1,933,925 research outputs found
Radio sources in the Chandra Galactic Bulge Survey
No full textWe discuss radio sources in the Chandra Galactic Bulge Survey region. By cross-matching the X-ray sources in this field with the NRAO VLA Sky Survey archival data, we find 12 candidate matches. We present a classification scheme for radio/X-ray matches in surveys taken in or near the Galactic plane, taking into account other multiwavelength data. We show that none of the matches found here is likely to be due to coronal activity from normal stars because the radio to X-ray flux ratios are systematically too high. We show that one of the source could be a radio pulsar, and that one could be a planetary nebula, but that the bulk of the sources are likely to be background active galactic nuclei (AGN), with many confirmed through a variety of approaches. Several of the AGN are bright enough in the near-infrared (and presumably in the optical) to use as probes of the interstellar medium in the inner Galaxy
HF pump-induced electron heating and artificial airglow at high latitudes: Aspect angle dependence
No full textLarge electron temperature increases (~3000 K) in the ionospheric F-region, produced by powerful HF wave injection, were measured using the EISCAT incoherent scatter radar near Tromsø, Norway, on 7 October 1999. Magnetic fieldaligned measurements showed HF-initiated ion outflows reaching several hundred ms-1 at 580 km. When scanning the radar antenna from field-aligned (77.2deg southward) to vertical, the strongest heating effects were always in the fieldaligned position, irrespective of the HF beam direction which was varied. The imaged 630 nm HF-enhanced airglow also remained localized near field-aligned. Why the strongest heating effects occur for HF rays transmitted along the magnetic field is unclear
Digital radio and its application in the HF (2-30 MHz) band
No full textThe propagation environment at high frequencies (HF, 2-30 MHz) has a significant impact on the performance of radio systems (especially data communications). However, the ability to communicate information over very long ranges using ionospheric propagation paths without any intermediate infrastructure makes the use of HF attractive for many applications. In order to increase the utility of HF communications there is a strong desire to increase HF data rates. Currently data rates of up to -2400 bps can be reliably achieved in standard 3 kHz HF channel allocations.
Whilst further increases in data rate within the confines of these narrowband frequency allocations is likely, the use of larger bandwidths (contiguous or otherwise) appears to offer potential for much greater throughputs. This requires a greater understanding of the characteristics of wideband channels and also requires transmitting and receiving equipment capable of wideband/multi-channel operation.
New waveforms have been proposed for the transmission of higher data rates in extended channel bandwidths (6 kHz). The results of laboratory measurements and analysis of data collected during on-air trials of a number of 16 kbps waveforms are presented. Analysis indicates that operation over surface wave and benign skywave channels is possible, demonstrating the benefit of exploiting greater channel
bandwidths.
Suitable architectures for the implementation of wideband and multi-channel digital HF radios (software radios) have been investigated. The work presented indicates that it is
now possible, for the first time, to construct high performance, direct sampling wideband digital HF receivers. In such a receiver the entire HF band is digitised and
then all subsequent processing is undertaken digitally. Conceptually this would allow an arbitrary number of channels to be simultaneously received using a single RF frontend and digitiser. With careful design performance comparable with that of the high performance conventional super-heterodyne single channel receivers can be obtained.
A prototype wideband multi-channel digital HF transceiver with this architecture has been implemented and its performance shown to agree with that predicted.
A particular challenge in complex systems such as software radios is the deployment of software across a number of heterogeneous processors. A new asynchronous, eventbased,
processing architecture which employs messaging to allow processing tasks to be effectively distributed across a multiple processors and buses is proposed. It has been
implemented on the digital transceiver platform and its effectiveness has been demonstrated.
