Contribution of thermospheric parameters to the formation of the Yakutsk layer F2 diurnal DST anomaly

Historically F2-the layer daytime DST anomaly is set an excess of midnight foF2 on the midday values. As far as known, the effect was first observed by Bellchambers and Piggott1 in 1958 at Halley Bay (76°S, 26°W, dip 64.6°). Similar results were confirmed using observations at Port Lockroy2. Later, when analyzing ground-based ionosonde observations from Antarctica, it was discovered3 that this effect took place in the Weddell Sea area and since then it is called the Weddell Sea Anomaly. However, using TOPEX TEC observations, it was shown4 the size of the anomaly area was actually located west of Faraday’s ionosonde station over the Bellinghausen Sea, so the correct name should be the Bellinghausen Sea Anomaly. Similar area with abnormal foF2 Diurnal variations are localized in the northern hemisphere around Yakutsk (62.0°N, 129.6°E, dip = 75.4°). Sato5 was perhaps one of the first to mention this fact. Later, a detailed morphological analysis of Yakutsk foF2 the anomalous variations were made by Mamrukov6.

A mechanism for such a foF2 diurnal variations were proposed immediately7,8,9. In summer at middle and upper latitudes F2– the region is sunny practically 24 hours a day and fresh plasma is produced even during the night. The upward plasma drift generated by the thermospheric wind towards the equator during the nighttime hours raises F2-layer of the strong recombination zone. This results in an accumulation of plasma at F2– heights of regions increasing foF2. The authors pointed out: “The ‘evening enhancements’ and ‘midnight maxima’ of foF2 that occur on certain regions of the Earth in summer are caused almost entirely by winds of neutral air” and also: “There seems little doubt that the diurnal variation at Port Lockroy is produced, as Kohl & Kingseven suggested by the vertical drifts of ionization. Of course, the greater the latitude of the station, the later the sunset in the summer and the later the night foF2 maximum occurs bearing in mind that the wind towards the equator reaches its maximum around midnight. Along with this, the authors correctly and carefully pointed out “more certain regions of the earth. This is due to the fact that not all stations located at the same latitudes (i.e. subject to the same solar ionization) show nocturnal foF.2 maximum. The authors did not take into account that the vertical drift of the plasma W depends on both the meridian Vnx and zonal Vny components of the thermospheric wind W = (Vnx cosD − Vny sinD) sinI cosI, where I and D — inclination and declination of the earth’s magnetic field. It should be emphasized that the effect of the zonal wind (via the magnetic declination D) on the F2-layer had already been discussed this timeten.

Since then, many mechanisms (some of which are purely speculative) have been suggested to explain daytime foF.2 anomaly but the initial idea tonight foF2 the midnight enhancement and maximum are due to upward plasma drift under direct solar ionization can be taken as commonly accepted4,11,12,13,14. Usually, the magnitude of the diurnal anomaly is estimated by the ratio r = (foF2)00LT/(foF2)12LT6,12,15. This means that r does not only depend on midnight foF2 improvement but also on midday depression and the processes involved may be different due to the different formation mechanisms of day and night F2-layer. This is a rather different level of analysis, not morphological but physical. The majority of analyzes devoted to the foF2 diurnal anomalies occur at the morphological level. The physical level requires knowledge of the aeronomic parameters responsible for the F2– the formation of the layers – first of all the thermospheric parameters, the solar ionizing radiation EUV and the vertical drifts of the plasma linked to the thermospheric winds. Attempts to blindly use global empirical models like MSIS and HWM93 without any external control have been undertaken13.16. Aeronomic parameters should be consistently related, but this consistency is questionable given how these empirical models were derived. An attempt to use a first-principle (physical) GSM TIP model in a comparison with observations from the best sounder IK-19 yielded unsatisfactory results15. Unlike the position observed with IK-19 of the anomaly centered on ~150°E with r ~ 1.5, the calculated anomaly is centered on ~80°–90°E with r ~ 1.2 (their Fig. 6). At 150°E, the calculated r ~ 0.7, i.e. less than twice that observed. Later in our article, it is shown that the Tunguska station located at 90.0°E shows no diurnal foF.2 anomaly. This means that the mechanism of Yakutsk foF2 the diurnal anomaly must be specified in the contribution part of the thermospheric parameters. Such an analysis, to our knowledge, has never been undertaken before.

The objectives of our article can be formulated as follows.

  1. 1.

