Long-term trends in foF2: A comparison of various methods

The terrestrial thermosphere and ionosphere form the most variable part of theEarth's atmosphere. Because our society depends on technological systems thatcan be affected by thermospheric and ionospheric phenomena, understanding,monitoring and ultimately forecasting the changes of the thermosphere–ionosphere system are of crucial importance to communications, navigation and the exploration of near-Earth space. The reason for the extreme variability of the thermosphere–ionosphere system isits rapid response to external forcing from various sources, i.e., thesolar ionizing flux, energetic charged particles and electric fields imposed via the interaction between the solar wind, magnetosphere and ionosphere, as well as coupling from below (“meteorological influences”) by the upward propagating, broad spectrum,internal atmospheric waves (planetary waves, tides, gravity waves) generated in thestratosphere and troposphere. Thunderstorms, typhoons, hurricanes, tornadoes andeven seismological events may also have observable consequences in the ionosphere.The release of trace gases due to human activity have the potential to cause changes inthe lower and the upper atmosphere.A brief overview is presented concerning the discoveries and experimentalresults that have confirmed that the ionosphere is subject to meteorologicalcontrol (especially for geomagnetic quiet conditions and for middle latitudes).D-region aeronomy, the winter anomaly of radiowave absorption, wave-liketravelling ionospheric disturbances, the non-zonality and regional peculiaritiesof lower thermospheric winds, sporadic-E occurrence and structure, spread-Fevents, the variability of ionospheric electron density profiles and Total ElectronContent, the variability of foF2, etc., should all be considered in connection withtropospheric and stratospheric processes. “Ionospheric weather”, as a part of spaceweather, (i.e., hour-to-hour and day-to-day variability of the ionospheric parameters)awaits explanation and prediction within the framework of the climatological, seasonal,and solar-cycle variations.

Effects on the ionosphere due to phenomena occurring below it

[1] Anthropogenic increases of greenhouse gases warm the troposphere but have a cooling effect in the middle and upper atmosphere The steady increase of CO2 is the dominant cause of upper atmosphere trends; other drivers are long-term changes of radiatively active trace gases such as CH4, O3, and H2O, secular change of solar and geomagnetic activity, and evolution of the Earth's magnetic field Observational and model studies have confirmed that in the past several decades, global cooling has occurred in the mesosphere and thermosphere; the cooling and contraction of the upper atmosphere has lowered the ionosphere and increased electron density in the E and F1 regions Trends of other parameters, including the F2 region, mesospheric clouds, and mesopause wave activity, have been more controversial Modeling investigations have demonstrated that both greenhouse gas forcing and secular change of the Earth's magnetic field can cause regional, diurnal, and seasonal variability of trends in F2 region density and height, which may contribute to discrepancies regarding ionospheric trends Recent studies also may have reconciled discrepancies between space-based and ground-based observations of mesospheric clouds: both types of observations do not find statistically significant trends in the ∼54°N–∼64°N latitude region, but space-based observations indicate that clouds may be increasing in frequency at higher latitude Limited observational studies have suggested possible trends in wave activity Changes in atmospheric dynamics, both as a consequence of global change in the lower and middle atmosphere and as a possible driver of trends in the upper atmosphere, is one of the critical open questions regarding trends in the upper atmosphere and ionosphere

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JA016317

Progress in observations and simulations of global change in the upper atmosphere

. We have applied the empirical orthogonal function (EOF) analysis to examine the climatology of the total ion density Ni at 840 km during the period 1996-2004, obtained from the Defense Meteorological Satellite Program (DMSP) spacecraft. The data set for each of the local time (09:30 LT and 21:30 LT) is decomposed into a time mean plus the sum of EOF bases Ei of space, multiplied by time-varying EOF coefficients Ai. Physical explanations are made on the first three EOFs, which together can capture more than 95% of the total variance of the original data set. Results show that the dominant mode that controls the Ni variability is the solar EUV flux, which is consistent with the results of Rich et al. (2003). The second EOF, associated with the solar declination, presents an annual (summer to winter) asymmetry that is caused by the transequatorial winds. The semiannual variation that appears in the third EOF for the evening sector is interpreted as both the effects of the equatorial electric fields and the wind patterns. Both the annual and semiannual variations are modulated by the solar flux, which has a close relationship with the O+ composition. The quick convergence of the EOF expansion makes it very convenient to construct an empirical model for the original data set. The modeled results show that the accuracy of the prediction depends mainly on the first principal component which has a close relationship with the solar EUV flux.

/pdf/statistical-characteristics-of-the-total-ion-density-in-the-173mt9yfhr.pdf

Statistical characteristics of the total ion density in the topside ionosphere during the period 1996-2004 using empirical orthogonal function (EOF) analysis

. An overall statistical study of night-time enhancements of NmF2 has been carried out. All available foF2 observations since 1955 at 53 ionosonde stations distributed worldwide in the latitude range f geom = 15° - 60° were used in the analysis. More than 200 000 station-nights of data were analysed. This large data base allowed us to study seasonal, solar cycle and spatial variations of the NmF2 nighttime enhancements. Both pre-midnight and post-midnight NmF2 peaks demonstrate distinct variations with geophysical conditions, indicating different physical mechanisms responsible for their formation. Key words. Ionosphere (mid-latitude ionosphere, ionosphere-magnetosphere interactions) Radio science (ionospheric physics)

/pdf/global-morphology-of-night-time-nm-f2-enhancements-1adj364n77.pdf

Global morphology of night-time Nm F2 enhancements

A statistical analysis of two peaks (pre-midnight and post-midnight) occurrence in NmF2 daily variations was made on a latitudinal chain of four ionosonde stations in the Eurasian longitudinal sector. Overall 6182 cases of the first and 5600 cases of the second peak occurrence were analyzed using all available foF2 observations for the years of solar maximum and minimum. Well-pronounced and systematic variations with season and solar activity were revealed in the occurrence probability of the peaks, their amplitude and timing. The pattern of both peaks occurrence is similar during winter and equinoxes for midlatitude stations implying one and the same mechanism of their formation. The pre-midnight summer peak shows specific variations in particular during solar maximum pointing to a different mechanism controlling its appearance. Possible mechanisms of both peaks formation are discussed.

/pdf/morphology-of-nmf2-nighttime-increases-in-the-eurasian-2m3i2grhm0.pdf

Morphology of NmF2 nighttime increases in the Eurasian sector

. A new approach to extract fo F2 long-term trends, which are free to a great extent from solar and geomagnetic activity effects, has been proposed. These trends are insensitive to the phase (increasing/decreasing) of geomagnetic activity, with long-term variations being small and insignificant for such relatively short time periods. A small but significant residual fo F2 trend, with the slope K r = - 2.2 × 10 -4 per year, was obtained over a 55-year period (the longest avail-able) of observations at Slough. Such small trends have no practical importance. On the other hand, negative (although insignificant) residual trends obtained at 10 ionosonde stations for shorter periods (31 years) may be considered as a manifestation of a very long-term geomagnetic activity increase which did take place during the 20th century. All of the revealed fo F2 long-term variations (trends) are shown to have a natural origin related to long-term variations in solar and geomagnetic activity. There is no indication of any manmade fo F2 trends.  Key words. Ionosphere (ionosphere-atmosphere interactions, ionospheric disturbances)

/pdf/a-revised-approach-to-the-fof2-long-term-trends-analysis-327taiek9u.pdf

T. Yu. Leschinskaya

Papers

Morphology of NmF2 nighttime increases in the Eurasian sector

A revised approach to the foF2 long-term trends analysis