area of the intersection of the 2 units of troughs, the issue of separation turns into significantly acute. Due to this fact, all instances of trough observations on this area have been analyzed totally. The highest panel in Fig. 1 exhibits the longitudinal variations within the magnitude of the poleward wall (PW) derived from the CHAMP knowledge for the quiet interval of August 15–24, 2000 (dots and approximation line). The longitudinal impact is detected confidently, which is kind of stunning, making an allowance for the extraordinarily irregular character of diffuse precipitation. The dashed line depicts the longitudinal variations within the common precipitation vitality flux derived at latitude of − 65° GMLat from the coloured Fig. S219. As one may count on, the variations within the magnitude of the PW utterly coincide with variations within the precipitation. Nonetheless, the excessive diploma of coincidence can be stunning. Electron precipitation is way stronger within the western hemisphere than within the jap hemisphere. Due to this fact, within the western hemisphere, the precipitation varieties a pronounced PW of the MIT, which is at all times clearly decided. This illustrates the latitudinal fp cross-section in Fig. second, which represents the MIT recorded on August 9, 2000, at longitude of 286° E at 0.6 LT and Kp = 2−. Within the jap hemisphere at problematic longitudes completely different situations might be realized. If the precipitation in zones I and II continues to be fairly intense, they kind (weak) peaks of electron density, and each troughs are noticed. If the precipitation in one of many zones could be very weak, then both the MIT or the HLT might be fashioned. For instance, curve 2 in Fig. second 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 exhibits weak electron density peak on the similar latitudes as profile 1, i.e. at latitudes of zone I of precipitation. Therefore, we will discuss concerning the formation of weakly expressed MIT. The latitudinal profile 3 was additionally recorded on August 9, 2000, however at longitude of 92° E. Right here is neither a peak nor a minimal of electron density on the latitudes of the MIT, subsequently the MIT will not be recognized on this case. The minimal of the electron density is noticed a lot poleward at latitude of − 68°, and it definitely belongs to HLT1 as a result of its PW is fashioned by the precipitation in zone II. Be aware that this trough might be simply confused with the MIT in a cursory evaluation. Lastly, if each zones haven’t any precipitation, then a monotonous lower is recorded within the electron density to the pole with out peaks and troughs. Such instances correspond to the fp values near 0 on the highest panel in Fig. 1.
The purple dots in Fig. 1 depict the RIT instances that have been noticed equatorward of the MIT. The RIT varieties in the course of the restoration section of a geomagnetic storm and even a weak substorm due to the decay of the magnetospheric ring present. The dynamics of this mid-latitude trough was described intimately earlier20,21. When the MIT and RIT are concurrently noticed, their identification will not be tough; the MIT place corresponds to the mannequin8 and precipitation in zone I, at the moment, the equatorward trough is the RIT (Fig. 2e). Nonetheless, throughout a storm, any scenario might be noticed: each troughs, one MIT, or one RIT. Furthermore, the MIT might be recognized on one path, and the RIT on the following path. Due to this fact, the principle technique of MIT and RIT separation is an evaluation of the prehistory of geomagnetic disturbance improvement20,21. Herein, even weak geomagnetic disturbances for the interval into account have been analyzed to separate the RIT from the MIT. An instance of such an evaluation is utilized under within the dialogue of Fig. 3.
Figures 2f,g present examples of constructions that may be outlined as quasi-troughs. Determine 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 minimal barely equatorward (at − 65.5°), and deep and extensive minimal at − 55°. How is the place of the MIT decided on this case? The latitude of − 65.5° for Kp = 1− corresponds slightly to the PW of the MIT, and the latitude of − 55° utterly goes past the existence area of “regular” MIT. Equally, the place of Ne minimal at latitude of − 60.5° for Kp = 1− in Fig. 2g is unquestionably decrease than the “regular” place of the MIT at longitude of 29° E (Fig. 1). The well-defined PW of the trough permits us to unravel this downside. Within the midnight hours, the bottom of the PW often coincides with the equatorward boundary of diffuse precipitation25. The MIT minimal is situated inside 5° equatorward of this boundary4, and the minimal distance is about 2°26; subsequently, the MIT minimal is often 3°–4° equatorward of the PW. The minimal of the trough decided on this manner in Fig. 2f,g coincides with the common place of the MIT (Fig. 1). As for the explanation for the formation of a further minimal of electron density, we must always be aware that the geomagnetic latitude of − 56° at longitude of 285° roughly corresponds to the geographical latitude of − 66°, that’s, the Polar Circle. The Polar Circle limits the world of the polar evening in winter situations, whereby there is no such thing as a photo voltaic ionization and the electron density decays. The affect of the polar evening impacts a reasonably wide selection of longitudes from 120° W to 30° E.
Lastly, Fig. 2h exhibits an instance of a clearly outlined minimal of electron density recorded on August 29, 2001, at latitude of − 50.2° and longitude 194° E. A number of well-expressed LLTs have been noticed at latitudes − 50° and equatorward (not proven in Fig. 1). They apparently belong to the category of LLTs found earlier27.