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.     

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.     

Annual variations in the electron content and height of the F layer in the northern and southern hemispheres,related to neutral composition

Total electron content (TEC) data is presented for similar sites at ±35° latitude, and conjugate sites at ±20°, for several years near solar maximum. Comparison with the MSIS atmospheric model shows that the large seasonal anomaly at 35°N (an increase of 80% in TEC from October to April) is fully explained by changes in neutral composition. The small seasonal anomaly at 35°S also agrees with the MSIS model. Composition changes fail to account for the generally higher TEC in the northern hemisphere; this suggests the presence of an overall south-to-north atmospheric wind. Eastern declinations also contribute to enhanced TEC in the northern hemisphere, in the Pacific zone. The MSIS model predicts a semiannual variation of about ±25% in TEC at all sites, while observed changes are only about ±8%; thus we require some enhanced loss process near the equinoxes, particularly in September and October.Peak height calculations assuming a constant pressure level give a large semiannual variation in the F2 region: this is replaced by an annual variation when h_m F2 is calculated from diffusion theory. Heights calculated from the MSIS model are similar to observed values at ±35° latitude on summer days. A decrease of about 20km in observed heights on winter days is attributed to a poleward neutral wind; this wind also reduces the observed TEC. At night the height changes correspond to an equatorial wind, which is largest in summer and equinox. Observed day time TEC is greater at 20°N than at 20°S at all times of year, suggesting a northward transequatorial wind which is strongest near January and gives increased TEC and decreased peak height at 20°N.     

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.     

Daytime mesospheric temperatures over the low-latitude station Thumba derived from rocket-borne solar Lyman-α absorption measurements

Molecular oxygen concentrations obtained from the rocket measurements of the absorption profile of solar Lyman-α radiation, in the upper atmosphere over Thumba (8°33′N, 76°52′E), have been used to derive gas temperatures in the 70–90 km altitude region. A mean reference temperature profile for the daytime mesosphere over Thumba obtained from these measurements shows (1) temperatures lower than those given by the CIRA model and (2) a mesopause around 83 km with a temperature of 175 K.     

Long period wind oscillations observed by the Kyoto meteor radar and comparison of the quasi-2-day wave with Adelaide HF radar observations

We have detected wind oscillations with periods ranging from 1.4 to 20 days at 80–110 km altitude using Kyoto meteor radar observations made in 1983–1985. Among these oscillations, the quasi-2-day wave is repeatedly enhanced in summer and autumn. We found that the period of the quasi-2-day wave ranges from 52 to 55 h in summer, and becomes as short as 46 to 48 h in autumn in 1983 and 1984. The change in the wave period seems to coincide with a decrease in the amplitude of the zonal mean wind. A quasi-2-day wave event was simultaneously observed in January 1984 at Kyoto (35° N, 136°E) and Adelaide (35° S, 138° E), which are located at conjugate points relative to the geographic equator. Amplitudes of the meridional component at Adelaide are approximately four times larger than those observed at Kyoto. Comparison observations clearly show that the meridional component is in phase and the zonal component is out of phase, respectively, implying antisymmetry of the quasi-2-day wave between the northern and southern hemispheres. Relative phase progressions with height are similar between the Kyoto and Adelaide results for both meridional and zonal components, and indicate the presence of an upward energy propagating wave with a vertical wavelength of about 100 km.     

Upper stratospheric and mesospheric temperatures derived from lidar observations at Aberystwyth

From lidar observations of relative atmospheric density above Aberystwyth (52.4°N, 4.1°W) upper stratospheric and mesospheric temperatures have been derived for a total of 93 nights between December 1982 and February 1985. Excellent agreement was found between radiances synthesised from these temperatures and those measured by satellite-borne instruments. Summer temperatures showed a smooth and regular variation with altitude and reasonably good agreement with the CIRA (1972) model atmosphere. By contrast, winter temperatures showed a much greater variability with altitude and greater changes from night to night, with the frequent occurrence of a large amplitude wave-like perturbation in the mesosphere with about 15 km vertical wavelength and amplitude about 20K between 60 and 80 km.Pronounced warmings of the stratosphere were observed during the three winters of observation. During the warming event occurring in early February 1983 the stratopause temperature increased to 303K at 43 km, while the major warming event of late December 1984/early January 1985 produced a stratospheric temperature gradient of 16K km⁻¹ between 34 and 36 km. During the latter event a distinct local temperature minimum at 32.6 km was observed on New Year's Eve, this descending to 29 km by the following night and being accompanied by a lowering of the stratopause from 43 to 38.5 km in the same period. These results demonstrate the ability of the present technique to resolve the high stratopause temperatures and steep stratospheric temperature gradients which occur during stratospheric warmings, in marked contrast to the limited resolution achieved by satellite experiments.     

