A. D. Bobra

Scattering of pulsar radiation and electron density turbulence in the interstellar medium

We report measurements, using the Culgoora circular array at 80 and 160 MHz and the Parkes 64 m telescope at 410 MHz, of the pulse broadening due to interstellar scattering on 33 pulsars. These results are added to published data on 52 other pulsars in order to investigate the interstellar turbulence levels over 85 paths in the Galaxy. We find a significant relation between the turbulence level C~ and dispersion measure, heliocentric distance, galactocentric distance, galactic latitude and galactic longitude. Our results are consistent with a distribution of turbulence that peaks near the galactic centre and extends out to past the solar circle, with the scale height perpendicular to the plane at least equal to that of the pulsars. The magnitude of the fluctuations in electron density responsible for the scattering is not proportional to the average electron density, but increases much more rapidly than the latter as the galactic centre is approached. The apparent dependence of C~ on heliocentric distance is preferentially interpreted as due to the presence of a highly clumped distribution of turbulence along all lines of sight.

Flux densities, spectra and variability of pulsars at metre wavelengths

We present the results of two-frequency flux density measurements of 74 pulsars with the Culgoora circular array. We show that the spectral index of a typical pulsar steepens markedly from 80 to 1400 MHz, but we found no significant relationship between the metre-wave spectral index and the published pulsar parameters

Power spectral analysis of enhanced scintillation of Quasar 3C459 due to Comet Halley

The radio source 2314+038 (3C459) showed enhanced scintillations on three days at a solar elongation of about 90° as the plasma tail of Halleys Comet swept across it on six days during 16-21 December 1985. If we assume that the plasma velocities in the tail were not constant everywhere, but increased linearly from about 50 kms-1 at the tail axis to the normal average solar wind velocity of 400 kms-1 at the edges where the tail merged with the solar wind, a power spectral analysis of the scintillations shows two ranges of the rms electron density variation t:..N and scale size a. In particular, these are a fine scale zone near the axis where a is in the range 9 to 27 km and t:..N in the range 2 to 5 cm -3 and a zone near the edges with a and t:..N in the ranges 100 to 265 km and 0·4 to 0·8 cm-3 respectively. The assumption of a single velocity of 100 kms-1 throughout the tail shows similar fine scales near the tail axis and large scales near the edges. The scale sizes in that case range from about 18 km at the axis to about 70 km at the edges, corresponding to t:..N of 3·3 and 0·85 cm-3 respectively. A comparison with the results obtained by Slee et al. (1987) shows that there is no radial variation of t:..N. The tail-lag is seen to playa crucial role in determining the correct occulting geometry and the path of the source through the tail.

Enhanced radio source scintillation due to Comet Austin (1989c1)

Enhanced scintillations in the direction of the quasar 2204+29 (3C441) were observed on 13 May 1990 when the tail of Comet Austin passed in front of it. Comparison with previous observations at 103, 327 and 408 MHz of Comet Halley and at 408 MHz of Comet Wilson show that proper occultation geometry is essential for observing enhanced scintillations. It has been shown that the solar elongation ? during such observations should be large, typically greater than 60· and in no case less than 30· at 103 MHz. At the time of the occultation the scintillation index (r.m.s./mean source flux) was greater than that expected for this source by a factor of 3. The r.m.s. electron density variation /IN, at a distance of 0·9 A.U. from the sun and 7·3· downstream of the nucleus, was found to be 6 cm-3 as compared with 1 cm-3 for the normal solar wind at 1 A.U. The corresponding scale sizes of the turbulence were found to be much finer than normally found in interplanetary scintillation (IPS) caused by the solar wind.

Radio and X-ray burst from PSR 0950+08

This article describes in detail a burst from PSR 0950+08 on July 29, 1992. This event was observed by two radio telescopes (separated by ~ 200 km) operating at 103 MHz. There exists a very convincing indirect evidence that at the same time the pulsar also emitted large X-ray flux. The X-ray flux during the event compares with that during a solar X-ray flare. During the event the Sun was extraordinarily quiet as the solar X-ray flux ≤ 3 · 10−7 W/m2 only was observed. The cause for the burst is quite unknown and may be complex. However, a possibility of accretion of a comet-like object by pulsar may provide reasonable explanation of the observations. These results open some interesting questions about the pulsar physics.

