NmF2 Variations Research Articles

Diurnal variation of NmF2 associated with ExB drift at the equator during solar maxima

Se analiza la variación diurna de la densidad máxima de electrones en la ionosfera, NmF2, medida en cinco estaciones, para diferentes estaciones del año, máxima actividad solar y baja actividad magnética (Kp ≤ 3) en función de la velocidad de drift ExB en el ecuador. Las estaciones analizadas son Kodaikanal, ubicada en el ecuador magnético, Tucumán y Okinawa, en las crestas de la anomalía de ionización ecuatorial (EIA), y Yamagawa y Kokobunji fuera de ellas. Se consideró la velocidad de drift de un modelo empírico basado en mediciones del satélite AE-E y la velocidad del viento meridional en la termosfera del modelo HWM93. El apartamiento de NmF2 de una función simple del ángulo zenital se explica en la mayoría de los casos cualitativamente en función de la contribución de electrones desde el Ecuador para las estaciones en el Ecuador magnético y en las crestas de la EIA. En el caso de las estaciones fuera de las crestas, NmF2 sigue una dependencia con el ángulo zenital parecida al comportamiento del viento meridional. Hay comportamientos en NmF2 que no pueden explicarse ni en función del drift ExB ni del viento meridional.

Structure Distribution of NmF2 during a Geomagnetically Quiet Period

AbstractUsing data from 48 ionosondes in the Northern Hemisphere and the model of IRI‐90, the latitude profiles of variations of NmF2 on Jan. 7, 1985 at sunspot minimum are investigated. The results show that in the equatorial anomaly F2 layer the “fountain effect” is the strongest at the Asia region and the “northern crest” in the range 20°N~30°N is the highest with a maximum value of NmF2 at noon, or a bit later. In the range of 30°N~50°N the F2 peaks decrease gradually and the peak times move to 10:00LT, while in the high latitude region (50°N~62°N) two peaks appear around noon, followed by a valley at dusk and the peak values increase again during the night. There are very few ionosondes at low latitudes in the European and American regions. Nevertheless, using the model of IRI‐90, it can be still found that the northern “crest” produced by the “fountain effect” is the weakest at longitude 270°N. The decreasing of daytime F2 peaks at higher latitudes is not evident in these regions. It means that the longitude change of latitude profiles of NmF2 diurnal variations is obvious. On the other hand, analysis of data measured by ionosondes in China and Japan (in a narrower range of longitude) shows that NmF2 diurnal variations in the region of 90°N~140°N has no great longitude difference, or so‐called longitude effect, at the “crest” region 20°N~30°N. However, at middle latitudes (30°N~50°N) the variation amplitudes of NmF2 are larger and steeper in the west China than that in the east of China and Japan.

The features and a possible mechanism of semiannual variation in the peak electron density of the low latitude F2 layer

Ionospheric data observed in 30 stations located in 3 longitude sectors (East Asia/Australia Sector, Europe/Africa Sector and America/East Pacific Ocean Sector) during 1974–1986 are used to analyse the characteristics of semiannual variation in the peak electron density of F2 layer ( NmF2). The results indicate that the semiannual variation of NmF2 mainly presents in daytime. In nighttime, except in the region of geomagnetic equator between the two crests of ionospheric equatorial anomaly, NmF2 has no obvious semiannual variation. In the high latitude region, only in solar maxima years and in daytime, there are obvious semiannual variations of NmF2. The amplitude distribution of the semiannual variation of daytime NmF2 with latitude has a “double-humped structure”, which is very similar to the ionospheric equatorial anomaly. There is asymmetry between the Southern and the Northern Hemispheres of the profile of the amplitude of semiannual variation of NmF2 and longitudinal difference. A new possible mechanism of semiannual variation of NmF2 is put forward in this paper. The semiannual variation of the diurnal tide in the lower thermosphere induces the semiannual variation of the amplitude of the equatorial electrojet. This causes the semiannual variation of the amplitude of ionospheric equatorial anomaly through fountain effect. This process induces the semiannual variation of the low latitude NmF2.