Oscillations in D-region absorption at periods of one to two months

One to two month oscillations in D-region absorption are found in seven years of daily ƒ-min data from low latitude stations at Singapore (1°N, 104°E) and Rarotonga (21°S, 160°W). Coherency (cross-spectral) analyses reveal that solar flux variations account for much of the ƒ-min variance at these periods. Over the range of periods from 10 to 200 days, statistically significant linear correlation is found between the ƒ-min time series and contemporaneous 10.7cm solar flux measurements at periods of 16–19 days, the 26–29 day solar rotation band, and a broad band covering 43–80 day periods.     

Asymmetries in mesospheric tidal structure

Radar wind measurements made at Adelaide (35°S, 138°E) and Kyoto (35°N, 136°E) are used to construct climatologies of solar tidal wind motions in the 80–185 km region. The climatologies, in the form of contour plots of amplitude and phase of the diurnal (24 h) and semidiurnal (12 h) tides, show that there are significant asymmetries between Adelaide and Kyoto. The amplitude of the diurnal tide is significantly larger at Adelaide than at Kyoto. At both stations the phase changes in a systematic way with lime such that the phases of the zonal wind components tend to be in anti-phase at the solstices. At Adelaide, there is more evidence of the propagating (1,1) diurnal mode. At both stations, the semidiurnal tide is strongest and has the longest vertical wavelengths (>100 km) in late summer; short vertical wavelength (~ 50–80 km) oscillations are most in evidence in winter. In order to place the Adelaide and Kyoto observations in context they are compared with observations made at other latitudes and with recent numerical simulations. There is encouraging agreement between the observations and models, especially for the semidiurnal tide.     

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.     

Solar flux and seasonal variations of the mesopause temperatures at 51°N

Observations of the OH (8-3) band airglow emission, using a multichannel tilting filter type photometer, have been carried out at Calgary (51°N, 114°W), Canada, since 1981. In this paper recent measurements of the nocturnal, seasonal and solar flux variations of the mesopause temperature, obtained from the rotational temperature of the OH (8-3) band observations, are presented. The data presented span the ascending phase of the present solar cycle viz. 1987–1988 (low solar activity) and 1990 (high solar activity). Good correlations (r = 0.73) between the OH (8-3) band rotational temperature and the 10.7 cm solar flux were observed. The mean temperature for the period investigated was about 210 K. The seasonally averaged nocturnal variations show only small irregular excursions, possibly associated with solar tides and the passage of gravity waves in the mesopause region. However, the observed rotational temperatures show considerable night-to-night changes.     

Variations of the gravity wave characteristics with height,season and latitude revealed by comparative observations

This paper reviews some recent observations of gravity wave characteristics in the middle atmosphere, revealed by co-ordinated observations with the MU radar in Shigaraki (35°N, 136°E) and nearby rocketsonde experiments at Uchinoura (31°N, 131°E). We further summarize the results of comparative studies on the latitudinal variations of the gravity wave activity, which were detected by additionally employing data obtained with MF radars at Adelaide (35°S, 139°E) and Saskatoon (52, 107W) and lidar observations at Haute Provence (44, 6E).The seasonal variation of gravity wave activity detected with the MU radar in the lower stratosphere showed a clear annual variation with a maximum in winter, and coincided with that for the jet-stream intensity, indicating a close relation between the excitation of gravity waves and jet-stream activity at middle latitudes. The long-period (2–21 h) gravity waves seemed to be excited near the ground, presumably due to the interaction of flow with topography, and the short-period (5 min 2 h) components had the largest kinetic energy around the peak of jet-stream.We found an increase with height in the vertical scales of dominant gravity waves, which can be explained in terms of a saturation of upward propagating gravity waves. The values of the horizontal wind velocity variance generally increased in the stratosphere and lower mesosphere, but they became fairly constant above about 65 km due to the wave saturation, resulting in the active production of turbulent layers.Although the gravity wave energy showed an annual variation in the lower atmosphere, it exhibited a semiannual variation in the mesosphere, with a large peak in summer and a minor enhancement in winter. Lidar observations reasonably interpolated the seasonal variations in the intermediate height regions.The gravity wave energy in the mesosphere, with periods less than about 2 h, was consistently larger in summer than in winter at all the stations, i.e. at 35N, 44N,52 N and 35 S. However, the values were generally larger at 35 N than at 52 N. which was found from a comparison of l-yr observations at Shigaraki and Saskatoon. Furthermore, a comparison between Shigaraki and Adelaide, located at the conjugate points relative to the equator, revealed that the gravity-wave energy in the mesosphere was found to be fairly similar, when we compared the values in summer/winter in each hemisphere.     

