Solar cycle variation of the total electron content around equatorial anomaly crest region in east asia

The monthly mean hourly values of total electron content data obtained at Lunping Observatory (geographic coordinates 25.00°N, 121.17°E; geomagnetic coordinates 14.3°N, 191.3°E) by using the ETS2 satellite beacon signal during the period from March 1977 to December 1990 have been used to analyze the solar cycle variations of total electron content (TEC) around equatorial anomaly crest region in East Asia. Positive, correlations were found between the 12 month running average of monthly mean TECs and sunspot numbers. By using the linear regression analysis method, the contour charts for real diurnal and seasonal variations of TEC at certain sunspot numbers were constructed and described. The diurnal variation of TEC was represented by the sum of its diurnal mean and first three harmonic components. The solar cycle variations of these components have also been discussed.     

Solar cycle variation of ƒoF2

For solar cycle 19 (1954–1964), the 12 monthly mean values of noon-time ƒoF2 at Ahmedabad (23°N, 73°E) show a large hysteresis effect when plotted against sunspot number or against geomagnetic Ap. However, a multiple regression analysis for the dependence of ƒoF2 on solar 2800 MHz flux and geomagnetic Ap, simultaneously, shows a better matching. Thus, long-term predictions need to take into account not just sunspot number but some solar index and geomagnetic index as two key parameters, simultaneously.     

A comparison of foF2 obtained from a time-dependent ionospheric model with Argentine Islands' data for quiet conditions

In this study a comparison is made of the Utah State University Time-Dependent Ionospheric Model (TDIM) and an ionosonde data set from Argentine Islands. This study is unique in that the Argentine Islands data set of foF2 spans complete diurnal, seasonal and solar cycle conditions for low geomagnetic activity. The TDIM reproduces these foF2 variations extremely well. Although the observed winter and summer solstice foF2 diurnal curves have opposite phases, they are readily modelled. At equinox where a sharp transition occurs from winter to summer, or vice versa, the monthly average is complicated by this feature and hence the TDIM does not reproduce the diurnal fine structure.The neutral wind induced vertical plasma drift is the only free parameter in this study. All the other inputs are fixed for the specific solar, seasonal and diurnal conditions. A vertical plasma drift variation is presented; although simplistic, it couples the geographic and geomagnetic frames. With additional information such as hmF2, it would be possible to deduce a unique vertically induced drift pattern.     

A search for a solar cycle-temperature relationship in the middle atmosphere over Volgograd

The variation of temperature in the middle atmosphere (15–80 km) at Volgograd (49°N, 44°E) during an 11-year solar cycle (1971–1982) has been studied. The temperature of the stratosphere did not show any significant influence of the sunspot cycle, but the temperatures of the mesosphere showed a strong in-phase relationship with the solar cycle. Computed correlation and regression coefficients were positive and highly significant in this region. At 60 and 70km the temperature variations were almost linearly related to the sunspot number. Seasonal studies indicated that solar activity has a much stronger influence on temperature during the winter than during the summer.     

The isobaric F2-layer

Cyclic diagrams, obtained by plotting the daily variation of the ionospheric electron density NmF2 against the height hmF2, are drawn for typical conditions at Slough (52°N) and Watheroo (30°S). Using the MSIS86 thermospheric model to relate the heights hmF2 to values of atmospheric pressure, the F2-peak is found to lie at nearly the same pressure-level at any given local time, over a wide range of geophysical conditions (season, solar cycle, magnetic disturbance). As local time varies, the pressure level corresponding to hmF2 varies in a way that is mainly determined by the local time variation of the thermospheric winds. This is verified for noon and midnight, using the MSIS86 model to compute the winds. The noon values of peak electron density (NmF2) are fairly consistent with theory (using values of solar ionizing flux as quoted in the literature), but with some discrepancies—particularly at sunspot maximum—that are probably due to uncertainties in chemical composition, or to the effects of vibrational excitation of molecular nitrogen. Overall, the analysis shows a remarkable consistency between ionospheric theory, the data and the MSIS model.     

