Some peculiarities of altitudinal distribution of atomic oxygen at low latitudes during magnetic storms

Several storms were examined during the period of the ESRO-4 mission to study the altitudinal behaviour of atomic oxygen and molecular nitrogen at low latitudes. A special method of data reduction was applied to distinguish composition changes due to thermal expansion and those which resulted from some non-thermal process. It was shown that in the majority of cases studied there exists some latitudinal interval where changes of the N₂/O ratio from quiet to storm days are small compared with that observed in the purely thermal case. This latitudinal interval was found to be displaced to the winter hemisphere (not always across the equator) during solstice cases and the corresponding altitudinal interval was sometimes equal to the whole height range covered by the satellite. The effective temperature of atomic oxygen turned out to be greater than that deduced from the altitudinal distribution of N₂. This implies that there exists a non-thermal component of the [O] increase on a storm day at low latitudes and thus [O] is distributed in a non-barometric way. A possible mechanism responsible for this behaviour is downwelling due to the disturbed global thermospheric circulation.     

Two-dimensional high-resolution imaging of the equatorial plasma fountain

The first visual representation of the reverse equatorial plasma fountain during night-time has been made using images obtained by an All Sky Imaging Fabry-Perot Spectrometer observing the OI 630.0 nm airglow emission line from the thermosphere; the intensity of this line emission is directly related to the F-region plasma densities during night-time. From the identifiable features when the enhanced airglow emitting region moves overhead and when it completely leaves the field-of-view, the equatorwards velocity of the EIA has been estimated to be ~150 km/h at the latitude of the measurements (Mt Abu, 20°20′ dip latitude, India). Lower limits for the latitudinal and longitudinal extents of the EIA crest have each been inferred to be 6°.     

Diurnal variations in the D-region during a storm after-effect

Measurements of electron concentration in the D- and lower E-regions of the ionosphere are reported for seven rocket flights from South Uist, Scotland, in April 1973. They took place during a 12-hour period starting 30 hours after the main phase of a severe geomagnetic storm. The principal feature of the results is that the electron concentrations below 85 km varied between 4 and 10 times the concentrations found on normal days. The variability was correlated with changes in radio wave absorption.The observed electron concentrations are compared with results of other workers for storm conditions and in particular with the model of Spjeldvik and Thorne (1975), and are found to be in reasonable agreement.The possibility of changes in positive ion composition occurring during the post-storm period is inferred.     

The effect of the electric field and neutral winds on E-region ion densities and conductivities at low latitudes

A model to calculate electron densities and electrical conductivities in the ionospheric E-region at low latitudes has been developed. Calculations have been performed under photochemical equilibrium and including plasma transport due to the electric field and neutral winds. Results have been compared with observations at Arecibo (18.15°N, 66.20°W), Thumba (8°32′N, 76°51′E) and SHAR (14.0°N, 80.0° E). Good agreement is obtained for Arecibo. For Thumba and SHAR agreement is satisfactory for altitudes above 110 km. Below 100 km, model predictions are too low in comparison with the observed data. The effect of plasma transport on electron densities and Hall and Pedersen conductivities is investigated in detail. A combination of neutral winds and a downward (or westward) electric field can compress the plasma into a thin layer. An upward electric field along with the neutral winds gives rise to a broad, multilayered profile. The ratio of height-integrated Hall to Pedersen conductivities changes from 1.2 to 2 in some cases.     

Travelling atmospheric disturbances as a possible explanation for daytime positive storm effects of moderate duration at middle latitudes

Using N-h profiles, critical frequency data, and the AE index, we find that daytime positive storm effects at middle latitudes are caused by uplifting of ionization and lag the onset of substorm activity by about 2.5 h. Since the increase in electron concentration lags the increase in layer height by about one hour, the perturbation responsible for positive storm effects should propagate with a high velocity (~500 m/s) from high to lower latitudes. Travelling atmospheric disturbances are offered as a possible explanation for short and medium duration events. A model calculation of the ionospheric response to such a perturbation supports our hypothesis.     

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.     

Case study of modelled and measured D-rmregion plasma densities

Two of the many sounding rockets launched as part of the MAC/EPSILON campaign are particularly suited to test the validity or quality of D-region models under night-time conditions. The measurements covered atomic oxygen, neutral temperatures and fluxes of charged particles. With these parameters as inputs, one is in the position to assess their relevance for the plasma parameters ion composition, transition heights of clusters and negative ions, and total plasma density. The two IOMAS payloads are believed to be the only night-time flights with associated measurements of atomic oxygen extending below the ledge at about 80 km. The relevance of atomic oxygen for the ion chemistry of the lower D-region could be experimentally demonstrated.     

