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.     

Associations between the 11-year solar cycle,the QBO and the atmosphere. Part I: the troposphere and stratosphere in the northern hemisphere in winter

Linear correlations between the three solar cycles in the period 1956–1987 and high-latitude stratospheric temperatures and geopotential heights show no associations. However, when the data are stratified according to the east or west phase of the quasi-biennial-oscillation (QBO) in the equatorial stratosphere significant correlations result: when the QBO was in its west phase the polar data were positively correlated with the solar cycle while those in middle and low latitudes were negatively correlated. The converse holds for the east phase of the QBO. Marked relationships existed throughout the troposphere too.No major mid-winter warming occurred in the west phase of the QBO during a minimum in the three solar cycles. In the east phase major warmings tended to take place in the minima of the cycle. Thus the signal of the quasi-biennial-oscillation in the extratropical stratosphere tends to be strengthened in solar minima, and weakened in solar maxima.     

Quasi-biennial oscillation in ionospheric parameters measured at Juliusruh (55°N, 13°E)

When seasonal variations were eliminated by evaluating 12-month running means, the ionospheric parameters foE, foF2 and hmF2 at Juliusruh (54.6°N, 13.4°E) showed large solar cycle variations. However, when further 3-yr running averages were evaluated and subtracted, QBO (Quasi-biennial oscillations) were noticed in all these parameters. Sunspot series did not reveal a QBO, but geomagnetic Ap did show a QBO. The peaks of the ionospheric QBO and QBO of Ap could be roughly compared, with lags or leads of a few months. Also, these compared roughly with the well-known QBO peaks of tropical stratospheric (50 mb) zonal winds. Similar analyses at other locations are warranted.     

Longitudinal differences and inter-annual variations of zonal wind in the tropical stratosphere and troposphere

A quantitative assessment has been made of the longitude-dependent differences and the interannual variations of the zonal wind components in the equatorial stratosphere and troposphere, from the analysis of rocket and balloon data for 1979 and 1980 for three stations near ±8.5° latitude (Ascension Island at 14.4°W, Thumba at 76.9°E and Kwajalein at 67.7°E) and two stations near 21.5° latitude (Barking Sands at 159.6°W and Balasore at 86.9°E). The longitude-dependent differences are found to be about 10–20 m s⁻¹ (amounting to 50–200% in some cases) for the semi-annual oscillation (SAO) and the annual oscillation (AO) amplitudes, depending upon the altitude and latitude. Inter-annual variations of about 10 m s⁻¹ also exist in both oscillations. The phase of the SAO exhibits an almost 180° shift at Kwajalein compared to that at the other two stations near 8.5°, while the phase of the AO is independent of longitude, in the stratosphere.The amplitude and phase of the quasi-biennial oscillation (QBO) are found to be almost independent of longitude in the 18–38 km range, but above 40 km height the QBO amplitude and phase have different values in different longitude sectors for the three stations near ±8.5° latitude. The mean zonal wind shows no change from 1979 to 1980, but in the troposphere at 8.5° latitude strong easterlies prevail in the Indian zone, in contrast to the westerlies at the Atlantic and Pacific stations.     

A relationship between equatorial lower stratospheric QBO and El Niño

Cycle-to-cycle evolution of zonal wind QBO is studied making use of meteorological balloon and M-100 rocket data at Trivandrum (8.5°N, 77°E) for a period of about 19 yr from 1971 to 1990. The height of occurrence of zonal wind QBO maxima is found to vary systematically from one cycle to the next. The successive QBO maxima (easterly or westerly) occur progressively at greater heights and, after reaching a particular height, the next maximum (easterly or westerly) occurs at a lower height. Thereafter, the upward progression of the successive maxima starts again. It is found that the upward progression of the QBO maxima is closely associated with the occurrence of an El Niño event. A simple physical mechanism is suggested for this relationship between the QBO and El Niño. It is hypothesized that if the vertical wavelength of the Kelvin and mixed Rossby-gravity (MRG) waves are smaller during an El Nino event, the observed upward progression of the QBO maxima can be explained.     

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.     

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.     