Simultaneous Observations of Large Enhancement In the Flux of PSR 0950+08 Over a 200 km Baseline at 103 MHz

Interplanetary Scintillation (IPS) is a diffraction phenomenon in which coherent electro-magnetic radiation, from a distant radio source, passes through the solar wind, which is a turbulent refracting medium, and suffers scattering. This results in random temporal variations of the signal intensity (scintillation) at the Earth. IPS observations to monitor the interplanetary medium (IPM), to measure solar wind velocities in the directions of a number of compact extra-galactic radio sources and to estimate the angular diameters of their compact components have been carried out for many years at 103 MHz using the IPS facility (Alurkar et al. 1989) of the Physical Research Laboratory (PRL), Ahmedabad, India. Over a period of approximately 10 years of such observations, it has been seen that PSR 0950+08 has always remained well within the noise level. We observed a sudden enhancement in its flux (Deshpande et al. 1994) on 29 July, 1992. The ionospheric observations which are being carried out on a regular basis at PRL using an ionosonde also recorded a steep increase in the absorption index F min , which is an indicator of the excess ionization produced due to X-rays. During this period, the Sun was exceptionally quiet, thereby raising the question about the origin of the X-ray flux responsible for the observed steep rise in F min during the transit of PSR 0950+08.

Interplanetary scintillation network for 3-dimensional space exploration in India☆

Abstract Recently, PRL commissioned three large radio telescopes operating at 103 MHz in order to record interplanetary scintillations (IPS) of compact radio sources from Thaltej (Ahmedabad), Rajkot and Surat, as a function of time for the study of solar wind velocity in three dimensions around the Sun between 0.3–1.0 AU. IPS are caused due to irregular scattering of radio waves from quasars by the electron density inhomogeneities in the solar wind that continually blow in three dimensions throughout the heliosphere. When one observes IPS from large number of sources each day, one can determine day-to-day changes in the average 3-dimensional structure of the solar wind velocity. The large IPS telescope at Thaltej would enable us to prepare the so called “ g -maps” of the sky that would indicate regions of enhanced interplanetary plasma turbulence and their evolution on day-to-day basis. This paper describes the 3-station IPS telescope network in India.

Interfacing Digital Magnetic Tape Recorder

With availability of computer system at most of the scientific research centres, it has become almost obligatory on the part of the scientists to acquire data directly on digital magnetic tapes. This helps in analysing the data quite accurately, rapidly and more imaginatively than in the past when data was entered into the computer through cards/ punched paper tapes after tiresome hand scaling. Although the digital magnetic tape units are not manufactured in India-they can be imported. Almost all the tape units need customer interfaces to be developed by the user according to the data formats suitable to him. This paper describes a general-purpose interface for PERTEC digital tape recorder. With slight modification, it can be used for most of the tape recorders. The interface generates computer compatible tape. The interface has been built and successfully used for many scientific experiments.

Time Synchronization for a 3-Site Interplanetary Scintillation (IPS) Experiment Using ATA Time Signal Transmission

For investigating the solar plasma and the solar wind velocity in the interplanetary space, the Physical Research Laboratory (PRL), Ahmedabad has undertaken a 3-site interplanetary scintillation (IPS) experiment. In order to determine the solar-wind velocity, simultaneous observations are to be made from three IPS radio telescopes situated at Ahmedabad, Rajkot and Surat. For computation of the solar-wind velocity, the relative time between any two stations has to be known within a few milliseconds. ATA time signals transmitted on 5, 10 and 15 MHz will be used for this purpose. In the first phase, a system incorporating an oscilloscope for manual time synchronization was developed. In the second phase, a semi-automatic electronic system for time synchronization was developed. A fully automatic time synchronization system is being integrated and is expected to be ready soon. This will enable the three clocks at the three sites to run synchronously with the required accuracy and will provide uninterrupted sy...

A 1-Bit Digital Autocorrelator

In this paper, we present a 1-bit method of computing autocorrelation functions. The experimental set-up of a 40-channel 1-bit autocorrelator developed at the Physical Research Laboratory, Ahmedabad is presented. Using noise input, the correlator was tested in conjunction with the 70-MHz IF amplifier, which forms a part of an RF spectral line receiver developed for the purpose of detecting OH emissions at 18-cm wavelength from interstellar hydroxyl molecules. Two sets, one of 7 and the other of 50 minutes of integration time, of test observations were analysed to compute autocorrelation functions and power spectra. It is concluded that the highest accuracy of spectral measurements would be achieved provided the latter are made in the flat region of the response of the band-pass filter.