The seasonal variations in the nighttime 6300 Å emission at midlatitudes from ISIS-II spacecraft

Optical limb observations at F-region heights from the ISIS-II satellite have been used to study the seasonal variations in the 6300 Å limb emission for nighttime conditions and the aeronomic implications. The observations were carried out over the American zone at northern midlatitudes, and refer mainly to the period 1973–1975 of low solar activity.The observed seasonal variations in the emission seem to be mainly controlled by the electron density at F-region heights for nighttime and quiet geomagnetic conditions. The winter minimum is found to be deeper than the summer minimum. The obervations give clear evidence of semiannual variation in the emission. The phase variations agree closely with that of the semiannual variations in electron density and neutral atmospheric density at F-region heights. However, the amplitude variations of the semiannual variations are found to be larger than suggested by the observed F-region electron density. The observations during highly disturbed conditions possibly show the presence of gravity waves with wavelengths around 500 km, which could transport auroral energy to lower latitudes. The midlatitude enhancements observed during disturbed conditions seem to be related to the inward movement of the plasmapause.     

Geomagnetic field variations at low latitudes and ionospheric electric fields

The solar cycle, seasonal and daily variations of the geomagnetic H field at an equatorial station, Kodaikanal, and at a tropical latitude station, Alibag, are compared with corresponding variations of the E-region ionization densities. The solar cycle variation of the daily range of H at either of the stations is shown to be primarily contributed to by the corresponding variation of the electron density in the E-region of the ionosphere. The seasonal variation of the ΔH at equatorial stations, with maxima during equinoxes, is attributed primarily to the corresponding variation of the index of horizontal electric field in the E-region. The solar daily variation of ΔH at the equatorial station is attributed to the combined effects of the electron density with the maximum very close to noon and the index of electric field with the maximum around 1030 LT, the resulting current being maximum at about 1110 LT. These results are consistent with the ionosphere E-region electron horizontal velocity measurements at the equatorial electrojet station, Thumba in India.     

Total electron content of the ionosphere at 31°S, 1967–1974

The total electron content (TEC) of the ionosphere at 31°S (geographic) has been calculated on the basis of Faraday rotation measurements made between September 1967 and January 1975 using geostationary satellites. The day-to-day, diurnal, seasonal and solar cycle variations of TEC are illustrated and discussed in relation to the maximum electron density of the F-layer, NMAX. A regression analysis is used to derive curves corresponding to fixed high and low levels of activity. The variations of slab thickness S = TEC/NMAX are also illustrated and discussed. The results overlap the observation periods of other published results and general agreement is found with these other results.     

Equatorial spread-F by electric fields and atmospheric gravity waves generated by thunderstorms

It is assumed that atmospheric gravity waves, resulting in travelling ionospheric disturbances (TIDs), and electric fields, generated by convective thunderstorms, have a reasonable influence on the large-scale structure of premidnight equatorial spread-F irregularities. The responsible mechanisms, viz the superposition of thunderstorm generated electric fields on the ionospheric electric fields being the determining factor for irregularity generation and the steepening of TID structures due to spatial resonance, are briefly outlined. It is recalled that convective activity is most pronounced in the intertropical convergence zone over the African and South American continents. A model based on the typical features of seasonal and geographical variation of tropical convection generating the TIDs is presented which can explain seasonal and geographical variations of premidnight equatorial spread-F occurrence.     

Control of diurnal and seasonal variation of particle precipitation by regular changes of the interplanetary magnetic field

Systematic changes of the position of the dipole axis of the Earth's magnetic field with respect to the solar axis induce distinct daily and seasonal variations of the vertical B_z-component in the solarmagnetospheric coordinate system (B_ZSM). Depending on the direction of the interplanetary magnetic field (IMF), negative B_ZSM- values are produced in spring by T polarity and in autumn by A polarity, whereas in the diurnal variation lowest B_ZSM-values have been calculated to occur near 23 UT for T, and near 11 UT for A polarity, respectively. In different ionospheric and geomagnetic parameters measured at high and midlatitudes increased precipitation of high energetic particles into the lower thermosphere and upper mesosphere has been detected during periods with negative B_ZSM-components. The seasonal variation of the parameters investigated, with maximum values near the equinoxes, as well as a part of their diurnal variations, can thus be explained by particle precipitation being markedly controlled by the IMF sector structure.     

