The effect of the mid-latitude ionospheric trough on whistler mode ducting during magnetic storms

Whistler-mode signals observed at Faraday, Antarctica (65° S, 64° W, Λ=50.8°) show anomalous changes in group delay and Doppler shift with time during the main phase of intense geomagnetic activity. These changes are interpreted as the effect of refracting signals into and out of ducts near L=2.5 by electron concentration gradients associated with edges of the mid-latitude ionospheric trough. The refraction region is observed to propagate equatorwards at velocities in the range 20–85 ms⁻¹ during periods of high geomagnetic activity (K_p ≥ 5), which is in good agreement with typical trough velocities. Model estimates of the time that the trough edges come into view from Faraday show a good correlation with the observed start times of the anomalous features. Whistler-mode signals observed at Dunedin, New Zealand (46° S, 171° E, Λ=52.5°) that have propagated at an average L-shell of 2.2 (Λ=47.6°) do not show such trough-related changes in group delay. These observations are consistent with a lower occurrence of the trough at lower invariant latitudes.     

A study of ionospheric electron density deviations during two great storms

A large foF2 data base has been collected for the two great storms occurring in March and October 1989. Plots of foF2 deviations versus time and latitude show the large depressions cannot be caused by a composition change alone; the storm induced dynamo electric field must also play an important role, especially for low latitudes. The observed ionospheric response is hemispherically asymmetric, with the autumnal hemisphere suffering a longer lasting, more latitudinally extended, and deeper depression.     

Observations and modelling of ionospheric and thermospheric disturbances during major geomagnetic storms: A review

The thermosphere is primarily energised by the combination of three sources of energy and momentum. Solar UV and EUV energy is absorbed globally on the dayside within the middle and upper thermosphere. There is a persistent, but highly variable, inflow of energy and momentum from the magnetosphere. These magnetospheric inputs are usually confined to high latitudes, except at times of very large geomagnetic disturbances. Tides and gravity waves upwell from their sources in the troposphere and stratosphere to deposit energy and momentum at levels from the middle mesosphere to the upper thermosphere. Solar EUV radiation between 120 ran and 250 nm photo-dissociates the molecules which dominate the composition of the lower thermosphere, in particular producing atomic oxygen which dominates the composition of the upper thermosphere. The combination of solar EUV radiation at wavelengths shorter than 120 nm, plus energetic (mainly) charged particles from the magnetosphere, also ionise the neutral constituents of the thermosphere, creating the ionosphere. Particularly at high latitudes, within the geomagnetic polar caps and auroral ovals, the energetic, dynamical and chemical coupling and interactions between the thermosphere and ionosphere dominate the structural and dynamical response of both the thermosphere and ionosphere to solar and geomagnetic inputs of energy and momentum.Comparisons between predictions using global thermosphere-ionosphere coupled models and comparable observational sets have shown encouraging agreement during periods of relatively quiet geomagnetic activity. This indicates that the major energetic, ionisation, chemical and dynamical processes and interactions can be described in models with reasonable accuracy. During periods of high geomagnetic activity, and particularly during major geomagnetic storms, large rapid disturbances of the thermosphere occur with extremely rapid variations. These disturbances are observed as large increases of temperature, density, major changes of neutral composition, and with the development of high speed wind flows and large amplitude waves which may propagate to affect the entire globe. Since the ionosphere is formed from thermospheric constituents and affected by thermospherc dynamics, the gross disturbances of the ionosphere during highly disturbed periods are related to contemporary changes of density, composition and flows of the thermosphere, as well as changes of ionisation sources and electric fields. Observations which describe the nature and scale of disturbances of the thermosphere during geomagnetic storms will be used, in combination with appropriate global numerical simulations, to aid interpretation of storm-time ionospheric phenomena. The role of energetic, dynamical and chemical coupling between the thermosphere and ionosphere is emphasised.     