A new low-power pulse-compression oblique HF ionospheric sounder, known as WHISPER, has been developed. This sounder has been implemented as a software application on the wideband HF digital transceiver. Waveforms suitable for making wideband (-80 kHz) measurements of the channel time varying complex impulse response have been designed. These have been used to make measurements on a 170 km path in the UK during Spring 2001. The results of these measurements have been analysed and confirm the ability of the sounding instrument to measure the channel scattering function and the amplitude and phase within individual modes. A
number of possible directions for further analysis, pertinent to the design of wideband HF modems, have been proposed
On the relationship between the velocity of E-region HF echoes and E B plasma drift
No full textInternational audienceIn this study, velocities of E-region HF echoes observed by the Stokkseyri HF radar are compared with ExB plasma drifts in the F-region measured by the DMSP satellites. Events were selected for which the DMSP track projected to the height of 110km was almost perpendicular to the central beams of the radar, resulting in a direct comparison of the cross-track component of the ExB drift and the line-of-sight HF velocity. We found that the typical ratio of HF velocity to the DMSP drift is ~0.35 in a range of DMSP drifts of 0-1700m/s. It is suggested that E-region HF velocities, observed both along the electrojet and at large flow angles, are significantly affected by scatter from the bottom of the electrojet layer where the irregularity phase velocity is expected to be strongly depressed with respect to the ExB flow
A compact portable HF terminal.
No full textThe paper describes the design philosophy and implementation of a compact, high-performance HF (2-30 MHz) long-range radio system. The system is automatic and can adapt to changes in the propagation and co-channel interference environment. In the design, extensive use is made of advanced real-time DSP algorithms and technology; this allows relatively simple RF equipment to be exploited, which leads to an economic system implementation. The system is readily transportable in a suitcase-sized containe
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Nightside studies of coherent HF radar spectral width behaviour
No full textA previous case study found a relationship between high spectral width measured by the CUTLASS Finland HF radar and elevated electron temperatures observed by the EISCAT and ESR incoherent scatter radars in the post-midnight sector of magnetic local time. This paper expands that work by briefly re-examining that interval and looking in depth at two further case studies. In all three cases a region of high HF spectral width (>200 ms-1) exists poleward of a region of low HF spectral width (<200 ms-1). Each case, however, occurs under quite different geomagnetic conditions. The original case study occurred during an interval with no observed electrojet activity, the second study during a transition from quiet to active conditions with a clear band of ion frictional heating indicating the location of the flow reversal boundary, and the third during an isolated sub-storm. These case studies indicate that the relationship between elevated electron temperature and high HF radar spectral width appears on closed field lines after 03:00 magnetic local time (MLT) on the nightside. It is not clear whether the same relationship would hold on open field lines, since our analysis of this relationship is restricted in latitude. We find two important properties of high spectral width data on the nightside. Firstly the high spectral width values occur on both open and closed field lines, and secondly that the power spectra which exhibit high widths are both single-peak and multiple-peak. In general the regions of high spectral width (>200 ms-1) have more multiple-peak spectra than the regions of low spectral widths whilst still maintaining a majority of single-peak spectra. We also find that the region of ion frictional heating is collocated with many multiple-peak HF spectra. Several mechanisms for the generation of high spectral width have been proposed which would produce multiple-peak spectra, these are discussed in relation to the data presented here. Since the regions of high spectral width are observed both on closed and open field lines the use of the boundary between low and high spectral width as an ionospheric proxy for the open/closed field line boundary is not a simple matter, if indeed it is possible at all
The location and kinematics of the emission line regions in Active Galactic Nuclei
