Monitoring Schumann resonances—I. Methodology

The complex demodulation as a spectral technique has been used for the quasi-continuous determination of the actual frequencies of Schumann resonances. Applying this method, the first three modes of the vertical electric component have been measured regularly in the Nagycenk Observatory (47.6°N, 16.7°E) since May 1993.     

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.     

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.     

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.     

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.     

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.     

Experimental estimates of electron density variations at the reflection height of VLF signals

To study the behaviour of the electron concentration at the reflection level of very low frequency (VLF) waves, two years of phase and amplitude records of the 12.9 kHz signals emitted from Omega-Argentina (43.20°S; 294.60°E) and received at Tucumán (26.90°S; 294.70°E) have been used. The experimental results are compared with values derived from the International Reference Ionosphere model (IRI-79). The experimental data show a seasonal variation not predicted by the model. Differences are explained in terms of changes of night-time atomic oxygen concentration, which control the electron density profile at the base of the night-time D-region, not taken into account in the IRI model. Values of atomic oxygen necessary to explain VLF data are comparable with published data.     

A study of equatorial waves in the Indian zone

An equatorial wave campaign was conducted at Trivandrum (8.5°N, 77°E), Minicoy (8.3°N, 73°E) and Port Blair (11.7°N, 92.7°E) during June-July 1988. The campaign provided balloon-measured daily wind profiles at all the three stations for 48 days in the 0–30 km altitude range and rocket-measured daily wind profiles at Trivandrum for 42 days in the 31–60 km altitude range. Using these daily wind data a study was made on different equatorial wave modes present in this region. The study revealed evidence of Kelvin waves with period 12–16 days and vertical wavelength ∼ 10 km in the lower stratosphere, with period 6–9.6 days and vertical wavelength of ∼ 10–15 km in the stratospheric-lower mesospheric region and MRG waves with periods 4–4.4 days and vertical wavelength of 10 km in the upper troposphere and lower stratosphere.     

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.     

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.     

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.     

Long-period motions in the equatorial mesosphere

The seasonal variations in winds measured in the equatorial mesosphere and lower thermosphere are discussed, and oscillations in zonal winds in the 3–10 day period range are examined. The observations were made between January 1990 and June 1991 with a spaced-antenna MF radar located on Christmas Island (2°N, 157°W). The seasonal variations are analyzed in terms of the mean, annual, and semiannual (SAO) harmonic components. The SAO is the dominant component in the zonal winds, with the amplitude and phase characteristics being in good agreement with earlier rocketsonde measurements at Kwajalien (9°N) and Ascension Island (8°S). The annual and semiannual oscillations combine to produce a stronger change in zonal wind strength in the first half-year (January–June) than in the second half-year (July–December). An annual cycle dominates the meridional winds with maximum velocities (5–10m s⁻¹) attained at about 90km. The meridional circulation at the solstices is consistent with a flow from the summer to the winter pole. Power spectral analyses indicate that motions in the 3–10 day period range occur mainly in the zonal winds, behavior which is interpreted as being due to eastward propagating Kelvin waves. Despite the intermittent nature there is an overall semiannual variation in Kelvin-wave activity. Maximum amplitudes are achieved at the mesopause in January/February and August/September which are times when the zonal winds are westward.     

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.     

The spatial coherence of Schumann activity in the polar cap

The spatial coherence of the first two Schumann resonant modes has been studied at two locations in the polar cap separated by 1100 km. Measurements were made at Assistance Bay and Mould Bay, which have geomagnetic latitudes of 83° and 79°, respectively, and satellite time-keeping was employed to accurately synchronise the field stations. The coherence was found to be high, typically 95% for the first Schumann mode, and was unaffected by changes in K_p, a storm sudden commencement or a solar flare event. Polarization rotations were observed between the two stations, which could most likely be attributed to the coastline effect. The results are consistent with a stable propagation of Schumann activity from mid to high latitudes that is relatively unaffected by changes in the polar cap ionosphere.     