Dependence of ionospheric response on the local time of sudden commencement and the intensity of geomagnetic storms

A study has been designed specifically to investigate the dependence of the ionospheric response on the time of occurrence of sudden commencement (SC) and the intensity of the magnetic storms for a low- and a mid-latitude station by considering total electron content and peak electron density data for more than 60 SC-type geomagnetic storms. The nature of the response, whether positive or negative, is found to be determined largely by the local time of SC, although there is a local time shift of about six hours between low- and mid-latitudes. The time delays associated with the positive responses are low for daytime SCs and high for night-time SCs, whereas the opposite applies for negative responses. The time delays are significantly shorter for mid-latitudes than for low-latitudes and, at both latitudes, are inversely related to the intensity of the storm. There is a positive correlation between the intensity of the ionospheric response and that of the magnetic storm, the correlation being greater at mid-latitudes. The results are discussed in the light of the possible processes which might contribute to the storm-associated ionospheric variations.     

The relation between ionospheric profiles and ELF propagation in the Earth-ionosphere transmission line

Based on an approximate wave solution, it is shown that, for reflexion of ELF waves from a given ionosphere described by a simple profile of ionization density, the phase-integral method may be used above a certain level, and the ionosphere may be abolished below this level. The height of ELF reflexion thus determined is independent of angle incidence, but not of frequency. The level is the one where the ionosphere passes from rapidly varying to slowly varying behavior, judged in relation to the local wavelength.An approximate solution is obtained to the mode problem in the Earth-ionosphere transmission line in terms of four crossing plane waves, one pair having O wave polarization and the other X wave polarization. The velocity of phase propagation is calculated, and also the rate of attenuation due to leakage of energy into the region above the level of reflexion. The attenuation rate due to collisional absorption below the level of reflexion is also calculated using a method similar to that employed for dielectric loss in an engineering transmission line.As the frequency descends through the ELF band, penetration of the D-region occurs in succession for the O and X waves, leading to reflexion from the E-region at the Schumann resonant frequency and penetration of the ionosphere at micropulsation frequencies. Under quiet day-time ionospheric conditions the penetration frequency-band for the D-region is around 20–60 Hz in middle and high latitudes, but around 75–100 Hz at the equator. At a frequency low enough to be reflected primarily from the E-region under quiet ionospheric conditions, an increase in D-region ionization that is just sufficient to transfer primary reflexion from the E-region to the D-region results in an increase in the rate of attenuation. On the other hand, when once reflexion is firmly established at the lower level, further increase of ionization in the D-region causes a reduction in the rate of attenuation. Similar effects are expected to occur at night in association with a sub-E-region ledge of ionization. Small variations in the ionization profile of such a ledge are the likely cause of night-time fluctuations of transmission at 45 and 75 Hz.     

Effects of neutral temperature on meridional winds estimated from ionospheric data

Indirect determination of meridional winds using ground-based ionosonde data from low latitude regions, under the assumption that the thermosphere and the F-region of the ionosphere behave as a closely coupled system, has been critically examined. The significance of neutral temperature and its variations in the above estimates has been demonstrated through individual case studies after duly validating the procedure adopted. Since the measured neutral temperatures have shown large deviations from the existing atmospheric models on many occasions and more so during high solar activity periods, it has been shown that the neutral temperature effects on the F-region heights should be properly accounted for before one attempts to estimate meridional winds. However, it has also been shown that during low solar activity periods, use of atmospheric models may still provide a fairly reasonable average picture. Examples of these effects are presented and discussed.     

Magnetic field of the magnetospheric ring current and its dynamics during magnetic storms

This review examines models existing in the literature which describe the magnetic field produced by the ring current (DR) at the Earth's surface based on the energy balance equation. The parameters of this equation, the injection function F and decay parameter τ are considered to depend on parameters of the interplanetary medium and the DR intensity. The existing models are shown to be able to describe the DR variations with sufficient accuracy (r.m.s. deviation δ between the experimental and modelled values of DR for 170 magnetic storms is 5 < δ < 15 nT and the correlation coefficient between the two is 0.85 <r<1). The models describe that part of the geomagnetic field variation at low latitudes during a magnetic storm that is controlled by the geoeffective characteristics of the interplanetary medium and which thus responds immediately to its variations (the driven part).The values of τ are significantly less during the main phase of a magnetic storm than during the recovery phase. This reflects the difference in the main mechanisms of ion loss from the ring current during the two phases of the storm. These are the interaction of ions with hydromagnetic waves during the main phase of the storm with its intervals of intense plasma injection into the inner magnetosphere and charge exchange with the cold hydrogen geocorona during the recovery phase.     

