foF2 Trends Research Articles

Trends in the F2-layer parameters to 2023

Diurnal variations in the long-term trends in the F2-layer critical frequency, foF2, are studied in detail based on the observations to 2022 and use of several solar activity (SA) indices to remove the SA effects in the foF2 variations. The new results confirm the conclusions derived earlier based on the data only to 2014. In the winter months, two well pronounced periods are observed: the nighttime hours when the trends are small by their magnitude and the daytime hours when the negative trends are maximal. In March, the picture is close to that in winter months, however the daytime negative trends are weaker than in winter. No pronounced diurnal variation in the foF2 trends is found in the summer month June. A search for long-term trends in foF2 is performed based on the observations at three stations in the Northern Hemisphere (Juliusruh, Boulder, and Moscow) and three stations in the Southern Hemisphere (Townsville, Hobart, and Canberra). The data for two winter months in each hemisphere for five near-noon LT moments are analyzed. Three solar activity proxies (F30, Ly-α, and MgII) are used to get rid of the SA effects. Negative trends are obtained for all considered situations (station, month, LT moment, SA proxy). The trends for different stations agree well with each other. A tendency in intensification in the negative trends during the recent years is seen in all months.

Evaluating F2-region long-term trends using the International Reference Ionosphere (IRI) model: is this a feasible approximation for experimental trends?

Abstract. The International Reference Ionosphere (IRI) is a widely used empirical ionospheric model based on observations from a worldwide network of ionospheric stations. Therefore, it would be reasonable to expect it to capture long-term changes in key ionospheric parameters, such as foF2 and hmF2 linked to trend forcings like greenhouse gas increasing concentrations and the Earth's magnetic field secular variation. Despite the numerous reported trends in foF2 and hmF2 derived from experimental data and model results, there are inconsistencies that require continuous refinement of trend estimation methods and regular data updates. This ongoing effort is crucial to address the difficulties posed by the weak signal-to-noise ratio characteristic of ionospheric long-term trends. Furthermore, the experimental verification of these trends remains challenging, primarily due to time and spatial coverage limitations of measured data series. Achieving these needs for accurate detection of long-term trends requires extensive global coverage and high resolution of ionospheric measurements together with long enough periods spanning multiple solar cycles to properly filter out variations of shorter terms than the sought trend. Considering these challenges, IRI-modeled foF2 and hmF2 parameters offer a valuable alternative for assessing trends and obtaining a first approximation of a plausible global picture representative of experimental trends. This work presents these global trend patterns, considering the period 1960–2022 using the IRI-Plas 2020 version, which are consistent with other model predictions. While IRI explicitly takes into account the Earth's magnetic field variations, the increase in the concentrations of greenhouse gases appears indirectly through the Ionospheric Global index (IG) which is derived from ionospheric measurements. F2-region trends induced by the first mechanism should be important only around the magnetic equator at the longitudinal range with the strongest displacement, and it should be negligible out of this region. Conversely, trends induced by the greenhouse effect, which are the controversial ones, should be dominant away from the geomagnetic equator and should globally average to negative values in both cases, i.e., foF2 and hmF2. Effectively, these negative global means are verified by trends based on IRI-Plas, even though not for the correct reasons in the hmF2 case. In addition, a verification was performed for more localized foF2 trend values, considering data from nine mid-latitude stations, and a reasonable level of agreement was observed. It is concluded that the IRI model can be a valuable tool for obtaining preliminary approximations of the Earth's magnetic-field-induced long-term changes in foF2 and hmF2, as well as of experimental trends only in the foF2 case. The latter does not hold for hmF2, even if the trends obtained are close to the expected values.

Dependence of long-term trends in foF2 at middle latitudes on different solar activity proxies

The most studied ionospheric parameter for long-term trends is foF2. The dominant factor of foF2 variability is the solar cycle, which is much stronger than the long-term trends. Therefore its effect in data must be removed. However, several decade long homogeneous measurements of the solar EUV fluxes are not available, so various solar activity proxies (solar activity indices) must be used. The aim of this paper is to study the impact of selection of different solar activity proxies on foF2 long-term trends and to find the best solar activity proxy for foF2 trends at middle latitudes. The results based on yearly average data of six midlatitude stations from four continents (1976–2014) and of six solar activity proxies show that the long-term trends in foF2 depend substantially on the solar activity proxy used, and the only solar proxy, which provides trends of the same sign for all stations and both sub-periods, is F30. Based on results of this paper and that of Laštovička and Burešová (2023), I can recommend F30 as the best solar proxy for studying long-term trends of foF2 at middle latitudes (at least for yearly average values).

