Inter-relations between current and electron density profiles in the equatorial electrojet and effects of neutral density changes

A simple model of the equatorial electrojet is used to try to reproduce observed current density profiles and it is found that an increase in neutral density is required. The effects of neutral density changes of various kinds are investigated. Changes in the electron density profile due to the j × B force are found to be fairly small and, in the cases studied here, are decreases at all heights.     

Vertical neutral wind in the equatorial F-region deduced from electric field and ion density measurements

DC electric field and ion density measurements near density depletion regions (that is, equatorial plasma bubbles) are used to estimate the vertical neutral wind speed. The measured zonal electric field in a series of density depletions crossed by the San Marco D satellite at 01.47-01.52 UT on 25 October 1988, can be explained if a downward neutral wind of 15–30 m s⁻¹ exists. Simultaneously, the F-region plasma was moving downward at a speed of 30–50 m s⁻¹ These events appear in the local time sector of 23.002̄23.15 in which strong downward neutral winds may occur. Indeed, airglow measurements suggest that downward neutral velocities of 25–50 m s⁻¹ are possible at times near midnight in the equatorial F-region.     

Computation of whistler wave normals using a combined matched filtering and parameter estimation technique

A new method is presented for computing the wave normal directions of deterministic VLF signals that can be approximated by plane waves. The a priori information about the frequency-time behaviour of the signal is exploited by a matched filtering and subsequent parameter estimation technique. The method was developed basically to determine wave normal directions of whistlers from wave field components measured on-board. After an outline of the method, results obtained for simulated whistlers are presented that demonstrate the superiority of the new method over the cross-product and Means methods.     

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.     

Influence of fluctuations in neutral gas density height distribution on F1 -layer dynamics

Model ionograms are analyzed, corresponding to conditions of disturbances to the diffusive-equilibrium gas density distribution. The results obtained show that the F1-layer clearly demonstrates a response to the dynamics of the vertical thermospheric structure. Features of the model virtual height-frequency curves correspond to the character of experimental ionograms recorded in the case of internal gravity waves.     

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.     

Correlation between the seasonal trends of planetary F2 layer ion density and intensity of the ring current

The paper reports a high degree of correlation between the seasonal trends of planetary F2 layer ion density and the intensity of the ring current. No interpretation is offered about the reasons or significance of this intriguing correlation in the present paper, but its possible implications are discussed.     

Incoherent scatter radar measurements of electron density and ion velocity during an isolated substorm

A study of the average pattern of F-region plasma densities and velocities measured by the Chatanika incoherent scatter radar has previously suggested that the main ionospheric F-region trough is formed in the evening sector by the westward transport of plasma under the influence of convective electric fields. This paper examines the role of convective electric fields on the electron density profile and the formation of the F-region density trough for a particular night. Incoherent scatter radar data from Chatanika are presented.On 25 May 1972 an isolated substorm occurred near 0900 UT after a long period of magnetic quiet. The substorm was manifested at Chatanika, in the evening sector, by a small positive bay and a concurrent onset of westward motion of plasma associated with a rapid decrease in the F-layer electron density in the region of the moving plasma. Analysis of plasma densities and velocities during this event indicates that

• 1.(1) temporal changes of plasma motion are associated with changes in the convective electric field pattern in response to substorm activity
• 2.(2) the electric field pattern created a north-south gradient in the F-layer electron density which is interpreted as the formation of the ionospheric trough near its equatorward edge, and
• 3.(3) large scale electron density fluctuations were observed in the evening sector resulting from westward travelling density variations originating in the midnight sector.

The study emphasizes the complexity, and difficulty in interpretation, of single station auroral zone measurements of the F-region ionosphere.     