A comparison of northern and southern hemisphere TEC storm behaviour

Changes in total electron content during magnetic storms are compared at stations with similar geographic and geomagnetic latitudes and eastward declinations in the northern and southern hemispheres.Mean patterns are obtained from 58 storms at ±35° and 28 storms at ± 20° latitude. The positive storm phase is generally larger (and earlier) in the southern hemisphere, while negative storm effects are larger in the north. These changes reduce the normal asymmetry in TEC between the two hemispheres. Composition changes calculated from the MSIS86 atmospheric model agree well with the maximum decreases in TEC in both seasons (when changes in the F-layer height are ignored). Recovery occurs with a time constant of about 35 h; this is 50% longer than in the MSIS86 model. There is a marked diurnal variation at 35°S, with a rapid overnight decay and enhanced values of TEC in the afternoon. This pattern is inverted (and weaker) at 35°N, where night-time decay is consistently slower than on undisturbed nights. These results require a diurnal change in composition of opposite sign in the two hemispheres, or enhanced westward winds at night changing to eastward near sunrise. There is some evidence for both these mechanisms. Following a night-time sudden commencement there is a large annual effect with daytime TEC increasing for storms near the June solstice and decreasing near December. Storms occurring between November and April tend to give large, irregular increases in TEC for several days, particularly at low latitudes. In summer and winter at both stations, the mean size of the negative phase does not increase for storms with K_p> 6. The size of the positive phase is proportional to the size of the change in a_p in winter, while in summer a positive phase is seen only for the larger storms.     

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.     

Simultaneous meteor radar observations at Monpazier (France, 44°N) and Punta Borinquen (Puerto Rico, 18°N). II—Mean zonal wind and long period waves

Meteor radar wind observations have been simultaneously conducted by the CNET/CRPE meteor radar group at Monpazier (France, 44°N) and at Punta Borinquen (Puerto Rico, 18°N) in 1977–1978. The Puerto Rico results yielded some characteristics of the mean zonal wind and long period motions at low latitudes which have been compared with the characteristics of the same components observed at middle latitudes. Some differences (both time and altitude shift) are seen for the winter and spring circulation between low and mid-latitudes and even high latitudes (Kiruna, 68°N) where a CNET radar was established in 1974–1975.More energy was observed at low latitudes in the long period wave range and with the systematic existence of peaks at periods 2.5 days and between 3 and 10 days. The long period motions can be generally considered as ‘evanescent’ waves at low and middle latitudes, except during the July–August campaign in Puerto Rico.The existence of two components of 6-day and 60 h periods at both stations in March 1978 allowed us to assume that these waves were planetary waves and to study their longitudinal and latitudinal structure.     

1987–1989 total ozone and ozone sounding observations in Northern Scandinavia and Antarctica,and the climatology of the lower stratosphere during 1965–1988 in Northern Finland

The total ozone observations of Tromsö (Northern Norway), Sodankylä (Northern Finland) and Murmansk (Northwestern Soviet Union) for 1987–1989 have been studied. Comparisons of the total ozone with stratospheric temperatures observed at Sodankylä have been made. These values have also been compared with the long-term mean total ozone at Tromsö and the long-term means of stratospheric temperatures at Sodankylä. No severe ozone depletions were observed. The exceptionally high total ozone values at these stations in February 1989 were connected to abnormally high stratospheric temperatures. The comparison of total ozone observed at roughly the same southern latitudes revealed great differences in the springtime.The 1989 ozone sounding observations of Sodankylä, Bear Island and Ny Ålesund (Spitzbergen) did not reveal any indications of pronounced ozone depletion. A comparative study of ozone, temperature and relative humidity indicated that the springtime variability of ozone in the lower stratosphere was clearly connected to meteorological variability. The lower tropospheric ozone had two distinct maxima, one in spring with large-scale photochemical causes and the other in summer connected with the emissions of hydrocarbons and oxides of nitrogen in Europe.Temperature observations made at Sodankylä over 24 yr revealed the existence of a potential for polar stratospheric cloud formation in the lower stratosphere in winter and early spring. A trend analysis of 50 hPa temperature revealed a negative trend of −0.16 K/yr in January and a positive trend of 0.15 K/yr in April; the annually-averaged trend was only −0.02 K/yr for this 24-yr period. When the January–February mean temperatures are separated according to the phase of the QBO in the tropical stratosphere, correlations between temperatures and sunspot numbers are found.     