Atmospheric transparency variations associated with geomagnetic disturbances

Geomagnetic storm-time variations of the atmospheric transparency in various latitudinal regions are considered. It is shown that the solar radiation measured at the Earth's surface at local noon increases by approximately 0.1 cal/cm² min at latitudes ϑ = 60–70° during geomagnetic disturbances. At middle latitudes (ϑ≈ 50°) this effect is not observed. The variation of the atmospheric transparency is shown to be associated with a simultaneous decrease of the galactic cosmic ray intensity.     

Solar cycle and long-period variations of mesospheric temperatures

Evidence for a temperature variation above about 55 km between years of high and low solar activity is found in rocket data of Volgograd (49°N, 44°E) 1969–1983, reaching a solar-cycle amplitude of 6K, whereas below 55 km no statistically significant solar cycle effect is detected. This mesospheric temperature variation is in qualitative agreement with a pressure variation at 80 km derived from lower ionosphere radio reflection heights near 51°N, 13°E, measured at Kühlungsborn/GDR, covering almost two solar cycles. When the solar cycle variation has been removed from these 80 km pressure data by means of a regression analysis, there remains a quasi-cycle of about 20 yr, which agrees well with observations of a general cooling of the northern mid-latitude stratosphere between 1965 and 1977, reported by other authors.     

The trouble with thermospheric vertical winds: geomagnetic,seasonal and solar cycle dependence at high latitudes

The vertical wind component is frequently used to determine the zero-velocity baseline for measurements of thermospheric winds by Fabry-Perot and other interferometers. For many of the upper atmospheric emission lines from which Doppler shifts are determined, for example for the OI 630 nm emission, available laboratory sources are not convenient for long-term use at remote automatic observatories. Therefore, the assumption that the long-term average vertical wind is zero is frequently used to create a baseline from which the Doppler shifts corresponding with the line-of-sight wind from other observing directions can then be calculated. A data base consisting of 1242 nights of thermospheric wind measurements from Kiruna (68°N, 20°E), a high-latitude site, has been analysed. There are many interesting short-term fluctuations of the vertical wind which will be discussed in future papers. However, the mean vertical wind at Kiruna also has a systematic variation dependent on geomagnetic activity, season and solar cycle. This means that the assumption that the average value of the vertical wind is zero over the observing period cannot be used in isolation to determine the instrument reference or baseline. Despite this note of caution, even within the auroral oval, the assumption of a zero mean vertical wind can be used to derive a baseline which is probably valid within 5 ms⁻¹ during periods of quiet geomagnetic activity (K_p < 2), near winter solstice. During other seasons, and during periods of elevated geomagnetic activity, a systematic error in excess of 10 ms⁻¹ may occur.     

Long-term relationships between sunspots,Ca-plages and the ionosphere

Smith and King (1981) recently found a relationship for Slough values of foE and foE2 using relative sunspot number (R) and area of photospheric faculae (A_F) from Greenwich publications. Since Greenwich stopped publishing the area of photospheric faculae, it is suggested in this paper that the area of Ca-plages (A_P) may be substituted for A_F for calculating foE and foF2. The values of foF2 calculated using the relationship given by Smith and King, with A_P and A_F separately, are found to have a good correlation (r=0.988). Similar equations showing the relationship between the foF2 values, sunspot numbers and Ca-plage area have been obtained for a few Indian stations. It has been found that in the regression equation, while the coefficients of the sunspot number and the Ca-plage area do not change significantly from one station to another, the constant term shows a latitude variation similar to the geomagnetic anomaly.     

Solar activity effects on equatorial plasma bubble zonal velocity and its latitude gradient as measured by airglow scanning photometers

Using a new mode of scanning 630-nm photometer operation the zonal velocities of ionospheric plasma depletions were measured over Cachoeira Paulista in Brasil in two east-west planes tilted 30°N and 30° S with respect to zenith. The measurements cover a time period of approximately 2 years, from January 1988 to January 1990, a period marked by significant increase in solar activity of the ongoing cycle. The results have permitted a rather detailed evaluation of the local time and latitude variations in the zonal plasma bubble velocity as a function of solar activity. Although the mean trend in the velocity local time variation is a decrease from early evening to post-midnight hours, a strong tendency for velocity peaks is observed near 21 LT and midnight. The velocities as well as their height (latitude) gradients show perceivable increases with solar activity represented as sunspot numbers. The present results are compared with the ambient plasma velocities measured using the Jicamarca radar by Fejer el al. (1985), J. Geophys. Res. 90, 12249, with that measured on board the DE 2 satellite on the equatorial latitudes by Coley and Heelis (1989), J. geophys. Res. 94, 6751, and with various theoretical calculations, in an attempt to bring out the salient features of the plasma dynamics of the equatorial ionosphere.     

