Comparative studies of scatterers observed by MF radars in tbe southern hemisphere mesosphere

Using data from three radar sites (low, mid and high latitude) we study the aspect sensitivity of mesospheric scatterers probed by MF radars operating at or near a frequency of 2 MHz. The aspect sensitivity (θ_s) is calculated by the spatial correlation technique. Calculations of θ_s arc further extended to estimate the lcngth-to-depth ratio of the scatterers which indicate that the scattcrers arc quasi-specular. We find that high and mid latitude sites exhibit clear seasonal behaviour, with maxima occurring in the local summer while the low latitude site shows no significant seasonal behaviour.     

Intense turbulence in the polar mesosphere : rocket and radar measurements

Simultaneous observations of polar mesospheric summer echoes (PMSE) have been made with two different frequency radars during the launch of a sounding rocket designed to measure the fluctuations in the electron density in the same height range. The cross-section for radar backscatter deduced from the rocket probe data under the assumption of isotropic turbulence is in reasonable agreement with the measured signals at both 53.5 MHz with the mobile SOUSY radar and 224 MHz with the EISCAT VHF radar, which correspond to backscatter wavelengths of about 3 and 0.75 m, respectively. Some controversy exists over the relative roles of turbulent scatter vs specular reflections in PMSE. A number of characteristics of the data obtained in this experiment are consistent with nearly isotropic, intense meter-scale turbulence on this particular day. Since equally compelling arguments for the importance of an anisotropic-type mechanism have been presented by other experimenters studying PMSE, we conclude that both isotropic and anisotropic mechanisms must operate. We have found the inner scale for the electron fluctuation spectrum, which corresponds to the diffusive subrange for that fluid, and have compared it to the inner scale for the neutral gas. The latter was found from the Kolmogorov microscale, which in turn depends on the energy dissipation rate in the gas. We found the dissipation rate from the spectral width of the 53.5 MHz backscatter signal and from the rocket electron density fluctuation data. The diffusive subrange was found to occur at a wavelength a factor of about 10 times smaller than the viscous subrange. This corresponds to a Schmidt number of about 100. High Schmidt numbers have been reported in recent measurements of the diffusion coefficient of the electrons in this height range made with the EISCAT incoherent scatter radar. About 15 min after the rocket flight an extremely high radar reflectivity was found with the SOUSY system. We have been able to reproduce this high level theoretically by scaling the rocket data with an increase in the neutral turbulence energy dissipation rate by a factor of 14 as deduced from the SOUSY spectral width, an increase in the electron density which is consistent with riometer data, and a 33% decrease in the electron density gradient scale length which is hypothesized. We also estimate the radar reflectivity at 933 MHz and conclude that signals in excess of thermal scatter levels would have occurred at the peak of the event studied, provided that the electron fluctuation spectrum decreases as k⁻⁷ in the viscous subrange. If the spectrum has an exponential form, however, a turbulent source cannot explain the enhanced 933 MHz echoes reported by EISCAT.     

VHF radar observations of nonlinear interactions in the summer polar mesosphere

VHF radar measurements of velocities and echo power in the summer polar mesosphere have been analysed using maximum entropy, bispectral and cross-spectral methods in order to study wave-wave interactions. The results show nonlinear interactions of second and even third order between the diurnal and semidiurnal tides and planetary waves with periods of 2 and 3 days in the velocity field. Similar analyses of time series of echo power suggest corresponding variations of the temperature field.     

Characteristics of polar mesosphere summer echoes (PMSE) observed with the EISCAT 224 MHz radar and possible explanations of their origin

We describe experiments carried out with the EISCAT VHF radar during the MAC/SINE campaign. These experiments included observations of the polar mesosphere summer echoes (PMSE), which were studied with a high spectral resolution program. The fine structure of the spectra imply that very thin and non-random transient structures of reflectivity occur frequently in the mesopause region. We find no clear relation between the echo power and the coherence time which could support the hypothesis of scatter from turbulence or partial reflection. In addition, the estimates of radar reflectivity let us discard incoherent scatter and pure turbulence scatter as the cause of the PMSE. We also discuss the relation of the PMSE and cluster ions, electric fields, charge accumulation and atmospheric gravity waves.     

Steepening of reflectivity structures detected in high-resolution Doppler spectra of polar mesosphere summer echoes (PMSE) observed with the EISCAT 224 MHz radar

Polar mesosphere summer echoes observed with the EISCAT 224 MHz radar frequently exhibit significant discontinuous offsets or jumps in the Doppler frequency. We can explain these frequency jumps as a result of a lifting of partially reflecting or scattering layers, which are distorted by bumps. These bumps can be caused by steepened refractivity variations, i.e. reflectivity structures. These suggestions are supported by model computations. We also notice that a relation exists between these structure shapes and gravity waves, which are steepened, but which do not necessarily break into enhanced turbulent velocity fluctuations.     