Types of ionospheric scintillations in southern mid-latitudes during the last sunspot maximum

A 5-yr study (1987–1992) has been undertaken at a southern mid-latitude station, Brisbane (35.6°S invariant latitude) on scintillation occurrences in radio-satellite transmission (at a frequency of 150 MHz) from polar orbit Transit satellites, within a sub-ionospheric invariant latitude range 20–55°S. Over 7000 recorded passes were used to define the spatial and temporal occurrence pattern of different types of scintillation events. Two predominant scintillation types were found: so-called type P (associated with a scintillation patch close to the magnetic zenith) and type S (characteristic of the equatorward edge of auroral scintillation oval). Type S was by far the most frequent during sunspot maximum (1988–1992), with sharp occurrence peaks in the summer-autumn period. Its seasonal occurrence showed a high degree of correlation (correlation coefficient r = 0.8) with the seasonally averaged 10.7 cm solar radio flux. This type occurred mainly at night-time except in austral summer where 40% of scintillations were detected in daytime, coinciding with the well-known summer peak of sporadic-E occurrence. Type P was more predominant during a year (1987) of ascending sunspot activity but decreased to a much lower level during the sunspot maximum.     

Midlatitude measurements of the ionospheric electric field during the ALADDIN programme

Three measurements of ionospheric electric field were made during the 24 h ALADDIN rocket programme at Wallops Island (37°50′N, 75°29′W) on June 29–30, 1974. The first of these used a double probe instrument, flown at 1500 Local Solar Time, and the second and third measurements were made by barium cloud releases at evening and morning twilight. These three electric field vectors have been compared with the predictions of a number of models of electric field due to the dynamo effects of various atmospheric tides, and also of a possible magnetospheric origin. On the assumption that the measurements were made at a location equatorward of the afternoon convergence and poleward of the morning divergence in the electric field patterns related to the Sq current cystem, Stening's model of the diurnal variation of the electric field induced by the (1, −2) tidal model at the time of the Summer solstice correctly predicts the directions of the observed electric field. Forbes and Lindzen's model, incorporating the three major propagating tidal modes as well as the evanescent (1, −2) mode, also bears an acceptable relationship to the ALADDIN electric field directions. The ALADDIN E-field magnitudes are comparable with those obtained by ground-based observations (incoherent scatter) from Millstone Hill and from Saint Santin but are about half of Stening's model values, and three times those of Forbes and Lindzen.While the Millstone Hill E-field directions are compatible with the ALADDIN observations, Saint Santin E-field directions, at the same latitude but 75° difference in longitude, are distinctly different from ALADDIN, implying that longitudinal differences are significant.     

Ion composition changes during F-region density depletions in the presence of electric fields at auroral latitudes

F-region density depletions in the afternoon/evening sector of the auroral zone are studied with the EISCAT UHF radar. Four case studies are presented, in which data from three experiment modes are used. In each case the density depletion can be identified with the main ionospheric trough. For the two cases occurring in sunlit conditions the electron densities recovered significantly after the trough minimum. Tristatic ion velocity measurements show the development of poleward electric fields of typically 50–100 m Vm⁻¹, which maximize exactly in the trough minimum. A special analysis technique for incoherent scatter measurements is introduced, based on the ion energy equation. By assuming that the ion temperature should obey this equation it is possible to fix this parameter in a second analysis and to allow the ion composition to be a free parameter. The results from two experiments with accurate velocity measurements indicate that the proportion of O⁺ near the F-region peak decreased from 100% in the undisturbed ionosphere to only 10% and 30%, respectively, in the density minimum of the trough. The loss of O⁺ is explained by the temperature dependence of recombination with nitrogen molecules. Temperatures derived from radar measurements are very sensitive to the assumed ion composition. For the above case of 10% O⁺ the deduced electron temperature in the trough was transformed from a local minimum of < 2000 K to a local maximum of 4000 K.     

