Historical F2-layered daily daylight saving time anomaly is defined as an excess of midnight foF2 about noon values. As far as is known, the effect was first observed by Bellchambers and Piggott1 1958 at Halley Bay (76°S, 26°W, tilt 64.6°). Similar results have been confirmed by observations at Port Lockroy2. It was later found when analyzing ground-based ionoprobe observations in Antarctica3 that this effect took place in the area of the Weddell Sea and has since been called the Weddell Sea Anomaly. However, it has been shown using TOPEX TEC observations4 how big the anomaly area was actually west of the Faraday ionoprobe station over the Bellinghausen Sea, so the correct name should be Bellinghausen Sea Anomaly. Similar area with abnormal FoF2 Diurnal variation is located in the northern hemisphere around Yakutsk (62.0°N, 129.6°E, inclination = 75.4°). Sato5 was perhaps one of the first to mention this fact. Later a detailed morphological analysis of Yakutsk foF2 Abnormal variations were performed by Mamrukov6.
A mechanism for such foF2 Daily fluctuations were immediately suggested7,8,9. In summer at middle and high latitudes F2-Region is exposed to the sun practically 24 hours a day and fresh plasma is also produced at night. Upward plasma drift generated during the nighttime hours by the equatorial thermospheric wind lifts F2layer from the region of strong recombination. This leads to accumulation of plasma at F2-Area heights increasingly foF2. The authors emphasized: “The ‘evening improvements’ and ‘midnight maxima’ of foF2 that occur by secure It shows that regions of the world in summer are almost entirely caused by neutral air winds” and also: “There seems little doubt that the diurnal variations are generated at Port Lockroy, as Kohl & King do7 suggested, by vertical ionization drifts.” Of course, the greater the latitude of the station, the later the summer sunset and later night time foF2 Maximum occurs considering that the equatorial wind reaches its maximum at midnight. The authors correctly and carefully emphasize “over secure regions of the world”. This is due to the fact that not all stations located at the same latitudes (i.e. exposed to the same solar ionization) have the nighttime foF2 maximum. The authors did not take into account that the vertical plasma drift W depends on both the meridional 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 zonal wind (via magnetic declination D) on the F2-Layer had already been discussed at the time10.
Since then, many mechanisms (some of which are purely speculative) have been proposed to explain daily foF2 Anomaly but the first idea this evening foF2 The amplification and the midnight maximum are due to the upward drift of the plasma in direct solar ionization and can be considered as generally accepted4,11,12,13,14. Usually, the size of the daily anomaly is given by the ratio r = (foF2)00LT/(foF2)12LT6,12,15. This means that r does not only depend on midnight foF2 Enhancement, but also on midday depression and the processes involved can be different due to the difference formation mechanisms of day and night F2-Layer. This is a whole different level of analysis – not morphological, but physical. The majority of the analyzes are dedicated to the foF2 Daily anomalies are performed at the morphological level. The physical level requires knowledge of aeronomic parameters responsible for the F2-Stratification – mainly thermospheric parameters, solar EUV ionizing radiation and vertical plasma drifts associated with thermospheric winds. Attempts have been made to blindly use global empirical models such as MSIS and HWM93 without external control13:16. The aeronomic parameters should be related consistently, but that 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 Top Sounder IK-19 observations led to unsatisfactory resultsfifteen. In contrast to the position of the anomaly observed with IK-19, which is centered at ~150°E with r~1.5, the calculated anomaly is centered at ~80°–90°E with r~1.2 (its Fig .6). At 150°E, the calculated r is ~ 0.7, twice smaller than the observed one. Later in our work it will be shown that the Tunguska station, located at 90.0° E, does not show daily FO2 Anomaly. It means that the mechanism of Yakutsk foF2 The daily anomaly should be given in the part of the contribution of the thermospheric parameters. As far as we know, no such analysis has been carried out before.
The goals of our work can be formulated as follows.
Consideration of the monthly median foF at lunchtime2 for ionoprobe stations located inside and outside the Yakutsk magnetic anomaly to check if they are statistically different.
To obtain a consistent set of the most important aeronomic parameters known for the F2-Regioning to check whether the thermospheric parameters for the stations inside and outside the anomaly area are different, using observations of the swarm’s neutral gas density for this comparison.
To show the controlling role of the thermospheric neutral composition in the observed noontime difference foF2 inside and outside the anomaly area.
To check if foF at night2 Maximum within the anomaly region and the absence of such a maximum outside the anomaly region are entirely due to different vertical plasma drifts in the two regions.
The Yakutsk ionospheric anomaly is undoubtedly related to the geomagnetic anomaly in that area. Fig. 1 shows a map of the Earth’s magnetic field declination, D (https://www.ngdc.noaa.gov/geomag/WMM/image.shtml) along with the ionospheric stations chosen 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 tilt and D – magnetic declination. The ionospheric stations have similar geodetic latitudes ~ 61°N, therefore they are exposed to the same insolation and three of them (Magadan, Yakutsk and Tunguska) have close magnetic latitudes Φ ~ 51°, while they have different magnetic declinations, D – negative ( west) at Magadan and Yakutsk and positive (east) at Tunguska and St. Petersburg.
Figure 2 shows the monthly median foF for June2 Daily fluctuations at ionospheric stations in the anomaly area (Yakutsk, Magadan) and outside this area (Tunguska, St. Petersburg) under conditions of solar maximum (1970, 1981) and solar minimum (1975, 1986). Historical foF2 Observations used in our work were mainly made by SPIDR, while more recent observations come directly from the ionospheric stations. A clear difference (also mentioned in previous posts) in foF2 Diurnal variations can be seen in June for the two groups of stations, both below solar maximum (1970 monthly F10.7 = 154.9 and 1981, F10.7 = 156.9) and solar minimum (1975, F10.7 = 69.7; 1986 f10.7 = 67.6). Inside the anomaly area (Yakutsk, Magadan) maximum in foF2 Diurnal variations occur near midnight, while outside of this area they occur around noon.
Figure 2 shows that stations within the anomaly area are not only characterized by larger nocturnal FoF2 but also by lower foF2 daily values. The latter feature was only mentioned in a few publications12 without its detailed analysis. However, this difference may be of fundamental importance since mean latitude is foF during the day2 directly reflects the state of the surrounding thermosphere and the observed difference in foF2 may indicate the peculiarities of the thermospheric parameters within the anomaly area.
Let’s check if foF is low2 inside the Yakutsk anomaly is an inalienable feature of this area.
Figure 3 gives foF2Ratio of Tunguska (outside the anomaly area) to Magadan and Yakutsk, which are inside the anomaly area. For comparison, the ratio of Magadan to Yakutsk is given.
We report ratios rather than observed foF2in order to remove variations in the solar cycle and make the display more visual. Figure 3 shows that Tunguska has larger foF at noon2compared to Magadan and Yakutsk, while the Magadan/Yakutsk ratio is centered around unity. Therefore, one can expect different thermospheric parameters inside and outside the anomaly area.