The winter anomaly as seen in the propagation of low frequency 40 kHz radio waves

An analysis of propagation data for LF 40 kHz radio waves shows that the field strength of the sky wave is enhanced during day-time on winter anomaly days (WAD), in striking contrast to the severe attenuation of HF radio waves. This peculiar enhancement of the field strength may be ascribed to an increase in the reflection coefficient. The analysis also demonstrates that the reflection height is lower on WAD, which seems to be associated with enhancements of ionization in the D-region. Moreover, it was found that WAD are characterized by an earlier occurrence in the morning and a delayed occurrence in the evening of pronounced interference maxima and minima, respectively.     

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.     

High-latitude tidal behavior in the mesosphere and lower thermosphere

The high-latitude structure of the mean winds and tides is described in this paper using climatologies prepared from radar data during the Atmospheric Tides Middle Atmosphere Program. The monthly evolution of the amplitude and phase of the tides is discussed. Comparison between the southern and northern hemispheres indicate that the diurnal tide is stronger in the southern hemisphere and that the antisymmetric diurnal tidal modes are dominant. The semidiurnal tide is larger than the diurnal tide. The vertical wavelength structure is significantly different between the southern and northern hemisphere. Comparisons with recent tidal models show several discrepancies.     

Coherent radar (SABRE) studies of dynamical processes in the auroral E-region

In the coherent radar technique, bacsccatter is obtained from plasma irregularities even though the radar frequency can greatly exceed the ionospheric plasma frequency maximum. From the velocity spectrum of the received signals an estimate of the flow velocity can be obtained and hence the electric field determined. Information regarding the irregularity scattering cross section is obtained from the amplitude of the backscatter return. Current radar studies of a range of geophysical phenomena are presented. In addition, attention is drawn to the problems of interpreting the radar observations in terms of the underlying geophysical processes.     

《诸司职掌》与明代会典的纂修

《诸司职掌》是明初的职官法典,与明代《会典》的纂修有着非常密切的联系。明代《会典》正是在其续编的过程中揭开了纂修的序幕,又是以其为蓝本完成了弘治、正德、嘉靖三朝的纂修、校订、刊行和续修工作。尽管在万历朝重修过程中其地位大大下降,而且由于史官的重大失误使其内容在时间标注上出现了一些谬误,但这并不影响其在明代《会典》纂修过程中所起的重要作用。     

The equatorial electrojet and associated currents as seen in Magsat data

Magsat data are re-examined with regard to the presence and character of fields due to the equatorial electrojet and meridional currents at dawn and dusk local times. Dip-latitude organized field variations at dawn are:

• 1.(1) extremely weak,
• 2.(2) extremely variable with longitude,
• 3.(3) inconsistent with the pattern expected from a line or narrow sheet current.

It is shown that the use of Magsat dusk data can ‘contaminate’ a main field model, introducing apparent equatorial electrojet effects into the dawn data.Fields due to the equatorial electrojet and (presumably) associated meridional currents are clearly present in the dusk data. They show a variation with longitude which is apparently associated with the longitudinal variation of the strength, or square of the strength, of the main field in the E-region. Also evident is a variation with time of the year, although data are available for only a six month period. The meridional currents are generally minimum during January and February and maximum either during November and December or March and April, depending upon longitude. The E-region horizontal currents are minimum in November and December and maximum in March and April, except for − 30° to −90° longitude when the maximum occurs in January and February.Assuming that field gradients in local time are considerably smaller than field gradients in dip-latitude, current densities are estimated to be 1–3.6μA/m² for the horizontal current at 110km and about 10–20 × 10⁻⁹ A/m² for the vertical currents at 400km altitude. These results confirm and extend earlier results of Takeda and Maeda.Most models of the electrojet system in the literature disagree severely with these measurements either because their scope is inadequate or because of the wind system they assume. Those models which best describe the data invoke an eastward wind and/or an eastward electric field at dusk local time.     

抗战时期的历史教育与徐文珊的《历史教育论》

抗战时期被称为中国的“大时代”,徐文珊的《历史教育论》则是这个“大时代”的产物。它是一部总结和阐发抗战时期中国历史教育动向的专门著作。它的出现,反映了当时历史教育由“考古”向“考今”的转向,也顺应了抗战的时代要求。它的内容比较丰富,具有鲜明的思想特色,主要包括首次明确地从理论上将历史教育区分为学校教育与社会教育两个不同领域、历史教育以情感教育为先等六个方面。而它的时代价值,则体现在推动了文化抗战工作,促进了历史教育向更高层次的发展。     

Observations of winds in the Antarctic summer mesosphere using the spaced antenna technique

Observations of winds in the 60–100 km height range were made at Mawson (68°S, 63°E) during December 1981 and January 1982 with the MF spaced antenna technique. The prevailing winds are in accord with other recent observations made at high latitudes and show a peak in the zonal wind near 80 km with westward winds of 30 m s ⁻¹. The meridional winds maximize near 90 km with an equatorward flow of 10 m s⁻¹. The diurnal tidal components are in reasonable agreement with recent model predictions, especially in phase. The amplitudes tend to be larger than the model values. The semidiurnal tide is not as stable as the diurnal tide and shows evidence for interference effects between different modes.     