Modelling F2-layer seasonal trends and day-to-day variability driven by coupling with the lower atmosphere

This paper presents results from the TIME-GCM-CCM3 thermosphere–ionosphere–lower atmosphere flux-coupled model, and investigates how well the model simulates known F2-layer day/night and seasonal behaviour and patterns of day-to-day variability at seven ionosonde stations. Of the many possible contributors to F2-layer variability, the present work includes only the influence of ‘meteorological’ disturbances transmitted from lower levels in the atmosphere, solar and geomagnetic conditions being held at constant levels throughout a model year. In comparison to ionosonde data, TIME-GCM-CCM3 models the peak electron density ( NmF2) quite well, except for overemphasizing the daytime summer/winter anomaly in both hemispheres and seriously underestimating night NmF2 in summer. The peak height hmF2 is satisfactorily modelled by day, except that the model does not reproduce its observed semiannual variation. Nighttime values of hmF2 are much too low, thus causing low model values of night NmF2. Comparison of the variations of NmF2 and the neutral [O/N 2] ratio supports the idea that both annual and semiannual variations of F2-layer electron density are largely caused by changes of neutral composition, which in turn are driven by the global thermospheric circulation. Finally, the paper describes and discusses the characteristics of the F2-layer response to the imposed ‘meteorological’ disturbances. The ionospheric response is evaluated as the standard deviations of five ionospheric parameters for each station within 11-day blocks of data. At any one station, the patterns of variability show some coherence between different parameters, such as peak electron density and the neutral atomic/molecular ratio. Coherence between stations is found only between the closest pairs, some 2500 km apart, which is presumably related to the scale size of the ‘meteorological’ disturbances. The F2-layer day-to-day variability appears to be related more to variations in winds than to variations of thermospheric composition.

Seasonal and solar cycle variations of the ionospheric peak electron density: Comparison of measurement and models

This paper examines the ability of empirical and physical models to reproduce the peak electron density of the midlatitude ionospheric F2 region (NmF2) from 1976 to 1980. The data from all midlatitude stations show a tendency toward a semiannual variation in noon NmF2 with peaks at the equinoxes for all levels of solar activity. The Southern Hemisphere semiannual variation is more pronounced than in the Northern Hemisphere primarily because the winter density is relatively low in the Southern Hemisphere. At most locations the equinox density peaks are approximately equal. However, the September peak is much weaker than the March peak at most Australian stations. This leads to a distinct longitudinal variation between the Australian and South American sectors. On the other hand, there is remarkably little longitudinal variation in the Northern Hemisphere. We present calculations from the field line interhemispheric plasma (FLIP) model from 1976 to 1980 at six representative midlatitude stations around the globe. The FLIP model reproduces the average seasonal and solar cyclical behavior of the measured NmF2 remarkably well most of the time. The greatest differences of 50% occur at the March equinox in the South American region and at the September equinox in the Australian region during September solstice solar maximum. The international reference ionosphere (IRI) model reproduces the average NmF2 even better than the FLIP model but, unlike the FLIP model, it has little day‐to‐day variation. A factor of 2 increase in the solar EUV ion production rate and in the atomic to molecular density ratio at the F2 region peak height (hmF2) produces a factor of 4 increase in NmF2 over a solar cycle. Most of this increase takes place before the average solar activity index (F10.7) reaches 175. At solar maximum in 1979 and 1980, there is little relationship between daily F10.7 and NmF2. Changes in the atomic to molecular density ratio at hmF2 are primarily responsible for the semiannual variation in the FLIP model NmF2. The inclusion of vibrationally excited N2 in the FLIP model improves the relative seasonal and solar cycle NmF2 variations in the FLIP model, but it causes the overall NmF2 to be too low at most stations.

Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion

Abstract. The companion paper by Zou et al. shows that the annual and semiannual variations in the peak F2-layer electron density (NmF2) at midlatitudes can be reproduced by a coupled thermosphere-ionosphere computational model (CTIP), without recourse to external influences such as the solar wind, or waves and tides originating in the lower atmosphere. The present work discusses the physics in greater detail. It shows that noon NmF2 is closely related to the ambient atomic/molecular concentration ratio, and suggests that the variations of NmF2 with geographic and magnetic longitude are largely due to the geometry of the auroral ovals. It also concludes that electric fields play no important part in the dynamics of the midlatitude thermosphere. Our modelling leads to the following picture of the global three-dimensional thermospheric circulation which, as envisaged by Duncan, is the key to explaining the F2-layer variations. At solstice, the almost continuous solar input at high summer latitudes drives a prevailing summer-to-winter wind, with upwelling at low latitudes and throughout most of the summer hemisphere, and a zone of downwelling in the winter hemisphere, just equatorward of the auroral oval. These motions affect thermospheric composition more than do the alternating day/night (up-and-down) motions at equinox. As a result, the thermosphere as a whole is more molecular at solstice than at equinox. Taken in conjunction with the well-known relation of F2-layer electron density to the atomic/molecular ratio in the neutral air, this explains the F2-layer semiannual effect in NmF2 that prevails at low and middle latitudes. At higher midlatitudes, the seasonal behaviour depends on the geographic latitude of the winter downwelling zone, though the effect of the composition changes is modified by the large solar zenith angle at midwinter. The zenith angle effect is especially important in longitudes far from the magnetic poles. Here, the downwelling occurs at high geographic latitudes, where the zenith angle effect becomes overwhelming and causes a midwinter depression of electron density, despite the enhanced atomic/molecular ratio. This leads to a semiannual variation of NmF2. A different situation exists in winter at longitudes near the magnetic poles, where the downwelling occurs at relatively low geographic latitudes so that solar radiation is strong enough to produce large values of NmF2. This circulation-driven mechanism provides a reasonably complete explanation of the observed pattern of F2 layer annual and semiannual quiet-day variations. Key words: Atmospheric composition and structure (thermosphere-composition and chemistry) - Ionosphere (mid-latitude ionosphere; modelling and forecasting)

Disturbed vertical E×B plasma drifts in the equatorial F2 region at solar minimum deduced from observed NmF2 and hmF2 variations

Abstract. Ground-based ionosonde and magnetic-field observations on the equatorial station Huancayo, ESRO4 neutral-composition measurements, and theoretical model calculations were used to analyze disturbed E×B vertical plasma drift during the phase of solar minimum in 1973. Vertical drifts calculated for disturbed days do not show the systematic decrease often mentioned in publications, and demonstrate strong dependence on IMF-Bz changes. It is confirmed with the help of our drift calculations that Bz turnings to a northward direction result in a decrease (up to reversal) of normal Sq (eastward during daytime and westward at nighttime) in the zonal component of electric field. Southward Bz excursions enhance normal Ey both in daytime and nighttime hours. Model predictions of Ey\\'s reaction to IMF-Bz changes are discussed.

A study of the undisturbed mid‐latitude ionosphere using simultaneous multiple site ionosonde measurements during the Sundial‐86 Campaign

The Sundial‐86 campaign obtained simultaneous ionosonde measurements of NmF2 from 41 mid‐latitude sites geographically dispersed around the world from September 21 to October 5, 1986. A three‐dimensional, time‐dependent model of Earth's ionosphere has been used to fit the variations in NmF2 obtained from these sites. The model successfully reproduced the measurements in both the northern and southern hemispheres using the vertical plasma drift as a free parameter. During geomagnetically quiet conditions this drift is primarily due to the meridional neutral wind. The vertical plasma drift required to fit the data is consistent from location to location and the deduced meridional neutral wind is also consistent with previous neutral wind measurements and models. However, it is shown that a lack of knowledge of the variation of the O+ flux at the top of the atmosphere introduces a large uncertainty in the deduced thermospheric winds.

Geomagnetic storms in the Antarctic F-region. II. Physical interpretation

The physical explanation of the changes of the maximum electron concentration of the F2 layer, NmF2. as a result of geomagnetic disturbances has proved elusive even after several decades of study. In this paper, we concentrate upon the main phase of the F-region storm which usually begins a few hours after the onset of a significant increase of geomagnetic activity. We present some new analysis of ionosonde data from the South Atlantic and Antarctica. There are major differences in storm phenology in the South compared with the North. In winter in the South, NmF2 is usually increased as the result of a geomagnetic storm whereas in the North it is depressed. In summer, both hemispheres normally show reduced values of NmF2 as a result of storm activity. It is these features which provide new insight into the problem of F-region storms. We interpret the observed seasonal and diurnal variation of NmF2 in terms of a mean (DC) effect and a diurnal variation (AC) effect, and discuss possible explanations for both. We conclude that there must be direct energy input at mid-latitudes during geomagnetic storms to cause the observed DC effect. We suggest that the energy sources are the precipitation of ring current ions, particularly O +, and the penetration of the cross-tail magnetospheric electric field to mid-latitudes in the first few hours of the storm. We attribute the AC effect to changes in the magnitude and direction of the thermospheric wind caused by alterations in momentum exchange between the neutral and ionized gas (ion drag). We suggest that longitudinal asymmetries in the DC sources may also contribute to the AC effect. Our interpretation and conclusions are consistent with previous in situ observations at mid-latitudes in which changes of the O N 2 ratio are observed. Further three-dimensional modelling of the coupled ionosphere and thermosphére, combined with additional F-region neutral wind observations at mid-latitudes, are necessary to test our suggestions.