Balloon measurements of stratospheric ion conductivities over the tropics

Conductivity measurements of negative and positive ions were made from about 20 to 35 km by two identical balloon-borne spherical probes at Hyderabad (17.5°N, 78.6°E), India on 22 April 1989 and 22 December 1990. One balloon was launched at 0158 h IST (Indian Standard Time) which reached its ceiling around 0330 h IST. After that time, it floated for about 3 h, 1.5 h before sunrise and 1.5 h after sunrise. Thus it gave data for both day- and night-time conditions at float altitude. The other balloon was launched at 0535 h IST. It gave data for daytime only. Several interesting results have been obtained at the float altitudes. During the night, in the flight of 22 April 1989 the conductivity values of positive ions were found to be about 1.5 times those of negative ions at the float altitude. During the day, in the flight of 22 April 1989, the positive ion conductivity values were found to increase with the increase of solar elevation angle at around 37.5 km altitude. The negative ion conductivity values, however, did not show any day-night variation. In the flight of 22 December 1990, these features were not seen. Instead, a pocket was found where conductivity values were very high (of the order of 10⁻¹¹ mho m⁻¹) at an altitude of about 32.5 km. Also in this flight, the positive ion conductivity was always found to be approximately equal to that of the negative ion conductivity.     

Simultaneous observations of travelling ionospheric disturbances by two-dimensional radio interferometry and the differential Doppler technique applied to satellite signals

The main object of the campaign reported here was to compare TID characteristics obtained from two essentially different observation techniques: (1) observation of the apparent angular position shifts of Virgo A by the Nançay radioheliograph (47.33°N, 2.15°E) gave azimuths and periods of travelling ionospheric disturbances (TIDs); (2) differential Doppler shifts of signals from NNSS-satellites recorded simultaneously at Tours (47.35°N, 0.70°E), Nançay and Besançon (47.32°N, 5.99°E) provided azimuths and latitudinal wavelengths. Observations were made during the period 10–30 November 1987, between 6 and 12 h UT. It is found that azimuths obtained from the two techniques are consistent if sufficient averaging over wave trains is performed: averaging over several hours for radio interferometry and averaging over the whole satellite trace for the differential Doppler technique. Averaging is necessary because of (1) the intrinsic dispersion in wave azimuth, (2) the broadness of observed wave spectra and the dispersive properties of gravity waves, and (3) the spatial separation of ionospheric points for the two techniques. Good agreement between the azimuths was achieved by setting the altitude of the TIDs, which is used in the differential Doppler analysis, to about 250 km, appreciably lower than the maximum in electron density (about 350 km). The mean azimuth of observed TIDs was 12° East from South with a standard deviation of about 30°. The dominant period and horizontal wavelength of the observed TIDs were 40 min and 450 km. The East-West coherence length of the TIDs was found to be only of the order of 200 km.     

VHF radio scintillations at Bombay

The simultaneous recordings of the amplitude scintillations of VHF radio signals from nearby geostationary satellites FLEETSAT (at 73°E long.) and SIRIO (at 65°E) received at Bombay (geog. lat. 19°N, geog. long. 73°E, mag. lat. 15°N) have revealed systematic time shifts in the starting and the ending of the individual scintillation events. The ionosphere crossover points of the two transmission paths were separated by only 80 km in the east-west direction, which was smaller than the average size of the irregularity patches. Scintillations normally started after 1930 h, reached a maximum at 2200 h and slowly decreased till 1000 h, after which no scintillations were observed. The speed of the irregularity patches computed from the time shifts of these events was about 150 m s⁻¹ in the early hours of the night, decreasing to about 100 m s⁻¹ by midnight and showing much lower velocities in the post-midnight hours.     