A comparison of plasma densities by EISCAT and the Dynasonde from auroral altitudes: evidence of intense structure

Under conditions of moderately-energetic particle precipitation typical of the equatorward side of the auroral oval, plasma densities obtained from routine analysis of EISCAT Common Program data are often a factor 2 to 5 smaller than those suggested by co-located digital ionograms. We consider the reasons for this disagreement, and in particular we reject the implications of diffractive and multiplyrefractive scatter as alternatives to the usual plasma-frequency interpretation of ionogram echoes. We examine the effects of the (5 min and shorter) temporal averaging applied to the EISCAT data and conclude that together with the evidently small size (perhaps as little as 20 km) and high velocity of these structures, this accounts for much, if not all, of the disagreement. We point out the significance of the higher plasma densities in the 100–150 km height range for estimates of Joule and particle heating.     

从《鄘风》、《邶风》、《卫风》探卫国之民风

《鄘风》、《邶风》、《卫风》三风中所反映的社会问题可以说是极其相似的,不仅仅是因为它们的地理位置关系,更是因为他们的相似的经济与政治问题,所以从此三风中,我们可以看出卫国的社会风貌与国人的生活之事。     

Theory of equatorial electrojet plasma waves: new developments and current status

There have been several important theoretical breakthroughs in the last few years that have substantially improved our understanding of the electrojet plasma instabilities. We now understand(1) the linear and nonlinear processes that control the longest wavelengths and probably also affect the electrojet current strength,(2) quite a bit about the two-dimensional turbulent cascade process that generates both the type 2 irregularities seen by radar and waves propagating even in the vertical direction, and(3) the anomalous diffusion process that limits the growth of the directly excited short wavelength type 1 waves and explains many of their properties. We finally really understand why type 1 and type 2 waves are different. In this paper we first briefly summarize the observed characteristics of the electrojet irregularities and then discuss the theory, devoting most of our attention to recent developments.     

Oblique HF radar studies of plasma instabilities in the equatorial electrojet in Africa

While the equatorial ionosphere was regularly investigated in South America by radar technique and a number of very interesting phenomena detected and associated with plasma instabilities, results in the other part of the world along the magnetic equator were relatively few. The preliminary results of new African experiments are presented. The experimental procedures and equipment and the first results at a.frequency of 21.3 MHz are described. The two types of instabilities previously detected at Jicamarca are regularly observed at 21.3 MHz and their characteristics are described in relation with different theories about plasma instabilities.     

Study of equatorial plasma bubble dynamics using GHz scintillation observations in the Indian sector

To study equatorial plasma bubble dynamics, telemetry signals (4 GHz) were recorded simultaneously from two geostationary satellites. INSAT-1B (74°E) and INSAT-1C (94°E) at Sikandarabad satellite Earth station (dip 42.0°) from January to December 1989 and at the Chenglepet satellite Earth station (dip 10.5°) during September–October 1989 along the same geomagnetic meridian. The characteristics and occurrence pattern of the scintillations suggest that these are equatorial plasma bubble induced events. Observations from the two satellites recorded simultaneously at each of these locations were utilized to estimate the east-west plasma bubble irregularity motion. Plasma bubble rise velocities over the magnetic equator were calculated from the systematic onset time differences observed between an equatorial and a low latitude station. The east-west plasma bubble velocity estimated at Sikandarabad, corresponding to 1200 km altitude in the equatorial plane, shows a night time variation pattern with a peak at around 2100 LT. The mean values over Chenglepet, which correspond to 400 km altitude, start decreasing right from 1900 LT and seem to be influenced by the plasma bubble rise velocities. The differences in magnitude observed between the present results and those reported elsewhere by other techniques are interpreted in terms of vertical shears in the plasma zonal flow over the equator. The near alignment of the two observing stations along a common geomagnetic meridian and the simultaneous use of two satellites located twenty degrees apart in longitude provided an excellent data base to study plasma bubble dynamics.     

Distortions in the height structure of the equatorial electrojet during counter electrojet events

This paper presents simultaneous observations made near the magnetic equator during counter electrojet events using a coherent VHF backscattcr radar, magnetometer and digital ionosonde to understand the physical processes that generate the counter electrojet conditions. The VHF backscatter radar gives the height structure of the drift velocity or the ionization irregularities, the equatorial electrojet current variations are obtained from the magnetometer and the digital ionosonde provides the presence of blanketing E-layers at the F-region heights which give rise to the backscatter signals. These observations have provided direct experimental evidence for the theoretically predicted distortions in the height structure of the polarization electric field in the equatorial electrojet due to the local effects of shearing zonal neutral winds.     