The effects of differential ion flows on EISCAT observations in the auroral ionosphere

During geomagnetic storms different partial pressure gradients in the auroral ionosphere may result in H⁺, He⁺, O⁺ and molecular ions drifting with different velocities along the Earth's magnetic field line. For relative drift velocities ⪡ 400 m s⁻¹ it is shown that differential ion flows may be identified by two signatures in the autocorrelation function (ACF) measured by EISCAT. For larger relative drifts numerical simulations show that these signatures still exist and may result in an asymmetry in the incoherent scatter spectrum for O⁺ and molecular ions. It is demonstrated that UHF data can be reliably analysed for k²λ_D² ≲ 1, but at high altitudes, where O⁺–H⁺ flows are expected, UHF observations will be restricted by large Debye lengths (k²λ_D² > 1). Examples of ACFs based on polar wind theory are presented and discussed for the VHF system and finally it is shown that large ion temperature ratios (T_i(H⁺) >T_i(O⁺)) can significantly affect the velocity determination.     

Non-homogeneous structure of neutral winds in the lower thermosphere during the MAC/EPSILON campaign

The paper presents the results of an investigation of the height variations of dynamic processes in the 80–110 km height region, carried out in Kazan, U.S.S.R. (56°N, 49°E) by the radiometeor method during the MAC/EPSILON campaign. Experimental results show that the largest values of vertical wind gradients, as well as zonal and meridional temperature gradients can be found at heights of ~ 83 km. At heights of 80 ⩽ h ⩽ 100 km, we can observe energy absorption of IGW and tides which are the major sources of turbulent energy in the above-mentioned height interval. Using the effects of IGW energy absorption, values of the turbulent eddy diffusion coefficient K_l ranging from 1600 to 4400 m²/s were calculated for October 1987. The energy dissipation rate ϵ was estimated to be from 0.1 to 0.4 W/kg.     

Parachute measurements of positive ion density of the middle atmosphere over the dip equator by spherical probe

A parachute-borne gridded spherical probe has been used to measure the total positive ion density. Two launches were made, using Soviet M-100 rockets, on 22 and 29 April 1987, at 1200 UT, from an equatorial station, Thumba (8°N, 76°E) India. Data were obtained for the altitude region 10 to 80 km. A broad maximum around 15 km and a broad minimum around 60 km have been noticed in the ion current profiles obtained in both flights. The theory of the operation of the probe has been given. A detailed discussion of the results obtained has also been included.     

The effects of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes

The influence of neutral winds on the propagation of medium-scale atmospheric gravity waves at mid-latitudes is investigated. A 3-dimensional neutral wind model is developed and used together with an atmospheric model in a gravity-wave ray-tracing analysis. It is demonstrated that the thermospheric wind can act as a filter for waves travelling at unfavorable angles to the mean flow, via the mechanisms of reflection and critical coupling. This wind filtering action rotates clockwise diurnally through 360° in the northern hemisphere. Observational evidence is presented which supports these predictions. Extensive modelling indicates that (a) faster and longer period waves are least affected by the neutral winds and (b) fixed-height (e.g. HF Doppler) observations of medium scale gravity waves is only likely to be possible for waves generated locally (within 500–1000 km). Waves generated at greater distances are probably dissipated before reaching the observation region.     

Reduction of the effects of non-stationarity in studies of amplitude statistics of radio wave backscatter

By studying the statistics of fluctuations in amplitude of radio signals backscattered from the atmosphere and lower ionosphere, it is possible to obtain information about the scatterers. This procedure has been applied previously, but often relatively long data series have been used (e.g. 10–30 min) in order to produce reliable amplitude distributions. Unfortunately, the nature of the scatterers can often change considerably over such a time interval and this can distort the amplitude distributions. An alternative approach, applied in this work, is to use much shorter data sets, derive a parameter representative of each data set (e.g. the Rice parameter) and then examine the statistics of this derived parameter over a longer period. Computer modelling was used to examine the statistics of such a parameter and some surprising results emerged, even for relatively long data series. Several examples of the application of this new method will be presented.     