Interannual fluctuations of the stratospheric temperature over the north polar region

The monthly means for the years 1964–1991 of 30 hPa temperatures over the North Pole and averaged over the 70–90°N region are analyzed. A multiple regression model is used to find long-term monthly trends and possible linear associations between these temperatures and the QBO, ENSO, and the 11-yr solar cycle. The model's residuals are examined for detection of other periodic interannual fluctuations in Arctic temperatures.It is found that the interannual variations of temperature at 30 hPa over the Arctic are a superposition of the oscillations due to the QBO, ENSO, the 11-yr solar cycle, and approximate 6-yr periodic fluctuations of unknown origin. The QBO, ENSO, and the solar cycle effects in the Arctic temperature explain about 35% of the total variance of the temperature monthly anomalies. In winter, the QBO, ENSO, and the 11-yr solar cycle signals in the temperature data depend on the phase of the equatorial QBO. The polar vortex seems to be warmer (colder) than normal when the West (East) phase of the equatorial QBO in a period of high solar activity. The monthly temperature trends over the Arctic show seasonal variations with positive trends in February and March. The year-round trends (sum of the monthly trends) are about −0.5 K per decade.     

Improved world-wide maps of monthly median foF2

New global maps of monthly median values of foF2 have been prepared using over 45,000 station months of foF2 observations, semi-empirical model values of foF2 in the mid-latitude ocean areas and empirical model values for the equatorial and high latitude regions. These observations have been carefully screened and mapped, using the Jones-Galley technique, to produce monthly median maps of foF2 for each hour, each month and for high and low levels of solar activity.     

Comparison of foF2 characteristics at similar low latitudes but different longitudes in the South American region

A comparison of the foF2 diurnal variation characteristics at Huancayo (12°S, 75°W, dip angle +2°)and Fortaleza(4°S, 38°W, dip angle −2° in 1976) showed in general larger midday foF2 at Fortaleza. In contrast, a comparison of foF2 at Tucuman (27°S, 65°W, dip angle − 22°) and Sào Paulo (24°S, 47°W, dip angle −22° in IGY period) showed smaller midday foF2 at São Paulo. Thus, a weaker fountain effect in the eastern part of the South American region is indicated.     

Equatorial night-time spread-F characteristics observed from stations in the Western Pacific

A comparative study of the ionospheric spread-F observed at an equatorial station. Port Moresby (9°24.5′S, 147°9.9′E; magn. dip lat. 18°S). Papua New Guinea, with corresponding data from the East Asian stations during a period 27 May–19 June 1983, as well as the same periods for 1981 and 1982, is presented. The Port Moresby spread-F condition is particularly outstanding and persistent at night and the occurrence of spread-F between 23 LT and 05 LT near June solstice is very high, reaching 100 % of all observed nights. The latitudinal variation of nocturnal spread-F, its duration and connection with sunspot number are analyzed. The relationship between long-term variation of night-time spread-F occurrence and solar activity during June and December solstices from 1954 up to 1984 at a typical lower magnetic latitudinal station, Wuhan (30°32.7′N, 114°21.5′E; magn. dip lat. 26°N). is also investigated.     

Faraday polarization fluctuations associated with amplitude scintillations at Lunping

The diurnal, seasonal and solar cycle variations of Faraday polarization fluctuations (FPF) associated with amplitude scintillations observed at Lunping, Taiwan (25.0°N, 121.2°E : geographic) during the period 1978–1981 are presented. The occurrence of polarization fluctuations is maximum in the premidnight hours. FPFs occur either simultaneously or with a time lag after the onset of amplitude scintillations. There is an increase in FPF activity with an increase in sunspot activity. Occurrence of FPF peaks in the equinoxes. There had been a moderate activity in summer while the winter occurrence is a minimum. The seasonal occurrence pattern compared with reports from other locations indicates a longitudinal control on FPF activity. The maximum probable duration of FPF ranges from 15 to 30 min. It was found that the association of FPF with range spread-F is much better than that with frequency spread-F. Large ambient ionization densities corresponding to plasma frequencies greater than 15 MHz appear to create a favourable environment for the occurrence of FPF.     