Trends in foF2 to 2022 and various solar activity indices

The trends in the F2-layer critical frequency foF2 are analyzed based on the observations at Juliusruh station to 2022. Five solar activity indexes (proxies): Ly-a, MgII, Rz, F10.7mon, and F30 are used to remove the SA effects. It is found that the conclusions obtained earlier on the diurnal (in the daytime stronger than at night) and seasonal (in winter stronger than in summer) variations in the amplitude of negative trends in foF2, k(foF2) are confirmed using all five proxies. The maximal negative values of k(foF2) are 0.03–0.04 MHz per year. Use of the “Delta” method confirms these conclusions. Indications are found that the negative trends in foF2 are stronger during the recent decade than during the entire considered period 1996–2022.

Long-Term Changes in Ionospheric Climate in Terms of foF2

There is not only space weather; there is also space climate. Space climate includes the ionospheric climate, which is affected by long-term trends in the ionosphere. One of the most important ionospheric parameters is the critical frequency of the ionospheric F2 layer, foF2, which corresponds to the maximum ionospheric electron density, NmF2. Observational data series of foF2 have been collected at some stations for as long as over 60 years and continents are relatively well covered by a network of ionosondes, instruments that measure, among others, foF2. Trends in foF2 are relatively weak. The main global driver of long-term trends in foF2 is the increasing concentration of greenhouse gases, namely CO2, in the atmosphere. The impact of the other important trend driver, the secular change in the Earth’s main magnetic field, is very regional, being positive in some regions, negative in others, and neither in the rest. There are various sources of uncertainty in foF2 trends. One is the inhomogeneity of long foF2 data series. The main driver of year-to-year changes in foF2 is the quasi-eleven-year solar cycle. The removal of its effect is another source of uncertainty. Different methods might provide somewhat different strengths among trends in foF2. All this is briefly reviewed in the paper.

Long-term trends of the F2-region at mid-latitudes in the Southern Hemisphere

The long-term variations in foF2 at Hobart (52.88°S, 147.32°E), Canberra (35.28°S, 149.13° E) and Christchurch (43.53°S, 172.64°E) stations, located in the mid-latitude zone in the Southern Hemisphere were analyzed using 1947–2006 years of the data. The saturation, solar and geomagnetic activity and seasonal effects were removed mainly by using 12-month running mean, linear and multiple regression (twofold regression) methods to find possible signatures of climate change in long-term trends in the foF2. The solar activity proxies, sunspot number, RZ, and F10.7 solar radio flux were used in regression to find the foF2 residuals at midday (12 LT) and midnight (00 LT) of the stations. The long-term trends obtained at 12 LT are more significant and consistent with the model results. The trends estimated with F10.7 solar flux are negative and the trends estimated with RZ are positive (small and not significant). The foF2 decreased by 0.1–0.4 MHz for the 5 solar cycles period which could be mainly due to enhanced CO2 in the troposphere that is cooling the upper atmosphere. Further research is needed to see if the foF2 trends are also affected by other factors such as thermospheric winds, neutral constituents, the secular variation of Earth's magnetic field, long-term changes in stratospheric ozone, solar and geomagnetic activities.

Behavior of F2-layer parameters and solar activity indices in the 24th cycle

The analysis of the behavior of the critical frequency foF2 during the 24th solar activity cycle (Danilov and Konstantinova, 2020a, c) is prolonged for two more months and the nighttime hours. In addition to the Rz and Ly-α indices used in the aforementioned papers for correction of the F10.7 index during the 24th cycle, the commonly used Mg II index is added. The results confirm the previous conclusions on the existence of the “vague” period with chaotic behavior of foF2 and the recovery of the negative trend in foF2 after 2008–2010. A comparison of the F10.7 index with three other SA indices (Ly-α, Rz, and Mg II) for the 22nd, 23rd, and 24th SA cycles is performed. It is shown that the relationship between F10.7 and other indices is close in the 22nd and 23rd cycles but differs from that in the 24th cycle. The corrected values of F10.7 in the 24th cycle are proposed for analysis of ionospheric trends during that cycle.

