foF2 Trends Research Articles

Applying artificial neural network to derive long‐term foF2 trends in the Asia/Pacific sector from ionosonde observations

An artificial neural network (ANN) method is first used for deriving long‐term trends of the F2‐layer critical frequency (foF2) at 19 ionospheric stations in the Asia/Pacific sector. It is found that the ANN method can eliminate the geomagnetic activity effect on foF2 more effectively than usual regression methods. Of the selected 19 stations, there are significant long‐term trends corresponding to a confidence level ≥90% at 14 stations and 12 of these stations present negative trends. An average trend of −0.05% per year in the selected area can be obtained if the 12 stations with significant negative long‐term trends be considered. No pronounced diurnal and latitudinal effects in trends and no uniform pattern of seasonal variation in most stations are detected. The long‐term trends for low latitude and equatorial stations differ from other stations suggest that some special dynamical processes may take effects in the equatorial anomaly region. Many factors which can influence ionosphere, such as the greenhouse effect, solar and geomagnetic activity, and neutral background gas, might contribute to the trend.

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

Results of various authors on long-term trends in foF2, which is equivalent to the maximum electron density in the ionosphere, and their interpretation do not reveal a consistent pattern. Therefore, a joint analysis of one carefully selected dataset was performed by six teams, which used different approaches to trend determination. High-quality data of station Juliusruh (54.6°N, 13.4°E) for noon (average from 10 to 14 UT) were used for the period of two solar cycles from minimum to minimum (1976–1996). Juliusruh is relatively sensitive to geomagnetic activity as an almost subauroral station, which might play some role in interpretation of trend results. Various methods provide results, which differ to some extent, even when one co-author applies different methods. Another source of differences is application of various ways of removal (or at least large suppression) of the effect of solar (and geomagnetic) activity. Nevertheless finally most teams obtained quite comparable results. Interpretation of the observed trends is not unique—co-authors consider either the long-term change in geomagnetic activity, or anthropogenic effects to be predominantly responsible for trends. There is some generally accepted output from the joint analysis. All trends are either negative or insignificant. Data corrections with sunspot number ( R), F10.7 adjusted to the Sun–Earth distance, observed F10.7, adjusted E10.7 and observed E10.7 result in somewhat different trends; the observed F10.7 and E10.7 appear to be the best correcting factor. The trends in foF2 are very small, of the order of −0.01 MHz/year, much smaller than the solar cycle effect and, therefore, sensitive to the solar activity correction. The Juliusruh dip angle increased very little over the period 1976–1996 and the possible impact of that increase on trends is negligibly small.

Progress in studies of the trends in the ionosphere F region

A detailed analysis of the hmF2 data obtained at the network of ionospheric stations is performed to reveal long-term trends independent of the geomagnetic activity variations during the recent decades (nongeomagnetic trends). The method developed and tested by the author earlier is applied. Unlike the results on foF2 published by the author earlier, the picture of hmF2 trends is not homogeneous. For 17 ionospheric stations positive significant trends are obtained. Six stations give negative trends in hmF2. For three stations no trends were derived. The behavior of the positive trends is considered. It is shown that these trends depend neither on geomagnetic latitude nor on local time, both facts confirming their independence of geomagnetic activity. The averaging of the positive trends over all 17 stations gives a relative trend of 0.0011 per year with the standard deviation of 0.0005. At the average height of the F2 layer of about 300 km this means an increase in hmF2 by 0.33 km per year. Possible interpretations of the obtained trends in foF2 and hmF2 from the point of view of anthropogenic changes in the thermosphere are considered.

FoF2 long-term trends at the southern crest of the equatorial anomaly

Long-term trends in the electron density of the ionosphere for the period 1957–1986 is studied using foF2 monthly median hourly data measured at Tucuman (26.9°S, 65.4°W), a station located at the southern crest of the equatorial anomaly. The linear trend for each hour and each month is estimated after filtering out the effects of solar activity. For the intervals 0–2 LT and 9–23 LT, during equinoxes and summer solstice, the trend is negative. Statistically null or slightly positive trends are observed for the interval 3–8 LT for every season, and for every hour of winter months. The daily amplitude of foF2 decreases since 1957 due to the decreasing trend in the maximum daily values and almost null-trended minimum daily values. A rough estimate, based on the dip angle trend (which in Tucuman has increased during the 30-year interval at a rate of 0.35%/year), indicates that negative foF2 trends should be expected during daytime hours, and positive trends during night-time hours, behaviour observed in the foF2 data here analyzed.

