Individual and integrated pulse properties of PSR B0943+10 involved in the mode-changing phenomenon

The characteristics of the “burst” (B) mode and “quiescent” (Q) mode pulse sequences–long known from studies at or below 103 MHz–are identified at 430 MHz for the first time. An 18-minute, Polarimetrie observation begins with a long Bmode sequence, which has a higher average intensity, regular drifting subpulses, and a preponderance of primary polarisationmode radiation. An abrupt transition to a Q-mode sequence is then marked by a) weaker average intensity, but occasional very bright individual subpulses, b) a complete cessation of drifting subpulses, with disorganized subpulses now being emitted over a much wider longitude interval, and c) near parity between the primary and secondary polarisation modes, resulting in pronounced depolarisation, both of individual pulses and the average profile. Careful study, however, of profile changes before and after this mode change reveals slower variations which both anticipate the abrupt transition and respond to it. A slow attenuation of the intensity level of the dominant component is observed throughout the duration of the B-mode sequence, which then accelerates with the onset of the Q-mode sequence. This slow variation appears to represent a “preswitching transition” process; and the combination of effects on slow and abrupt time scales are finally responsible for the formation of the characteristic B and Q-mode average profiles.     

On the polarisation and emission geometry of pulsar 1929+10: Does its emission come from a single pole or two poles?

Pulsar B1929+10 is remarkable on a number of grounds. Its narrow primary components exhibit virtually complete and highly stable linear polarisation, which can be detected over most of its rotation cycle. Various workers have been lured by the unprecedented range over which its linear polarisation angle can be determined, and more attempts have been made to model its emission geometry than perhaps for any other pulsar. Paradoxically, there is compelling evidence to interpret the pulsar’s emission geometryboth in terms of an aligned configuration whereby its observed radiation comes from a single magnetic-polar emission regionand in terms of a nearly orthogonal configuration in which we receive emission from regions near each of its two poles. Pulsar 1929+10 thus provides a fascinating context in which to probe the conflict between these lines of interpretation in an effort to deepen our understanding of pulsar radio emission. Least-squares fits to the polarisation-angle traverse fit poorly near the main pulse and interpulse and have an inflection point far from the centre of the main pulse. This and a number of other circumstances suggest that the position-angle traverse is an unreliable indicator of the geometry in this pulsar, possibly in part because its low level ‘pedestal’ emission makes it impossible to properly calibrate a Polarimeter which correlates orthogonal circular polarisations. Taking the interpulse and main-pulse comp. II widths as indicators of the magnetic latitude, it appears that pulsar 1929+10 has anα value near 90‡ and thus has a two-pole interpulse geometry. This line of interpretation leads to interesting and consistent results regarding the geometry of the conal components. Features corresponding to both an inner and outer cone are identified. In addition, it appears that pulsar 1929+10–and a few other stars–have what we are forced to identify as a ‘furtherin’ cone, with a conal emission radius of about2.3‡/P _1/2 Secondarily, 1929+10’s nearly complete linear polarisation provides an ideal opportunity to study how mechanisms of depolarisation function on a pulse-to-pulse basis. Secondary-polarisation-mode emission appears in significant proportion only in some limited ranges of longitude, and the subsequent depolarization is studied using different mode-separation techniques. The characteristics of the two polarisation modes are particularly interesting, both because the primary mode usually dominates the secondary so completely and because the structure seen in the secondary mode appears to bear importantly on the question of the pulsar’s basic emission geometry. New secondary-mode features are detected in the average profile of this pulsar which appear independent of the main-pulse component structure and which apparently constitute displaced modal emission. Individual pulses during which the secondary-mode dominates the primary one are found to be considerably more intense than the others and largely depolarised. Monte-Carlo modeling of the mode mixing in this region, near the boundary of comps. II and III, indicates that the incoherent interference of two fully and orthogonally polarised modes can adequately account for the observed depolarisation. The amplitude distributions of the two polarisation modes are both quite steady: the primary polarisation mode is well fitted by a χ² distribution with about nine degrees of freedom; whereas the secondary mode requires a more intense distribution which is constant, but sporadic.     

2d-Spectroscopy with the Image Scanner of the Gregory-Coudé-Telescope on Tenerife

In summer 1995 a new system for spectropolarimetry in two spatial dimensions was installed and tested at the Gregory-Coudé-Telescope (GCT) at the Observatorio del Teide (Izaña, Tenerife). The system consists of a scanning device, which is free of instrumental polarization, and a fast CCD camera. During scanning the solar image is moved across the entrance slit of the spectrograph and at a selected number of positions spectra are taken. Fast spectropolarimetry is the main goal of the scientific applications.     

Interstellar polarization: New approximation

A new, physically valid, approximation of the wavelength dependences of the degree of linear interstellar polarization P _λ observed in a wavelength range from 0.2 to 4 μm is suggested. The data obtained from the ground-based observations and the HPOL and WUPPE space missions on P _λ of 104 stars located in a galactic latitude range B from ?20° to +20° are used. This approximation provides better agreement with the observational P _λ data than the models by Serkowski (1973) and Whittet et al. (1992) proposed before.     