Estimating the likelihood of fallacious ecological inference: linear ecological regression in the presence of context effects

In situations where the only reliable data source is electoral data at the aggregate level for a geographic unit such as voting precincts, social scientists have sought to use ecological regression techniques to recreate the voting behavior of particular groups without committing the ecological fallacy of the sort warned of by Robinson (1950). Until quite recently, the most common use of ecological regression techniques was for the analysis of historical data, e.g., determining the social groups that supported the Nazis. In the US, however, in a development little known to political geographers, within the past decade, social science expert witnesses have testified in hundreds of cases involving minority voting rights—cases where a central issue has been to ascertain the extent of white/Anglo support for black or Hispanic (or more recently, Asian-American) candidates in elections involving candidates of more than one race/ ethnic background. One issue in several recent voting rights cases has been whether linear regression methods can validly be used to model racial voting patterns, with experts for defendant jurisdictions claiming that the presence of contextual effects may invalidate the assumptions underlying linear models and give rise to an ecological fallacy. For data on voting patterns in small geographic units such as voting precincts, we show that the presence of a plausible type of race-related contextual effect will, in general, lead to a quadratic relationship between minority share of the electorate and the share of the vote received by the minority candidate. However, when either a substantial proportion of the electorate is located in racially homogeneous or near homogeneous precincts (as often occurs given the patterns of residential concentration in US cities), or when the contextual effects are small, we show that the estimates of the key parameters of interest in voting rights litigation, namely the proportions of minority and non-minority voters who voted for the minority candidate(s), will be very well approximated by a bivariate linear model. In such circumstances, fallacies of ecological inference can be avoided with near certainty, despite the fact that the linear model that has been used to fit the data omits potentially important variables and regression diagnostics for it would show heteroskedasticity, while a quadratic regression would provide coefficients that are essentially uninterpretable in terms of the parameter of interest. This result has potentially broader implications for the reliability of ecological inference because it demonstrates that, under plausible assumptions, a simple ecological model can yield accurate results despite theoretical errors in model specification—because those errors prove to be of little or no practical significance.     

Absence/presence and the ontological politics of heritage: the case of Barrack 57

19 July 2009. A barn burns down in a small Dutch town. Afterwards, this invisible and insignificant ‘barn’ became widely known as ‘Barrack 57’. The destruction triggered attention and led to the barn’s association with a Nazi Second World War transit camp and with Anne Frank. Its material destruction made this barn/barrack both present and absent in various networks. We use the case of Barrack 57 to study the interplay between presence/absence and non-existence of objects in these networks, an exercise which connects to and contributes to the development of constructivist perspectives on object formation in heritage studies. Our analysis of presence/absence and non-existence therefore is based on different concepts developed in actor network theory and Niklas Luhmann’s theory of social systems. Of particular importance is Luhmann’s distinction between first- and second-order observation. We argue that heritage objects themselves are the result of different enactments of (non) human properties in various relational configurations. With this view, a new task for critical heritage scholars emerges. Understanding the dynamics of presence/absence and non-existence of heritage objects in different networks deepens insight into the broader issues of the formation of heritage objects and their delineating technologies and the policies of normalisation and naturalisation.     

Role of chemical effects in the formation of electron concentration depletions during HF heating of the ionosphere

A chemical mechanism which reduces the electron concentration in the upper ionosphere during HF heating is presented. It is based on the excitation of nitrogen vibrational levels by fast electrons which have appeared as a result of absorption of a radio wave. The vibrational excitation of nitrogen leads to an increase in ion-molecular exchange, followed by a depletion of the electron concentration because the positive molecular ions are very effective in electron -ion recombination. Two different models arc discussed. In the equilibrium model, the vibrational temperature has been established in the region disturbed by the radio wave. In the nonequilibrium model, the fast electrons are moving inside a thin duct where the time of vibrational-vibrational relaxation is greater than the time required by the excited molecules to leave the channel due to diffusion, so that the vibrational temperature cannot be established.