Diurnal variations in the flux of ionisation above the F2 peak in the northern and southern hemispheres

The flux of ionisation at 850 km height is calculated using the MSIS atmospheric model, a simplified form for the continuity equation at the peak of the F2-layer, and observed values of NmF2. Results are given for stations at latitudes of 32°N, 21°N, 21°S and 37°S during 1971 and for Tahiti (18°S) in 1980. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at mid latitudes. Increased recombination due to N 2 vibrational excitation produces a large afternoon decrease in NmF2 in summer, near solar maximum, and an increased downward flux. At all stations the day-time flux has a much larger downward component in winter than in summer. Because of the eastward magnetic declination, zonal winds produce opposite effects on the diurnal variations of hmF2, NmF2 and flux in the northern and southern hemispheres. Downward fluxes are largest in the morning in the southern hemisphere and in the late afternoon and evening in the north. At ± 21° latitude, neutral winds have a major effect on the distribution of ionisation from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than in summer. Averaged over both hemispheres, the total flow at 21° latitude is approximately the same for solstice and equinox conditions. At mid latitudes there is a downwards flux of about 1–2 × 10 12 m 2 s −1 into the night ionosphere.

Diurnal variations in F2 peak density and ionization flux above Tahiti

The effects of composition and ionization fluxes on the diurnal variation of NmF2 at an equatorial anomaly zone station (Tahiti) are separated. The calculated diurnal variation of the fluxes agrees well with what would be expected from published equatorial E × B drift observations and global neutral wind models. A correlation analysis shows that lower hmF2 is often accompanied by larger NmF2, in spite of a much larger recombination rate. This illustrates the dominance of the fountain effect and neutral wind induced interhemispheric transport at this station.

Electron content and its variation over Lindau

Electron content and its variations over Lindau (51.6° N, 10.1° E) have been studied utilizing the 150 and 400 MHz transmissions from U.S. NNSS satellites during the year 1971. The general and characteristic features of diurnal and seasonal variations of total electron content (TEC) are outlined and compared with corresponding NmF2 variations. The experimental values have been compared with those calculated theoretically (after Stubbe, 1970). The appearance of nocturnal winter and evening summer maximum at Lindau has been studied in detail utilizing TEC measurements, obtained from the satellite S-66 transmissions over the period 1964–1968 and the foF2 values. The general features of these phenomena are outlined and the causes of their appearance discussed. This is found that night maximum occurs even in equinox 1 period (March–April) in TEC. This, however, does not appear in foF2 values though a very weak trend of a maximum around 02–03 hr could be noted in March 1965 and 1966.

The influence of disturbed conditions and increased solar activity on geomagnetic distortion of the equatorial ionospheric F2 region

The principal diurnal anomalies in the maximum electron density of ionospheric F2 layer over Kodaikanal are discussed. From an analysis of data obtained over half a solar cycle, it has been found that the phenomena of rapid increase in the electron densities following sunrise, the diurnal asymmetry and the maintenance of abnormally high ionic densities after sunset undergo systematic changes during stormy conditions. In order to examine whether these changes are accounted for by additional movements during disturbed conditions, magnitudes of movement terms of the continuity equation at three heights, for the quiet as well as the disturbed days, have been computed and discussed. The changes in the pattern of diurnal variation of NmF2 with solar activity and the mechanism responsible for these changes are also discussed. From an analysis of published F2 layer data obtained at a number of stations with magnetic dip between ± 40°, it has been shown that the ratio of morning to afternoon peak densities yields a fair measure of equatorial F2 layer distortion anomaly. From solar cycle variation in diurnal asymmetry, an attempt has been made to estimate the extent by which the anomalous belt widens during sunspot maximum.