Trends in Parameters of the F2 Layer and the 24th Solar-Activity Cycle

Trends in foF2 are analyzed according to the data of Juliusruh and Boulder ionospheric stations to 2018. It is shown that the use of the traditional solar-activity index, F10.7, leads to an unexplainable foF2 trend if the data for the 24th solar-activity cycle are included in the analysis. It is suggested that the F10.7 index does not correctly describe the solar ultraviolet-radiation variation in that cycle. Correction of that index with the Rz (sunspot number) and Ly-α (intensity of the Lyman-alpha line in the solar spectrum) indices is performed. It is shown that reasonable values of the foF2 trends are then obtained. A similar analysis for hmF2 trends leads to the same results.

The Contribution of Geomagnetic Activity to Ionospheric foF2 Trends at Different Phases of the Solar Cycle by SWM

Solar activity dominates the temporal variability of ionospheric properties, which makes it difficult to identify and isolate the effects of geomagnetic activity on the ionosphere. Therefore, the latter effects on the ionosphere are still unclear. Here, we use the spectral whitening method (SWM)—a proven approach to extract ionospheric perturbations caused by geomagnetic activity—to directly obtain, in isolation, the effects of geomagnetic activity. We study its contribution to the ionosphere for different phases of the solar cycle. The time lag between the solar and geomagnetic activities provides an opportunity to understand the contribution of geomagnetic activity to the perturbation of the ionosphere. The results suggest that this contribution to the ionosphere is significant when geomagnetic activity is at its maximum level, which usually happens in the declining phase of the solar cycle, but the contribution is very weak at the solar minimum and during the ascending phase. Then, by analyzing the contributions in different months, we find that the role of geomagnetic activity is larger around winter but smaller around summer.

Trends in hmF2 and the 24th solar activity cycle

The measurements of the F2-layer height, hmF2, at Juliusruh station during 1958–2018 are analyzed by the method developed by the authors earlier and used to determine long-term trends in parameters of the F2 layer. It is found that, in the same way as for the behavior of the critical frequency described by the authors in the previous publication, peculiarities in the hmF2 behavior are observed after 2002–2003. If the solar activity index F10.7 is used, an increase in the ΔhmF2 value is observed and provides a positive trend in hmF2 that contradicts to the available ideas on the continuing process of cooling and contraction of the upper atmosphere. If the F10.7 index corrected by other SA indices (the sunspot number Rz or the Lyman-alpha line intensity) is used, as it has been done for the foF2 trends in the previous publications by the authors, the picture of changes in the hmF2 trends is obtained similarly to the changes in the foF2 trends. A well pronounced negative trend in hmF2 is observed to 2002–2003, and then it is substituted by a chaotic change in ΔhmF2 during the “vague” period, whereas during the last analyzed years it becomes negative again. Analysis of the slope s of the foF2(hmF2) dependence shows that there is a systematic decrease in s with time. That confirms the conclusion of the authors on a decrease in the oxygen amount in the thermosphere.

Trends in foF2 and the 24th solar activity cycle

The trends in foF2 are analyzed based on the data of Juliusruh and Boulder ionospheric stations. It is shown that using the traditional solar activity index F10.7 leads to an impossible trend in foF2 when the data for the 24th solar activity cycle are included into the analysis. It is assumed that the F10.7 index does not describe correctly the solar ultraviolet radiation variations in that cycle. A correction of this index using the Rz (sunspot number) and Ly (intensity of the Lyman-α line in the solar spectrum) is performed, and it is shown that in that case reasonable values of the foF2 trends are obtained.

Effect of the Inclusion of Solar Cycle 24 in the Calculation of foF2 Long-Term Trend for Two Japanese Ionospheric Stations