ELF/VLF propagation measurements in the Atlantic during 1989

The vertical electric field component was measured by a group of the Ukrainian Institute of Radio Astronomy on board the Professor Zubov scientific vessel during April 1989 at latitudes from 30°S to 50°N. Results of the amplitude measurements in the Atlantic of natural ELF radio signals and those from the VLF navigation system “Omega” at its lowest frequency of 10.2 kHz are given. Characteristics were obtained of the moving ship as the field-site for the ELF observations. Variations in the ELF radio noise amplitude recorded at tropical latitudes agree with the computed data for the model of three continental centres of lightning activity. The VLF results were obtained by the “beat” technique providing the simplest narrow-band amplitude registration. Range dependencies of the field amplitudes from A (Norway), B (Liberia) and F (Argentina) stations have been analysed. The VLF attenuation factor was estimated for the ambient day conditions along the four cardinal directions. This allowed the detection of a statistically significant attenuation difference between the east-west and west-east propagation paths. The VLF radio signal was also used as a probe to evaluate the effective height of the vertical electric antenna and to calibrate the ELF noise amplitudes.     

Antarctic polar cap total electron content observations from Casey Station

In early 1990 a modified JMR-1 satellite receiver system was installed at Casey Station, Antarctica (g.g. 66.28°S, 110.54° E, -80.4°A, magnetic midnight 1816UT, L = 37.8), in order to monitor the differential phase between the 150 and 400 MHz signals from polar orbiting NNSS satellites. Total electron content (TEC) was calculated using the differential phase and Casey ionosonde foF2 data, and is presented here for near sunspot maximum in August 1990 and exactly one year later. The data are used to investigate long-lived ionization enhancements at invariant latitudes polewards of − 80° A, and the ‘polar hole’, a region from −70 to − 80° A on the nightside of the polar cap where reduced electron densitiy exists because of the long transport time of plasma from the dayside across the polar cap. A comparison is made between the Casey TEC data and the Utah State University Time Dependent Ionospheric Model (TDIM) which uses as variables the solar index (F 10.7), season (summer, winter or equinox), global magnetic index (K_p), IMF B_y direction, and universal time (UT) [sojkaet al. (1991) Adv. Space Res.11(10), 39].     

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.     

Climatology of gravity waves in the middle atmosphere

An attempt is made at the statistical analysis of small-scale disturbances in the stratosphere and mesosphere with the aid of meteorological rocket observations at many stations from 77°N to 8°S for several years.By applying a high-pass filter to daily rocket data in the height range 20–65 km, wind and temperature fluctuations with characteristic vertical scales close to or less than 10 km are obtained, which are considered to be due to internal gravity waves. Results are expressed in terms of parameters which tend to emphasize smallscale vertical fluctuations and which should provide qualitative measures of gravity wave activity.It is found that the gravity wave activity shows a notable annual cycle in higher latitudes with the maximum in wintertime, while it shows a semiannual cycle in lower latitudes with the maxima around equinoxes. It is also found from the standard deviation around the monthly mean that the temporal variability of gravity waves is very large.     

Modelling of transequatorial propagation of HF radio waves

In the geometrical optics approximation, a synthesis oblique ionogram of ionospheric and magnetospheric HF radio wave signals propagating between magnetic conjugate points has been carried out. The magnetospheric HF propagation is considered for a model of the waveguide formed by field-aligned irregularities with depleted electron density. The characteristic peculiarities of the magnetospheric mode have been determined: (i) strong disperion of the group delay with a frequency at 14–18 MHz, from − 1.4 to 0.6 ms/MHz for magnetically conjugate points at geomagnetic latitudes φ = 30°, 40° and 50°, respectively, (ii) spreading ∼ 1–2 ms, and (iii) a possibility of propagation between magnetic conjugates points at moderately low geomagnetic latitudes φ₀ ∼ 30–40° at frequencies exceeding 1.5 times the maximum usable frequency (MUF) of multi-hop ionospheric propagation.     