Phase identification and attribute analysis of broadband seismograms at far-regional distances

A trained analyst can frequently provide a rapid assessment of a seismic record and provide identification for many seismic phases. For digital data a challenge is to find methods (or combinations of methods) which can provide equivalent levels of phase identification and attribute analysis. Until now, there have been few discussions on phase attribute analysis for broadband records, even though the character of the major phases has been recognised several decades ago. We introduce a combination of four simple methods into the analysis of broadband seismograms so as to provide a means of improving phase recognition and the full use of broadband information for far-regional distances where the seismograms are particularly complex (because of the influence of the upper mantle discontinuities). For arrival detection we can employ the energy ratios of the short term behaviour to the long-term trend, using the vertical component and horizontal components of unrotated seismic records. We also use auto-regressive analysis to endeavour to separate broadband records into three parts: the seismic signal, microseismic noise and white noise. The higher order auto- and cross-correlation coefficient (representing the similarity of waveform) can be used to identify the presence of seismic phases, by avoiding the influence of the relatively low order correlation of microseismic noise. For each broadband 3-component record a set of complex traces are constructed and then a variety of definitions of instantaneous phase and frequency can be exploited to separate the behaviour of signal and noise. The complex traces can also be used for polarisation analysis. The changes in the character of the eigenvectors are particularly helpful for recognising the phases of broadband records in the far-regional range. The individual methods are quite powerful but when used in combination can provide a very effective means of phase characterisation.     

Whose neighbourhood is it? Ethnic diversity in urban spaces in Germany

This article focuses on the reciprocal effects that exist between the physical environment and the social interactions between ethno-cultural groups in disadvantaged neighbourhoods. Biographical narrative interviews and mental map analyses were carried out with German and Turkish residents in two disadvantaged neighbourhoods in Germany to define the resident's own experience of their physical surroundings.The findings of the research verify that the changes in the environment were not only physical and structural changes: they also led to changing identities having a symbolic meaning for the established German residents. As the Turkish residents work to create a sense of identity, the established residents view these changes more as a threat than as an enrichment. They do not so much feel threatened by the cultural differences between the groups, but they sense that their own `old-time' German socio-cultural position could be affected by the establishment of Turkisch residences and institutions in `their' neighbourhood. The empirical research revealed differences in the ethno-cultural perceptions between Duisburgh-Marxloh where there is a pronounced small-scale segregation between the groups, whereas there is a small-scale mix of different ethnic groups even at the block level in the other area investigated in Wuppertal-Osterbaum. Although both areas have a high percentage of ethnic minorities the small-scale mix in Wuppertal-Ostersbaum seems to have prevented a polarisation in the population.     

Time domain electromagnetic‐induced polarisation: extracting more induced polarisation information from grounded source time domain electromagnetic data

Electrical induced polarisation surveys are used to detect chargeable materials in the earth. For interpretation of time domain electrical‐induced polarisation data a common procedure is to first invert the direct current data (electric current on time) to recover conductivity and then invert the induced polarisation data (current off‐time) to recover chargeability. This direct current‐induced polarisation inversion procedure assumes that the off time data are free of secondary electromagnetic induction effects. To comply with this, early time data are often discarded or not recorded. For mid‐time data, an electromagnetic decoupling technique, which removes electromagnetic induction in the observations, needs to be implemented. Usually, responses from a half‐space or a layered earth are subtracted. Recent capability in three‐dimensional time domain electromagnetic forward modelling and inversion allows to revisit these procedures. In a Time domain electromagnetic‐induced polarisation survey, a high sampling rate allows early time channels of the electromagnetic data to be recorded. The recovery of chargeability then follows a three‐step workflow: (i) invert early time channel time domain electromagnetic data to recover the three‐dimensional conductivity; (ii) use that conductivity to compute the time domain electromagnetic response at later time channels and subtract this fundamental response from the observations to extract the induced polarisation responses, and (iii) invert the induced polarisation responses to recover a three‐dimensional chargeability. This workflow effectively removes electromagnetic induction effects in the observations and produces better chargeability and conductivity models compared with conventional approaches. In a synthetic example involving a gradient array, we show that the conductivity structure obtained from the early time channel data, which are usually discarded, is superior to that obtained from the steady state direct current voltages. This adds a further reason to collect these electromagnetic data.     

Complex resistivity of mineral rocks in the context of the generalised effective‐medium theory of the induced polarisation effect

This paper develops the generalised effective‐medium theory of induced polarisation for rock models with elliptical grains and applies this theory to studying the complex resistivity of typical mineral rocks. We first demonstrate that the developed generalised effective‐medium theory of induced polarisation model can correctly represent the induced polarisation phenomenon in multiphase artificial rock samples manufactured using pyrite and magnetite particles. We have also collected representative rock samples from the Cu–Au deposit in Mongolia and subjected them to mineralogical analysis using Quantitative Evaluation of Minerals by Scanning Electron Microscopy technology. The electrical properties of the same samples were determined using laboratory complex resistivity measurements. As a result, we have established relationships between the mineral composition of the rocks, determined using Quantitative Evaluation of Minerals by Scanning Electron Microscopy analysis, and the parameters of the generalised effective‐medium theory of induced polarisation model defined from the laboratory measurements of the electrical properties of the rocks. These relationships open the possibility for remote estimation of types of mineralisation and for mineral discrimination using spectral induced polarization data.     

Electromagnetically induced transparency based six-wave mixing in a Doppler-broadened folded four-level system

We study phase-conjugate six-wave mixing spectroscopy based on electromagnetically-induced-transparency in a Doppler-broadened folded four-level system. It is found that the six-wave mixing spectrum can be either Doppler-free or very broad, depending on whether the interference between the polarisations of atoms with different velocities is constructive or destructive. To obtain the Doppler-free six-wave mixing spectrum in the folded four-level system, the conditions are more stringent in comparison with those in the cascade and N-type four-level systems. This polarisation interference can be controlled in the presence of a strong coupling field.