Critical Frequency Of F2 Region Research Articles

Progress in investigating long-term trends in the mesosphere, thermosphere, and ionosphere

Abstract. This article reviews main progress in investigations of long-term trends in the mesosphere, thermosphere, and ionosphere over the period 2018–2022. Overall this progress may be considered significant. The research was most active in the area of trends in the mesosphere and lower thermosphere (MLT). Contradictions on CO2 concentration trends in the MLT region have been solved; in the mesosphere trends do not differ statistically from trends near the surface. The results of temperature trends in the MLT region are generally consistent with older results but are developed and detailed further. Trends in temperatures might significantly vary with local time and height in the whole height range of 30–110 km. Observational data indicate different wind trends in the MLT region up to the sign of the trend in different geographic regions, which is supported by model simulations. Changes in semidiurnal tide were found to differ according to altitude and latitude. Water vapor concentration was found to be the main driver of positive trends in brightness and occurrence frequency of noctilucent clouds (NLCs), whereas cooling through mesospheric shrinking is responsible for a slight decrease in NLC heights. The research activity in the thermosphere was substantially lower. The negative trend of thermospheric density continues without any evidence of a clear dependence on solar activity, which results in an increasing concentration of dangerous space debris. Significant progress was reached in long-term trends in the E-region ionosphere, namely in foE (critical frequency of E region, corresponding to its maximum electron density). These trends were found to depend principally on local time up to their sign; this dependence is strong at European high midlatitudes but much less pronounced at European low midlatitudes. In the ionospheric F2 region very long data series (starting at 1947) of foF2 (critical frequency of F2 region, corresponding to the maximum electron density in the ionosphere) revealed very weak but statistically significant negative trends. First results of long-term trends were reported for the topside ionosphere electron densities (near 840 km), the equatorial plasma bubbles, and the polar mesospheric summer echoes. The most important driver of trends in the upper atmosphere is the increasing concentration of CO2, but other drivers also play a role. The most studied one was the effect of the secular change in the Earth's magnetic field. The results of extensive modeling reveal the dominance of secular magnetic change in trends in foF2 and its height (hmF2), total electron content, and electron temperature in the sector of about 50∘ S–20∘ N, 60∘ W–20∘ E. However, its effect is locally both positive and negative, so in the global average this effect is negligible. The first global simulation with WACCM-X (Whole Atmosphere Community Climate Model eXtended) for changes in temperature excited by anthropogenic trace gases simultaneously from the surface to the base of the exosphere provides results generally consistent with observational patterns of trends. Simulation of ionospheric trends over the whole Holocene (9455 BCE–2015) was reported for the first time. Various problems of long-term-trend calculations are also discussed. There are still various challenges in the further development of our understanding of long-term trends in the upper atmosphere. The key problem is the long-term trends in dynamics, particularly in activity of atmospheric waves, which affect all layers of the upper atmosphere. At present we only know that these trends might be regionally different, even opposite.

Critical frequency and maximum electron density of F2 region over four stations in the North American sector

Ionosonde measurements obtained at College (64.9°N, 212.0°E), Goose Bay (53.39°N, 299.7°E), Boulder (40°N, 254.7°E) and Wallops Island (37.9°N, 284.5°E) during low solar activity years were used to check the performance of International Reference Ionosphere model to predict the critical frequency of the F2 region (foF2) of the ionosphere. Data were obtained from the University of Massachusetts Lowell DIDBase through internet and correspond to low solar activity years. CCIR and URSI options were used to model calculations. The results show that the modeled values given for Goose Bay, Boulder and Wallops Island are better than those given for the other station. For the middle latitude stations, in general, the deviations between modeled and measured foF2 are lower than 20%. Cases with very good agreement between predictions and measurements have been obtained. The worst predictions are given for the high latitude station. The study of maximum electron density of the F2 region (NmF2) shows cases where the deviation between modeled and measured NmF2 reaches values as high as 40% or more. The effects of these disagreements on ionospheric modelling are stressed. Additional studies considering other solar activities are needed.

On the time of minimum ionization in the F2 region

Using measurements of the critical frequency of F2 region (f oF2) the validity of the International Reference Ionosphere model to predict the time of minimum ionization is checked. Data obtained at different ionospheric stations have been considered. The CCIR and URSI options are used to model calculations. For CCIR option the results show that good predictions were obtained for about 40% of the considered cases. For the rest of the considered data, the model predicts the minimum at times earlier than that observed in the measurements. The percentages of good predictions obtained with URSI option are lower than those corresponding to CCIR one.

F2 region maximum electron density height predictions for South American latitudes

Values of the F2 region maximum electron density height (hmF2) calculated using ground ionosonde data at South American latitudes are used to check the validity of the International Reference Ionosphere (IRI) to predict this variable. With this in mind we compare hmF2 predictions given by the model when measurements of critical frequency of F2 region and propagation parameter M(3000)F2 were used as input parameter in IRI (hmF2IRI‐Exp), against those obtained using the standard International Radio Consultative Committee (CCIR) option (hmF2IRI‐CCIR). In this paper we used hmF2IRI‐Exp values because hmF2 measurements were not available for the considered cases. Moreover, a comparison of the measured M(3000)F2 values with the CCIR predictions have been done. The results show that, in general, the standard predictions follow the diurnal tendency observed in the hmF2IRI‐Exp values. At low latitudes the hmF2IRI‐Exp values show oscillations not reproduced by the standard option. Cases with disagreements for 24 hours have been observed at high latitudes. Other cases with good agreement have been also obtained. The results suggest that, in general, the standard option of the model gives good hmF2 predictions at South American latitudes. Few cases showed deviation between 15 and 25%. As we expected, the obtained results suggest that the deviation between predicted and measured M(3000)F2 values is the main contribution for the deviation between hmF2IRI‐CCIR and hmF2IRI‐Exp. The comparison with the results obtained in previous work shows that the IRI performance in predicting M(3000)F2 and hmF2 is better than in predicting foF2.

Some Remarks on the Critical Frequencies of the F2-Region

Equatorial anomaly is investigated. Monthly average critical frequency of F2-region plotted against observatories co-ordinates does not show equatorial trough though the seasonal effect is well pronounced. Mitra's suggestion for the investigation of Ionospheric Equator is emphasized in this article and possible nodes of equator have been located.