Climatologies of semi-diurnal and diurnal tides in the middle atmosphere (70–110 km) at middle latitudes (40–55°)

The radars utilized are meteor (2), medium-frequency (2) and the new low-frequency (1) systems: analysis techniques have been exhaustively studied internally and comparatively and are not thought to affect the results. Emphasis is placed upon the new height-time contours of 24, 12 h tidal amplitudes and phases which best display height and seasonal structures; where possible high resolution (10 d) is used (Saskatoon) but all stations provide monthly mean resolution. At these latitudes the semi-diurnal tide is generally larger than the diurnal (10–30 m s⁻¹ vs. < 10 ms⁻¹), and displays less month to month variability. The semi-diurnal tide does show significant regular seasonal structure; wavelengths are generally small (⩽50 km) in winter, large in summer (≲ 100 km), and these states are separated by rapid equinoctial transitions. There is some evidence for less regularity toward 40°C. Coupling with mean winds is apparent. The diurnal tide has weaker seasonal variations; however there is a tendency for vertical wavelengths and amplitudes to be larger during summer months. On occasions in winter and fall wavelengths may be less than 50 km. Again the seasonal transitions are in phase with reversals of the zonal wind. Agreement with new numerical models is to be shown encouraging.     

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.     

Seasonal and tidal variations of neutral temperatures and densities in the high latitude lower thermosphere measured by EISCAT

Measurements of ion temperature, ion-neutral collision frequency and ion drift in the E-region from the period December 1984 to November 1985 are used to derive neutral temperatures, densities and meridional winds in the altitude intervals 92–120 km, 92–105 km and 92–120 km, respectively. Altitude profiles of temperature and density and their seasonal variations are compared with the CIRA 1972 and MSIS 1983 models and the effects of geomagnetic activity are demonstrated. Semi-diurnal tidal variations in all three parameters are derived and the comparison with lower latitude measurements is discussed.     

Annual and semiannual variations of the geomagnetic field at equatorial locations

For a year of quiet solar-activity level, geomagnetic records from American hemisphere observatories located between about 0° and 30° north geomagnetic latitude were used to compare the annual and semiannual variations of the geomagnetic field associated with three separate contributions: (a) the quiet-day midnight level, MDT; (b) the solar-quiet daily variation, Sq; (c) the quiet-time lunar semidiurnal tidal variation, L(12). Four Fourier spectral constituents (24, 12, 8, 6 h periods) of Sq were individually treated. All three orthogonal elements (H, D and Z) were included in the study.The MDT changes show a dominant semiannual variation having a range of about 7 gammas in H and a dominant annual variation in Z having a range of over 8 gammas. These changes seem to be a seasonal response to the nightside distortions by magnetospheric currents. There is a slow decrease in MDT amplitudes with increasing latitude.The Sq changes follow the patterns expected from an equatorial ionospheric dynamo electrojet current system. The dominant seasonal variations occur in H having a range of over 21 gammas for the 24 h period and over 12 gammas for the 12 h period spectral components. The higher-order components are relatively smaller in size. The Sq(H) amplitudes decrease rapidly with increasing latitude. Magnetospheric contributions to the equatorial Sq must be less than a few per cent of the observed magnitude.The L(12) variation shows the ionospheric electrojet features by the dominance of H and the rapid decrease in amplitude with latitude away from the equator. However, the seasonal variation range of over 7 gammas has a maximum in early February and minimum in late June that is not presently explainable by the known ionospheric conductivity and tidal behavior.     

Plasmaspheric zonal electric fields and coupling fluxes from the Dunedin VLF Doppler experiment,for 180°E,L ≈ 2.3, at solstice and equinox

Group delays and Doppler shifts from ducted whistler-mode signals are measured using the VLF Doppler experiment at Dunedin, New Zealand (45.8°S, 170.5°E). Equatorial zonal electric field and plasmasphere-ionosphere coupling fluxes are determined for L ≈ 2.3 at June solstice and equinox during magnetically quiet periods. The general features of the electric field measured at Dunedin agree with those predicted from ionospheric dynamo theory with a (1,−2) tidal component. Some seasonal variations are observed, with the electric field measured during equinox being smaller and predominantly westward during the night. The electric field at June solstice is also westward during the evening and for part of the night, but turns sharply eastward during the pre-dawn and dawn period at the duct entry site. The June electric field appears to follow a diurnal variation whereas the equinox electric field shows a possible 4-hourly periodic variation. Seasonal variations in the neutral wind pattern, altering the configuration of the ionospheric dynamo field, are the probable cause of the seasonal differences in the electric field. The seasonal variation of the coupling fluxes can be explained by the alteration of the E x B drift pattern, caused by the changes in the electric field.     