First summer results on winds in the upper mesosphere derived from the 843 nm hydroxyl emissions measured from the Bear Lake Observatory,Utah

The Imaging Fabry-Perot Interferometer (IFPI) at the Bear Lake Observatory (BLO), Utah (41.9°N, 111.4°W) is used for studies of the aeronomy of the middle and upper atmosphere. Wind and temperature structure can be determined from observations of the Doppler shift and Doppler broadening of the airglow and auroral emissions from the mesosphere and thermosphere. The mesospheric winds recorded at the end of August, September and early October 1992 are consistent with a semi-diurnal tidal variation. The amplitude of this variation is approximately 30 ms⁻¹ at the end of August and early September and approximately 20 ms⁻¹ at the end of September and early October. However, during June and July, the semi-diurnal tidal variation, if present, is weak, with amplitude < 5 ms⁻¹. No consistent semi-diurnal tidal variation is observed during late October 1992. During the solstice period, antisymmetric tidal components may be preferentially generated in such a way that they can result in destructive interference with the normally dominant symmetric modes, resulting in a decrease of tidal variation. This is consistent with the observed decrease in tides during the June, July and late October periods. Near the equinoxes, however, the excitation of these antisymmetric modes is expected to be weaker, possibly explaining why a pronounced and consistent semi-diurnal tidal variation has been observed during the August, September and early October periods. In contrast, the mesospheric winds derived from the Sheffield Meteor Wind Radar (53.4°N, 1.5°W) reveal a clear semi-diurnal tidal variation throughout the year, with an amplitude that may vary between 15 ms⁻¹ and 50 ms⁻¹, being about 25 ms⁻¹ on average. The IFPI records winds from a region of the atmosphere centred at 87 km, whereas the Sheffield Meteor Wind Radar measures winds centred at 95 km. Therefore, the two regions may experience different tidal modes due to the different latitude, longitude and altitude of the observed regions and/or the different topography of the observing sites. Some proposed reasons for these differences are presented.     

The investigation of long-distance HF propagation on the basis of a chirp sounder

Experimental data on round-the-world HF radio signals near the terminator are given. The critical frequency of the ionospheric waveguide is found to be F_c ∼ 16–17 MHz. At frequencies F < F_c the group delay has a negative dispersion $\text{τ}\text{dot}{}_{\mathrm{<mtext>g</mtext>}}\text{= ∂τ/∂F ⋍ −100}$ μs/MHz and $\text{τ}\text{dot}{}_{\mathrm{<mtext>g</mtext>}}\text{⋍ 80}$ μs/MHz for frequencies f > f_c. Ray-tracing calculations are carried out. It is found that the low frequency branch of round-the-world signals (F < F_c) is formed mainly by waveguide modes and the high frequency branch (F > F_c) by 0 ricochet and hop modes.Experiments on waveguide modes escaping from the ionospheric channel due to field-aligned scattering by artificial ionospheric turbulence are carried out. The conditions for trapping of radio waves in the ionospheric waveguide are investigated. It is shown that if the gradient of the critical frequency F₀F2 is less than minus 2 × 10⁻² MHz/100 km radio wave trapping takes place in the ionospheric waveguide at frequencies exceeding by 1–2 MHz the maximum observed frequency of the hop mode. The frequency time characteristics of the mode and the geophysical conditions for the effective control of radio waves escaping from the waveguide are defined.     