Eiscat observations of the ionospheric D-region during auroral radio absorption events

Electron densities in the D-region have been observed with EISCAT during energetic electron precipitation events. Sample results are presented which demonstrate the value of the technique in studying variations of electron density with fine temporal and spatial resolution. Different types of absorption event can be characterized in terms of the changes in the incoming electron spectrum inferred from profiles of electron density. We contrast the D-region behaviour of night- and day-time events in terms of precipitating spectrum and absorption profile. A softening of the electron spectrum during the course of a morning event is clearly seen.     

Double structure of ionospheric conductivity in the midnight auroral oval during a substorm

Measurements of precipitating particles on board DMSP F7 spacecraft are used to analyze the distribution of ionospheric conductance in the midnight auroral zone during substorms. The distribution is compared with the meridional profile of ionospheric currents calculated from magnetic data from the Kara meridional chain. Two regions of high Hall conductance are found; one of them is the traditional auroral zone, at latitudes 64–68°, and the other is a narrow band at latitudes 70–73°. The position of high conductance zones is in agreement with the location of the intense westward currents. The accelerated particle population is typical of electrons E_e > 5 keV in the high conductance region.     

The effect of rapid changes in ionospheric flow on velocity vectors deduced from radar beam-swinging experiments

The effects on the horizontal ionospheric velocity vectors deduced from radar beam-swinging experiments, which occur when changes in the flow take place on short time scales compared with the experiment cycle time, are analysed in detail. The further complications which arise in the interpretation of beam-swinging data, due to longitudinal gradients in the flow and to field-aligned flows, are also considered. It is concluded that these effects are unlikely to seriously compromise statistical determinations of the response time of the flow, e.g. to changes in the north-south component of the IMF, such as have been recently reported by Etemadi et al. (1988, Planet. Space Sci. 36, 471), using EISCAT ‘Polar’ data.     

Changes of the electron concentration profile during local heating of the ionospheric plasma

Using numerical simulation of a non-stationary problem of thermodiffusion and diffusive spreading of the electron component of the dense cold ionospheric plasma, the processes of formation and relaxation of strong disturbances of the electron temperature and concentration in the E- and F-regions of the middle-latitude ionosphere are examined, taking into account the altitudinal distribution of the electron transport coefficients. The cases of local heating and heating at separated altitudes of the ionospheric plasma by powerful radio waves generated from ground-based HF-facilities are numerically investigated. The numerical simulations of the non-stationary problem are compared with the analytical evaluations carried out for the stationary and quasi-stationary heating models. Results obtained from numerical experiments give good explanations of the experimentally observed deformation of the altitudinal ionospheric plasma density profile and the creation of negative cavities in the upper ionosphere and positive cavities in the lower ionosphere during the process of plasma heating.     

EISCAT observations of ion composition and temperature anisotropy in the high-latitude F-region

The papers by Winseret al. [(1990) J. atmos. terr. Phys.52, 501] and Häggström and Collis [(1990) J. atmos. terr. Phys.52, 519] used plasma flows and ion temperatures, as measured by the EISCAT tristatic incoherent scatter radar, to investigate changes in the ion composition of the ionospheric F-layer at high latitudes, in response to increases in the speed of plasma convection. These studies reported that the ion composition rapidly changed from mainly O⁺ to almost completely (>90%) molecular ions, following rapid increases in ion drift speed by >1 km s⁻¹. These changes appeared inconsisent with theoretical considerations of the ion chemistry, which could not account for the large fractions of molecular ions inferred from the obsevations. In this paper, we discuss two causes of this discrepancy. First, we reevaluate the theoretical calculations for chemical equilibrium and show that, if we correct the derived temperatures for the effect of the molecular ions, and if we employ more realistic dependences of the reaction rates on the ion temperature, the composition changes derived for the faster convection speeds can be explained. For the Winser et al. observations with the radar beam at an aspect angle of ϕ = 54.7° to the geomagnetic field, we now compute a change to 89% molecular ions in < 2 min, in response to the 3 km s⁻¹ drift. This is broadly consistent with the observations. But for the two cases considered by Häggström and Collis, looking along the field line (ϕ = 0°), we compute the proportion of molecular ions to be only 4 and 16% for the observed plasma drifts of 1.2 and 1.6 km s⁻¹, respectively. These computed proportions are much smaller than those derived experimentally (70 and 90%). We attribute the differences to the effects of non-Maxwellian, anisotropic ion velocity distribution functions. We also discuss the effect of ion composition changes on the various radar observations that report anisotropies of ion temperature.     