The influence of ionospheric absorption on mid-latitude whistler mode signal occurrence from VLF transmitters

Whistler mode signals from VLF transmitters received at Faraday, Antarctica (65° S, 64° W) during 1986–1991 show an annual variation in the number of hours over which signals are observed, with a maximum in June and a minimum in December. The variation was larger at solar minimum than at maximum and can be understood in terms of changes in absorption of VLF signals in the D-region, where the high geographic latitude of Faraday plays an important role in producing low attenuation levels during the austral winter. In contrast, very little such variation was observed at Dunedin, New Zealand (46° S, 171° E) in 1991. Nighttime whistler mode signals have start and end time trends that are consistent with the influence of F-region absorption. Increases in whistler mode occurrence appear to be associated with periods of high geomagnetic activity at solar maximum but not during solar minimum. A possible mechanism involving decreased F-region absorption is discussed.     

Variability of the solar cycle length during the past five centuries and the apparent association with terrestrial climate

Solar data have been used as parameters in a great number of studies concerning variations of the physical conditions in the Earth's upper atmosphere. The varying solar activity is distinctly represented by the 11-yr cycle in the number of sunspots. The length of this sunspot period is not fixed. Actually, it varies with a period of 80–90 yr. Recently, this variation has been found to be strongly correlated with long-term variations in the global temperature. Information about northernhemisphere temperature based on proxy data is available back to the second half of the sixteenth century. Systematic monitoring of solar data did not take place prior to 1750. Therefore, a critical assessment of existing and proxy solar data prior to 1750 is reported and tables of epochs of sunspot minima as well as sunspot cycle lengths covering the interval 1500–1990 are presented. The tabulated cycle lengths are compared with reconstructed and instrumental temperature series through four centuries. The correlation between solar activity and northern hemisphere land surface temperature is confirmed.     

Inter-annual variability of tides in the mesosphere and lower thermosphere

The inter-annual variation in diurnal and semi-diurnal atmospheric tides between 85 and 95 km has been studied for various years between 1978 and 1988. Observations comprised wind measurements from the medium frequency SA mode wind radars at Adelaide (35°S), Christchurch (44°S) and Saskatoon (52°N) and the meteor wind radar at Durham (43°N). Although the observations include the interval between solar maximum and solar minimum, there is in general no correlation between tidal amplitudes and solar activity. In contrast with earlier studies there does appear to be a positive correlation between solar activity and the amplitude of the semi-diurnal tide, but only during the southern summer and simultaneous northern winter.     

Effects of composition and fountain effect on the annual variation of the day-time F-layer at low latitudes

The annual variation of the daytime F2-layer peak electron density (NmF2) is studied at two low latitude stations, Okinawa and Tahiti (geomagnetic latitudes ± 15°) for the sunspot maximum years 1979–1981. Observed values are compared with those calculated using the MSIS model and a simplified version of the continuity equation for day-time equilibrium conditions. Summer-winter differences imply an intensification of the fountain effect on the winter side of the equator at the expense of the summer side. This could be explained by a summer to winter neutral wind. Semi-annual variations, however, appear to be mainly due to changes in neutral composition.     

Observations of neutral winds in the polar cap during northward IMF

Long term remote observations of neutral winds at F-region altitudes have been performed at Thule Air Base (lat. 76.5°N, long. 69.0°W), Greenland, and Søndre Strømfjord (lat. 67.0°N, long. 50.9°W), Greenland. The former site is very close to the geomagnetic pole, while the latter site is within the polar cap for several hours each night on either side of geomagnetic midnight. Wind data corresponding to clear sky conditions and Kp ⩽ 4 were sorted according to the sign of the IMF B_z component. The averaged maximum poleward flow near midnight LST was reduced by approximately one third during B_z northward conditions. If the magnitude of B_y was less than the magnitude of the northward B_z component, then the averaged poleward flow was further reduced by one half. In addition, if B_z > 5 nT, then sunward directed horizontal neutral winds were observed at the very highest latitudes near noon LST.