OI 630 nm imaging observations of equatorial plasma depletions at 16° S dip latitude

Equatorial ionospheric irregularities in the F-layer have been the subject of intensive experimental and theoretical investigations during recent years. The class or irregularities which continues to receive much attention is characterized by large scale plasma depletions, generally referred to as ionospheric plumes and bubbles. The OI 630.0 nm F-region night-glow emissions arising from recombination processes can be used to observe the dynamics of transequatorial ionospheric plasma bubbles and smaller scale plasma irregularities. In a collaborative project between the Center for Space Physics of Boston University and Brazil's National Institute for Space Research (INPE), an all-sky imaging system was operated at Cachoeira Paulista (22.7° S, 45.0° W, dip latitude 15.8° S), between March 1987 and October 1991. In addition to the imager, photometer and VHP polarimeter observations were conducted at Cachoeira Paulista, with ionospheric soundings carried out at both C. Paulista and Fortaleza, the latter at 3.9° S, 38.4° W, 3.7° S dip latitude. For this longitude, the observed seasonal variation of the airglow depletions shows a maximum from October through March and a very low occurrence of airglow depletions from April through September. This long series of OI 630.0 nm imaging observations has permitted us to determine that when there are extended plumes, the altitudes affected over the magnetic equator often exceed 1500 km and probably exceed 2500 km at times, the maximum projection that can be seen from Cachoeira Paulista. This holds true even during years of low solar flux.     

山东省泰安市农业空间生产资本化特征及模式研究

农业生产空间是农村基本功能的载体。从资本“三重循环”视角出发,将“空间的生产”理论引入农业研究,提出并构建农业空间生产资本化的作用逻辑及其特征表征,并以山东省泰安市村庄为研究单元,研究其农业空间生产资本化特征、模式表征及空间规律。发现,泰安市农业生产具有资本投入集约化、空间生产集中化、组织分工专业化等特征;农业空间生产资本化模式共有四种,在空间上呈现出以中心城区为圆心,圈层式向外拓展的规律性,农业空间生产资本化度和三重特征都表现出由内向外先增后缓降的格局。在乡村振兴背景下,识别村庄农业发展模式,根据要素条件,分类指导农业种植结构调整,有利于促进农村多功能有机融合,实现乡村农业产业兴旺发展目标。     

HF Doppler observations of medium-scale travelling ionospheric disturbances at mid-latitudes

From 1972 to 1975 F-region medium-scale travelling ionospheric disturbances (MSTIDs) were observed at Leicester, U.K. (52°32′N 1°8′W) by means of the HF Doppler technique. Most of the features of the disturbances previously reported in the literature are confirmed, with the exception of the apparent seasonal variation in the propagation direction. The measured wave azimuth rotates clockwise through 360° in 24 h, supporting theoretical predictions concerning the filtering effect of the neutral wind in the northern hemisphere. The most commonly observed direction of wave propagation, however, is displaced from the antiwind direction and is located at an azimuth of 130–140° relative to the wind. A periodic variation of the direction of wave propagation with respect to the anti-wind direction is evident, which may indicate that lower atmospheric winds can have a greater influence on waves at thermospheric heights than previously supposed.A synoptic survey of the data set reveals little correlation between wave occurrence and auroral processes, and it is unlikely that high-latitude sources are responsible for many of the MSTIDs observed at mid-latitudes.     

Effect of the electric field on the equatorial ionospheric plasma distribution

It is known that on a counter electrojet day the noontime electron density at the equator shows enhanced values with no bite-out. The consequences of the absence of the normal equatorial electrojet on the electron density distribution at the equatorial station Kodaikanal (dip latitude 1.4°N, long. 77.5°E) and at an anomaly crest location Ahmedabad (dip latitude 18°N, long. 73°E) are discussed for a strong electrojet (SEJ) day and a counter electrojet (CEJ) day. The electron density distribution with height for a pair of SEJ and CEJ days at the two equatorial stations Kodaikanal and Huancayo (dip latitude 1°N, long. 75°W) are studied. The F-region peak height, hm and the semi-thickness parameter ym on the SEJ day followed a similar variation pattern. On the CEJ days ym exhibited a substantially low and mostly flattened daytime variation compared to the peaked values on the SEJ day. An attempt is made to interpret these differences in terms of the changes in the vertical drift pattern resulting from the E × B drift of plasma at the equator and the varying recombination rate β, which is also a height dependent and a local time dependent parameter.