Travelling ionospheric disturbances and the effectiveness of powerful HF transmitters in ionospheric modification and radio location of the Moon

Using ray tracing we investigate, on a qualitative level and in the linear approximation, the effects of medium-scale travelling ionospheric disturbances (MS TIDs) arising when powerful HF radio transmitters are operated in conjunction with antenna arrays designed for ionospheric modification (heating) and for radio location of the Moon. It is shown that the HF radio wave focusing effect, arising during the movement of the MS TIDs, can give rise to a strong inhomogeneous and nonstationary modulation of the space-time distribution of the field intensity of a powerful radio transmitter both at heights near the reflection region (in heating experiments) and at the exit from the ionosphere (in radio location of the Moon). The excess of intensity over an unperturbed value for typical parameters of MS TIDs in experiments on ionospheric modification can reach values of hundreds of percent: a ‘spot’ of increased intensity of the wave field can have the size of about 1–10 km, and can move with a velocity close to the MS TID phase velocity.In the case of lunar radio location, the inhomogeneity and nonstationarity of the wave field intensity distribution at the exit from the ionosphere substantially complicates the evaluation of the corresponding distribution on the Moon's surface and the interpretation of the Moon-reflected radio signal characteristics.     

实验法探讨瓷胎元素组成和烧成温度对热膨胀法测温的影响

高温技术是我国古陶瓷科技发展史上最重要的技术成就之一,而古陶瓷测温是古陶瓷研究领域中的核心研究内容。热膨胀法具有比较直接和精确判定古陶瓷烧成温度的特点,然尚存在部分标本测量误差较大等问题。鉴于此,本研究充分考虑我国南北方古瓷胎组成配方和结构性能特点,运用实验考古学方法模拟制备具有我国古陶瓷典型瓷胎配方和不同烧结状态的标准参考样,采用热膨胀仪科学化表征瓷胎重烧热膨胀曲线上转折点与其化学组成、烧成温度间的耦合关系,并深入剖析其影响规律及内在机理,同时确立瓷胎化学组成中Al₂O₃含量和自身烧结状态是影响热膨胀法判定古陶瓷烧成温度准确性和适用性的关键技术参数。本研究有助于科学化认知现有热膨胀测温方法,补充和完善使用热膨胀仪判定古陶瓷烧成温度的方法和理论体系。     

Relative changes in consistency of winter surface air temperature during ENSO events across western Canada

Previous studies established a connection between El Niño–Southern Oscillation (ENSO) and winter surface air temperature (SAT) in western Canada. This paper compares the year-to-year variation of winter SAT across the region under ENSO and ENSO-free (neutral) conditions. The comparison is based on the ratio of the difference between the standard deviation (σ) of mean monthly ENSO SAT signals and σ of mean monthly ENSO-free signal to σ of the mean monthly ENSO-free SAT. The signal is defined as the difference between mean monthly temperature during ENSO and the mean monthly ENSO-free SAT. During El Niño December and February, SAT variability is lower by 13 and 18 percent, respectively, than in an equivalent ENSO-free period. In January, variability under El Niño is 24 percent higher than its ENSO-free counterpart. During La Niña, decrease in variability is observed during all three months by 3, 21 and 24 percent from December through February, respectively. The lower variability experienced in five of the six ENSO months underscores greater winter SAT consistency during ENSO. Variability difference is lowest along the Pacific Coast and highest in the Prairies. Except in January under El Niño, those areas, which normally experience SAT variability higher than the regional average (mostly the Prairies), achieve greater temperature stability (reduced variability) during ENSO. January variability is higher in the Prairies during El Niño. In general, in western Canada, SAT regimes look more similar under ENSO than they do under ENSO-free conditions. ENSO tends to stabilise winter temperatures more effectively in the Prairies than it does along the coast.