Long-term trends of <i>fo</i>F2 independent of geomagnetic activity

Abstract. A detailed analysis of the foF2 data at a series of ionospheric stations is performed to reveal long-term trends independent of the long-term changes in geomagnetic activity during the recent decades (nongeomagnetic trends). The method developed by the author and published earlier is used. It is found that the results for 21 out of 23 stations considered agree well and give a relative nongeomagnetic trend of -0.0012 per year (or an absolute nongeomagnetic trend of about -0.012 MHz per year) for the period between 1958 and the mid-nineties. The trends derived show no dependence on geomagnetic latitude or local time, a fact confirming their independence of geomagnetic activity. The consideration of the earlier period (1948–1985) for a few stations for which the corresponding data are available provides significantly lower foF2 trends, the difference between the later and earlier periods being a factor of 1.6. This is a strong argument in favor of an anthropogenic nature of the trends derived.Key words. Ionosphere (ionosphere-atmosphere interactions; ionospheric disturbances; mid-latitude ionosphere)

Long-term trends of <i>f</i>oE and geomagnetic activity variations

Abstract. A relationship between foE trends and geomagnetic activity long-term variations has been revealed for the first time. By analogy with earlier obtained results on the foF2 trends it is possible to speak about the geomagnetic control of the foE long-term trends as well. Periods of increasing geomagnetic activity correspond to negative foE trends, while these trends are positive for the decreasing phase of geomagnetic activity. This "natural" relationship breaks down around 1970 (on some stations later) when pronounced positive foE trends have appeared on most of the stations considered. The dependence of foE trends on geomagnetic activity can be related with nitric oxide variations at the E-layer heights. The positive foE trends that appeared after the "break down" effect may also be explained by the [NO] decrease which is not related to geomagnetic activity variations. But negative trends or irregular foE variations on some stations for the same time period require some different mechanism. Chemical pollution of the lower thermosphere due to the anthropogenic activity may be responsible for such abnormal foE behavior after the end of the 1960s.Key words. Ionosphere (ionosphere-atmosphere interactions; ionospheric disturbances)

A revised approach to the ƒ<i>o</i>F2 long-term trends analysis

Abstract. A new approach to extract foF2 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 foF2 trend, with the slope Kr = - 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 foF2 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 foF2 trends. Key words. Ionosphere (ionosphere-atmosphere interactions, ionospheric disturbances)

The geomagnetic control concept of the F2-layer parameter long-term trends

The geomagnetic control concept relates F2-layer parameter trends with long-term variations of geomagnetic activity and allows one to explain both foF2 and hmF2 trends consistently. Periods with negative and positive trends correspond to the periods of increasing or decreasing geomagnetic activity. The applied method gives the trends which are significant at the 95–99% confidence level for most of local time moments. Daytime foF2 trends show a pronounced dependence on geomagnetic (invariant) latitude with the largest negative (positive) trends for rising (falling) periods of geomagnetic activity at high latitudes and small trends at lower latitudes. No latitudinal dependence was found for the hmF2 trends, which are positive at most of the stations considered for the period of increasing geomagnetic activity. This difference is due to different NmF2 and hmF2 dependencies on main aeronomic parameters. For the period of increasing geomagnetic activity the revealed daytime foF2 and hmF2 trends may be related with neutral composition and temperature perturbations while meridional thermospheric wind at lower and middle latitudes and soft electron precipitation in the auroral zone determine to a great extent the mechanism of the nighttime trends formation. Thus main morphological features of the foF2 and hmF2 trends may be explained in the framework of contemporary F2-layer storm mechanisms, i.e., they have a natural origin rather than an artificial one.