Integrated seismic tomography and ground-penetrating radar (GPR) for the high-resolution study of burial mounds (tumuli)

The study primarily aims at providing adequate imaging resolution of large and prominent targets of archaeological interest, such as pyramids and tumuli, at all depth levels. We implemented an integrated seismic tomography and georadar (STG) technique to perform high-resolution imaging and characterization of tumuli (burial mounds). We tested the proposed technique on a preserved late Bronze Age burial mound in northern Italy, for which STG succeeded in performing an accurate 3-D reconstruction of the structure and stratigraphy as proved by later archaeological excavations. We completed two transmission seismic tomography measurements, at present ground level and at 1.5 m elevation, with a 24-channel seismograph and 15° angular separation between geophones. The ground-penetrating radar (GPR) dataset encompasses 12 250 MHz radial profiles and 12 common mid point gathers for velocity analysis. Shallow layers of the mound are successfully imaged by GPR, whilst the structure of the deep central part of the tomb is reconstructed from seismic traveltime inversion. In particular, GPR images lenses and layers of sediments forming the external part of the tumulus, evidences of a looting attempt, peripheral structures associated with later exploitation of the mound (furnaces) and, in the external sector of the tumulus, the top of the deep layer of silty sediments covering the funeral chamber. Tomographic results reveal seismic velocity anomalies of potential archaeological interest at ground level, which were successively validated by archaeological excavations. The integration of GPR and tomographic datasets is an effective strategy to overcome the imaging and interpretation problems related to the structure of such peculiar funeral monuments. STG can be applied to a virtually unlimited dimensional range and requires a limited data acquisition, processing and inversion effort. The results of the study allowed the identification of the funeral chamber and a detailed imaging of layering and structural details.     

Redfern as the Heart(h): Living (Black) in Inner Sydney

Before the arrival of the ‘white fella’ over 200 years ago, the Gadigal people and others of the Eora Darug occupied the place where the city of Sydney now stands. At the heart of this second tier global city, the inner‐city suburb of Redfern has become a mainstay of urban Aboriginal identity. Yet, this troubled and stigmatised focal point of populist media representations and government policy does not reflect the diversity of urban Aboriginal life in inner Sydney. This paper draws on a range of sources about living in Redfern, from the difficult politics of establishing and retaining an Aboriginal urban space and place in the contemporary gentrifying city – achieved in large part through the establishment of now long‐standing service provision – through to the rise of alternate visions and lives and many more ‘ordinary’ ways of living in the city. This paper seeks to highlight that Aboriginal people variously inhabit, occupy, and sometimes thrive in Australia's first colonial city and the site of invasion. It also provides several of the author's personal experiences of engagement with some of these processes.     

Measurements of noctilucent cloud heights: a bench mark for changes in the mesosphere

Recent studies of noctilucent clouds indicate a secular increase in their frequency of occurrence which may be associated with changes either in the water vapour concentration or in the mean temperature of the mesopause. Noctilucent clouds are expected to be seen very close to the level where the relative humidity is 100% and changes in temperature of the mesosphere or in its water vapour content might be expected to alter the height at which the clouds occur. Comparison of our height measurements with those obtained over the past 100 yr shows that the suggested changes since 1885 in temperature (a decrease of approximately 7°C) and water vapour mixing ratio (an increase of approximately 1.7 ppmv) have not been sufficient to affect the height of the clouds by an observable amount. Future height measurements of noctilucent clouds may provide a practical indicator of changes taking place in the mesosphere which could be associated with global-scale alterations of the lower atmosphere.     

Spread-F-like irregularities observed by the Jicamarca radar during the day-time

Until now the presence of F-region irregularities responsible for spread-F (sp-F) traces in ionograms has been considered as a purely night-time phenomenon extending sporadically to the early morning hours. We herein report that, on two occasions (26 March 1974 and 1 February 1984) similar irregularities were observed between 1400 and 1600 hours local time with the Jicamarca radar. These irregularities caused enhancements in the power of the radar echo of as much as two orders of magnitude, were found over a region of a few hundred kilometers on the topside of the F-region extending from around 600 to 1000 km altitude, and persisted for 1–2 h. The irregularities were aspect sensitive (aligned with the magnetic field) and produced echoes with a fading rate of the order of one to a few seconds. The background zonal electric field, inferred from the vertical drift velocity, was fairly constant in altitude, with values smaller than 0.1 mV m⁻¹. During the duration of the events, zonal components of both signs occurred, with the component passing through zero several times. We have no information on the vertical component of E. These irregularities could not be observed with ground-based ionosondes, since they are on the topside of the F-region. They may be related to fossil bubbles that are responsible for HF ducting observed by satellites.