Of Winds,Worms and Mana: the Traditional Calendar of the Torres Islands,Vanuatu

This paper explores how the expression of ‘capacity’ (mena, more commonly known as mana in other Oceanic settings) and the evocation of abundance are played out in relation to seasonal change and human‐environmental relations, as observed in the Torres Islands, Vanuatu. It considers the importance for communities of North and Central Vanuatu of the appearance of a ubiquitous sea worm known as the Palolo, in combination with the regular motions of the sun and the moon, the prevaling winds and various other ecological patterns. While human‐environmental relations are often mediated by the idea of mena, this also constitutes a basic social value that informs local senses of place and belonging. The study of temporality and environmental knowledge can help to de‐centre territorial notions of ‘place’ by situating this concept within the broader context in which people experience it. Finally, because these data bear on macro comparisons that span the Western Pacific, this article is aimed in part at putting the interpretation of time and calendrics within the Indo‐Pacific frame of reference that they should begin to take.     

Atomic oxygen determination from a nitric oxide point release in the equatorial lower thermosphere

Atomic oxygen density values in the 80–105 km altitude equatorial region have been obtained by analyzing the chemiluminescence of nitric oxide point releases from three CENTAURE II-C rockets. The light emission produced by the NO—O chemiluminous recombination was sufficiently high to render the artificial clouds observable only by ground-based instruments. The difficulties associated with these kind of experiments have been greatly avoided by a new technique ejecting the NO gas into the backward direction of the flight. It has been found that below 90 km the derived atomic oxygen densities are in relatively good agreement with those reported by other workers. At approximately 105 km the measured value is about two times higher than the n(O) density obtained by averaging a set of data from a great number of other nights but coincides rather well with the measurements of Dickinsonal. (1980).     

The influence of sunspot number and magnetic activity on the diurnal variation of the geomagnetic field at mid to high latitudes

The variations of the diurnal range of the geomagnetic field with sunspot number and with magnetic activity was studied at mid and high latitude stations in the northern hemisphere at different seasons. The effect of increasing sunspot number is small at lower latitudes and increases with geomagnetic latitude, while the effect of increasing magnetic activity is to increase the range at all latitudes, very greatly at the higher geomagnetic latitudes.     

Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set

The magnetic field expressions from the current ribbon and thick current versions of the continuous distribution of current density model and their merits have been presented. For the first time both the latitudinal and vertical parameters of the equatorial electrojet (EEJ) have been derived from the same set of data. The local noon and daytime means of certain key parameters of the EEJ are shown to be in good agreement with those from other sources. Selected local noon means include: peak current density j_o, 10.58 ± 0.34 A/km²; peak current intensity j_o, 224 ± 9 A/km; total eastward current I₊, 74 ± 5 kA ; EEJ current focal distance w, 300 ± 5 km ; half thickness at half of peak current density p, 7.0 ± 0.1 km; peak westward current location x_m, 5.13 ± 0.08° dip latitude; and EEJ latitudinal extent L₁, 12 ± 1° dip latitude. The problem of model calculated landmark distances of EEJ being consistently shorter than observations, encountered by Onwumechiliet al. [J. geomagn. Geoelecl. 41, 443 (1989)] has been solved.     

Kelvin-Helmholtz instability in a compressible plasma with a magnetic field and velocity shear

The development of the Kelvin-Helmholtz instability (KHI) in a compressible plasma containing a magnetic field is studied for a finite thick layer with a velocity shear in a linear two-dimensional MHD approximation. Approximate analytical expressions for the phase speed, growth rate and wavenumber of the fastest growing unstable mode are derived for the velocity profile across the layer with the velocity shear having a sudden onset, termed a sharp elbow. The Dispersion equation is obtained from boundary conditions on the sharp elbow where the logarithmic derivative of the total pressure (or normal component of velocity) plays the main role.The KHI, driven by velocity shear, is considered as a possible generator of plasma oscillations in magnetospheric regions such as at the boundary between the inner plasma sheet and the plasmasphere, the boundary between the plasma sheet and the tail lobes and the boundary between the plasma sheet and the magnetopause. Analytical expressions for the phase speed, growth rate, period and wavenumber of the growing unstable mode obtained with some simplifying assumptions leads to values similar to the results of numerical analyses.     

An observational study of the D-region winter anomaly in lower latitudes during sudden stratopsheric warmings

An observational study of the D-region winter anomaly of HF radio wave absorption in lower latitudes has been made during the period of a sudden stratospheric warming of the 1967/1968 winter. By means of large-scale isopleth analysis of the absorption index, ƒ_min, and of meridional winds near 70 km height along 60°N, it is found that there exists a winter anomaly in lower latitudes which is comparable in order to that in middle latitudes, resulting from a nitric oxide (NO) increase due to southward transport from higher latitudes by well-developed planetary wave winds. From the daily changes of absorption in the equatorial region, it is found that the enhanced absorption reveals an oscillation with a period of about 2 weeks and has its maximum in the region south of 20°N. The period is similar to that of planetary wave amplitudes in the winter stratosphere and mesosphere, suggesting that an effect of planetary waves could contribute to the equatorial anomaly of the absorption in the D-region.