QBO and solar activity effects on temperatures in the mesopause region

This statistical correlation study is based on two upper mesospheric temperature data sets for the region around 90 km, on the phase of the quasi-biennial oscillation (QBO) and on the solar 10.7-cm radiation. The temperatures are measurements from passive OH1-spectrometers and lidar sounding experiments carried out between 1980 and 1988 at geographic latitudes between 50°N and 70° N. An anticorrelation exists betwen the 10.7-cm flux and the temperatures. Two different types of significance tests were used to check the confidence level of the correlation. The anti-correlation is highly significant (confidence ⪢ 99.99%) for latitudes around 50°N if the QBO is in its east phase and for the spring/summer temperatures. The anti-correlation is not significant for the QBO west phase and for fall/winter temperatures (confidence < 95%). For higher latitudes, around 79°N, no significant correlation was found.     

foF2 hysteresis variations and the semi-annual geomagnetic wave

Seasonal and solar cycle variations of the foF2 hysteresis magnitude are investigated. Data for the noon foF2 monthly medians for Slough (51.48°N, 0.57°W), the monthly means for the sunspot numbers, and for the geomagnetic activity index aa(N) for the northern hemisphere for the period 1933–1986, covering solar cycle from 17 to 21, are used. It is found that: (1) the greatest negative amplitudes of the foF2 hysteresis variation are near the equinoxes, and (2) the solar cycle average noon foF2 hysteresis magnitude is linearly correlated with the solar cycle average semi-annual geomagnetic amplitude of the aa-index. These results support the hypothesis that the foF2 hysteresis is due to the geomagnetic activity variation during the sunspot cycle.     

Results inferred from electron density measurements at Saskatoon,Canada (L = 4.4) by a partial reflection technique—I. Variations of nitric oxide in the D-region during quiet periods

Measurements of electron densities at Saskatoon (52°N, 106° W, L = 4.4) from 1976 to 1979 reveal seasonal variations which cannot be explained solely by solar zenith angle variations. These profiles have been used to infer the variations of nitric oxide density with season and solar activity for quiet-time conditions. It is found that while nitric oxide varies with season, it remains unchanged with the change in solar activity. The summer and spring profiles are much lower than the measured values of Baker et al. (1977) for heights below 85 km, while the winter estimated values show differences from the measured values in the height range 77.5–85 km. Above 85 km the values for all the seasons are close to the measured values. A dip in the nitric oxide distribution is obtained in all the cases around 80–82 km and the values of nitric oxide at the minimum are less than those measured by Baker et al. (1977) or Meira (1971).     

Diurnal tide in the Antarctic and Arctic mesosphere/lower thermosphere regions

The behaviour of the diurnal tide at 95 km over various years between 1965 and 1986 is studied using radar data from Heiss Island (81°N), Mawson (67°S), Molodezhnaya (68°S) and Scott Base (78°S). The observations are also compared with the model results of FORBES and HAGAN [(1988) Planet. Space Sci. 36, 579] for the same latitudes. There are substantial fluctuations in amplitude and phase at all stations, particularly in winter. Phase fluctuations can be as large as a uniform random distribution over the 24-h cycle. In summmer the phases of the meridional components are well defined and suggest the presence of a dominant symmetric mode. The meridional amplitudes are larger in summer whereas the zonal components have a greater variation and show no significant variation with season.     

Recent measurements of middle atmospheric electric fields and related parameters

In 1989, two series of rocket measurements were carried out to investigate middle atmosphere electric fields. The measurements were taken both in the Northern Hemisphere on Heiss Island (80°37′N and 58°03′E) and in the Southern Hemisphere in the Indian Ocean (40–60°S and ~45°E) on board the research vessel ‘Akademik Shirshov’. Along with the vertical electric fields, aerosol content and positive ion density were also measured. Some of the rocket launches were made during the extremely strong solar proton events (SPE) of October 1989. The experiments showed the strong variability of the electric fields in the middle atmosphere at polar and high middle latitudes. In all the measurements the maximum of the vertical electric field height profile in the lower mesosphere was observed to be more than ~ 1 V/m. The electric field strength and the field direction at maximum varied considerably among the launches. A maximum value of + 12 V/m was detected at a height of about 58 km at 58°30′S on 21 October 1989 during the SPE. The simultaneous measurements of the electric field strength, positive ion density and aerosols point out both an ion -aerosol interaction and a connection between the mesospheric electric fields and aerosol content.