    Consider the monthly median foF at noon2 for ionosonde stations inside and outside the Yakutsk Magnetic Anomaly to check if they are statistically different.

  2. 2.

    Extract from the midday observations of the ionosondes a coherent set of the main aeronomic parameters responsible for the F2-region formation to check if thermospheric parameters are different for stations inside and outside the anomaly area using Swarm neutral gas density observations for this comparison.

  3. 3.

    Show the controlling role of thermospheric neutral composition in the observed noon foF difference2 inside and outside the anomaly zone.

  4. 4.

    To check if night foF2 maximum inside the anomaly zone and the absence of such a maximum outside the anomaly zone are totally due to different vertical drifts of the plasma in the two regions.

Comments

The Yakutsk ionospheric anomaly is undoubtedly related to the geomagnetic anomaly located in this area. Figure 1 shows a map of the declination of the Earth’s magnetic field, D (https://www.ngdc.noaa.gov/geomag/WMM/image.shtml) along with the ionospheric stations selected for our analysis: Magadan (60, 1°N, 151.0°E, Φ=50.7°, I=71.0°, D=−8.3°), Yakutsk (62.0°N, 129.5°E, Φ=51 ,0°, I = 75.4°, D = −11.9°), Tunguska ( 61.6°N, 90.0°E, Φ = 50.7°, I = 77.5°, D = 7.5°) and St. Petersburg (60.0°N, 30.7°E, Φ = 56.2°, I = 72.6°, D = 5.1°), where Φ—magnetic latitude, I—magnetic inclination and D—magnetic declination. The ionospheric stations have similar geodetic latitudes ~ 61°N so they are subject to the same solar illumination and three of them (Magadan, Yakutsk and Tunguska) have magnetic latitudes close to Φ ~ 51°, whereas they have a different magnetic declination, D—negative (westward) at Magadan and Yakutsk and positive (eastward) at Tunguska and St. Petersburg.

Figure 1

Declination of the magnetic field, D stations and ionosondes analyzed (asterisks).

Figure 2 gives the monthly median foF for June2 diurnal variations at ionospheric stations located in the area of ​​the anomaly (Yakutsk, Magadan) and outside this area (Tunguska, St. Petersburg) under conditions of solar maximum (1970, 1981) and solar minimum (1975 , 1986). Historical FoF2 the observations used in our article come mainly from SPIDR, while recent observations come directly from ionospheric stations. A well-pronounced difference (also mentioned in previous publications) in foF2 diurnal variations are observed in June for the two groups of stations under solar maximum (1970 F monthly10.7 = 154.9, and 1981, F10.7 = 156.9) and solar minimum (1975, F10.7 = 69.7; 1986, F10.7 = 67.6). Inside the anomaly zone (Yakutsk, Magadan) maximum in foF2 diurnal variations occur around midnight, while outside this zone they occur around noon.

Figure 2
Figure 2

June monthly median FoF2 diurnal variations at ionospheric stations located in the area of ​​the anomaly (Yakutsk, Magadan) and outside this area (Tunguska, St. Petersburg) under conditions of solar maximum (1970, 1981) and solar minimum (1975 , 1986).

Figure 2 shows that the stations inside the anomaly zone are distinguished not only by a greater nocturnal foF2 but also by lower foF2 daytime values. This last feature has only been mentioned in some publications12 without any detailed analysis. However, this difference may have fundamental significance because the day at mid-latitudes foF2 directly reflects the state of the surrounding thermosphere and the observed difference in foF2 can indicate peculiarities of thermospheric parameters within the anomaly zone.

Check if low foF2 the interior of the Yakutsk anomaly is an inalienable feature of this area.

Figure 3 gives foF2ratios for Tunguska (outside the anomaly zone) to Magadan and Yakutsk located inside the anomaly zone. The Magadan to Yakutsk ratio is given for comparison.

picture 3
picture 3

Midday June and July monthly median foF2ratios for Tunguska/Magadan (triangles), Tunguska/Yakutsk (diamonds) and Magadan/Yakutsk (circles) calculated over the period (1968-1991).

We give ratios rather than observed foF2remove the variations of the solar cycle and make the plot more visual. Figure 3 shows that Tunguska exhibits greater foF at noon2compared to Magadan and Yakutsk while the Magadan/Yakutsk relationship is centered around unity. Therefore, different thermospheric parameters can be expected inside and outside the anomaly region.

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