Tidal winds from the MLT global radar network during the first LTCS campaign—September 1987

Winds and tides were measured by a number of MLT (Mesosphere, Lower Thermosphere) radars with locations varying from 43–70°N, 35–68°S, during the first LTCS (Lower Thermosphere Coupling Study) Campaign, 21–25 September 1987. The mean winds were globally westerly, consistent with early winter-like (NH) and late winter (SH) circulations.The semi-diurnal tide had vertical wavelengths near or less than 100 km at most locations, with some latitudinal variation (longer/shorter at lower latitudes in the NH/SH)—amplitudes decreased at high latitudes. The global tide was closer to anti-symmetric, with northward components being in phase at 90 km. Numerical model calculations [Forbes and Vial (1989), J. atmos. lerr. Phys. 51, 649] for September have rather similar wavelengths and amplitudes; however, the global tide was closer to symmetric, and detailed latitudinal trends differed from observed.The diurnal tide had similar wavelengths in each hemisphere, with short values (~30 km) at 35°, long (evanescence) at 68–70°, and irregular phase structures at mid-latitudes. The tide was neither symmetric nor anti-symmetric. Model calculations for the equinox [Forbes. S and Hagan (1988), Planet. Space Sci. 36, 579] were by nature symmetric, and showed the short wavelengths extending to mid-latitudes (43–52°). Southern hemisphere phases were significantly (6–8 h) different from observations. Amplitudes decreased at high latitudes in model and observation profiles.     

Temperatures and tides in the high latitude mesopause region as observed in the OH night airglow emissions

The OH (6-2) band night airglow emissions have been observed from two sites at 60 and 70°N, respectively, in Norway during the December–April periods 1985–1986 and 1986–1987. Variations in rotational temperatures at ~90 km on time scales from tens of minutes to days show similar patterns as at 80°N. The semi-diurnal tide is dominant with average peak to peak amplitudes of ~5 K over the observing periods. There is a negligible difference in average tidal amplitude at 60 and 70°N. The phase of the tide is changing slowly through the December–February period. The mid-winter to early spring average temperatures are ~10 K higher than predicted by the CIRA 1972 90 km model atmosphere for the respective sites.     

Optical interferometer measurements of thermospheric temperature at Kavalur (12.5°N, 78.5°E), India

First results on the behaviour of thermospheric temperature over Kavalur (12.5°N, 78.5°E geographic; 2.8°N geomagnetic latitude) located close to the geomagnetic equator in the Indian zone are presented. The results are based on measurements of the Doppler width of O(¹D) night airglow emission at 630 nm made with a pressure-scanned Fabry-Perot interferometer (FPI) on 16 nights during March April 1992. The average nighttime (2130-0430 IST) thermospheric temperature is found to be consistently higher than the MSIS-86 predictions on all but one of the nights. The mean difference between the observed nightly temperatures and model values is 269 K with a standard error of 91 K. On one of the nights (9/10 April 1992, Ap = 6) the temperature is found to increase by ~250 K around 2330 IST and is accompanied by a ‘midnight collapse’ of the F-region over Ahmedabad (23°N, 72°E, dip 26.3°N). This relationship between the temperature increase at Kavalur and F-region height decrease at Ahmedabad is also seen in the average behaviour of the two parameters. The temperature enhancement at Kavalur is interpreted as the signature of the equatorial midnight temperature maximum (MTM) and the descent of the F-region over Ahmedabad as the effect of the poleward neutral winds associated with the MTM.     

Spread-F and ionization anomaly belt

The present investigation attempts to bring out the dynamics of the F-region at magnetic equatorial and low latitudes in the American zone. Data are examined for two sets of nights, one with strong range-type spread at Huancayo another with complete absence of spread-F. A prominent bulge of the F-region was observed within and below a latitude 10°N in the evening hours of the spread-F nights. Contours of electron distribution during post-sunset hours at the equatorial latitude, Huancayo (Dip 2°N); low latitude, Talara (dip 13°N); and a location near the anomaly crest location, Panama (dip 38°N), indicated a much steeper gradient in electron density at fixed heights on spread-F nights compared to a rather low gradient on the nonspread-F nights. Enhanced concentration of electrons at the anomaly crest location Panama, and a lower density at the equatorial location Huancayo, were observed on spread-F present nights. This is attributed to the phenomena of an evening plasma fountain in operation at equatorial latitudes on spread-F nights.