One of the main ionization sources of the F2 region of the Earth’s ionosphere is the solar EUV irradiance accounting for ~ 90% of its variability during quiet time. Consequently, prior to long-term trend estimations solar activity must be filtered out. The last two solar activity cycles present low activity levels, and particularly solar cycle 24 is the lowest in the last ten solar cycles. The effect of the inclusion of this last solar cycle on foF2 trend estimation is analyzed for two mid-latitude ionospheric stations: Kokubunji (35.7°N, 139.5°E) and Wakkanai (45.4°N, 141.7°E). Filtering is done considering the residuals of different regressions between foF2 and Rz and also between foF2 and F10.7. In both cases, foF2 trends become less negative when solar cycle 24 is included in trend estimations since foF2 residuals systematically exceeds the values predicted by a linear, quadratic or cubic fit between foF2 and F10.7 or Rz from 2008 onwards. In addition, the Earth’s magnetic field secular variation at both stations would induce a positive foF2 trend during daytime that could counteract the greenhouse gases decreasing trend. It is interesting to think that including the latest solar cycles does not necessarily imply incorrect results in the statistical analysis of the data, but simply that solar activity is decreasing on average and also the trend.

Ionospheric Trend Over Wuhan During 1947–2017: Comparison Between Simulation and Observation

AbstractSince Roble and Dickinson (1989), who drew the community's attention about the greenhouse gas effect on the ionosphere, huge efforts have been implemented on ionospheric climate study. However, direct comparison between observations and simulations is still rare. Recently, the Wuhan ionosonde observations were digitized and standardized through unified method back to 1947. In this study, the NCAR‐TIEGCM was driven by Mauna Loa Observatory observed CO2 level and International Geomagnetic Reference Field (IGRF) geomagnetic field to simulate their effects on ionospheric long‐term trend over Wuhan. Only March equinox was considered in both data analysis and simulation. Simulation results show that the CO2 and geomagnetic field have comparable effect on hmF2 trend, while geomagnetic field effect is stronger than CO2 on foF2 trend over Wuhan. Both factors result in obvious but different diurnal variations of foF2/hmF2 long‐term trends. The geomagnetic field effect is nonlinear versus years since the long‐term variation of geomagnetic field intensity and orientation is complex. Mean value of foF2 and hmF2 trend is (−0.0021 MHz/yr, −0.106 km/yr) and (−0.0022 MHz/yr, −0.0763 km/yr) for observation and simulation, respectively. Regarding the diurnal variation of the trend, the simulation accords well with that of observation except hmF2 results around 12 UT. Overall, good agreement between observation and simulation illustrates the good quality of Wuhan ionosonde long‐term data and the validity of ancient ionosphere reconstruction based on realistic indices driving simulation.

Long-term changes in the Delta foF2 parameter based on the data of two European ionospheric stations

Analysis of changes in the critical frequency foF2 in recent decades has been performed by determination of “Delta foF2” parameter. These values determine the mean change of foF2 values from the “etalon period” (1958–1980) to later periods. The results are compared with the determination of foF2 trends, which was performed in a series of previous publications of the authors. The data of two most reliable ionospheric stations of the European region, Slough and Juliusruh, are analyzed. The results confirm all principal conclusions obtained earlier, which were based on analysis of the trends. A systematic decrease of foF2 with time occurs (which corresponds to a negative trend), and the character of changes in the Delta values with season and local time on the whole agrees with the character of changes in the trend. It is shown that the results based on the data of both considered stations show good agreement.

New results in studying foF2 trends

It is shown that there are three causes of the difference in values of trends in the F2-layer parameters obtained by different groups of authors: using initial data series of different length, the absence of the allowance for seasonal changes in trends and the absence of the allowance for diurnal changes in trends. The problem how the deep minimum of solar activity of 2008—2009 influenced the trends in the critical frequency of the F2 layer and whether are the negative trends in foF2 conserved in the following five years is considered. It is obtained that in the vast majority of cases, the monotonous change in the curves used to derive the foF2 trend was distorted in the years close to the anomalously low minimum of solar activity in 2008—2009. That reduced the magnitude of the obtained negative trend in the critical frequency as compared with the results based on the data to 2009. The change in the dependence of the F2-layer critical frequency on its height hmF2 is considered based on two sources of initial data. It is obtained that the slope k of the foF2 dependence on hmF2 systematically decreases from the earlier (“etalon”) period 1958–1980 to later periods 1988–2010, 1998–2010, and 1998–2014. As far as the foF2 value depends on the atomic oxygen concentration at heights of the F region much stronger than hmF2, the found decrease in k confirms the concept of a decrease in the atomic oxygen concentration in the thermosphere with time formulated by the authors earlier.