黄山风景区客流波动的多时间尺度特征分析

基于黄山风景区1987年1月至2010年12月的月度客流数据,运用TRAMO/SEATS方法和小波分析方法分析了黄山风景区客流的季节性变化和多时间尺度特征,结果表明：黄山风景区客流季节性变化显著,不仅存在月度波动性,而且季节性波动呈现年度变化性,客流淡季过长,淡季与平季、旺季落差过大是黄山风景区面临的经营难题;具有6个月、12个月、29个月和60个月不同时间尺度的周期变化,其中12个月的周期振荡最为明显,具有全域性;另外,6个月和12个月的时间特征尺度体现了黄山风景区客流由于季节性因素形成的年内季节性变化和年间的周期性变化,而29个月和60个月的时间特征尺度初步判断为由我国旅游业波动周期、景区外围区域经济周期和危机事件冲击影响共同作用所形成.最后,本文提出了一些对应性策略.     

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).     

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.     

Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data

The daily variations of the meridional wind at ±18° latitude have been obtained for summer and winter between 1977 and 1979 using the in situ measurements from the Atmosphere Explorer-E (AE-E) satellite. The AE-E altitude increased from about 250 to about 450 km during this period, with solar activity increasing simultaneously. Data are presented at three altitudes, around 270, 350 and 440 km. It was possible to average the data to obtain the 24 h variations of the meridional wind simultaneously at northern and southern latitudes and thereby study the seasonal variation of the meridional wind in the altitude range covered. Two features are found showing significant seasonal variation: (a) a late afternoon maximum of the poleward wind occurring only in winter at 1800 LT at all three altitudes; (b) a night-time maximum in the equatorward wind—the summer equatorward wind abating earlier (near 2130 LT) and more rapidly than the winter wind (after 2300 LT). Furthermore, in summer the night-time wind reaches higher amplitudes than in winter. The night-time feature is consistent with the observed seasonal variation of the equatorial midnight temperature maximum, which occurs at or before midnight in summer and after midnight in winter, showing a stronger maximum in summer. The observed night-time abatement and seasonal variations in the night-time winds are in harmony with ground based observations at 18° latitude (Arecibo). The time difference found between summer and winter abatements of the night-time equatorward wind are in large part due to a difference between the phases of the summer and winter diurnal (fundamental) components, and diurnal amplitudes are larger in summer than in winter at all threee altitudes. However, the higher harmonics play an important role, their amplitudes being roughly 50% of the diurnal and in some instances larger. The 24 h variation is mainly diurnal at all altitudes in both summer and winter, except in winter around 2700 km altitude where the semi- and ter-diurnal components are approximately equal to or larger than the diurnal.     

In situ oxygen isotope micro-analysis of faunal material and human teeth using a SHRIMP II: a new tool for palaeo-ecology and archaeology

A Sensitive High Resolution Ion MicroProbe (SHRIMP II) has been used to make high spatial resolution in situ micro-analyses of oxygen isotopes in fish otoliths, and teeth from fossil herbivores and a Neanderthal. Large intra-tooth variations in the oxygen isotopic composition (up to 9‰) were observed in the enamel of herbivores from the Neanderthal fossil site of Payre, consistent with preservation of seasonal cyclicity. The range of isotopic compositions observed in Neanderthal tooth enamel was much smaller (∼3‰), possibly the result of a longer enamel maturation time averaging out variability. An archaeological otolith from a Preceramic site in Northern Peru exhibited marked changes in δ¹⁸O over life, due either to the fish occasionally migrating from the sea to a lower salinity habitat, or to short-lived rises in sea water temperature. A fish otolith from Australia's Willandra Lakes World Heritage Area showed clear seasonal variations, but also a general trend towards isotopically heavier and more saline water, as indicated by higher δ¹⁸O and Sr/Ca values resulting from increased evaporation. The results of these case studies are compared to results of oxygen isotope analysis using more conventional methods and demonstrate the ability of the SHRIMP II to provide precise high spatial resolution in-situ oxygen isotope analyses of a variety of biogenic materials. This approach has major advantages over conventional methods. It can provide rapid, micro-scale isotopic analyses of sub-permil precision without the need for chemical preparation of the sample.     