No full textThis thesis contains the results from a study of the optical emission lines of various samples of active galactic nuclei (AGN). We have used three separate techniques to determine the precise kinematics and the relative location of the emission line regions of AGN: a detailed study of a small sample (10) of AGN, a study of the general emission line properties of two larger samples, and the interpretation of the results from observations using numeric photoionisation models. Although we present results on both the broad line and narrow line regions of AGN the main drive behind this work is the study of the poorly defined intermediate line region using emission from highly ionised species (high ionisation emission lines). Our results derived from observations indicate that a significant proportion of the emission from these species is produced within the central void of the putative dusty torus and that emission from more highly ionised species increases at radii closer to the central engine of the AGN. Furthermore, we find strong evidence that the intermediate region traced by these high ionisation emission lines is generally outflowing from the central engine but there is little evidence of any significant acceleration or deceleration of this material between the inner and outer regions traced by the higher and lower ionisation lines, respectively. Our results derived from numerical photoionisation codes support a model in which this outflowing material responsible for the high ionisation line emission is launched from the inner edge of the the dusty torus
Persistent enhancement of the HF pump-induced plasma line measured with a UHF diagnostic radar at HAARP
No full textPlasma lines excited by a powerful, high-frequency (HF) radio wave are studied using data obtained with an ultrahigh frequency (UHF) radar at HAARP (High Frequency Active Auroral Research Program) from 3 to 5 February 2005. Of particular interest is persistent enhancement of the radar backscatter power during HF on at several HF frequencies. The persistent enhancement is induced with the HF frequency slightly lower than foF2 by a few hundred kHz; by contrast the persistent enhancement does not appear when the HF frequency is equal to and higher than foF2 or lower than foF2 by more than 500 kHz. When persistent enhancements of the radar backscatter power appear, two case studies show that the local plasma frequency at the reflection height of the O-mode polarization wave is close to the second or third electron gyroharmonic frequencies, but one case study shows that the local plasma frequency at the reflection height is significantly different from the third electron gyroharmonic frequency
Mercer 5: A probable new globular cluster in the Galactic bulge
No full textWe present a detailed study of a dust-obscured Galactic star cluster Mercer 5 ([MCM2005b] 5) in an extremely crowded field in the Milky Way. Near-infrared (near-IR) photometry from United Kingdom Infrared Digital Sky Surveys (UKIDSS) and the Son of ISAAC on the New Technology Telescope (SofI/NTT), combined with near-IR spectroscopy also from SofI, indicates that it is almost certainly a Galactic globular cluster, located at the edge of the Galactic bulge. The cluster suffers ~9 mag of visual extinction, with strong evidence for an extinction gradient across the cluster. A simulation of the differential reddening in the cluster using empirical data from NGC 6539 (chosen because it had high signal-to-noise ratio data and low field star contamination) as a template mimics the observations extremely well. This simulation and other arguments are used to indicate that the most prominent clump of stars in the colour-magnitude diagrams is a horizontal branch clump. On this basis we conclude that the cluster is at a distance of ~5.5kpc and suffers from visual extinction ranging from ~8.5 to ~12.5 mag. Alternative explanations for its nature, such as a young cluster or an old open cluster, are much less likely, on the grounds of no visible main sequence or stars with IR excesses for the former and location versus lifetime arguments for the latter. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS
Superdarn radar HF propagation and absorption response to the substorm expansion phase
No full textInternational audienceCoherent scatter HF ionospheric radar systems such as SuperDARN offer a powerful experimental technique for the investigation of the magnetospheric substorm. However, a common signature in the early expansion phase is a loss of HF backscatter, which has limited the utility of the radar systems in substorm research. Such data loss has generally been attributed to either HF absorption in the D-region ionosphere, or the consequence of regions of very low ionospheric electric field. Here observations from a well-instrumented isolated substorm which resulted in such a characteristic HF radar data loss are examined to explore the impact of the substorm expansion phase on the HF radar system. The radar response from the SuperDARN Hankasalmi system is interpreted in the context of data from the EIS-CAT incoherent scatter radar systems and the IRIS Riometer at Kilpisjarvi, along with calculations of HF absorption for both IRIS and Hankasalmi and ray-tracing simulations. Such a study offers an explanation of the physical mechanisms behind the HF radar data loss phenomenon. It is found that, at least for the case study presented, the major cause of data loss is not HF absorption, but changes in HF propagation conditions. These result in the loss of many propagation paths for radar backscatter, but also the creation of some new, viable propagation paths. The implications for the use of the characteristics of the data loss as a diagnostic of the substorm process, HF communications channels, and possible radar operational strategies which might mitigate the level of HF radar data loss, are discussed
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