Overview of the trends in the ionospheric E and F2 regions

A review of the studies of long-term trends in the ionospheric E and F regions is given. It is shown that there are two principal methods of trend determination: absolute and relative. The main details of both methods are described and illustrated by particular results. It is noted that there are some disadvantages in the absolute trend method which might have led to significant differences of the results obtained by this method from those obtained by the relative trend method. The most significant result of the studies by the relative method is determination of a strong relation of the foF2 and hmF2 trends derived upon geomagnetic activity. Nevertheless one cannot explain all the trends in foF2 and hmF2 obtained by this method only in the scope of a geomagnetic control concept. An assumption that there is a “nongeomagnetic” component of the F2-region parameter trends seems inevitable. In the E region the results obtained by various authors using the absolute method give positive trends of foE which agree with the trends of the ion composition.

An interpretation of the <i>ƒo</i>F2 and <i>hm</i>F2 long-term trends in the framework of the geomagnetic control concept

Abstract. Earlier revealed morphological features of the foF2 and hmF2 long-term trends are interpreted in the scope of the geomagnetic control concept based on the contemporary F2-layer storm mechanisms. The F2-layer parameter trends strongly depend on the long-term varying geomagnetic activity whose effects cannot be removed from the trends using conventional indices of geomagnetic activity. Therefore, any interpretation of the foF2 and hmF2 trends should consider the geomagnetic effects as an inalienable part of the trend analysis. Periods with negative and positive foF2 and hmF2 trends correspond to the periods of increasing or decreasing geomagnetic activity with the turning points around 1955, and the end of 1960s and 1980s, where foF2 and hmF2 trends change their signs. Such variations can be explained by neutral composition, as well as temperature and thermospheric wind changes related to geomagnetic activity variations. In particular, for the period of increasing geomagnetic activity (1965–1991) positive at lower latitudes, but negative at middle and high latitudes, foF2 trends may be explained by neutral composition and temperature changes, while soft electron precipitation determines nighttime trends at sub-auroral and auroral latitudes. A pronounced dependence of the foF2 trends on geomagnetic (invariant) latitude and the absence of any latitudinal dependence for the hmF2 trends are due to different dependencies of NmF2 and hmF2 on main aeronomic parameters. All of the revealed latitudinal and diurnal foF2 and hmF2 trend variations may be explained in the frame-work of contemporary F2-region storm mechanisms. The newly proposed geomagnetic storm concept used to explain F2-layer parameter long-term trends proceeds from a natural origin of the trends rather than an artificial one, related to the thermosphere cooling due to the greenhouse effect. Within this concept, instead of cooling, one should expect the thermosphere heating for the period of increasing geomagnetic activity (1965–1991).Key words. Ionosphere (ionosphere-atmosphere interactions; ionospheric disturbances)

F2-layer parameters long-term trends at the Argentine Islands and Port Stanley stations

Abstract. The ionospheric sounding data at two southern hemisphere stations, the Argentine Islands and Port Stanley, are analyzed using a method previously developed by the authors. Negative trends of the critical frequency foF2 are found for both stations. The magnitudes of the trends are close to those at the corresponding (close geomagnetic latitude) stations of the northern hemisphere, as considered previously by the authors. The values of the F2 layer height hmF2 absolute trends ΔhmF2 are considered. The effect of ΔhmF2 dependence on hmF2 found by Jarvis et al. (1998) is reproduced. A concept is considered that long-term changes of the geomagnetic activity may be an important (if not the only) cause of all the trends of foF2 and hmF2 derived by several groups of authors. The dependence of both parameters on the geomagnetic index Ap corresponds to a smooth scheme of the ionospheric storm physics and morphology; thus, a principal cause of the foF2 and hmF2 geomagnetic trends is most probably a trend found in several publications in the number and intensity of ionospheric storms.Key words. Ionosphere (ionosphere-atmosphere interaction; ionospheric disturbances)

Do ionospheric trends indicate to the greenhouse effect?