Altitude and latitude dependence of the equatorial electrojet

The effects of day-to-day or seasonal variation of altitude and latitude profiles of the Elayer plasma density in the equatorial ionosphere on equatorial electrojet (EEJ) structure are examined numerically using a self-consistent and high resolution dynamo model. It is found that variations in the E-layer peak altitude and amplitude and its gradient below significantly affect EEJ structure. For any realistic shape, the EEJ peak appears at or below the E-layer peak altitude. Distinct double peaks appear in the EEJ structure, such as revealed by rocket measurements, if the E-layer peak is above 105 km or the gradient is large, as when sporadic-E is present. The influence of the latitudinal variation of ionospheric field line integrated conductivities upon the amplitude and altitude of the EEJ peak is demonstrated.     

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.     

A tidal wind model for 85–120 km based on CIRA-86: a feasibility study

This paper examines the feasibility of deriving a climatology of the diurnal variations of the wind in the 85–120 km region from the tidal components of temperature, density, and composition contained in the new COSPAR International Reference Atmosphere, CIRA-1986, Part I: Thermosphere Models [(1988), Adv. Space Res.8, 9]. To derive the wind field, we used the zonal and meridional momentum equations which have been modified from the characteristic scales of the tidal components observed in the 85–120 km region. The CIRA temperature and density model was used to derive the eastward (westerly) and northward (southerly) pressure gradient forces which serve as the forcing functions in the coupled momentum equations. Ground-based wind data from the Mesosphere-Lower Thermosphere (MLT) radar network is used as an independent data set to check the accuracy of the derived tidal wind model. At midlatitudes, the model reproduces some of the general features observed in the radar tidal data, such as the dominant semidiurnal tide with increasing amplitude with height and clockwise (counterclockwise) rotation of the velocity vector observed in the northern (southern) hemisphere. The model overestimates the semidiurnal amplitudes observed by radar by 50–75% during most seasons with the best agreement found during the equinoctial months. The model exhibits little phase variation with height or season, whereas the radar data exhibit a downward phase progression during most seasons (other than summer) characteristic of upward propagating tidal waves, and large seasonal phase variations associated with seasonal changes in vertical wavelengths. The diurnal tidal amplitudes, which are generally 5–20 m s⁻¹ at mid-latitude radar stations and are dominant over the semidiurnal amplitudes at lower latitudes, are less than 5 m s⁻¹ at all latitudes in the model.     

Atmospheric winds calculated from diurnal changes in the mid-latitude ionosphere

Diurnal variations in the electron content (N_t) and peak density (N_m) of the ionosphere are calculated using a full time-varying model which includes the effects of electric fields, interhemispheric fluxes and neutral winds. The calculation is iterated, adjusting the assumed hourly values of neutral wind until a good match is obtained with mean experimental values of N_t and N_m. Using accurate ionospheric data for quiet conditions at 35°S and 43°S, winds are derived for summer, equinox and winter conditions near solar maximum and solar minimum. Solar maximum results are also obtained at 35°N. Changes in the neutral wind are found to be the major cause of seasonal changes in the ionosphere, and of differences between the two hemispheres. Calculated winds show little variation with latitude, but the winds increase by about 30% at solar minimum (in equinox and winter). The HWM90 wind model gives daytime winds which are nearly twice too large near solar maximum. The theoretical VSH model agrees better with observed daytime variations, and both models fit the observed winds reasonably well at night. Results indicate that modelling of the quiet, mid-latitude ionosphere should be adequate for many purposes when improved wind models are available. Model values for the peak height of the ionosphere are also provided; these show that wind calculations using servo theory are unreliable from sunrise to noon and for several hours after sunset.