Trends in the critical frequency foF2 after 2009

The problem of how the deep minimum of solar activity of 2008–2009 influenced trends in the critical frequency of the F2 layer and whether the negative trends in foF2 are conserved in the following five years is considered in the paper. The initial data series used in previous publications by the authors are completed by data to 2014 (2015). Methods of searching for trends that were described in the past several times by the authors were applied. It was found that, in the vast majority of cases, the monotonous change in the curves used to derive the foF2 trend was distorted in years close to the anomalously low minimum of solar activity in 2008–2009. That decreased the obtained trend in the critical frequency as compared with the results based on the data to 2009. However, subsequent years again lead to a tendency toward monotonous decrease in foF2. The causes of the anomalies indicated above lie apparently in the inadequacy of the solar activity index F10.7 for the description of changes in the aeronomical parameters determining the foF2 behavior in the period of a very deep minimum of activity that is widely described in some publications.

Comparison of trends in parameters of the F2 layer obtained by various authors

A comparison of the results of determination of long-term trends in foF2 and hmF2 in publications of various authors is performed. Causes of the discrepancies in the absolute values of negative trends obtained by different researchers are analyzed. Three main causes are formulated: a wrong choice of the length of the data series used for deriving trends and the absence of the allowance for the diurnal and seasonal variations in trends. The role of all three factors leading to underestimation of the amplitude of the negative trends in foF2 and hmF2 is considered with specific examples.

Seasonal and diurnal variations in foF2 trends

AbstractMedian values of the F2 layer critical frequencies observed at ionospheric stations Slough, Juliusruh, and Rome are analyzed in detail. Long‐term trends in foF2, k(fo), for the period 1985–2009 are found using the method described earlier by authors. The dependence of the trend value k(fo) on local time and month of the year is studied. For all three stations, similar seasonal and diurnal behaviors of k(fo) are obtained. The maximum values of the negative trends in foF2 fall on the noon period of the day (1000–1600 LT). In the seasonal behavior, two maxima of the absolute value of k(fo) (at the end of winter and beginning of spring and middle of fall) are observed. The absolute values of the trends are small in the summer months. Possible relation to trends in thermospheric parameters which follow from the obtained conclusions on variations in the foF2 trends are discussed. Arguments are presented in favor of a decrease in the atomic oxygen concentration due to an increase in the eddy diffusion as the main cause of negative foF2 trends. It is shown that a negative trend in the atomic oxygen concentration on the order of 10% per decade is able to explain the negative trend in foF2 in winter. Three possible causes of the difference in foF2 and hmF2 trends obtained by different authors are formulated and discussed. They are the following: the use of the entire series of data and the absence of the allowance for diurnal and seasonal variations in the trends.

Choice of series of initial data in deriving trends in the ionospheric F2-layer parameters

It is shown that the use of initial data series of different lengths is one of the main causes of the difference in the values of trends in the F2-layer parameters, foF2 and hmF2, obtained in a series of publications of the authors and published by other researchers. Arguments are presented in favor of the statement that systematic changes in the F2 layer began around 1980. Searching for trends, the authors consider periods after 1980. At the same time the majority of other scientists use the entire available array of data beginning in 1957 and 1958 (and sometimes even earlier) to search for trends. In that case, the analyzed time interval automatically includes the period before 1980, when the trends were absent. This leads to a substantial underestimation of the magnitude of the obtained negative trends in foF2 and hmF2. Particular examples based on the data of both the authors and other scientists confirming the statement are presented.

Variations in foF2 trends with season and local time

The median values of critical frequencies observed at ionospheric stations Slough, Juliusruh, and Rome are analyzed in detail. Long-term trends in foF2 for the period 1985–2010 were found using the method previously described numerous times by the authors. The dependence of the trend value k on local time and month of the year was studied. For all three stations, a similar seasonal and diurnal behavior of k is obtained. The maximum values of the negative trends in foF2 fall on the noontime period of the day (1000–1600 LT). In the seasonal behavior, two maximums of the absolute value of k (in the end of winter-beginning of spring and middle of fall) are observed. The absolute values of the trends are small in the summer months. The conclusion regarding the seasonal behavior of k coincides with the conclusion of the authors in an earlier publication: negative trends in foF2 are better pronounced in winter than in summer. The possible conclusions on the character of trends in the thermospheric parameters, which follow from the obtained conclusions on variations in the foF2 trends, are discussed.