There are several groups of ionospheric data which have been analyzed during the recent decades in search for long-term trends in the upper atmosphere and ionosphere. The data on the radio wave absorption in the D region indicate to some systematic trends which may be considered as a manifestation of a temperature decrease around the mesopause. Analysis of rocket measurements of the electron concentration in the D region reveals strong enough positive trend in the upper D region. Several groups claimed positive trends of the E-layer critical frequency, foE. Also negative trends of the ratio of two principal ions in the E region ([NO +]/[O 2 +]) were found. These trends of three independently registered parameters may be interpreted as an indication to the existence of pronounced trends of dynamical processes at altitudes of the D and E regions. In many publications trends of the F2-layer parameters have been considered. Trends in both, the critical frequency foF2 and maximum height hmF2, were found. It was also found that the foF2 trends depend strongly on geomagnetic latitude and geomagnetic activity. This fact makes it possible to assume that the trends are not a consequence of the «greenhouseå cooling of the upper atmosphere (as has been numerously suggested) but manifest secular changes of geomagnetic activity, which influences F2-layer parameters though ionospheric storms. Trends in ionospheric storm number and intensity have also been claimed. However, even if trends of a geomagnetic origin input considerably into the observed long-term changes of foF2 and hmF2, there still may be a «nongeomagneticå component in these changes manifesting «greenhouseå (or other antropogenic) trends. The question is still open.

Geomagnetic control of the <i>fo</i>F2 long-term trends

Abstract. Further development of the method proposed by Danilov and Mikhailov is presented. The method is applied to reveal the foF2 long-term trends on 30 Northern Hemisphere ionosonde stations. Most of them show significant foF2 trends. A pronounced dependence of trend magnitude on geomagnetic (invariant) latitude is confirmed. Periods of negative/positive foF2 trends corresponding to the periods of long-term increasing/decreasing geomagnetic activity are revealed for the first time. Pronounced diurnal variations of the foF2 trend magnitude are found. Strong positive foF2 trends in the post-midnight-early-morning LT sector and strong negative trends during daytime hours are found on the sub-auroral stations for the period with increasing geomagnetic activity. On the contrary middle and lower latitude stations demonstrate negative trends in the early-morning LT sector and small negative or positive trends during daytime hours for the same period. All the morphological features revealed of the foF2 trends may be explained in the framework of contemporary F2-region storm mechanisms. This newly proposed F2-layer geomagnetic storm concept casts serious doubts on the hypothesis relating the F2-layer parameter long-term trends to the thermosphere cooling due to the greenhouse effect.Key words: Ionosphere (ionosphere-atmosphere interactions; ionospheric disturbances)

Geomagnetic control of the foF2 long-term trends

Further development of the method proposed by Danilov and Mikhailov is presented. The method is applied to reveal the foF2 long-term trends on 30 Northern Hemisphere ionosonde stations. Most of them show significant foF2 trends. A pronounced dependence of trend magnitude on geomagnetic (invariant) latitude is confirmed. Periods of negative/positive foF2 trends corresponding to the periods of long-term increasing/decreasing geomagnetic activity are revealed for the first time. Pronounced diurnal variations of the foF2 trend magnitude are found. Strong positive foF2 trends in the post-midnight-early-morning LT sector and strong negative trends during daytime hours are found on the sub-auroral stations for the period with increasing geomagnetic activity. On the contrary middle and lower latitude stations demonstrate negative trends in the early-morning LT sector and small negative or positive trends during daytime hours for the same period. All the morphological features revealed of the foF2 trends may be explained in the framework of contemporary F2-region storm mechanisms. This newly proposed F2-layer geomagnetic storm concept casts serious doubts on the hypothesis relating the F2-layer parameter long-term trends to the thermosphere cooling due to the greenhouse effect.Key words: Ionosphere (ionosphere-atmosphere interactions; ionospheric disturbances)

Atmospheric greenhouse effect and ionospheric trends

Theoretical calculations predict that CO2 doubling would produce a 50 K decrease in the thermospheric temperature which can result in about 20 km decrease in the F2 peak height (hmF2) and a minor decrease in the F2 layer critical frequency (foF2) [Rishbeth and Roble 1992]. In this paper we analyze ionosonde data for some 31 stations to study the long term trends in hmF2 and foF2. Regression coefficients for hmF2 and foF2 as a function of solar activity, are obtained for each station and departures (anomalies) from expected values derived for both these parameters. An analysis of hmF2 and foF2 anomalies indicates negative trends for some stations and positive trends for others. These varied between +29 to −20 km for hmF2 and +0.7 to −0.6 MHz for foF2 in the 34 year period since 1957, the International Geophysical Year (IGY). We conclude that the present data do not provide a definitive evidence of any global long term trend in the ionosphere.